# ROSETTA/OSIRIS observations of the 67P nucleus during the April 2016 flyby:
high-resolution spectrophotometry

(accepted in A&A, DOI:10.1051/0004-6361/201833807)

C. Feller1 [Corresponding author: clement.feller-at-obspm.fr], S. Fornasier1,
S. Ferrari2, P.H. Hasselmann1 , A. Barucci1, M. Massironi2, J.D.P Deshapriya1
, H. Sierks3, G. Naletto4,5,6, P. L. Lamy7, R. Rodrigo8,9, D. Koschny10,
B.J.R. Davidsson11 , J.-L. Bertaux7, I. Bertini4, D. Bodewits12, G.
Cremonese13, V. Da Deppo6, S. Debei14, M. De Cecco15, M. Fulle16, P. J.
Gutiérrez17, C. Güttler3, W.-H. Ip18,19, H. U. Keller20,21, L. M. Lara17, M.
Lazzarin12, J. J. López-Moreno17, F. Marzari4, X. Shi3, C. Tubiana3 , B.
Gaskell22, F. La Forgia4 , A. Lucchetti13,S. Mottola21, M. Pajola13, F.
Preusker21, and F. Scholten21

1LESIA, Observatoire de Paris, PSL Research University, CNRS, Univ. Paris
Diderot, Sorbonne Paris Cité, Sorbonne Université, 5 Place J. Janssen, Meudon
Cedex 92195, France

2Center of Studies and Activities for Space (CISAS) G. Colombo, University of
Padova, Via Venezia 15, 35131 Padova, Italy

3Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg, 3,
37077, Goettingen, Germany

4University of Padova, Department of Physics and Astronomy “Galileo Galilei”,
Via Marzolo 8, 35131 Padova, Italy

5University of Padova, Center of Studies and Activities for Space (CISAS) “G.
Colombo”, Via Venezia 15, 35131 Padova, Italy

6CNR-IFN UOS Padova LUXOR, Via Trasea, 7, 35131 Padova, Italy

7LATMOS, CNRS/UVSQ/IPSL, 11 boulevard d’Alembert,78280, Guyancourt, France

8Centro de Astrobiologia, CSIC-INTA, 28850 Torrejon de Ardoz, Madrid, Spain

9International Space Science Institute, Hallerstrasse 6, 3012 Bern,
Switzerland

10Science Support Office, European Space Research and Technology Centre/ESA,
Keplerlaan 1, Postbus 299, 2201 AZ Noordwijk ZH, The Netherlands

11Jet Propulsion Laboratory, M/S 183-401, 4800 Oak Grove Drive, Pasadena, CA
91109, USA

12Auburn University, Physics Department, 206 Allison Laboratory, Auburn, AL
36849, USA

13INAF, Astronomical Observatory of Padova, Vicolo dell’Osservatorio 5, 35122
Padova, Italy

14University of Padova, Department of Industrial Engineering, Via Venezia 1,
35131 Padova, Italy

15University of Trento, Faculty of Engineering, Via Mesiano 77, 38121 Trento,
Italy

16INAF Astronomical Observatory of Trieste, Via Tiepolo 11, 34143 Trieste,
Italy

17Instituto de Astrofísica de Andalucía (CSIC), c/ Glorieta de la Astronomia
s/n, 18008 Granada, Spain

18Graduate Institute of Astronomy, National Central University, 300 Chung-Da
Rd, Chung-Li 32054, Taiwan

19Space Science Institute, Macau University of Science and Technology, Avenida
Wai Long, Taipa, Macau

20Institut für Geophysik und extraterrestrische Physik, Technische Universität
Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig, Germany

21Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für
Planetenforschung, Asteroiden und Kometen, Rutherfordstraße 2, 12489, Berlin,
Germany

22Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ,
85719, USA

## Abstract

Context:
From August 2014 to September 2016, the Rosetta spacecraft followed comet
67P/Churyumov-Gerasimenko along its orbit. After the comet passed perihelion,
Rosetta performed a flyby manoeuvre over the Imhotep-Khepry transition in
April 2016. The OSIRIS/Narrow-Angle-Camera (NAC) acquired 112 observations
with mainly three broadband filters (centered at 480, 649, and 743nm) at a
resolution of up to 0.53 m/px and for phase angles between 0.095∘ and 62∘.

Aims:
We have investigated the morphological and spectrophotometrical properties of
this area using the OSIRIS/NAC high-resolution observations.

Methods: We assembled the observations into coregistered color cubes. Using a
3D shape model, we produced the illumination conditions and georeference for
each observation. We mapped the observations of the transition to investigate
its geomorphology. Observations were photometrically corrected using the
Lommel-Seeliger disk law. Spectrophotometric analyses were performed on the
coregistered color cubes. These data were used to estimate the local phase
reddening.

Results:
The Imhotep-Khepry transition hosts numerous and varied types of terrains and
features. We observe an association between a feature’s nature, its
reflectance, and its spectral slopes. Fine material deposits exhibit an
average reflectance and spectral slope, while terrains with diamictons,
consolidated material, degraded outcrops, or features such as somber boulders,
present a lower-than-average reflectance and higher-than-average spectral
slope. Bright surfaces present here a spectral behavior consistent with
terrains enriched in water-ice. We find a phase -reddening slope of
0.064$\pm$0.001%/100nm/∘ at 2.7 au outbound, similarly to the one obtained at
2.3 au inbound during the February 2015 flyby.

Conclusions:
Identified as the source region of multiple jets and a host of water-ice
material, the Imhotep-Khepry transition appeared in April 2016, close to the
frost line, to further harbor several potential locations with exposed water-
ice material among its numerous different morphological terrain units.

Keywords: comets: individual: 67P/Churyumov-Gerasimenko, space vehicles:
ROSETTA, space vehicles: instruments: OSIRIS, methods: data analysis,
techniques: image processing

##  1 Introduction

As part of the HORIZON 2000 perspective, the ROSETTA mission has been the
European Space Agency’s cornerstone for the study of the small bodies of the
solar system (Bar-Nun et al., 1993). For 26 months, the Rosetta spacecraft
followed comet 67P/Churyumov-Gerasimenko (67P) along its orbit from $\sim$4.3
au inbound to perihelion to $\sim$3.8 au outbound. During this period, its
instruments extensively characterized under different observations conditions
the nucleus and the inner coma. After dropping the Philæ probe, which
performed measurements directly on the surface of the nucleus, the spacecraft
instruments notably monitored the nucleus for changes as the comet approached,
went through and moved away from its perihelion (reached on 13 August 2015).
In particular, during these 26 months, the OSIRIS instrument, which is the
Rosetta scientific imaging system (Keller et al., 2007), acquired a vast
number of observations of the comet in the 200-1000 nm wavelength domain. Most
notably, during low-altitude flyby manoeuvres over the nucleus, performed in
February 2015 and April 2016, the OSIRIS instrument imaged the nucleus surface
at different wavelengths with a sub-meter spatial resolution (Feller et al.,
2016 and Hasselmann et al., 2017).

The ROSETTA mission and the OSIRIS instrument have especially shown that the
nucleus surface is exceedingly dark (pv,649nm $\sim$6.7%), its visible
spectrum does not exhibit absorption bands, and its reflectivity notably
increases with the wavelength, that is, it presents a red spectral behavior
(e.g., Sierks et al., 2015; Fornasier et al., 2015). Furthermore, the two
lobes of the nucleus present the same range of morphologic, spectroscopic,
spectrophotometric and photometric properties overall, although some subtle
differences can be observed at the centimeter to hectometer scale in terms of
colors, spectra, and composition (e.g., Capaccioni et al., 2015; Filacchione
et al., 2016b; Fornasier et al., 2015; Pommerol et al., 2015; Poulet et al.,
2016, or Fornasier et al., 2016).
In the appraisal of the pre-perihelion data gathered by the Rosetta
spectrometer VIRTIS Coradini et al. (2007) and Quirico et al. (2016) indicated
that the spectrum and low albedo of the average nucleus surface can be
accounted for as a mixture of opaque minerals with dark refractory
polyaromatic carbonaceous components bearing methyl, alcohol, ammonium, and
ester groups.
While assessing the nature of the nucleus surface and investigations for
relevant surface analogs are still an ongoing subject of research (e.g., Jost
et al., 2017a, b and Rousseau et al., 2017 and references therein), the
examination of the remaining parts of the trove of images acquired by the
OSIRIS instrument is also underway. In this study, we present the results of
the spectrophotometric analysis from some of the most striking features of the
region observed during the April 2016 flyby.
In the next section, we present the observational dataset, the data reduction
procedures, and the methods used in this analysis, before we briefly present
the morphological properties of the flyby area in section 3. In section 4 we
present the results of this spectrophotometric analyses before we discuss our
findings in section 5.

##  2 OSIRIS/NAC observations of the April 2016 flyby

The OSIRIS/NAC instrument. The scientific imaging system on board the Rosetta
spacecraft, OSIRIS, comprised two cameras: the Narrow-Angle Camera (NAC) and
the Wide-Angle Camera (WAC). The NAC had a 2048x2048 px CCD array, each pixel
being a square with an 13.5 $\mu$m edge. The optical system associated with
the NAC gave it a field of view of 2.35∘x2.35∘ and an angular resolution of
18.6 micro-radians per px ($\mu$rad / px). The NAC also comprised a set of 12
broadband filters optimized for the study of the nucleus mineralogy in the
250-1000 nm wavelength domain. A selection of the NAC filters relevant to this
study is listed in Table 1. For a detailed description of the instrument
specifications and hardware, we refer to Keller et al. (2007).

Filter | $\lambda_{c}$ | $\Delta\lambda$ | Filter | $\lambda_{c}$ | $\Delta\lambda$
---|---|---|---|---|---
| [nm] | [nm] |  | [nm] | [nm]
F15 | 269.3 | 53.6 | F28 | 743.7 | 64.1
F16 | 360.0 | 51.1 | F51 | 805.3 | 40.5
F24 | 480.7 | 74.9 | F41 | 882.1 | 65.9
F23 | 535.7 | 62.4 | F61 | 931.9 | 34.9
F22 | 649.2 | 84.5 | F71 | 989.3 | 38.2
F27 | 701.2 | 22.1 |  |  |
Table 1:  OSIRIS/NAC filters: the table lists the filters of the OSIRIS/NAC
instrument with their associated central wavelength and bandwidth.

Context of the observations. At the time of the April 2016 flyby, comet 67P
was outbound from perihelion and its heliocentric distance increased from 2.76
au to 2.78 au between 9 and 10 April. In order to perform this particular
flyby manoeuvre, the Rosetta spacecraft was moved more than 950 km away from
the comet two weeks before the manoeuvre was executed. It was then
progressively approached again, and flown above the transition area between
the Imhotep and Khepry morphological regions at less than 30 km from the
nucleus surface, at the moment of closest approach.
The position of this transition on the nucleus, the area common to all OSIRIS
images acquired during this manoeuver as well as that from the February 2015
flyby, are shown in the left panel of fig. 1. The temporal evolutions of the
median distance between spacecraft and the imaged surfaces as well as the
median phase angle for each observation taken during the flyby are plotted in
the right panel of fig. 1.
The diagram displayed in the right panel of fig. 1 corresponds to the
evolution of the median phase angle (blue squares) within an observation’s
frame, around the time of closest approach.

Figure 1: Context of 9-10 April 2016. Left (inset): Position of the Imhotep
depression (in pink) with respect to the comet large lobe (LL) and small lobe
(SL). (Main) Cylindrical projection centered on the Cheops boulder (gold
square) of the Imhotep region, see main text for more details. Right: Curves
of the median distance to the surface and median phase angle around the moment
of closest approach during the flyby. The uncertainties correspond to the
standard deviation of the values across each observation.

The left panel of Fig. 1 was produced using the “SPG SHAP7 v1.0” shape model
with five millions facets and a horizontal spacing of about 2 meters (Preusker
et al., 2017). In this figure, which we only produced for illustrative
purposes, the brighter a facet, the smaller the angle between its normal and
the direction from the nucleus center of mass to its barycenter. This means
that the relative brightness of a facet in the left panel of fig. 1 is
indicative of the local tilt. Additionally, the longitudes and latitudes are
given here relative to the Cheops boulder (gold square) in order to properly
accommodate the common area to the observations taken during the 14 February
2015 flyby (defined by the green dashed line), which lay just around the
-180∘/+180∘ border in the Cheops reference frame (Preusker et al., 2015). In
this frame, the coordinates of the Cheops boulder are +142.35∘ E, -0.28∘ S.
Additionally, in the left panel of fig. 1, the area common to all NAC
observations taken during the flyby manoeuvre is delimited by the dashed
orange line, while the sub-spacecraft position is marked by the continuous
blue line. In this figure, the projection of the boresight of the OSIRIS NAC
on the surface of the comet is indicated by the continuous red line.
Alongside the boresight of the OSIRIS/NAC, around the moment of closest
approach, the median distance between the spacecraft and the nucleus surface
varied between 28.6 and 29.2 km, as illustrated by the red dots in the right
panel of Fig. 1. Hence, OSIRIS/NAC resolved the observed terrain with a
resolution of about 0.53 m/px.

Observations. We list in Table 6 the OSIRIS/NAC observation sequences used in
this study, as well their corresponding NAC filter image combination. We also
list the median distance of the spacecraft to the surface elements, the
corresponding spatial resolution for the NAC, the median phase angle, and the
phase angle range as well as the position in longitude and latitude of the NAC
boresight.
These observations were acquired around the moment of closest approach to the
nucleus and either on an inbound or outbound trajectory from the moment of
closest approach, which was reached during the first minutes after midnight on
10 April 2016.
For the analyses of this study, we chose to consider not only the observations
acquired just around the moment of closest approach as a sequence of 3 NAC
filter images (those centered at 480, 649, and 743 nm), but also two other
sets of observations acquired shortly before and after the moment of closest
approach using all of the 11 NAC filters (thus spanning the 269.3-989.3 nm
wavelength domain), which also imaged the area in question under notably
different illumination conditions (see the first and last lines in Table 6).
When these two additional sets of observations were included in our analysis,
we found no evidence of morphological and structural differences that might
indicate a major activity event in this area around the moment of closest
approach.
As described in detail in Küppers et al. (2007) and Tubiana et al. (2015),
when they were uploaded to Earth, these OSIRIS observations were reduced using
the standard OSIRIS pipeline (the 1.0.0.34 version of the “OsiCalliope”
software). In short, all these images were uncompressed, calibrated,
radiometrically corrected, and converted from DN/s to radiance factor111The
radiance factor (or RADF, Hapke, 1993) is also denoted hereafter as I/F. We
recall that the radiance factor and the reflectance factor (or REFF, Hapke,
1993), often used in laboratory experiments and for instance in Jost et al.
(2017a), are related as follows: REFF = RADF/cos(i), where i is the solar
incidence angle. , and they were also corrected for geometric distortion.

Similarly to Fornasier et al. (2015); Feller et al. (2016) and Hasselmann et
al. (2017), two further steps were added before the analyses: (1) we computed
and retrieved the illumination conditions of the surface elements intercepted
by each pixel of each image of the dataset, (2) we coregistered each image
from each observation sequence to its corresponding F22 image, so as to force
a correspondence between the pixels of each and every image of an observation
sequence with the intercepted surface elements.
These two steps were achieved using software developed within the OSIRIS team.
The first was assembled based partially on Gaskell et al. (2008) and Jorda et
al. (2010). The latter step was written based on van der Walt et al. (2014) as
a two-part process in which first, image features are detected with the
“Oriented FAST and Rotated BRIEF” (ORB) algorithm (Rublee et al. (2011) or
through an image segmentation and the use of an optical-flow algorithm (as
based on Lucas and Kanade, 1981; Shi and Tomasi, 1994 and Bugeau and Pérez,
2009), and then, when the features were matched, each image was coregistered
to the reference image using a homographic transformation.

In order to produce mappings of the illumination conditions, we used the “SPG
SHAP7 v1.0” shape model of the nucleus, as detailed before. Furthermore, since
at the time of these observations, the comet was only at 2.7 au, the Sun
cannot be considered as a point-like source here. As noted in Shkuratov et al.
(2011), the trigonometric relation expressing the angular diameter of Sun
across the nucleus surface is arcsin(R⊙/r), where R⊙ is the radius of the
Sun’s photosphere and r is the distance of the observed object from the Sun.
Hence, we can only sample phase angles greater than 0.095∘. Therefore the
phase angle ultimately ranges in our dataset from 0.095∘ to 61.7∘. In Table 6,
phase angle ranges that have been truncated thus are denoted by a star.

##  3 Morphology of the flyby area

Throughout the ROSETTA mission, the nucleus of comet 67P has been shown to
comprise a variety and a complexity of aspect and morphological structures, as
discussed at length in Thomas et al. (2015a); El-Maarry et al. (2015); Auger
et al. (2015); Massironi et al. (2015) and El-Maarry et al. (2016). The
depression that is the Imhotep physiographical region is surrounded by the
Apis, Ash, Bes, and Khepry regions.
As the spacecraft flew over this part of the comet (see the left panel of Fig.
1), OSIRIS/NAC acquired observations of the Imhotep depression and of the
nearby regions Ash, Aten, Bes, and Khepry (see the right panel of fig. 2).
Given the path of the spacecraft in these observations, we hereafter refer to
the area of the April 2016 flyby as the Khepry-Imhotep transition.

Figure 2: Left: RGB mapping of the flyby area overlaid with the limits of the
corresponding morphological regions (see El-Maarry et al., 2015). Right:
Geomorphological mapping of the transition area between Khepry and Imhotep.
The peculiarities of the bright spots are discussed in the following sections.

Figure 3: OSIRIS NAC observations of the Khepry-Imhotep transition region.
Left (NAC 2016-06-09T08.47.10.761Z F22) arrows point toward degraded outcrops,
talus, and gravitational deposits at the boundary between the Aten, Imhotep,
and Khepry regions.
Right (NAC 2014-09-02T15.44.22.578Z F22): Between niches, consolidated
materials form an ordered staircase structure, which from the deeper terrace
labeled 1 rises up to the highest terrace labeled 5.

As detailed in El-Maarry et al. (2015) and El-Maarry et al. (2016), these
regions all present different aspects and structure, but also common
characteristics. While Ash, Aten, Bes, and Khepry present common features such
as exposed consolidated material in the form of terraced and layered units and
smooth deposits, Ash has been shown to be an area of airfall material deposit
(Thomas et al., 2015b), and Bes hosts fractures and a pit.
Since the unconsolidated terrains of Imhotep are located in wide flat areas
that correspond to the local gravitational lows, Auger et al. (2015) defined
the region as a real accumulation basin for boulders and fine materials. In
addition, along the side of the region that is being investigated and just
before the perihelion passage, they observed that consolidated materials
displayed bright patches despite their long-lasting exposure to illumination.
This regeneration of bright patches on scarps has been associated with a
progressive retreat driven by sublimation.

In the Khepry-Imhotep transition, as illustrated in the right panel of fig. 3,
most of the surface terrains consist of consolidated materials that form
terraces and niches because the Bes, Khepry, and Ash regions decline either
toward the Imhotep or the Aten depressions (Giacomini et al., 2016) and (Lee
et al., 2016). Imhotep in particular is indeed at a deeper structural level
with respect to all these regions (Penasa et al., 2017), and on this specific
side, is mostly filled by deposits of fine unconsolidated materials comprising
low-density boulder fields or isolated megaclasts (Auger et al., 2015) and
nucleating scarps. Outcropping layered terrains that surround depressions and
niches form an ordered staircase structure of terraces (see the right panel of
fig. 3).
The localized erosion of these terraces provides coarse surfaces (i.e.,
degraded outcrops, see the area indicated to by the yellow arrow in the right
panel of fig. 3) or more evolved clast fields (i.e., diamictons) on their flat
top, or clast deposits at the cliff base. The latter can be ascribed to a
specific source and are distinguished in talus eposits, which consist of well-
sorted clasts and gravitational deposits, which in constra imply well-graded
unconsolidated materials supported by fine materials (see the left panel of
fig. 3, the pink and brown arrows, respectively).
Terraces are partly covered by shallow deposits of fine material (i.e.,
unresolved regolith), which could likely be either the result of in-situ
erosion or of airfall deposits, whereas their overhangs and steep ramps host
most of the observed bright patches (bright spots in the right panel of fig.
2).

##  4 Global and local spectrophotometry

In the following section, we present the results of the spectrophotometric
analysis of the region and of particular surface elements from the OSIRIS/NAC
observations that were taken just around the moment of closest approach and
were acquired using the filters centered at 480, 649, and 743 nm.

###  4.1 Global spectrophotometry

As shown in the right panel of fig. 1 and in Table 6, most of the NAC images
of this dataset were acquired with a pixel scale of $\sim$0.53 m/px. In these
observations, the phase angle densely samples the range between 0.1∘ and 10∘.
Most of the shadows are therefore absent in these panels, and the contents of
the niches, which are usually shadowed, are visible.
False-color images (hereafter RGBs), assembled from the NAC observations that
wer acquired during the flyby area, are presented in the top panels of figs. 4
and 5. The spectral slope mappings (computed in the 535-743 nm range and
normalized at 535 nm) corresponding to the RGB figures are shown in the bottom
panels of figs. 4 and 6. As described in Fornasier et al. (2015), these
spectral slopes were computed using the formula in Eq. 1,

$S[\%/100nm]=\frac{R_{743nm}-R_{535nm}}{R_{535nm}}\cdot\frac{10^{4}}{(743.7[nm]-535.7[nm])}.$
(1)

As in Feller et al. (2016), when absent, the radiance factor mappings at 535
nm were estimated in this study by interpolating between those observed at 480
nm and those at 649 nm. The resulting differences are discussed in section
4.4.
The spectral normalization was performed with the filter centered at 535 nm as
no mineralogical bands are expected there, and in order to emulate the
normalization using the V filter (Bessell, 1990) in the small-body literature.

In the RGB panorama222Assembled from 2016-02-10 15h20 and 15h28 OSIRIS/NAC
images taken with a 0.9 m/px resolution and at a phase angle of $\sim$65∘, and
as such comparable to fig. 12. (fig. 4) of Hasselmann et al. (2017), the
transition area between the Khepry and Imhotep regions appeared to only
slightly vary in terms of colors and spectral slopes, in the considered
dataset and in the top panels in figs. 4 and 5 for instance, given the higher
resolution and the lower range of phase angles, several additional structures
and finer variations are evident.

Figure 4: Top: Panel of RGBs produced from 480nm, 639nm, and 743nm NAC filter
images taken during the flyby. For clarity, the 2016-04-10T00h01 image has
been rotated anticlockwise by 90∘ . The positions of the bright surfaces
discussed in the main text are denoted by red circles, while locations of
groups of somber boulders are marked by green triangles. Bottom: Panel of the
spectral slopes in the 535–743 nm range for the NAC images presented in fig.
8. The black circle in the right mapping encloses the surface elements for
which the phase angle is lower than 0.095∘. The symbols used in each image
correspond to the surface elements investigated in fig. 8. See main text for
details regarding the black and pink crosses and for the central cuesta
structure encircled here in red. The red arrow points to the position of
bright spot 44 in Deshapriya et al. (2018). Figure 5: RGB from NAC filter
images acquired at 23h46, before the moment of closest approach (see fig. 4).
The red circles point to the location of bright surfaces, while the green
triangles indicate groups of somber boulders. This figure encompasses all of
the features investigated in this paper. Figure 6:  Spectral slope in the
535–743 nm range associated with fig. 5. The symbols used here and in the
bottom panel of fig. 4 refer to the same surface elements. See fig. 8 for the
corresponding measures. The red polygon defines the outcropping of spectrally
blue consolidated material that is also marked in fig. 4 and is discussed in
the main text.

The most striking features in this area are the bright blue-white patches
located either in niches, underneath overhangs, or also among, at the tops of,
or behind boulders. These features all present spectral slopes that are
smaller than those of their surroundings. The position of the bright spots we
investigated is marked by red circles in the top panels of figs. 4 and 5. A
cursory investigation of the surfaces that are located close to overhangs and
have a spectral slope smaller than 14 %/100 nm at these low phase angles (see
fig. 6) points to circumferences that vary between 2 and at least 12 pixels
(i.e., between $\sim$ 1 and 6 meters). The widest bright surface measured is
located below the black upward triangle in the bottom right panel of fig. 4-b.
Additionally, within the niche indicated by the pink square in the bottom left
panel of fig. 4, the area of the whole bright surface is $\sim$ 160 m2.

In the bottom right panels of figs. 4 and 6, another striking feature is the
apparent unit of outcropping consolidated material with the inclined top
(cuesta) that is delimited by the red polygon. The overall spectral behavior
of this feature is smaller than its surroundings, which means that it might
indicate an enrichment in water-ice rich material on the whole surface of the
outcrop. The overall spectral slope of this unit is $\sim$ 17 %/100 nm at 4∘
phase angle, while its surroundings are at $\sim$ 18 %/100 nm. This outcrop is
the only such large unit of consolidated and stratified material in the flyby
area that shows this behavior.
We furthermore observe that the surface of this terrain presents small
variations of spectral slopes that closely follow the nature of the terrain:
in contrast to terrains with a smooth aspect or those that lie at the top of
the local cliffs, terrains presenting a rougher aspect or that have a higher
declivity exhibit a slightly stronger spectral slope.

We also note that the spectral behavior of areas that are covered with fine
material deposits (see the right panel of fig. 2) is smaller than that of
their surroundings, but only slightly. For instance, in the bottom right panel
of fig. 4, the material found at the top of the scarp (its location is
indicated by the center of the pink cross) has a reflectance of 6.81$\pm$0.02%
at 649 nm and a spectral slope of 17.9$\pm$0.1 %/100 nm, while the crown of
the scarp directly next to it (its location is found just below the right arm
of the pink cross, 13 pixels, about $\sim$6.9 m away from its center) has a
corresponding reflectance of 6.54$\pm$0.02% and a spectral slope of
18.2$\pm$0.2 %/100 nm. This amounts to a mere 4.0% difference of the
reflectance and only 1.7% disparity in spectral slope.
Similarly, between the red cross that marks the position of a fine material
deposit (also referred to hereafter as UR-01), and the black cross just below
it, which indicates the top of a consolidated material area whose surface is
unresolved and apparently smooth, we find that the fine material deposit has a
reflectance of 6.37$\pm$0.03% at 649nm (i.e., 3.8% lower than the consolidated
material surface) for a spectral slope of 18.2$\pm$0.2 %/100 nm (i.e., 5.5%
lower than the consolidated material).
These two examples illustrate the contrast and differences between these
surface elements of the nucleus and that they can be perceived, even though
they are weak.

Following this correlation between spectral behavior and terrain nature, we
furthermore observe that in terrains that are covered with diamicton and areas
where consolidated material emerges, talus and gravitational accumulation
deposits are particularly distinguishable in the RGBs through their ochre-
orange tinge, and in the spectral slopes mappings, they have values higher 18
%/100 nm.
A close inspection of the mappings further indicates that their variations are
associated with a particular morphological feature. Most notably among the
boulder fields, gravitational accumulation deposits, and outcrops of
consolidated material, we also observe a widespread presence of large somber
boulders and megaclasts among the different types of terrains in this area
(see the right panel in fig. 2). The most obvious of the somber features is
the 60 x 100 m2 sized megaclast visible in the left panel of fig. 4 and in
fig. 5.
Fig. 8 depicts the radiance factor in the orange filter images corresponding
to the RGB presented in figure 4. We recall here that these images were
photometrically corrected using the Lommel-Seeliger disk law (Seeliger, 1885
and Fairbairn, 2005). In the bottom right panels of figs. 4 and 8, the black
circle at the bottom of the 2016-04-10T00h01 mappings delimits surface
elements with phase angles smaller than 0.095∘, as discussed in section 2.

Time (UTC) | 23:34 | 23:46 | 00:01
---|---|---|---
Median phase angle (degrees) | 4.0$\pm$1.3 | 3.0$\pm$1.0 | 1.2$\pm$0.6
Median I/F (at 649nm) | 5.3$\pm$0.2% | 5.8$\pm$0.2% | 6.3$\pm$0.30%
Median S535-743nm (%/100nm) | 18.0$\pm$0.6% | 18.2$\pm$0.6% | 18.3$\pm$0.6%
Brightest I/F value (at 649 nm) | 20$\pm$3% | 12.0$\pm$1.3% | 14.0$\pm$1.7%
ID of measure | BF-01* | BF-08 | BF-06
Symbol associated | red arrow | $\boldsymbol{\triangle}$ | $\boldsymbol{\bigcirc}$
S535-743nm (%/100 nm) | 8$\pm$11 | 4$\pm$1 | 5$\pm$3.7
Darkest I/F value (at 649 nm) | 5.1$\pm$0.1% | 5.5$\pm$0.1% | 5.7$\pm$0.1%
ID of measure | SB-05 | SB-05 | SB-05
Symbol associated | - | $\diamond$ | -
S535-743nm (%/100 nm) | 19.5$\pm$0.3 | 19.7$\pm$0.2 | 19.5$\pm$0.3
Table 2:  Details of the median and extreme radiance factor values in the
measurements: the symbols are those used in Figs. 4, 8, and 8. The BF-01
measure is marked because this particularly bright surface has also been
investigated as the ID-44 bright surface in Deshapriya et al. (2018); it
survived for several months. The positions and the values of other particular
measurements are reported in Table 7.

We assembled in Table 2 a summary of the median and some extreme values of the
reflectance at 649 nm (as well as the corresponding spectral slopes). These
latter values correspond to the extrema found among the surface elements we
investigated, and they are further discussed in the next sub-section.
In fig. 8 a progressive overall decrease in contrast can be perceived as the
spacecraft approached the moment of opposition, which indicates the
observation of the opposition effect (e.g., Seeliger, 1885 and Shkuratov et
al., 2011). Nevertheless, many features whose reflectances deviate by more
than a sigma from the median I/F value are distinguishable from their
surroundings.
This the case, for instance, for bright surfaces, which, as discussed
previously, are found in niches, at the bottom of underhangs, or at the top of
boulders. They are easily identifiable in fig. 8 as the white areas.
Meanwhile, at the other end of reflectances, most of these easily identifiable
surface elements are the somber boulders discussed previously that we indicate
with green triangles in the top panel of figs. 4 and 5.

The values listed in table 2 show that the BF-01 measure is in particular
distinguishable because it is close to four times the median reflectance of
the observed area and has a standard deviation of 3%. This particular highest
value measured for the radiance factor corresponds to one peculiar bright
surface, referred to as BF-01 and as ID-44 in Deshapriya et al. (2018). It is
located in a field of boulders and diamicton of the Khepry region, and its
position is indicated by a red arrow in the upper left corner of figs. 4 and
8. This particular feature covers part of the boulder top. As it is extended
well beyond the integration box, the measurement we report was performed
around the brightest pixel (I/F $\sim$ 24%). We note that the DN/s values of
the corresponding pixels are similar to but do not reach the saturation levels
of the detector. This particular detail was also noted in most of the
measurements performed over the course of several months that were reported in
Deshapriya et al. (2016). This surface element is further discussed in the
section on the 11-filter spectrophotometry.
Nevertheless, this feature is the brightest observed in the region of this
flyby. For reference, in that same image, when we measure the radiance factor
of BF-08 (the large bright patch close to an overhang, noted by a black
triangle), we find a value of 12$\pm$1.3%/100 nm. As discussed below, the same
bright spot was also observed on the next day at 11:50, and still had, at a
phase angle of $\sim$64∘, a radiance factor of 3.6$\pm$0.7%, while its
surroundings appeared far darker, with a radiance factor of only about 1%.

Feature | UR-00 | BF-02 | BF-05 | BF-06 | BF-08 | BF-010 | SB-05
---|---|---|---|---|---|---|---
23h34 |  17.7$\pm$0.2 |  7.0$\pm$1.5 |  5.0$\pm$5.0 |  7.0$\pm$2.9 |  2.0$\pm$1.6 |  |  19.5$\pm$0.3
23h46 |  18.0$\pm$0.5 |  |  6.0$\pm$2.0 |  5.0$\pm$4.6 |  4.0$\pm$1.1 |  |  19.7$\pm$0.2
00h01 |  18.3$\pm$0.2 |  |  9.0$\pm$3.0 |  5.0$\pm$3.7 |  |  6.8$\pm$0.6 |  19.5$\pm$0.3
Table 3:  Selection of spectral slope values in the 535–743 nm range:
selection of values from the measurements marked in the right panel of figs.4,
8, and 8. The spectral slopes of the bright features (BF) are almost one-third
of those of the smooth regolith (UR) and just less than one-fourth of those of
the somber boulders (SB).

In a similar manner but at the opposite end of reflectances, the boulders
exhibiting the red spectral behavior that we discussed above also distinguish
themselves from their surroundings in these reflectances mappings through
their lower-than-average reflectance. They present the same behavior
independently of the phase angle at which they are observed.

###  4.2 Spectrophotometric analysis of local features

Figure 7: Panel of the photometrically corrected radiance factor images taken
with the orange filter. From left to right: Image taken at 23h34, 23h46 and
00h01. The average phase angle is 4∘, 3∘ , and 1∘, respectively. See fig. 4
for the symbol keys. The red square in the central figure points to the area
observed in August 2016 that is shown in fig. 14.

Figure 8: Panel of the result of the three-filter spectrophotometry
measurements. Top: Reflectance (I/F) measurements. The bars indicate the
dispersion of the values measured within the box and their propagation for the
relative reflectance. Bottom: I/F measurements normalized at 535 nm. The key
for the measurements IDs is, again, as follows: smooth, unresolved-looking,
regolith (UR), bright feature (BF), and somber boulder (SB). The letter after
the underscore indicates the 23:34 image (A), the 23:46 image (B), or the
00:01 image (C). Finally, the last three digits identify the feature among its
particular type. See fig. 8 for an illustration of the measurement locations.

As this way, several dozens of surface elements were identified in the images
as particular features of interest, for instance, bright patches, somber
boulders, consolidated material, and unresolved regolith. From this collection
of elements, we selected six features appearing in figs. 4, 8 and 8.
The radiance factor and spectral slope measurements were performed by
integrating the signal in a 3x3 pixels box (i.e., 1.6 x 1.6 m2), unless
otherwise noted. These measurements were made on both common and image-unique
features. All images presented here have three measurements in common: a
stretch of smooth regolith (red cross), a bright spot underneath a overhang
(blue circle), and a somber boulder (green star). These measurements are
identified by a tag: UR for very smooth unresolved-looking regolith, BF for
bright feature, or SB for somber boulder (see fig. 8 and tables 2 and 3).
We show the results of the three-filter spectrophotometry in fig. 8.

Based on these measurements, we pursued the previously discussed notable
distinction in behavior between the selected bright features and the other
regions of interest. While somber boulders and smooth unresolved-looking
regolith exhibit a similar behavior in both radiance factor and relative
reflectance, all the selected bright features have a radiance factor at least
twice that of the somber boulders or the unresolved regolith (see fig. 8 and
table 7). Moreover, the bright surfaces also display smaller and less red
spectral slopes than those of other surfaces and differ more widely from the
slope of the average terrain. For the investigated terrains, for instance,
while the spectral slopes of the somber boulders are between 1% and 19% higher
than the slopes of the smooth regolith, the spectral slopes of the bright
surfaces are between 10% and 87% smaller than those of the smooth-looking
regolith. All corresponding values are assembled in table 3.

In the flybly area of February 2015 (encompassed by the green dashed line in
the left panel of fig.1), some small bright surfaces were observed along the
decline that leads toward the center of the Imhotep depression. However, these
bright surfaces had reflectivities that would never be higher than 50% than
the local average, and their spectral slopes were as high or greater than
those of neighboring smooth terrains. These surfaces were interpreted to be
partially richer in refractive materials and coated by deposits of organics
(Feller et al., 2016).
Given the wider range of variations of the reflectance and the flatter spectra
exhibited by the bright surfaces observed here, we interpret these surfaces to
be of a similar nature to some of the surfaces investigated in Pommerol et al.
(2015); Filacchione et al. (2016b); Barucci et al. (2016) and Oklay et al.
(2016b), which were observed with the OSIRIS and VIRTIS instruments and were
shown to be slightly richer in water-ice content, but only by a few percent.
In this understanding, the more water-ice a surface element of the nucleus
contains, the flatter its spectrum. A comprehensive study of such surfaces and
of tentative modeling of their composition has been presented in Raponi et al.
(2016).
Following the conclusions of these papers, we consider the surfaces to be
appropriate candidates as locations enriched in water-ice material at the time
of these observations. These locations should be further investigated and
compared with possible VIRTIS observations.

Figure 9: Georeferenced RGB of the April 2016 flyby region, assembled using
photometrically corrected NF84, NF82, and NF88 images, and centered around the
Cheops boulder. This mapping has a 0.1∘x0.1∘ resolution in longitude and
latitude. The region shown correspond to areas visible in at least three of
the considered observations. The locations where resurfacing processes were
observed through the evidence of mass-wastings and moving scarps that were
discussed in e.g. Groussin et al. (2015) and Deshapriya et al. (2018) are
indicated. For clarity, only the largest bright spots are indicated, and the
given positions of somber boulders point to their presence in all type of
terrains, with the exception of the central depression of Imhotep.

###  4.3 Georeferenced RGB mapping

As the previously presented RGBs only allowed us to consider part of the flyby
region as it was around the moment of closest approach, we have projected each
photometrically corrected image onto a grid and assembled the results in a
georeferenced RGB of the area (see fig. 9).
To produce this mapping, we have generated the georeference of each image to
the SPG SHAP7 v1.0 model using ray-tracing and the reconstructed trajectory of
the Rosetta spacecraft with respect to landmarks on the nucleus, computed
during the generation of the SPC shape model.
We then projected the images onto a grid with a 0.1∘x 0.1∘ resolution in
longitude and latitude, and averaged the reflectance by the number of times a
grid element appeared in the observations. This resolution in longitude and
latitude corresponds to a resolution of (2.7$\pm$0.3)x(2.7$\pm$0.3) m2.

In this figure, the areas encircled by continuous lines correspond to some of
the morphological units defined in fig. 2, the red circles and green triangles
identify some of the bright surfaces and somber boulders, respectively. Scarps
are noted here by the dotted yellow line, while a well-defined fine-dust
deposit surrounded by consolidated material is indicated by the red arrow.
Furthermore, the colored stars indicate areas pointed out by Groussin et al.
(2015) as origins of resurfacing processes in the weeks before the comet
passage at perihelion, while the blue circle indicates the position of pre-
perihelion bright spots first discussed in Pommerol et al. (2015), where
water-ice was detected (Filacchione et al., 2016a).
The variations in color and hue displayed in fig. 9 are a compelling
indication of the multiplicity of terrain types. We recall that in a figure
like this, a relative difference in brightness between two surface elements is
indicative of a variation in geometric albedo, while a relative difference in
color is indicative of a distinction of spectral behavior. We selected here
and designated a few elements discussed previously, as well as in previous
studies of this area.

While not all bright spots are apparent with this resolution, some of the
bright surfaces (red circles in fig. 9 and in the previous figures) are still
easily distinguishable. Those pointed here are the largest and located at the
bottom of underhangs, except for two: one located around (-32∘, +14∘), found
at a base of a niche of the Ash region, and another located close to (-46∘,
-22∘) at the feet of a megaclast.
We note here that these two surface elements are both part of the source
regions of two activity events that occurred around perihelion in August 2015.
Both events are discussed in Fornasier et al. (ssue), and are numbers 42 and
133 in table A1.

Additionally, we also refer to Oklay et al. (2016a) for the investigation of
the (-32∘, +14∘) bright feature (denoted there ROI7) in pre-perihelion images
(NAC filter sequence acquired at 2014-09-25 06:46:25 UTC). This bright feature
was then notably compared to the area marked by a blue circle (-26∘, +14∘),
which was also found to harbor particularly bright boulders and surface
elements whose spectral slope was lower than their surroundings (Pommerol et
al., 2015 and Oklay et al., 2016a). This particular area, observed by the
VIRTIS instrument in October 2014 and labeled BAP-1 in Filacchione et al.
(2016a), was found to have spectral properties that are best fit with a water-
ice content that varies between 1.2% and 3.5%, depending on the mixing
scenario the authors considered (areal or intimate mixing), and are therefore
one of the very first nucleus regions where emerging water-ice-rich material
was ascertained.

The two bright surfaces, located around (-44∘, +8∘) and (-38∘, +20∘), are both
particularly evident in the spectral slope mappings of the bottom left panel
in fig. 4, and BF-010 and BF-011 have geometric albedos at 649 nm of 11.3% and
8.9%, respectively (i.e., $\sim$ 66% and 31%, respectively, higher than the
nucleus average of 6.8%) and spectral slopes of 6.8% and 9.0% (i.e., $\sim$
62% and $\sim$51%, respectively, lower than the region average of 17.98 %/100
nm at $\alpha$ = 0∘, see fig. 13). These two surfaces are found at the bottom
of large declivities, and the nature of the topography between the
morphological regions Ash and Aten is only well rendered in fig. 9 by the
actual absence (areas in black) of observations of these cliffs during the
flyby manoeuvre.

For similar reasons, the bright surface located around -55∘of longitude and
-2∘of latitude, identified in this study as BF-08, also stands out because
during the 23h34 and 23h46 observations, its reflectance was at higher than
11%, while its spectral slope ranged between 0.8 and 5.1%/10 nm (i.e., between
$\sim$72% and $\sim$96% lower than the region average).
Following the previous remarks, it is therefore most likely that these bright
surfaces harbor water-ice-rich material.

Similarly, in this mapping, the largest somber boulders are also still
distinguishable in different parts of the region (see the green triangles in
fig. 9). They are found to be on most types of morphological units: diamictons
(green polygons), fine-material deposits covered with boulder fields, fine-
material deposits (e.g., the lone boulder in Aten in between the two bright
surfaces), as well as among the gravitational accumulation deposits (red
polygon) and the degraded outcrops (black polygon). We note, however, that
they are absent from the fine-material deposits that cover the smooth central
region of the Imhotep depression, as well as from two of the taluses of the
area.
We note here that such boulders with a lower-than-average reflectance and a
red spectral behavior have also been observed in the area of the February 2015
flyby (Feller et al., 2016).

Other areas of this flyby region present a contrast as clear as that of somber
boulders and their neighboring terrains, for instance, the area indicated by
the red arrow in fig. 9, where previous fine-material deposits cover the top
of a series of terraces whose fronts are formed by the outcropping
consolidated material. Moreover, scarps within fine-material deposits are
clearly visible. They are denoted by the yellow arrows in the corresponding
figure. Such features were evident in pre-perihelion images, for instance, the
red star marks the linear feature shown in Thomas et al. (2015b). During the
approach to perihelion, however, some new scarps appeared or progressed across
the surface, exposing resurfacing processes of the smooth central area of
Imhotep (Groussin et al., 2015), with evidence of mass-wasting processes
around the location marked by the yellow, green, and red stars as well. These
resurfacing processes are also discussed in Deshapriya et al. (2018). We also
note here that the area around the red star was the source from another
activity event observed in NAC observations acquired on 12 August 2015 (see
entry 151 in table A1 of Fornasier et al., ssue).
While the contrast between the top and bottom of these scarps is well marked
in this figure and the outline of the scarp close to (-20∘, -10∘) is even
evident at a low phase angle (see the right panel of fig. 8), the differences
in the corresponding spectral slope mapping are notably small (see fig. 6) as
it presents itself as a variation of less than 2 %/100 nm around the local
average spectral slope value.

The flyby area is distinguished by the diversity of morphological units, as
well as by the variety of colors of these different terrains and of particular
features such as bright spots and somber boulders. Furthermore, this
particular region of the comet can be deemed of a particular interest as it
encompasses numerous surfaces elements where cometary activity and surface
evolution have been observed as the comet approached, passed through
perihelion, and moved away from it. The source locations of jets and outbursts
observed around perihelion are shown in fig. 1 of Fornasier et al., submitted.
For instance, the area where niches hosts two large bright surfaces located
around (-32∘, 14∘) was observed on 1 August 2015 to be the source of one
particular large outburst. Such bright surfaces were not evident in OSIRIS/NAC
images acquired in September 2014 (Auger et al., 2015 and Oklay et al.,
2016b), but further indicate compositional heterogeneities on the nucleus
immediately below the dust mantle, that are revealed as the insolation is
sufficient to pierce through this insulating layer and trigger a violent
activity event, such as has been observed for cliff collapse (Pajola et al.,
2016 and Fornasier et al., 2016).

###  4.4 NUV, Vis, and NIR spectrophotometry

We present here the results acquired from the two sequences of observations
taken before and after the moment of closest approach using the 11 NAC filters
between 269 and 989 nm.
These two sequences were found to be best coregistered when segmentation and
optical flow algorithms were combined. Considering the difficulty of
correcting the parallax effect for the full image, we chose to integrate the
signal in 5x5 pixels boxes, which corresponds to a surface element of 24.5 m2.
We used both sequences to investigate some particular surface features as well
as to constrain the differences of spectral slopes described previously.

We present in fig. 10 the reflectance mappings and mappings of the spectral
slope computed in the 535-882 nm range for the 2016-04-10T11h50 observation
sequence. As in previous observations, we note the same apparent
anticorrelation between the brightness (in radiance factor) of an area, its
morphological type, and the steepness of its spectral slope.
In these figures, areas that were previously identified as fine-material
deposits (such as terraces and surfaces close to the bottom of underhangs)
appear slightly brighter and present a smaller spectral slope. Similarly, the
consolidated material structure encircled in red in the bottom panel of fig.
4, although it has a reflectance that is almost average, still exhibits, at
$\sim$ 51∘of phase angle, a spectral slope slightly lower than 17 %/100 nm.
This observation supports the previously stated conclusion that this feature
likely is a large-scale (of about 100 m) compositional heterogeneity.
Meanwhile, the structures that are identified as eroded consolidated material,
diamictons, or boulder fields here also present a reflectance that is slightly
lower than average and a higher spectral slope.
In these figures, one feature that is not visible in other NAC images is the
large bright surface close to the underhang with a low spectral slope. This
area is also distinguished through its particularly smooth appearance. This
feature was one of the bright spots whose photometric properties were
investigated in Hasselmann et al. (2017) and were found to best match the
remnants of a sublimated intimate mixture of water-ice, carbon black, and
tholins studied in Jost et al. (2017a). We further investigate its
spectrophotometric properties below.

In this sequence, we investigated the bright spot on the boulder that were
previously identified (BF-01, blue circle), a neighboring unresolved-appearing
terrain (orange square, UR-02), a bright smooth surface underneath an overhang
(green star, BS-01), the previously investigated smooth-looking regolith (red
cross, UR-00), a smooth area in a degraded outcrop of consolidated material
(black upward triangle), and a somber feature in fine-material deposits on
Imhotep (magenta downward triangle, SF-01). We report the results of the
spectrophotometric analysis in fig. 11.

Figure 10: 2016-04-10T11h48 sequence ($\alpha$ $\sim$ 51∘): Maps of the
photometrically corrected radiance factor (left) and of the spectral slope in
the 535–882 nm range (right). The symbols refer to the measurements made and
presented in fig. 11. Under these illumination conditions ($\alpha$ $\sim$
51∘), the shadows are cast on the niches and areas where bright material was
previously observed (see fig. 4). Figure 11: Spectrophotometry measurements
for the 2016-04-10T11h48 sequence. The images were photometrically corrected
using the Lommel-Seeliger disk law. The symbols correspond to the measurements
made at the locations indicated in fig. 10a. We note here that the measures
for BF-01 were close to the detector saturation levels. These values therefore
correspond to lower limits.

We report here the values of the spectral slopes in the 535–743 nm range for
the measurements that were made without correcting for the phase-reddening
phenomenon (see following section). These measurements are (in %/100 nm)
20.4$\pm$0.4 for UR-00, 20.0$\pm$0.9 for UR-02, 20.9$\pm$0.4 for CM-01,
3$\pm$6 for BF-01, 12$\pm$2 for BS-01, and 19.2$\pm$0.7 for SF-01.

Figure 11 shows that the smooth-looking terrain and the consolidated material
exhibit a similar red spectral behavior, as has been observed previously in
the high-resolution images of the February 2015 flyby. This spectrum for the
average smooth-looking regolith terrain (UR-00) is typical of the average dark
terrain of the comet nucleus (see, e.g., fig. 2 in Fornasier et al., 2016).
However, contrary to what was observed in February 2015, the bright surfaces
of this area (BF-01 or BS-01) have spectra sensibly different from smooth
surfaces or consolidated material. Moreover, the selected bright areas are at
least twice as bright (in terms of radiance factor) as the average smooth-
looking regolith terrain.
We indicate here that in all of these observations, the reflectances of the
bright feature at the top of a boulder (called BF-01 in this study and ID-44
in Deshapriya et al. (2018)) were again similar to but did not reach the
saturation levels of the NAC. This is denoted by the blue arrows in the left
panel of fig. 11.
We observe, as noted previously with the three-filters observations, that the
smooth unresolved-looking regoliths, the surfaces among the eroded outcrop,
and the somber feature exhibit similar characteristics: the reflectance is
considerably lower than those of the bright features, and they share a very
similar spectral behavior. On the other hand, the two identified bright
features have very low spectral slopes at $\sim$51∘ phase angle and noticeably
fainter normalized reflectances. We interpret these surfaces to be
fractionally enriched in water-ice material (Filacchione et al., 2016b;
Barucci et al., 2016; Deshapriya et al., 2016; Fornasier et al., 2016 and
Oklay et al., 2016b) , and the observed differences between these two surfaces
might reflect a further difference in composition, as considered in Deshapriya
et al. (2018).

We also considered these two sequences with 11 filters in order to constrain
the differences in spectral slopes using different computing methods.
In previous studies, spectral slopes have been computed in the 535-882nm range
when the corresponding OSIRIS images were available. Spectral slopes have
otherwise been computed in the 535-743 nm range, using either the F23 filter
image, or an interpolation using the F24 and F22 filter images. We present in
fig. 12 the corresponding mapings.

We have investigated the global variations of the spectral mappings assembled
from the 2016-04-09T12h13 and 2016-04-10T11h50 observations, as well as some
local variations between the assembled 2016-04-09T12h13 spectral slope
mappings.
For each observation, we applied a Gaussian fit to the different spectral
slope mappings to determine the center of the distribution and its full width
at half-maximum (FWHM). The results are assembled in table 4.

Observation |  $\Delta\lambda$ (nm) | S (%/100 nm)
---|---|---
9th/04 - 12h13 | 535 - 882 | 17.8$\pm$2.2
$\alpha$ $\sim$ 62∘ | 535 - 743 | 21.6$\pm$2.9
| 535* - 743 | 21.2$\pm$2.6
10th/04 - 11h50 | 535 - 882 | 17.6$\pm$2.3
$\alpha$ $\sim$ 51∘ | 535 - 743 | 21.0$\pm$3.0
| 535* - 743 | 20.4$\pm$2.6
Table 4:  Differences in spectral slopes: spectral slope values (S, center,
and FWHM) of the Gaussian fitting for each spectral slope mapping considering
their wavelength range ($\Delta\lambda$). The asterisk denotes that the
corresponding 535 mappings were interpolated from the associated 480 nm and
649 nm filter images.

We note that based on these results for the 12h13 observations, the spectral
mapping in the 535-882 nm range is $\sim$21% and $\sim$19% lower than the
535-743 nm and the 535* -743 nm spectral mappings. Similarly, for the 11h50
observations, the spectral slopes mapping in 535-882 nm range is $\sim$19.5%
and $\sim$16.5% lower than the other two spectral mappings.

Location | Spectral slope range (nm)
---|---
| 535 - 882 | 535 - 743 | 535* - 743
ROI - A | 17.7 $\pm$ 0.7 | 21.2 $\pm$ 0.5 | 20.8 $\pm$ 0.5
ROI - B | 17.2 $\pm$ 0.2 | 20.3 $\pm$ 0.4 | 20.0 $\pm$ 0.4
ROI - C | 18.5 $\pm$ 0.5 | 22.3 $\pm$ 0.5 | 21.6 $\pm$ 0.7
ROI - D | 18.1 $\pm$ 0.2 | 21.6 $\pm$ 0.2 | 21.7 $\pm$ 0.3
ROI - E | 14.7 $\pm$ 0.2 | 19.5 $\pm$ 0.7 | 20.8 $\pm$ 0.5
ROI - F | 15.2 $\pm$ 0.3 | 19.1 $\pm$ 0.4 | 18.9 $\pm$ 0.4
Table 5:  Comparison of the spectral slope ranges: Different spectral slope
values for the different regions of interests indicated in fig. 12.

We further list in Table 5 local measurements of the spectral slopes around
the Imhotep region, including areas outside the Imhotep-Khepry transition.
Following the previous remark, we find once again that the 535-882 nm spectral
slope measurements are on average 20$\pm$5% lower than the 535-743 nm spectral
slopes, or 18$\pm$9% lower than those computed between 535nm (interpolated)
and 743nm.
In both cases, the extreme values come from the measurements of the strikingly
blue regions (i.e., those with a lower slope than average) in fig. 12. These
535-882nm measurements differ by 32% and 26% from the corresponding 535-743nm
values and by 41% and 24% from the corresponding 535*-743nm values.
These areas are part of a region of the nucleus where water-ice-rich material
and outbursts have been observed (Knollenberg et al., 2016; Deshapriya et al.,
2018; Oklay et al., 2016b and Agarwal et al., 2017. They also encompass the
feature described as “blues veins” in Hasselmann et al. (2017), whose
photometric properties were best matched by those of the remnants of an
intimate mixture of water-ice, carbon black, and tholins after sublimation.
The noted spectral difference is therefore very likely to derive from a
compositional difference. As there appears to be an overall shift down of the
535-882 nm spectral slopes in the bottom left corner of the mapping with
respect to the others, however, we note here that this local difference in
values might be due to a new flat-field correction for the NAC F24 filter
(centered at 535nm) that was implemented in-flight in late 2015. While this
artifact might slightly overestimate the noted particularity of this area (by
$\sim$9% compared to the other two measurements), we still correctly observe
that this area is different from the other neighboring areas (see the other
mappings in fig. 12).

Figure 12: From bottom to top: Spectral slopes computed in the 535-882 nm
range, in the 535-743 nm range, or in the 535-743 nm range with an
interpolation at 535 nm. Each spectral slope mapping has its own color bar,
adjusted within $\pm$7$\sigma$ around the median value.

###  4.5 Phase reddening

Figure 13: Phase reddening observed above the Khepry-Imhotep region during the
flyby. Using the density of each bin as weights, the linear best-fit solution
gives a spectral slope at 0∘ and a phase reddening slope similar to that
measured in Feller et al. (2016).

Phase reddening is the increase in spectral slope with the phase angle, first
observed on the lunar surface (Gehrels et al., 1964). As discussed at length
in Jost et al. (2017a), this phenonemon has also been observed in laboratory
settings (Gradie et al., 1980) as well as on asteroids (Clark et al., 2002 and
Sanchez et al., 2012). The ROSETTA mission parameters have allowed us soon
after rendezvous with the comet to provide the first definite phase-reddening
measurement on a comet Fornasier et al. (2015). As in our previous studies
(Fornasier et al., 2016 Feller et al., 2016), we have sought here to constrain
the measure of the phase reddening in the Imhotep-Khepry transition as the
comet was at 2.7 au away from the Sun.
To this effect, we computed the spectral slope mappings for each sequence of
images listed in table 6 and then ran a linear regression.

The density plot of the spectral slopes computed for this dataset and their
linear best-fit solution are shown in fig. 13.
Around the morphological transition between Khepry and Imhotep, in the 535-743
nm range, and between 0.1∘ and 62∘ of phase angle, these spectral slopes are
best fit by a slope of 0.064$\pm$0.001 %/100 nm/∘ and a spectral slope at
$\alpha$=0∘ of 17.985$\pm$0.004 %/100 nm.

We note that these results are quite similar to those obtained for the part of
the Ash-Imhotep transition flyby in February 2015, which were 0.065$\pm$0.001
%/100 nm/∘ and 17.9$\pm$0.1 %/100 nm, as the comet was 2.3 au inbound to
perihelion. We recall that the flyby discussed here took place while the
nucleus was 2.7 au outbound.
While the values of these 535-743 nm spectral slopes are $\sim$ 17$\pm$5 %
higher than their 535-882 nm counterparts, the phase-reddening slopes are
nevertheless consistent with those measured in the 535-882 nm range over one
rotational period of the nucleus, between August 2014 and February 2016, as
listed in Fornasier et al. (2016).
As shown in that study, the phase-reddening slope of the nucleus abated from
0.104 to 0.041 %/100 nm/∘between August 2014 and August 2015 (inbound to
perihelion), before a further increase between August 2015 and February 2016
(outbound from perihelion) at least up to the initial phase-reddening slope of
0.104%/100 nm/∘. Thermal modeling by Keller et al. (2015) and analysis of
detailed nucleus observations by El-Maarry et al. (2016) have lead to the
interpretation of this variation as a likely consequence of the global
thinning of the dust mantle across the nucleus that is observed around
perihelion (on an order of magnitude of 1 m during this passage), and the
change in surface properties, such as dust roughness and the composition of
the uppermost layer.

This area, which has been the source region of several jets during that
passage of the comet through the inner solar system, is also the area where
most of the fine-material deposits of the Imhotep central area went through a
resurfacing process in the weeks before perihelion. Likewise, this is the area
where evidence of diurnal and seasonal water cycles were observed (Fornasier
et al., 2016), and where more surfaces likely containing water-ice material
are exposed on the surface by April 2016 than in August or September 2014\.
These surfaces were then, at 2.7 au, likely to survive until the approach to
the next perihelion passage.
We therefore interpret the increase in phase-reddening slope outbound from
perihelion to reflect the complex nature of the changes that occurred and were
occurring on this part of the nucleus surface and the transition in terms of
cometary activity as its heliocentric distance grew.

##  5 Discussion

The OSIRIS/NAC observations taken during the April 2016 low-altitude low-phase
angle flyby show in great detail the complexity of the transition between the
morphological regions Imhotep and Khepry. While this area is similar to most
of the overall nucleus surface, it contains a variety of morphological
features as well as a diversity of behaviors in terms of colors and spectral
properties.
Moreover, it shows some striking differences with the part of the transition
area between the Ash and Imhotep regions that the spacecraft crossed during
the February 2015 flyby (Feller et al., 2016) and that was surveyed with a 12
cm/px resolution.

In the Ash-Imhotep transition area of the February 2015 flyby (see also table
8), the part belonging to the Ash morphological unit appeared to be coated
with fine-material deposits and peppered with decimeter- and meter-sized
boulders, while the 400 m slope, leading toward the center of the Imhotep
depression, consisted of strata of consolidated material covered with sparse
boulders and a few small localized surfaces where fine material and pebbles
are visible.
On the other hand, the Imhotep-Khepry transition presents a terraced
topography where several extended fine-material deposits are visible on
Imhotep, Khepry, and part of Aten. It also hosts large features of
consolidated material, as well as areas of degraded outcrops, diamicton,
taluses, gravitational accumulation deposits, and boulder fields.

These two transitions further differ from one another in their global
spectrophotometric properties. Some degree of difference was already indicated
by August 2014 observations where at $\sim$ 1.3∘of phase angle, the Ash-
Imhotep transition exhibited reflectances at 649nm lower than 5.1%, and a
553-882 nm spectral slope higher than 13.5 %/100 nm, whereas the Imhotep-
Khepry transition presented reflectances of about 5% and higher, and
corresponding spectral slopes between 11 and 14 %/100 nm (see fig. 9 in
Fornasier et al., 2015). Similarly, in fig. 13 of Fornasier et al. (2015), at
around 10∘ phase angle, the former transition only displayed strongly elevated
spectral slope values, while the latter presented a range of moderate and
elevated spectral slopes values. Based on these observations, the Ash-Imhotep
transition could then be classified as belonging to a group of terrains with
high spectral slopes, and the Imhotep-Khepry to the group of terrains with
intermediate spectral slopes.

This group of terrains also included the regions close to the top of the small
lobe of the comet, such as Agilkia. This particular region, investigated in La
Forgia et al. (2015), shows clear variations in the assembled normal albedo
and spectral slope mappings. As indicated by the authors, the comparison of
these mappings with the corresponding morphological map points to a
correspondence between the reflectance, spectral slope, and morphological
nature of a terrain. In particular, it was observed that around the Agilkia
area, the fine-material deposits exhibited on average a slightly higher
reflectance and a lower spectral slope (computed following Jewitt and Meech
(1986) in the 480-882nm range) than their locally surrounding diamicton
fields, taluses, gravitational accumulation deposits, or neighboring
outcropping layered terrains.
As apparent in the mappings of the previous section, a similar observation can
be made for the Imhotep-Khepry transition. While in the reflectance mappings
uneven and rough surfaces (e.g outcropping stratified terrains or degraded
outcroppings) appear only slightly darker in this area than fine-material
deposits, a clearer contrast arises from the spectral slope mappings in which
smooth areas of fine-material deposits stand out from their rougher
surroundings (e.g., diamictons or taluses). Other outstanding features add
themselves to this general picture, such as the somber boulders, bright
surfaces in niches, or close to underhangs and the large consolidated material
feature encircled in red in the bottom panel of fig. 4, which presents lower-
than-average spectral slopes at its head and flanks.

While investigations into the nature of somber boulders are still ongoing, the
bright surfaces displaying low spectral slopes that are essentially found in
niches or at the bottom of underhangs, exhibit a behavior that is typically
associated with areas that are enriched in water-ice material, as explained
previously.
Any enrichment in water-ice material would be only fractional, however, as
discussed in previous studies (Sunshine et al., 2006; Filacchione et al.,
2016b; Barucci et al., 2016; Oklay et al., 2016b and Fornasier et al., 2016).
In particular, the analysis of surfaces observed by the VIRTIS infrared
spectrometer, in which bright surfaces were visible, led by Raponi et al.
(2016), has indicated that areas presenting a spectral slope lower than 10
%/100 nm in the 500-1000 nm range at a phase angle of 95∘ could be composed of
just over 1% of pure water-ice in an areal mixing scenario and of well over 5%
in an intimate mixture scenario.
Additionaly, the area denoted by a blue circle in fig. 9, where bright
surfaces have been identified by Pommerol et al. (2015), was considered to
host material enriched by up to 6% in water-ice according to infrared spectrum
modeling (Filacchione et al., 2016b). For some of the surface elements of the
same area, Oklay et al. (2017) have obtained corresponding values ranging from
6% to 25% based on the spectrophotometric properties of these spectral regions
and thermal modeling. Similarly, the analyses conducted in the Anhur and Bes
on some of the singular very large compositional heterogeneities have
indicated local $\sim$ 20% and $\sim$ 30% enrichment in water ice (Fornasier
et al., 2016 and Fornasier et al., 2017).

At this time, we cannot report any constraint on the water-ice enrichment of
the observed bright surfaces from the considered dataset. We can report,
however, that some of the bright surfaces have been observed several times
during the mission and that investigations of a temporal variation in their
spectrophotometric properties is currently being undertaken. For instance, we
have found the bright feature BF-06 and another bright feature close to UR-0
to clearly present once again in August 2016 at 3.5 au a spectral behavior
that is consistent with a water-ice material enrichment (see fig. 14).

Figure 14: RGB reflectance properties of BF-06 and of another neighboring
bright surface as observed in the NAC 2016-08-06T11:47 sequence of
observations. The red star and blue asterisk indicate the bright feature at
the bottom of an underhang close to UR-00 and BF-06, respectively. Other
symbols mark neighboring smooth surfaces and consolidated material. In these
observations, the phase angle is 71.4∘and the resolution is 16 cm/px. For
clarity, the area visible in this image is roughly delimited by the red square
in fig. 8.

In this observation, the reflectance of the bright surfaces is clearly between
4 and 10 times higher than that of neighboring smooth surfaces of fine-
material deposits, and it clearly exhibits a spectral behavior with a neutral
slope with respect to the same neighboring surfaces. This observation
therefore indicates the persistence of these bright surfaces, and in the
paradigm of a spectral behavior consistent with a water-ice material
enrichment, the lack of sufficient insolation to sublimate this material
between April and August 2016. As the comet was at 3.5 au, it would then be
quite likely that these surfaces would survive until the next approach to
perihelion.

In the bright surfaces investigated conjointly by the OSIRIS and VIRTIS
instruments, water-ice material is absolutely certain. However, the exact
nature of the average terrain material that surrounds these bright features
and composes the top-layer of the nucleus surface (also referred to as “dark
terrain” in Fornasier et al., 2017) still remains to be defined.
Comparison of the photometric results from the February 2015 flyby with
laboratory measurements has found that intra-mixture of carbon, tholins, and
water ice (Jost et al., 2017a) matched the phase curve better than the inter-
mixture counterpart. A similar observation was made regarding the nature of
bright spots for this flyby observed with the OSIRIS/WAC camera (Hasselmann et
al., 2017).
However, the comparison of these samples with the measurements of smooth fine-
material deposits of the Ash-Imhotep transition had indicated a mismatch of
these particular samples in terms of the spectral slopes, as tholinsinduced a
stronger spectral slope than had been observed (see fig. 16 of Feller et al.,
2016).

Recent researches into the spectrophotometric properties of cometary analogs
have been notably reported in Jost et al. (2017a, b) and Rousseau et al.
(2017). While the first study furthered the analysis of carbon, tholins, and
water-ice mixtures, the latter investigated the properties of coal mixed with
silicates and/or with pyrrhotites (iron sulfides).
In both studies, the authors have found mixtures that provided a satisfactory
match of the comet spectrum in the 250-1000nm range. Additionally, Jost et al.
(2017b) also reported mixtures that matched the observed phase-reddening
phenomenon. However, Jost et al. (2017b) also noted that the investigated
mixtures did not provide a simultaneous match of the comet albedo, spectrum,
phase curve, and of the phase reddening.
Furthermore, Rousseau et al. (2017) noted that although they found mixtures
that matched the comet reflectance, these mixtures did not match the observed
spectral slope of the average comet terrain either. They pointed out that the
organic compound they used induced a lower spectral slope than the one of the
comet.

Consequently, investigations for an appropriate cometary analog for the
surface of 67P in terms of spectrophotometric and physical properties, as
pointed in part in Kaufmann and Hagermann (2018), for instance, are still
ongoing.
In addition to considering the albedo and reflectance spectrum of a mixture,
including phase-reddening measurements might prove a pertinent additional
comparison factor. However, the grounds for this phenomenon are still the
subject of research and remain to be well defined. This phenomenon has
tentatively been attributed to the increased contribution of the multiple
scattering at large phase angles as the wavelength and albedo increase, and to
a contribution of surface roughness effects (Hapke, 2012; Sanchez et al., 2012
and Schröder et al., 2014). Jost et al. (2017a) further underlined that the
nature of a surface phase reddening also appears to depend on its composition,
and that in their surface analog mixtures, both the darkening agent (carbon
particles) and the organic compound (tholins) had a distinctive influence on
the presence and strength of the phase reddening.
While additional theoretical investigation and laboratory experiment
validation are crucial to determine the frame of the phase-reddening
phenomenon within a low temperature, irradiated and Van-der-Waals forces
dominated environment, considering the phase reddening in current laboratory
experiments would nevertheless constitute a further valuable comparison tool
for determining an appropriate analog of the surface of 67P.

##  6 Conclusions

We have presented here the results of the geomorphological mapping and of the
spectrophotometric analysis based on the OSIRIS/NAC images acquired during the
April 2016 flyby manoeuvre above the Imhotep-Khepry transition area on 67P.

  * •

We have identified and mapped the host of morphological units that are visible
in the Khepry-Imhotep transition, which, without marking this region as
unique, indicate its diversity and relative complexity.

  * •

We have performed spectrophotometric analyses of the transition in general as
well as of peculiar surface elements. While the smooth-looking regolith of
this region is similar to the average dark terrain of the nucleus in terms of
reflectance and spectral slope, rough terrains such as diamictons, degraded
outcrops, and consolidated material exhibit a lower-than-average reflectance
and a higher-than-average spectral slope.
One particular outcrop of consolidated material roofed by a cuesta presents a
slightly higher-than-average reflectance and a slightly lower-than-average
spectral slope, which likely indicates a local compositional heterogeneity at
a scale of some tens of meters.
Additionally, some meter-sized features also present a peculiar
spectrophotometric behavior. Sombre boulders, which are ubiquitous in the
Imhotep-Khepry transition, show a similar spectral behavior as the neighboring
smooth terrains, and their reflectance is $\sim$20% lower than average. The
bright surfaces of this area are unmistakable, their reflectance is at least
at twice the local average, and their spectral behavior is repeatedly less
steep than the average surface. These features both indicate small-scale
compositional heterogeneities across the surface of this region.

  * •

We have identified that the bright features that exhibit moderate to neutral
spectral slopes are likely candidates for locations that are enriched in
water-ice material. We have also shown that some of the bright surfaces
investigated in April 2016 that are likely enriched in water-ice material have
persisted up to and beyond the frost line, and at least up to August 2016.
This further supports evidence for long-term survival of water-ice-rich
material and frosts on the nucleus surface.

  * •

We have measured the local phase-reddening parameters as the comet was at 2.7
au outbound from perihelion, and have found them to be on the same order as
those obtained for the Ash-Imhotep transition observed during the February
2015 flyby, which further indicates seasonal variations on the surface of the
nucleus.

Accordingly, this transition area displays a diversity in reflectances and
spectral slopes that is as great as it is in morphological terms. This
indicates that the variety in the surface top-layer composition was greater
than in the flyby area on 14 February 2015.

## Acknowledgements

OSIRIS was built by a consortium of the Max-Planck-Institut für
Sonnensystemforschung, Göttingen, Germany, CISAS–University of Padova, Italy,
the Laboratoire d’Astrophysique de Marseille, France, the Instituto de
Astrofísica de Andalucia, CSIC, Granada, Spain, the Research and Scientific
Support Department of the European Space Agency, Noordwijk, The Netherlands,
the Instituto Nacional de Técnica Aeroespacial, Madrid, Spain, the Universidad
Politéchnica de Madrid, Spain, the Department of Physics and Astronomy of
Uppsala University, Sweden, and the Institut für Datentechnik und
Kommunikationsnetze der Technischen Universität Braunschweig, Germany. The
support of the national funding agencies of Germany (DLR), France (CNES),
Italy (ASI), Spain (MEC), Sweden (SNSB), and the ESA Technical Directorate is
gratefully acknowledged.
Rosetta is an ESA mission with contributions from its member states and NASA.
Rosetta’s Philae lander is provided by a consortium led by DLR, MPS, CNES and
ASI.
The SPICE libraries and PDS resources are developed and maintained by NASA.
The authors thanks the referee and editors for their questions, remarks and
advices for the improvement of this manuscript.

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##  Appendix A Additional material

April 2016 (UTC) | NAC Filters | D (km) | Res. (m/px) |  $\alpha_{m}$ (∘) |  $\alpha_{min}$ \- $\alpha_{max}$ (∘) | Longitude | Latitude
---|---|---|---|---|---|---|---
09T12:15:17.916 | All filters | 47.0$\pm$0.6 | 0.87$\pm$0.05 | 60.6 | 59.5-61.7 | 149.89∘ | -5.76∘
09T23:01:09.919 | F84, F82, F88 | 28.8$\pm$0.2 | 0.54$\pm$0.02 | 7.80 | 6.67-8.94 | 121.33∘ | -17.09∘
09T23:08:09.910 | ” | 28.8$\pm$0.2 | 0.54$\pm$0.02 | 6.99 | 5.85-8.14 | 118.12∘ | -16.77∘
09T23:14:09.894 | ” | 28.8$\pm$0.2 | 0.54$\pm$0.02 | 6.29 | 5.16-7.45 | 115.04∘ | -16.64∘
09T23:19:08.951 | ” | 28.8$\pm$0.2 | 0.53$\pm$0.02 | 5.72 | 4.58-6.87 | 112.54∘ | -16.50∘
09T23:22:09.905 | ” | 28.7$\pm$0.1 | 0.53$\pm$0.02 | 5.37 | 4.23-6.53 | 111.24∘ | -16.29∘
09T23:25:09.886 | ” | 28.7$\pm$0.1 | 0.53$\pm$0.02 | 5.02 | 3.88-6.19 | 109.92∘ | -16.11∘
09T23:28:09.912 | ” | 28.7$\pm$0.1 | 0.53$\pm$0.02 | 4.68 | 3.53-5.85 | 108.70∘ | -15.87∘
09T23:31:09.942 | ” | 28.7$\pm$0.1 | 0.53$\pm$0.01 | 4.34 | 3.19-5.50 | 107.51∘ | -15.62∘
09T23:34:09.904 | ” | 28.7$\pm$0.1 | 0.53$\pm$0.01 | 3.99 | 2.84-5.18 | 106.47∘ | -14.71∘
09T23:37:09.883 | ” | 28.7$\pm$0.1 | 0.53$\pm$0.01 | 3.65 | 2.48-4.88 | 105.35∘ | -13.30∘
09T23:40:09.927 | ” | 28.6$\pm$0.1 | 0.53$\pm$0.01 | 3.30 | 2.10-4.58 | 104.30∘ | -11.19∘
09T23:43:09.884 | ” | 28.6$\pm$0.1 | 0.53$\pm$0.01 | 2.94 | 1.69-4.26 | 103.33∘ | -8.71∘
09T23:46:09.901 | ” | 28.6$\pm$0.1 | 0.53$\pm$0.01 | 2.57 | 1.25-3.93 | 102.74∘ | -5.78∘
09T23:49:09.982 | ” | 28.6$\pm$0.1 | 0.53$\pm$0.01 | 2.19 | 0.80-3.56 | 103.48∘ | -2.47∘
09T23:52:10.005 | ” | 28.6$\pm$0.1 | 0.53$\pm$0.01 | 1.82 | 0.39-3.17 | 104.65∘ | 0.87∘
09T23:55:09.984 | ” | 28.6$\pm$0.1 | 0.53$\pm$0.01 | 1.48 | 0.095*-2.79 | 106.05∘ | 3.32∘
09T23:58:09.911 | ” | 28.6$\pm$0.1 | 0.53$\pm$0.01 | 1.24 | 0.095*-2.47 | 107.85∘ | 5.11∘
10T00:01:09.929 | ” | 28.6$\pm$0.1 | 0.53$\pm$0.01 | 1.21 | 0.095*-2.43 | 110.21∘ | 5.88∘
10T00:04:09.927 | ” | 28.6$\pm$0.1 | 0.53$\pm$0.01 | 1.42 | 0.095*-2.72 | 112.57∘ | 5.78∘
10T00:07:10.009 | ” | 28.6$\pm$0.1 | 0.53$\pm$0.01 | 1.75 | 0.31-3.09 | 114.98∘ | 4.60∘
10T00:10:09.944 | ” | 28.7$\pm$0.1 | 0.53$\pm$0.01 | 2.11 | 0.70-3.49 | 117.11∘ | 2.31∘
10T00:13:10.092 | ” | 28.7$\pm$0.1 | 0.53$\pm$0.01 | 2.48 | 1.14-3.85 | 118.13∘ | -0.52∘
10T00:16:08.998 | ” | 28.8$\pm$0.1 | 0.53$\pm$0.01 | 2.84 | 1.57-4.18 | 118.36∘ | -3.40∘
10T00:19:09.893 | ” | 28.8$\pm$0.1 | 0.54$\pm$0.01 | 3.19 | 1.98-4.48 | 117.89∘ | -6.17∘
10T00:22:09.912 | ” | 28.8$\pm$0.2 | 0.54$\pm$0.01 | 3.54 | 2.36-4.78 | 116.89∘ | -8.22∘
10T00:25:09.908 | ” | 28.9$\pm$0.2 | 0.54$\pm$0.02 | 3.88 | 2.72-5.08 | 115.50∘ | -9.78∘
10T00:28:09.931 | ” | 28.9$\pm$0.2 | 0.54$\pm$0.02 | 4.21 | 3.06-5.38 | 113.83∘ | -10.48∘
10T00:31:09.977 | ” | 28.9$\pm$0.2 | 0.54$\pm$0.02 | 4.54 | 3.40-5.71 | 112.01∘ | -10.51∘
10T00:34:09.905 | ” | 28.9$\pm$0.2 | 0.54$\pm$0.02 | 4.88 | 3.73-6.04 | 110.17∘ | -10.44∘
10T00:37:09.911 | ” | 28.9$\pm$0.2 | 0.54$\pm$0.02 | 5.21 | 4.07-6.37 | 108.31∘ | -10.26∘
10T00:40:09.924 | ” | 29.0$\pm$0.2 | 0.54$\pm$0.02 | 5.54 | 4.40-6.70 | 106.39∘ | -9.98∘
10T00:43:09.896 | ” | 29.0$\pm$0.2 | 0.54$\pm$0.02 | 5.87 | 4.74-7.03 | 104.49∘ | -9.66∘
10T00:46:09.944 | ” | 29.0$\pm$0.2 | 0.54$\pm$0.02 | 6.21 | 5.07-7.46 | 102.60∘ | -9.41∘
10T11:50:16.568 | All filters | 53.8$\pm$0.5 | 1.00$\pm$0.05 | 51.2 | 49.9-52.2 | 87.31∘ | -14.82∘
Table 6:  List of OSIRIS/NAC observations used in this study: in the table above, D is the median distance between spacecraft to imaged surface, $\alpha_{m}$ is the median phase angle, and the next column gives the amplitude of the phase angle values. The longitudes and latitudes are those of the NAC boresight in the Cheops frame (see the main text for details).  Feature | Long. | Lat. | 23h34 | 23h46 | 00h01 | 23h34 | 23h46 | 00h01
---|---|---|---|---|---|---|---|---
|  $\pm$0.1 |  $\pm$0.1 | I/F at 649 nm (%) | Spectral slope (%/100 nm)
UR-00 | -46.7 | -8.3 | 5.3$\pm$0.1 | 5.6$\pm$0.1 | 6.4$\pm$0.1 | 17.7$\pm$0.2 | 18.0$\pm$0.5 | 18.3$\pm$0.2
BF-01 | -56.5 | -8.3 | 20.0$\pm$3.0 |  |  | 8.0$\pm$11.0 |  |
BF-02 | -43.1 | -27.8 | 9.0$\pm$0.6 |  |  | 7.0$\pm$1.5 |  |
BF-03 | -46.0 | -20.3 | 7.0$\pm$0.2 | 7.6$\pm$0.1 |  | 11.0$\pm$2.0 | 10.8$\pm$0.4 |
BF-04 | -45.9 | -20.4 |  | 8.8$\pm$0.3 |  |  | 8.5$\pm$0.6 |
BF-05 | -45.7 | -12.6 | 9.0$\pm$1.6 | 11.0$\pm$1.4 | 11.0$\pm$1.2 | 5.1$\pm$5.0 | 6.0$\pm$2.0 | 9.0$\pm$3.0
BF-06 | -41.8 | -6.9 | 9.0$\pm$1.0 | 11.0$\pm$1.2 | 14.0$\pm$1.7 | 7.0$\pm$3.0 | 5.0$\pm$5.0 | 5.0$\pm$4.0
BF-07 | -39.0 | -6.7 | 6.4$\pm$0.2 | 7.0$\pm$0.3 | 8.0$\pm$0.3 | 15.9$\pm$0.5 | 16.2$\pm$0.6 | 15.0$\pm$1.0
BF-08 | -51.6 | -4.3 | 12.0$\pm$1.3 | 11.0$\pm$1.1 |  | 2.3$\pm$1.5 | 4.0$\pm$1.1 |
BF-09 | -54.2 | -3.3 |  |  | 8.1$\pm$0.1 |  |  | 10.6$\pm$0.5
BF-10 | -32.5 | 13.2 |  |  | 11.3$\pm$0.6 |  |  | 6.8$\pm$0.6
SB-00 | -51.4 | -24.9 | 4.2$\pm$0.1 |  |  | 20.1$\pm$0.7 |  |
SB-01 | -43.9 | -18.4 | 4.9$\pm$0.1 | 5.3$\pm$0.1 |  | 20.2$\pm$0.5 | 20.1$\pm$0.7 |
SB-02 | -45.5 | -11.7 | 4.8$\pm$0.1 | 5.2$\pm$0.1 |  | 21.1$\pm$0.5 | 21.8$\pm$0.4 |
SB-03 | -27.7 | -3.8 | 5.3$\pm$0.1 | 5.8$\pm$0.1 |  | 18.4$\pm$0.3 | 18.5$\pm$0.1 |
SB-04 | -20.8 | 7.0 | 5.0$\pm$0.1 | 5.4$\pm$0.1 |  | 18.5$\pm$0.2 | 18.8$\pm$0.1 |
SB-05 | -20.0 | 7.5 | 5.1$\pm$0.1 | 5.5$\pm$0.1 | 5.7$\pm$0.1 | 19.5$\pm$0.3 | 19.7$\pm$0.2 | 19.5$\pm$0.2
SB-06 | -42.6 | 6.5 |  | 5.0$\pm$0.1 | 5.6$\pm$0.1 |  | 19.7$\pm$0.5 | 18.5$\pm$0.4
SB-07 | -17.4 | 9.6 |  |  | 5.4$\pm$0.1 |  |  | 19.0$\pm$0.1
Table 7:  Extended list of features investigated: In the table above, the first column indicates the ID of the feature, the second and third columns give the features’ positions relative to the Cheops boulder, the two sets of three columns list their reflectance at 649 nm and their spectral slopes in the 535-743 nm range for the three sequences of observations acquired close to the moment of opposition and discussed in the main text.  Properties | February 2015 | April 2016
---|---|---
# NAC images | 91 | 121
Filters used | 24 sequences with F84,F82,F88 | 33 sequences with F84,F82,F88
| 2 sequences with all filters | 2 sequences with all filters
Sequences cadence (seconds) | 36 - 600 | 180 - 420
Best pixel scale | 0.11 m/pxl | 0.53 m/pxl
Phase angle range | 0∘\- 33∘ | 0∘-12∘\+ $\sim$ 51∘\+ $\sim$ 60∘
Phase reddening | S(0∘) = 17.9%/100 nm | S(0∘) = 17.9%/100 nm
parameters |  $\beta$ = 6.52 $\cdot$ 10${}^{-}4$/100nm/∘ |  $\beta$ = 6.40 $\cdot$ 10${}^{-}4$/100nm/∘
Median pv(0∘, 649 nm) | 6.3%$\pm$0.6% | 6.3%$\pm$0.6%
Max pv(0∘, 649 nm) | 10.4% | 16.0%*
Peculiar features | Strata heads, | bright patches,
| bright patches, | somber boulders
| somber boulders |
Investigated surfaces |  |
Bright patches | Spectral slope† > 17%/100nm | Spectral slope† < 10%/100nm
Sombre boulders | Spectral slope†> 17%/100nm | Spectral slope† > 17%/100nm
Spectral differences | Slight | Strong
(bright/somber features) |  |
Bright features’ spectral | average dark terrain | water-ice rich material
behavior similar to |  |
Favored locations of bright features | None obvious | Bottom of niches and underhangs
Table 8:  Comparison of the main characteristics of the February 2015 and
April 2016 flybys: Abridged comparative of the two flybys discussed in Feller
et al. (2016) and in the main text.
$\dagger$: Spectral slope close to 0∘of phase angle.