# The Spatial Distribution of the Unidentified 2.07 µm Absorption Feature on
Europa and Implications for its Origin

M. Ryleigh Davis Division of Geological and Planetary Sciences, California
Institute of Technology, Pasadena, CA 91125, USA  Michael E. Brown Division of
Geological and Planetary Sciences, California Institute of Technology,
Pasadena, CA 91125, USA  Samantha K. Trumbo Cornell Center for Astrophysics
and Planetary Science, Cornell University, Ithaca, NY 14853, USA

(Received March 31, 2023; Revised July 21, 2023; Accepted August 4, 2023)

###### Abstract

A weak absorption feature at 2.07 µm on Europa’s trailing hemisphere has been
suggested to arise from radiolytic processing of an endogenic salt, possibly
sourced from the interior ocean. However, if the genesis of this feature
requires endogenic material to be present, one might expect to find a
correlation between its spatial distribution and the recently disrupted chaos
terrains. Using archived near-infrared observations from Very Large
Telescope/SINFONI with a $\sim$1 nm spectral resolution and a linear spatial
resolution $\sim$130 km, we examine the spatial distribution of this feature
in an effort to explore this endogenic formation hypothesis. We find that
while the presence of the 2.07 µm feature is strongly associated with the
irradiation pattern on Europa’s trailing hemisphere, there is no apparent
association between the presence or depth of the absorption feature and
Europa’s large-scale chaos terrain. This spatial distribution suggests that
the formation pathway of the 2.07 µm feature on Europa is independent of any
endogenous salts within the recent geology. Instead, we propose that the
source of this feature may simply be a product of the radiolytic sulfur cycle
or arise from some unidentified parallel irradiation process. Notably, the
2.07 µm absorption band is absent from the Pwyll crater ejecta blanket,
suggesting that radiolytic processing has not had enough time to form the
species responsible and placing a lower limit on the irradiation timescale. We
are unable to find a plausible spectral match to the 2.07 µm feature within
the available laboratory data.

Galilean satellites(627) — Europa(2189) — Planetary surfaces(2113) — Surface
composition(2115) — Surface ices(2117) — Infrared spectroscopy(2285) — Very
Large Telescope(1767)

††journal: PSJ††facilities: VLT:Yepun (SINFONI)††software: Astropy (Astropy
Collaboration et al., 2013, 2018, 2022), Cartopy (Met Office, 2022), GeoPandas
(Jordahl et al., 2022), SciPy (Virtanen et al., 2020), Shapely (Gillies et
al., 2023), SpecUtils (Earl et al., 2022)

##  1 Introduction

Beneath its icy shell, Jupiter’s moon Europa hosts a deep global salty ocean
that is likely in direct contact with the silicate mantle. When combined with
a possible supply of radiolytically produced oxidants from the surface, this
water-rock interaction makes Europa a prime target for astrobiological
consideration (e.g. Anderson et al., 1998; Kivelson et al., 2000; Chyba, 2000;
Hand et al., 2009). Our current best window into Europa’s largely
unconstrained ocean chemistry is through the surface composition.
Specifically, the recently geologically active chaos terrains, bands, and
ridges show clear spectroscopic evidence for the presence of salt hydrates
that are likely sourced from the subsurface ocean (e.g. McCord et al., 1999;
Johnson et al., 2002; Leblanc et al., 2002; Dalton et al., 2005; Shirley et
al., 2010; Trumbo et al., 2019a, 2020). These large scale chaos terrains,
characterized by polygonal iceberg like blocks of ice within a matrix (Carr et
al., 1998), are interpreted to be areas of focused heat flow and possibly
local melting which may have exposed oceanic material (e.g Greenberg et al.,
1999; Collins & Nimmo, 2009). However, exogenic sulfur ions from Io’s
volcanoes are continuously deposited onto Europa’s trailing hemisphere, and
bombardment by magnetospheric ions, protons, and electrons (Pospieszalska &
Johnson, 1989; Cooper et al., 2001; Paranicas et al., 2001, 2009) drives a
radiolytic sulfur cycle. This cycle chemically alters the surface composition
and creates a “bull’s-eye” pattern of hydrated sulfuric acid centered at the
trailing hemisphere apex (270∘W, 0∘N) (Carlson et al., 1999, 2002, 2005).
Understanding the source and nature of exogenic material and radiolytic
processing on Europa’s surface and disentangling the original state of the
endogenic material is therefore critical for interpreting Europa’s surface
composition and inferring the properties of its underlying ocean.

Europa’s surface composition includes crystalline and amorphous water ice
(Hansen & McCord, 2004), radiolytically produced hydrated sulfuric acid on the
trailing hemisphere (Carlson et al., 1999, 2005), and an additional hydrated
salt component that is largely constrained to the recently geologically active
chaos terrains, bands, and ridges (McCord et al., 1999; Dalton et al., 2005;
Dalton III et al., 2012). The distribution of this additional salt hydrate is
consistent with Europa’s recent geologic features containing endogenic
material sourced from the subsurface. Irradiation produced O2 and hydrogen
peroxide (H2O2) (Johnson et al., 2003; Loeffler et al., 2006; Trumbo et al.,
2019b) as well as CO2 (McCord et al., 1998; Hansen & McCord, 2008; Carlson et
al., 2009) have also been confirmed on Europa’s surface. Recent observations
with HST/STIS revealed the presence of absorption features at 230 and 450 nm,
spatially constrained to the leading hemisphere chaos terrains (Trumbo et al.,
2019a, 2022), which closely match color center features produced in laboratory
data of irradiated sodium chloride (NaCl) (Hand & Carlson, 2015; Denman et
al., 2022; Brown et al., 2022). Together, these results provide strong
evidence for the presence of endogenous NaCl in Europa’s leading hemisphere
chaos.

Additional endogenic salt species, which are spectroscopically distinct from
the hydrated sulfuric acid, may also exist within Europa’s chaos terrains
(e.g. Fischer et al., 2015; Trumbo et al., 2020), however their precise nature
is widely debated. Some analyses of Galileo/NIMS spectra have suggested that
Europa’s endogenous hydrate may be dominated by sulfate salts (e.g. McCord et
al., 1998; Dalton et al., 2005; Dalton III et al., 2012), while more recent
studies with higher spectral resolution data sets suggest a chloride dominated
composition (Brown & Hand, 2013; Ligier et al., 2016). Further complicating
the picture, the hydrated material of the leading hemisphere chaos is
spectroscopically distinct from the hydrated material in the trailing
hemisphere chaos, presumably due to the significant exogenous alteration which
occurs on the trailing hemisphere (Fischer et al., 2015, 2016; Trumbo et al.,
2020). Trumbo et al. (2020) demonstrated that a 360 nm absorption feature and
700 nm slope change strongly correlate to the trailing hemisphere chaos
terrains and reflect a combination of endogenous and exogenous processes. They
suggest that while relatively unaltered endogenic material may persist in the
comparatively sheltered leading hemisphere chaos terrains, it may become
progressively more chemically altered towards the trailing hemisphere apex due
to radiolytic processing.

A weak absorption feature at 2.07 µm is also present on Europa’s trailing
hemisphere. It was first noted in the high resolution Keck/OSIRIS observations
of Brown & Hand (2013), who suggested epsomite (MgSO4 . 7 H2O) or magnesium
sulfate brine as a plausible spectral match. With no sign of a sulfate
absorption on Europa’s leading hemisphere, they suggested that magnesium
chloride (MgCl2) sourced from Europa’s interior may be radiolytically
converted into epsomite, a hydrated magnesium sulfate (MgSO4), when in the
presence of Iogenic sulfur ions deposited onto the trailing hemisphere. While
it is possible that this conceptual model of the alteration of chloride-rich
endogenic salts into sulfates via sulfur radiolysis could be consistent with
the visible-wavelength data of the trailing hemisphere (Hibbitts et al., 2019;
Trumbo et al., 2020), more laboratory data is needed to test this hypothesis.
Ligier et al. (2016), on the other hand, used VLT/SINFONI observations to show
that the 2.07 µm absorption feature can also be reproduced by certain
combinations of MgCl2 and perchlorate (Mg(ClO4)2) salts, despite neither one
having a distinct absorption on their own. The authors suggested that
endogenic magnesium-chloride may be radiolytically converted into magnesium
chlorate and perchlorate, rather than sulfate, on Europa’s trailing
hemisphere.

Both proposed origins for the 2.07 µm absorption feature (magnesium-bearing
sulfates versus chlorine salts) share one important characteristic. They
require a contribution of endogenic material, presumably tied to the ocean
chemistry, which is chemically altered into the species responsible for the
absorption by the trailing hemisphere irradiation environment. In both of
these scenarios we would expect to find a spatial correlation between the
presence and depth of the 2.07 um feature and the large-scale geologic units
or chaos terrains, where we would expect geologic resurfacing to have emplaced
endogenic, salty material. The trailing hemisphere chaos terrains are
spectroscopically distinct from their leading hemisphere counterparts (Trumbo
et al., 2020), presumably due to irradiation induced chemical alteration of
this endogenic material. Indeed, the best fit spectral models from Ligier et
al. (2016) show an enhancement of MgCl2 in the chaos terrains on both
hemispheres and magnesium perchlorate confined mainly to the trailing
hemisphere chaos, consistent with the 2.07 µm feature arising from a mixture
of endogenic chloride and its irradiation product. However, their models do
not map the 2.07 µm absorption feature directly, but rather rely on linear
mixture modeling of the largely featureless near infrared continuum, and
degeneracies amongst the various salt hydrates make positive identification
difficult. A more recent study by King et al. (2022) combined VLT/SPHERE and
Galileo/NIMS observations in an attempt to model Europa’s surface composition
across the near-infrared. Their best fit models include sodium chloride and
show distributions for both magnesium sulfate and magnesium bearing chlorine
species, which are broadly consistent with both hypotheses. However, they
point out that the uncertainties in the model abundances and degenerate fit
results for various compositional mixtures mean confident detection of any
individual salt species is not feasible with the existing data sets.

Previous observations indicate that irradiated NaCl is likely present and
spatially concentrated in the leading hemisphere chaos terrains (Trumbo et
al., 2022), and confirmation of endogenic MgCl2 or its radiolytic products
could provide crucial information for constraining Europa’s ocean and surface
chemistry. While atomic sodium and potassium, as well as chlorine ions, likely
sputtered from the surface, have been detected in Europa’s atmosphere or a
nearby pickup cloud (Brown & Hill, 1996; Brown, 2001; Volwerk et al., 2001),
attempts to measure magnesium in Europa’s atmosphere were unsuccessful. Hörst
& Brown (2013) provide an upper limit on the atmospheric Na/Mg ratio of at
least a factor of two less than meteoritic or cosmic abundances, and the
presence of magnesium bearing salts on Europa’s surface remains unproven.
Thus, the detection of MgCl2 along with NaCl on Europa’s surface may have
important implications for the relative balance of various cation and anion
species within Europa’s ocean. In the relatively simple freezing models of
Johnson et al. (2019), for example, the presence of MgCl2 could suggest a
sulfate-poor, sodium, magnesium and chloride rich ocean with a low pH due the
presence of magnesium ions.

In this paper, we use high spatial and moderate spectral resolution archived
VLT/SINFONI data to explore the spatial distribution of the 2.07 µm feature
and determine whether its presence is associated with the large-scale chaos
terrains, which could confirm its suspected relationship to endogenic salts
and possibly Europa’s ocean chemistry. With two possible origins of Europa’s
2.07 µm absorption feature arising from radiolytic processing of endogenic
MgCl2 and no clear resolution for the genesis of the feature from spectral
modeling, we turn to the spatial distribution of the absorption across
Europa’s trailing hemisphere in an attempt to gain insight.

Figure 1: Example VLT/SINFONI spectra of Europa’s leading and trailing
hemispheres showing the 2.07 µm absorption feature, which is only present on
Europa’s more heavily irradiated trailing hemisphere. The data are normalized
to 1.0 at 2.2 µm and the trailing hemisphere spectrum is offset by +0.5 for
clarity. The vertical dotted line marks the location of the 2.07 µm band
center and the continuum-removed absorption (multiplied by a factor of 3) is
included above each spectrum.

##  2 Observations & Data Reduction

We use archived spatially resolved, moderate-resolution, ground-based near-
infrared spectra from VLT/SINFONI acquired with adaptive optics during ESO
program 088.C-0833(A) at Paranal Observatory and originally published in
Ligier et al. (2016). The data were obtained by Ligier et al. (2016) over five
nights between October 2011 and January 2012 in the H+K band setting, covering
a wavelength range of 1.452 - 2.447 µm at a spectral resolution of $\sim$1 nm
($R\sim\frac{\lambda}{\Delta\lambda}=1500$). Europa’s angular diameter on-sky
ranged from 0.897” to 1.083”, so to cover the full disk with the 0.8” $\times$
0.8” field of view, integrations were taken in five offset frames and co-added
into a single mosaic across the full disk. The on-sky spatial sampling of the
SINFONI instrument is 12.5 $\times$ 25 mas, resulting in spatial pixels of
$\sim$ 35 $\times$ 70 km at the center of Europa’s disk. We note that the
spatial scale of the SINFONI pixels over-samples the diffraction-limited
spatial resolution of the VLT at the wavelengths of interest, which is $\sim$
130 km at the center of Europa. More detailed information on these
observations can be found in Table 1 in Ligier et al. (2016).

We downloaded the raw data and associated calibration files from the ESO
Science Archive Facility and followed a similar reduction routine as Ligier et
al. (2016). As a first step in the data reduction process, we used the ESO
SINFONI data reduction pipeline v3.3.3 with EsoRex (Modigliani et al., 2007).
The pipeline identified bad, non-linear and hot pixels, performed standard
dark and flat lamp corrections, corrected for optical distortions in the image
slicer slitlets, determined a calibrated wavelength solution, and computed the
2D to 3D mapping to create a 3D data cube with two spatial dimensions and one
spectral dimension for each observation. We interrupted the pipeline before
the five offset frames were combined into a single mosaic to remove correlated
regions of bad-pixel patches, which were not identified by the pipeline, and
which were also noted by Ligier et al. (2016). We identified these bad pixel
regions using the spectral angle mapping criterion (SAM) (Kruse et al., 1993),
which describes the degree of similarity between two spectra. We masked out
pixels with a SAM angle that differed by more than $\sim$10% from the median
SAM angle between all individual spectra and the combined median in a
10$\times$10 pixel bounding box centered on the bad pixel. These bad pixel
regions, which makeup $\sim$7% of the total detector pixels, were excluded
from the final co-added data cube created after running the remainder of the
ESO pipeline. While Ligier et al. (2016) do not indicate what method they used
to identify these remaining bad pixel patches, we find that our identified bad
pixel patches closely match the pixels which appear to be masked in Figure 3
of Ligier et al. (2016).

Next, we used the standard star observations of the solar-type star HD18681,
which were taken immediately before or after the Europa observations on each
night, to correct for the instrument response, remove telluric absorption
features, and derive a relative reflectance calibration. Due to the range of
air masses over which the five Europa cubes and the calibration star were
observed each night, an on-sky angular separation between Europa and HD18681
of $\sim$17∘, and possibly changing atmospheric conditions, we found a simple
division by the telluric standard was not sufficient. The quality of the
telluric correction is particularly important for our analysis of the 2.07 µm
absorption, which is just beyond telluric CO2 and O2 absorptions between 2.0
and 2.05 µm. We therefore followed a similar telluric correction procedure as
described in Brown & Hand (2013) and continuum divided the observed telluric
spectrum by a best-fit polynomial to construct an atmospheric transmission
spectrum. We then empirically determined an exponential scaling factor which
adjusts the depths of the telluric absorptions in the transmission spectrum to
most closely match the telluric signature in each of the Europa observations,
paying particular attention to the quality of the telluric removal of the CO2
and O2 features between 2.0 and 2.05 µm. Each observation consists of hundreds
of spectra, which allowed us to determine the optimal scaling factor to very
high precision. The original stellar continuum fit was multiplied by this
best-fit atmospheric transmission spectrum to create a new standard star
spectrum with an appropriately scaled telluric signature, which was then
divided from the Europa observations. As described in Ligier et al. (2016), we
used a Lambertian surface to calculate the geometrically corrected reflectance
spectrum of each pixel and corrected for small offsets in the relative
reflectance of overlapping regions on Europa’s surface obtained during
different nights by scaling the band-integrated flux of each observation cube
so that the integrated reflectance matched in these overlapping regions. The
final data product is a series of data cubes, one for each hemispherical view
on Europa (centered at 30∘W, 55∘W, 130∘W, 225∘W, and 315∘W) with the
corresponding relative reflectance spectrum for each pixel 111The full,
reduced dataset is permanently archived at
https://doi.org/10.22002/agga7-v3393. A characteristic leading and trailing
hemisphere spectrum, at (90∘W, 0∘N) and (270∘W, 0∘N) respectively, are shown
in Figure 1, with the 2.07 µm absorption feature present only in the trailing
hemisphere spectrum.

Figure 2: (a) A map of Europa’s trailing hemisphere, showing a selection of
four of the 29 considered “rings of constant radiolysis” outlined in black,
which are used to determine whether the weak 2.07 µm absorption feature is
associated with Europa’s large-scale geologic units. The chaos terrains are
shown in red and Pwyll crater and its ejecta are orange. For each ring,
projected detector pixels are shown and their assigned geologic unit indicated
by color, where chaos terrains are dark red, Pwyll crater and its ejecta are
orange, and the ridged plains are gray. (Europa map image credit: NASA/JPL/
Björn Jónsson). (b) Summed spectra, by geologic unit, for all of the pixels
within each example ring. Spectra are normalized to 1.0 at 2.12 µm and offset
for clarity. The best-fit polynomial continuum is indicated by a black dashed
line. (c) Continuum removed absorption feature, by geologic unit, for the four
selected example constant radiolysis rings. The depth of the feature decreases
noticeably as we move from Ring 1 outwards, however there is no apparent
difference in the depth of the 2.07 µm feature between the chaos terrains
(red) and ridged plains (gray) for a given ring. This suggests that the
absorption feature is unaffected by the compositional difference between the
chaos terrains and ridged plains, and it is therefore unlikely to be related
to any of the proposed irradiation products of endogenic material. Instead,
the 2.07 µm feature appears to arise from the radiolytic processing of water
ice with exogenic material, possibly related to the sulfur radiolytic cycle.
One notable exception is that the 2.07 µm absorption feature is entirely
absent from Pwyll crater and its ejecta blanket. Figure 3: (a) Expected
trailing hemisphere irradiation patterns as a function of the angular distance
from the trailing hemisphere apex for cold sulfur ions (solid) (Cassidy et
al., 2013), radiolytically produced hydrated sulfuric acid (dashed) (e.g.
Carlson et al., 2002; Ligier et al., 2016), and 0.1 - 25 MeV electrons
(dotted) (Paranicas et al., 2009). The estimated energy fluxes are normalized
to 1.0 at the trailing hemisphere apex. (b) Calculated band area of the 2.07
µm absorption feature for each of the 29 “rings of constant radiolysis”
considered. We find no measurable difference in the absorption depth between
the chaos terrains and ridged plains. However, the absorption feature is
essentially absent from Pwyll crater and its ejecta blanket. (c) Calculated
2.07 µm band area of the chaos terrains from panel b, with the chaos terrains
further subdivided by type as described in Leonard et al. (2023). There is no
measurable difference in the 2.07 µm band area between the different types of
chaos terrain.

##  3 Spatial Distribution

With no clear resolution of the source of the 2.07 µm feature from previous
studies (Brown & Hand, 2013; Ligier et al., 2016), we turn to its spatial
distribution in order to determine whether its presence is associated with the
large-scale chaos terrains, which could confirm its suspected relationship to
endogenic salts and possibly Europa’s ocean chemistry. We note that endogenic
salts are expected within Europa’s bands and ridges as well as within the
chaos terrains (e.g. Dalton III et al., 2012). Indeed, while the exact
formation mechanism of these bands and ridges is not known, many proposed
mechanisms require a direct link between the ice shell and ocean (e.g. Head et
al., 1999; Fagents et al., 2000; Prockter & Patterson, 2009; Johnson et al.,
2017; Howell & Pappalardo, 2018). However, ridges on Europa are typically a
few hundred meters to a few kilometers wide (Collins & Nimmo, 2009) and bands
are no wider than 30 km (Prockter et al., 2002). At the spatial resolution of
the VLT ($\sim$130 km) the fractional coverage of a band or ridge feature
within a single pixel is small. If irradiation of endogenic salts were
responsible for the 2.07 µm feature, we would therefore expect to measure a
difference in the band strength between the large-scale chaos terrains which
fully cover a single VLT spatial resolving element, and the background plains,
which are overlaid by bands and ridges that only cover a small fraction of the
spatial resolving element and are therefore unresolved. Indeed, the spectral
signature of irradiated endogenic material correlated with the trailing
hemisphere chaos terrains is seen in the maps of Trumbo et al. (2020) which
have a comparable spatial resolution of $\sim$150 km. The 360 nm absorption
feature attributed to radiolytically processed endogenic material, for
example, is strongest within the large scale chaos terrains and is
significantly weaker or absent from the non-chaos pixels on the trailing
hemisphere. If confirmed, a similar association of the 2.07 µm absorption band
with Europa’s large-scale chaos terrains would therefore support the
hypotheses of Brown & Hand (2013) and Ligier et al. (2016), which suggest the
absorption arises from some radiolytic product of endogenic material.

One common way to examine the spatial distribution of an absorption feature is
to map its depth or area. However, the continuum shape between 2.0 and 2.2 µm
changes significantly across Europa’s surface, largely due to variation in the
strength of the 2.0 µm water absorption band. As can be seen in Figure 1, the
change in the slope, concavity, and overall spectral shape between the leading
and trailing hemispheres is larger than the depth of the 2.07 µm feature. The
shape of the spectrum in this region varies gradually across Europa’s surface,
and we find that challenges in precisely defining this changing continuum,
combined with significant pixel-to-pixel noise inherent in the IFU data, lead
to large uncertainties in the measured band strength. Another option is to
compare the depth of the 2.07 µm feature between the chaos and non-chaos
terrains integrated over the entire trailing hemisphere. However, the
irradiation bullseye is symmetric about the trailing apex while the
distribution of the chaos terrains on Europa’s trailing hemisphere is not. As
a result, the integrated chaos terrain spectrum samples areas on Europa with a
sufficiently different continuum spectrum than is sampled by the integrated
non-chaos pixels, such that it is difficult to determine whether these
continuum differences wash out the signal of a geologic association or if the
feature simply does not correlate with Europa’s geology. We therefore
construct a new technique wherein we consider rings of approximately constant
radiolysis, where the overall continuum between $\sim$2 to 2.2 µm is much more
constant, and compare the strength of the 2.07 µm feature for integrated
spatial pixels that are within different large-scale geologic units and
receive a similar irradiation dose.

To perform this spatial analysis, we first define the large-scale geologic
units using the new United States Geologic Survey (USGS) global geologic map
of Europa, which maps geologic units including craters, chaos terrains, bands,
and regional plains at a scale of 1:15M (Leonard et al., 2023). Because the
width of the bands and ridges are much smaller than our $\sim$130 km spatial
resolution, we exclude these from our large-scale geologic units. We also
exclude ten small craters with diameters less than $\sim$100 km and several
small patches of discontinuous crater ejecta which are unresolved at the
spatial resolution of the VLT. This leaves us with three distinct large-scale
geologic units on Europa’s trailing hemisphere - chaos terrains, ridged
plains, and Pwyll crater including its large ejecta blanket. The chaos
terrains can be further broken down into three sub-types - low albedo chaos,
mottled chaos, and knobby chaos as defined in Leonard et al. (2023).

We then define a set of “constant radiolysis rings” which cover the trailing
hemisphere, where each ring contains pixels that are expected to receive a
similar irradiation dose. While the idea is simple, the exact irradiation
geometry on Europa is complex and remains an active area of research
(Paranicas et al., 2009; Cassidy et al., 2013; Bagenal & Dols, 2020). The
location dependent irradiation dose depends on a number of factors, including
the type of particle, its energy, and the complex interaction between
Jupiter’s magnetosphere, the plasma torus, and Europa’s ionosphere and a
variety of irradiation patterns is possible (Bagenal & Dols, 2020). However,
sulfuric acid production via the sulfur radiolytic cycle relies on the
presence of sulfur ions from the plasma torus as well as radiolytic processing
from energetic electrons and light ions, which are important sources of
ionization energy that drive surface chemistry changes on Europa (Paranicas et
al., 2001, 2002, 2009; Cassidy et al., 2013). The shape of the sulfuric acid
bullseye is therefore a simple proxy for the total radiolytic processing from
the combined effects of the plasma torus and magnetospheric particle
bombardment occurring at a given location on Europa’s trailing hemisphere. To
match the shape of the sulfuric acid bullseye, we start with a series of
overlapping rings centered on the trailing hemisphere apex and defined by
$\cos(\theta)$, where $\theta$ is the angular distance from the trailing
hemisphere apex. We then empirically determine a latitudinal flattening
coefficient of 0.6 so that the shape of the rings matches the sulfuric acid
bullseye from Figure 10 in Ligier et al. (2016) and set the width of the rings
to match the diffraction-limited spatial resolution of the VLT. Figure 2 shows
a selection of four example rings distributed across the trailing hemisphere.

Once we have defined our constant radiolysis rings and large-scale geologic
units, we assign each data pixel to its respective ring and geologic unit.
Data pixels which overlap, typically because they were observed on two
separate nights, are separated into multiple polygons and intersecting regions
are averaged. In this way, we preserve the maximum possible spatial
information from the dataset. In order to account for the wings of the point
spread function (PSF), we exclude any spatial pixels whose central coordinates
are within one spatial resolving element of a boundary between geologic units.
Spatial pixels which cross the boundary of a ring are assigned to whichever
ring intersects the largest fraction of the pixel. Pixels with more than 40%
of their area in two separate rings are duplicated and assigned to both. Once
all of the pixels have been categorized, we sum the spectra from all of the
pixels assigned to a given geologic unit and constant radiolysis ring. We then
normalize these integrated spectra at 2.12 µm, just past the absorption
feature of interest, and plot the spectra for each geologic unit in order to
compare the depth of the 2.07 µm feature. Most of the rings only have
integrated spectra for the chaos terrains and ridged plains, however a few
rings also contain pixels from Pwyll crater and its ejecta blanket. The
irradiation pattern is expected to be symmetric about the trailing hemisphere
apex (Paranicas et al., 2009; Hendrix et al., 2011; Cassidy et al., 2013), so
we do not expect the east-west dichotomy in the terrain types, with
significantly more chaos towards the sub-Jovian hemisphere and more plains
towards the anti-Jovian hemisphere, to affect the results of our analysis. We
also fit a fifth order polynomial continuum to each of the integrated spectra
between 1.98 and 2.16 µm, excluding the region between 2.04 and 2.1 µm which
contains the absorption feature and compare the continuum removed absorptions
for the different geologic units. We choose a fifth order polynomial because
it is able to provide a fair visual match to the continuum near the center of
the trailing hemisphere as well as in the more icy regions where a third order
polynomial produces an artificially deep absorption feature because the
polynomial is not able to account for the concavity of the continuum shape,
which is strongly influenced by the 2.0 µm water ice absorption feature. We
then continuum divide each spectrum and integrate the residual absorption
between 2.05 and 2.1 µm to compute an integrated band area.

As can be seen for a selection of four constant radiolysis rings in Figure 2,
visual inspection of the integrated and continuum removed spectra (panels (b)
and (c)) reveals that there is no discernible difference in the depth of the
2.07 µm feature between the chaos terrains (red) and surrounding ridged plains
(gray). Indeed, we find no discernible difference in the absorption band
between the chaos and ridged plains in any of the 29 rings of constant
radiolysis, which completely cover the trailing hemisphere. Likewise, we do
not find a measurable difference in the integrated band areas between the
chaos and ridged plains units for each ring, as seen in Figure 3. We do not
include error bars on the measured band areas in Figure 3 because we are
unable to accurately calculate these uncertainties with the unknown effects of
the changing continuum. However, we estimate a rough error of $\sim 10^{-4}$
on the integrated band area based on the scatter in the measured band areas of
adjacent, overlapping rings. Figure 3(c) shows the measured band areas for
each chaos sub-type, when present in each of the 29 constant radiolysis rings.
Separately considering each chaos sub-type (low albedo chaos, mottled chaos,
and knobby chaos) identified by Leonard et al. (2023) also reveals no
discernible difference in the depth of the 2.07 µm feature between the chaos
terrain sub-types. Most of the trailing hemisphere chaos terrains are
comprised of low albedo chaos ($\sim$64% fractional coverage) and mottled
chaos ($\sim$32%), with only $\sim$4% of the chaos at larger angular distances
from trailing center classified as knobby chaos.

It is interesting to note that the 2.07 µm absorption feature is clearly
absent from Pwyll crater and its ejecta blanket, even though Pwyll is located
in a region with relatively high amounts of radiolysis. Ring 2 in Figure 2
highlights the absence of this feature at Pwyll, and also illustrates that
significant differences in the depth of the 2.07 µm feature between different
large-scale geologic units are identifiable with our spatial analysis
technique. The absence of a 2.07 µm absorption feature within the Pwyll crater
ejecta blanket is also clearly seen in Figure 3(b), where the measured band
areas for Pwyll are very small compared to the band areas of the chaos
terrains and ridged plains within the same constant radiolysis rings.

While the strength of the 2.07 µm absorption feature appears to be constant
between the chaos terrains and ridged plains for each constant radiolysis
ring, a clear decrease in the strength of the feature is seen as we move from
the innermost rings with the highest expected radiolysis rates to the
outermost rings where the 2.07 µm feature is no longer seen. This decrease in
band area is consistent with radiolytic production of the species responsible
for the absorption feature. As a third and final check, we examined the
division of the integrated ridged plains by the chaos terrain spectra and
found that for some of the rings there was a broad dip in the ratioed spectrum
between $\sim$ 2 to 2.2 µm, which is a clear spectral match to water ice. This
subtle difference in the continuum shape seen between the chaos terrains and
ridged plains is consistent with a slight excess of water ice and therefore a
deeper 2.0 µm water ice absorption band in the ridged plains relative to the
chaos terrains. We find no sign of any additional structure in the ratioed
spectra between 2.05 and 2.1 µm, further confirming that the 2.07 µm
absorption feature is unaffected by the compositional differences between the
large-scale chaos terrains and ridged plains on Europa’s trailing hemisphere.

##  4 Discussion

###  4.1 Chaos Terrains and Ridged Plains

As evidenced by its absence on Europa’s leading, less-irradiated hemisphere,
the species responsible for the 2.07 µm feature is most likely a radiolytic
product formed via sulfur radiolysis or processing via magnetospheric particle
bombardment. Our spatial distribution analysis reveals that the depth of
Europa’s 2.07 µm feature is spatially associated with the sulfuric acid
bullseye on the trailing hemisphere, which we use as a proxy for the amount of
radiolytic processing due to the combined effects of the plasma torus and
magnetospheric particle bombardment. This distribution is consistent with a
radiolytic origin for the absorption feature. However, we find no evidence of
any spatial association with the large-scale chaos terrains which would be
expected if the formation pathway required radiolytic processing of endogenic
salty material. For any given ring of constant radiolysis, we find no
measurable difference in the band area of the 2.07 µm absorption feature
between the large-scale chaos terrains and the icier ridged plains suggesting
that the irradiation process which creates the 2.07 µm absorption is
independent of any endogenic material within Europa’s recent geology. With
both the epsomite hypothesis of Brown & Hand (2013) and the magnesium
chlorates hypothesis of Ligier et al. (2016) relying on the presence of
irradiated endogenic magnesium chloride (MgCl2), and no known exogenic source
of magnesium ions to Europa (Bagenal & Dols, 2020), we find that the spatial
distribution of the 2.07 µm absorption band is inconsistent with both of these
proposed origins for the absorption feature. Instead, our results suggest that
the species responsible for Europa’s 2.07 µm absorption feature is a
radiolytic product of water ice and exogenically sourced material and is not
affected by the presence of whatever endogenic salt component exists within
the chaos terrains.

###  4.2 Pwyll Crater

Pwyll crater has an $\sim$26 km diameter with a bright ray ejecta blanket that
extends over 1000 km (Greeley et al., 1998). Pwyll is thought to be young,
with estimates for its age ranging from $\sim$ 3 - 18 Myr (Bierhaus et al.,
2001). The dark, red crater itself has apparently excavated endogenic material
from a depth of $\sim$1 km (Garozzo et al., 2010) and high spatial resolution
Galileo/NIMS observations show evidence of asymmetric bands interpreted as
salt-rich material at very high concentrations (Fanale et al., 2000). However,
the composition sharply transitions to be very ice-rich at the crater edge and
throughout the ejecta blanket (Fanale et al., 2000). At the spatial resolution
of the VLT, we do not resolve the dark salty crater itself, and are dominated
by the spectral signature of the bright ice-rich ejecta blanket. The distinct
lack of a 2.07 µm absorption feature within the Pwyll crater ejecta blanket,
despite its presence in both the chaos terrains and ridged plains, implies
that radiolytic processing has not yet had enough time to form the species
responsible. The lack of a 2.07 µm absorption within Pwyll may simply mean
that the irradiation timescale for the process which produces the absorption
feature is longer than the age of Pwyll crater. Or alternatively, there may
not have been time for enough exogenic material from the plasma torus or
magnetosphere to build up at Pwyll thus limiting the production of the species
responsible for the absorption. In either case, the absence of the absorption
within the Pwyll crater ejecta blanket places a strong lower limit of the age
of Pwyll on the timescale over which the species responsible for the 2.07 µm
absorption forms on Europa.

###  4.3 Comparison with Irradiation Patterns

Figure 3(b) shows the measured band area for each ring of constant radiolysis
as a function of the angular distance due north from the trailing hemisphere
apex. Panel (a) shows the expected shape of several distinct irradiation
patterns on Europa’s trailing hemisphere for comparison with the measured band
areas. As previously noted, the absorption feature is absent from the Pwyll
crater ejecta blanket, but both the chaos terrains and background ridged
plains show a clear decrease in the band area with increasing angular
distance. The measured fall-off in band area for the 2.07 µm absorption
feature with increasing distance from the trailing hemisphere apex is broadly
consistent with a formation pathway driven by the irradiation pattern of the
sulfur plasma, energetic ions and electrons, or the measured sulfuric acid
bullseye. Cold sulfur plasma bombards the trailing hemisphere with a fall-off
described by the cosine of the angle from trailing center (Hendrix et al.,
2011; Cassidy et al., 2013), which we would expect to see reflected in the
measured 2.07 µm band depths if cold sulfur plasma controlled the formation of
the absorption. If the formation pathway is instead driven by energetic
electrons, we would expect a fall-off consistent with the shape of the power
per unit area contours for 10 keV - 25 MeV electrons from Figure 8 in
Paranicas et al. (2009). Or, if the formation pathway is driven by a
combination of these irradiation patterns as expected for the sulfur
radiolytic cycle, we might expect to find a fall-off consistent with the
flattened cosine of the sulfuric acid bullseye.

As can be seen in Figure 3(a), the differences in the shapes of these
distributions are small near the center of the trailing hemisphere, where the
2.07 µm absorption feature is strongest, and become more pronounced farther
out where the feature has all but disappeared. Uncertainties in measuring the
band depths for the various rings and a significant change in the continuum
shape moving outwards from trailing center are sufficiently large so as to
obscure the subtle differences which could differentiate between these
irradiation signatures. The measured band areas for the chaos terrains and
ridged plains in Figures 3(b) and (c) show a plausible match to all three
irradiation patterns shown in panel a. Our results remain consistent with a
variety of radiolytic production pathways and we are unable to determine
whether the overall irradiation flux or the presence of any specific exogenic
material is the limiting factor in the formation of the 2.07 µm absorption.

###  4.4 Potential Origins for the 2.07 µm Absorption

If the 2.07 µm absorption feature does indeed arise from the radiolytic
processing of water ice and exogenically sourced material on Europa’s trailing
hemisphere, the products of the known sulfur cycle offer an obvious first
place to search for a plausible spectral match. Laboratory experiments have
shown that various irradiation products can be created in electron or ion
irradiated water ice bearing sulfur ions, or via sulfur ion bombardment of
pure water ice. However, the specific products produced depend strongly on the
temperatures, energies, and projectiles involved (Moore et al., 2007;
Strazzulla et al., 2007; Loeffler et al., 2011). While hydrated sulfuric acid
(H2SO4) is expected to be the dominant irradiation product on Europa, some of
the possible intermediate products include sulfur dioxide (SO2), hydrogen
sulfide (H2S), and various sulfate anions (SO4^2-, SO3^2-, HSO4^-) which can
combine to form species such as sulfonic acid (SO2H2) or hydrogen disulfide
(H2S2). Tribbett & Loeffler (2022) demonstrated that H2O + H2S + O3 ice
mixtures at Europa temperatures can undergo thermal oxidation reactions on
laboratory timescales, which may affect the steady state composition and
intermediary products of the radiolytic sulfur cycle.

Calculations based on laboratory experiments of sulfur ion implantation in
water ice have shown that radiolysis can produce the expected concentration of
sulfuric acid hydrate on Europa during $\sim 10^{4}$ years (Strazzulla, 2011).
Therefore, if the feature arises from an intermediary of the sulfur cycle or a
parallel irradiation process, it is somewhat surprising that we do not see any
evidence for the 2.07 µm absorption feature in Pwyll crater. However, Europa’s
UV to visible albedo ratio, which is anti-correlated with irradiation induced
discoloration on the trailing hemisphere, also shows an enhancement at Pwyll
suggesting that the discoloration timescale is longer than the age of Pwyll
crater (Burnett & Hayne, 2021) and consistent with the observed absence of the
2.07 µm absorption feature. If the 2.07 µm feature does arise from an
intermediary of the radiolytic sulfur cycle, the lack of a feature at Pwyll
could suggest that Pwyll crater is much younger than expected, that it has not
yet had enough time to build up a sufficient amount of sulfur for sulfur
radiolysis to occur, or alternatively that the calculations of Strazzulla
(2011) significantly underestimate the timescale over which the sulfur cycle
should reach an equilibrium state. Future laboratory work may be crucial for
understanding this discrepancy.

Additionally, the 2.07 µm absorption feature may also be explained by a
parallel radiolytic cycle or some other unknown irradiation product. For
example, the addition of CO2 ice into various sulfur radiolysis studies
produced additional products such as carbonyl sulfide (OCS), carbon disulfide
(CS2), carbonic acid (H2CO3), and other carbon and sulfur bearing species
(Garozzo et al., 2010; Mahjoub et al., 2017). While CO2 has been detected on
Europa (McCord et al., 1998; Hansen & McCord, 2008; Carlson et al., 2009), its
spatial distribution across the trailing hemisphere is largely unconstrained.
It is therefore uncertain whether an irradiation product of a combined carbon-
sulfur radiolytic cycle is consistent with our observed spatial distribution,
but nevertheless worth investigating.

We completed an extensive literature search of these potential sulfur- and
carbon- bearing intermediary species and other possible irradiation products
but were unable to find a plausible spectral match for the 2.07 µm absorption
amongst existing data sets. We are therefore unable to identify the source of
the 2.07 µm absorption feature, highlighting the need for additional
irradiation experiments at Europa-like temperatures, particularly those
including spectra across the full near-infrared wavelength range, in order to
better understand the sulfur radiolytic cycle and determine whether any
generated products can explain the 2.07 µm absorption feature.

##  5 Conclusions

Using archived VLT/SINFONI H+K band spectra, we have shown that the presence
and band area of the 2.07 µm absorption feature on Europa’s trailing
hemisphere is not spatially associated with the large-scale geology, except
for Pwyll crater and its ejecta blanket which lacks the absorption feature.
There is, however, a spatial association between the 2.07 µm absorption band
and the trailing hemisphere sulfuric acid bullseye suggesting that the
formation of the 2.07 µm feature on Europa is independent of the endogenous
salts thought to be present within the chaos terrains and is most likely an
irradiation product of water ice and exogenic material. Thus, we find that
neither epsomite nor a combination of magnesium chloride and magnesium
perchlorate, as proposed by Brown & Hand (2013) and Ligier et al. (2016),
respectively, are likely to explain the spatial distribution of the 2.07 µm
absorption feature and we consider an alternative hypothesis. We propose that
the source of this feature may be an intermediary product of the radiolytic
sulfur cycle, or something formed during the bombardment of water ice by
electrons or the remaining (non-sulfur) ion species. Current laboratory data
of relevant radiolytic species is fairly sparse in the 2.0 - 2.2 µm range and
we are unable to identify any plausible spectral matches. This highlights the
need for more laboratory data at these wavelengths and at Europa-like
conditions in order to identify radiolytic product(s) which may be responsible
for the 2.07 µm absorption feature and provide insights into the nature of the
sulfur- and possibly carbon- radiolysis occurring on Europa.

Acknowledgments

This research has made use of the services of the ESO Science Archive Facility
and is based on observations collected at the European Southern Observatory
under ESO program 088.C-0833(A). M.R.D. would like to thank Dr. Erin Leonard
for providing up-to-date shape files from the new United States Geologic
Survey (USGS) global geologic map of Europa, which we used to define the
various geologic units discussed in the text. S.K.T. is supported by the
Heising-Simons Foundation through a 51 Pegasi b postdoctoral fellowship.

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