Space Science Reviews (2024) 220:51
https://doi.org/10.1007/s11214-024-01072-3
Radar for Europa Assessment and Sounding: Ocean to
Near-Surface (REASON)
Donald D. Blankenship
·
Alina Moussessian
·
Elaine Chapin
·
Duncan A. Young
·
G. Wesley Patterson
·
Jeffrey J. Plaut et al.
[full author details at the end of the article]
Received: 1 August 2023 / Accepted: 29 April 2024
© The Author(s) 2024
Abstract
The Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) is a
dual-frequency ice-penetrating radar (9 and 60 MHz) onboard the Europa Clipper mission.
REASON is designed to probe Europa from exosphere to subsurface ocean, contributing the
third dimension to observations of this enigmatic world. The hypotheses REASON will test
are that (1) the ice shell of Europa hosts liquid water, (2) the ice shell overlies an ocean and is
subject to tidal flexing, and (3) the exosphere, near-surface, ice shell, and ocean participate
in material exchange essential to the habitability of this moon. REASON will investigate
processes governing this material exchange by characterizing the distribution of putative
non-ice material (e.g., brines, salts) in the subsurface, searching for an ice–ocean interface,
characterizing the ice shell’s global structure, and constraining the amplitude of Europa’s
radial tidal deformations. REASON will accomplish these science objectives using a com-
bination of radar measurement techniques including
altimetry
,
reflectometry
,
sounding
,
in-
terferometry
,
plasma characterization
,and
ranging
. Building on a rich heritage from Earth,
the moon, and Mars, REASON will be the first ice-penetrating radar to explore the outer so-
lar system. Because these radars are untested for the icy worlds in the outer solar system, a
novel approach to measurement quality assessment was developed to represent uncertainties
in key properties of Europa that affect REASON performance and ensure robustness across
a range of plausible parameters suggested for the icy moon. REASON will shed light on a
never-before-seen dimension of Europa and – in concert with other instruments on Europa
Clipper – help to investigate whether Europa is a habitable world.
Keywords
Europa
·
Ice shell
·
Ice-penetrating radar
·
Europa Clipper
1Introduction
1.1 Europa Clipper Summary and Science Motivation for REASON
The icy landforms of Europa, some familiar and others enigmatic and unique in the Solar
System, indicate a complex and recently (
100 Myr) active history for its ice shell (Bier-
haus et al.
2009
; Doggett et al.
2009
; Kattenhorn and Hurford
2009
). The formation of these
Europa Clipper: A Mission to Explore Ocean World Habitability
Edited by Haje Korth, Bonnie J. Buratti and David Senske
51
Page 2 of 91
D.D. Blankenship et al.
landforms, as well as the nature of the ice shell and character of the deep interior, have been
driven by the analysis of data acquired during the Voyager 1 and 2 spacecraft encounters
with Jupiter (1979) and the Galileo mission (1995–2003) (Alexander et al.
2009
). These
data include direct measurements in two dimensions of Europa’s surface (e.g., imaging and
spectroscopy) and indirect measurements in a single dimension of Europa’s global subsur-
face properties (e.g., radio science and magnetometry).
In particular, analyses of these data have provided insights into the subsurface domains
within and bounding Europa’s ice shell. Key among these insights was the presence of a
global ocean hidden beneath the ice shell, inferred from the Galileo magnetometer data,
(Carr et al.
1998
; Khurana et al.
1998
; Pappalardo et al.
1999
;Kivelsonetal.
2000
)and
a rocky mantle of unknown complexity, inferred from radio science data (Anderson et al.
1998
). However, direct measurements of the near-surface (i.e., the upper few hundred me-
ters) and subsurface of Europa’s ice shell, including its properties, structure, and bounding
interfaces, are required to address the daunting framework of working hypotheses arising
from interpretations of previous Voyager and Galileo mission data.
The key example of this are the many working hypotheses for the exchange of material
between the surface and subsurface, and their implications for habitability,
1
which made Eu-
ropa a top candidate for exploration in both the 2003 and 2011 Planetary Decadal Surveys
(National Research Council
2003
,
2011
). Assessing Europa’s potential for habitability re-
quires understanding the structure and evolution of its ice shell as well as its coupling to the
ocean/rocky mantle system and exosphere. The Radar for Europa Assessment and Sound-
ing: Ocean to Near-surface (REASON) is designed to operate in conjunction with other
investigations on Europa Clipper to accomplish this objective (see Pappalardo et al.
2024
,
this collection). REASON is an ice-penetrating radar
2
optimized to probe Europa from its
exosphere to its subsurface ocean. If successful, the REASON investigations will revolu-
tionize our understanding of Europa’s ice shell by providing the first direct measurements
of the structure and properties of these subsurface domains and their bounding interfaces
at scales critical for understanding exchange processes that govern Europa’s potential for
habitability.
REASON baseline science investigations of Europa require four types of measurement
techniques: radar
altimetry
to determine surface topography, radar
reflectometry
to study
surface roughness and near-surface
3
structure and composition as well as radar
sounding
to
probe both the shallow and full depth subsurface domains of the ice shell coupled with radar
interferometry
to discriminate nadir subsurface features from radar returns of cross-track
surface features. Two additional measurement techniques, radar
plasma characterization
(to
1
Habitability in this context refers to the conditions necessary to support life as we know it (see Vance et al.
2023
, this collection).
2
Here we use the terms radar sounder and ice-penetrating radar to refer specifically to pulse radar systems on
airborne or space-based platforms (Schlegel et al.
2022
). Radar sounder is the term typically used in planetary
radioglaciology, whereas Radio Echo Sounder (RES) and ice-penetrating radar are terms commonly used in
terrestrial radioglaciology.
3
Formally, near-surface refers to depths
<
300 m, shallow subsurface refers to depths
≤
3 km, and full depth
subsurface refers to depths from the surface to
≤
30 km. The shallow and full depth subsurface depths are
defined consistent with the thin and thick ice shell modes described in Sect.
4.2.6
. The near-surface is defined
as the region close to the surface where subsurface structure cannot be directly resolved by the radar (i.e., the
vertical resolution). In practice, the vertical resolution is governed by the bandwidth, windowing function,
and dielectric properties of the material; however, here we adopt the required HF vertical resolution in ice as
a conservative upper bound (Table
2
).
Radar for Europa Assessment and Sounding: Ocean to Near-Surface...
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51
derive the total electron content (TEC) in the ionosphere) and radar
ranging
4
(to measure
the differential range between the spacecraft and the surface through time at groundtrack
crossover points) provide, respectively, a critical framework for understanding the boundary
conditions for surface/near-surface exchange and the energetics of tidal deformation across
the ice shell’s full breadth and depth.
1.2 Radar Sounding (and Assessment) of Ice from the Inner to the Outer Solar
System
Radar sounding refers to a technique where electromagnetic energy is directed and transmit-
ted into the subsurface, producing reflections which are recorded as signals. This is distinct
from side-looking imaging radars, where energy is directed and transmitted off-nadir and
backscattered energy is recorded as signal. Consequently, smooth topographic features that
are highly reflective and specular are “bright” (i.e., high signal-to-noise ratio, SNR) in radar
sounding data and “dark” (i.e., low SNR) in imaging radar data. Ice penetrating radars have
the unique capability to penetrate deep into the subsurface of icy bodies due to the high
transparency of ice at MHz frequencies (Warren
1984
; Blankenship et al.
2009
;Kofman
et al.
2010
; Pettinelli et al.
2015
). As these transmitted radio waves travel through the sub-
surface, their reflections, referred to as “returns” or “echoes”, vary in power and character
as they interact with subsurface horizons and structures that are associated with dielectric
contrasts (i.e., changes in their relative dielectric permittivity
5
). A critical consideration for
radar studies of icy worlds across the solar system is the leverage provided by the thermo-
physical transformation of ice to water, which is characterized by a contrast in permittivity
of over an order of magnitude.
From its early origins measuring the thickness of glaciers and the extent of floating ice
shelves on Earth, ice penetrating radar has proven to be a versatile geophysical technique
used to constrain the presence of sub-ice water (e.g., water channels, lakes, ocean incur-
sion), properties of the entire ice column (e.g., porosity, thermal state, composition), basal
conditions (e.g., frozen vs. thawed, floating vs grounded ice, topographic heterogeneity),
and ice dynamics (e.g., variations in accumulation/melting and flow/fracture) (Schroeder
et al.
2020
). Application of radar sounding as a ground based and airborne geophysical
technique has been extended to orbital studies of bodies beyond Earth, with extensive cov-
erage of the Moon (Porcello et al.
1974
) and Mars (Picardi et al.
2004
; Seu et al.
2007
;
Jordan et al.
2009
). Multiple mission concepts to study the icy Galilean moons of Jupiter
have highlighted subsurface sounding of the ice as a necessary technique for accomplishing
proposed science goals (Greely and Johnson
2004
) or explicitly included ice-penetrating
radar as part of the model payload (Ludwinski et al.
1998
; Clark et al.
2009
; Lewis et al.
2016
). For Europa in particular, early feasibility studies supported radar sounding with an
ice penetrating radar as capable of characterizing the three-dimensional distribution of sub-
surface water, including the potential detection of a subsurface ocean (Chyba et al.
1998
;
Blankenship et al.
1999
). Ultimately, these early mission concepts evolved into the National
Aeronautics and Space Administration (NASA) Europa Clipper mission (Brown
2021
)and
the European Space Agency (ESA) JUpiter ICy moons Explorer (JUICE) mission (Grasset
4
REASON’s
ranging
measurement technique should not be confused with ranging as used by Gravity/Radio
Science (see Mazarico et al.
2023
, this collection). The term
ranging
was adopted to distinguish the measure-
ment technique from
altimetry
.
5
Permittivity is a complex quantity which describes the polarizability of a material (i.e., energy storage and
dissipation in response to an applied electromagnetic field) (Sihvola
1999
).
51
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D.D. Blankenship et al.
Fig. 1
Radargram generated from ice-penetrating radar data collected over Totten Ice Shelf, Antarctica
(TOT/JKB2d/X16a) modified from Lindzey (
2015
)
et al.
2013
), both of which include ice penetrating radars. The radar data collected in the
Jovian system by REASON on Europa Clipper and by the Radar for Icy Moons Exploration
(RIME) on JUICE (Bruzzone et al.
2013
) will mark the ultimate extension of our capacity
for radar sounding and assessment of ice from the inner to the outer Solar System.
1.3 Earth’s Cryosphere
Radar sounding and assessment of Earth’s cryosphere represents the foundation from which
planetary radioglaciology has been and will continue to be built. As such, terrestrial data
sets, analysis techniques, and confounding factors represent critical historical context for
interpreting future radar sounding data collected on other worlds.
Radar surveys of the terrestrial cryosphere, both floating and grounded (Fig.
1
), were
originally pursued to generate maps of ice thickness and surface/basal topography (Bailey
et al.
1964
; Gudmandsen
1969
) – an effort that continues to date (Lythe and Vaughan
2001
;
Fretwell et al.
2013
; Frémand et al.
2023
). Surveys in recent decades have expanded their
scope from identification to characterization of the bed (Schroeder et al.
2020
and references
therein), where bed characteristics are typically quantified using the power of the basal return
(Peters et al.
2005b
; Christianson et al.
2016
; Chu et al.
2021
). Radar sounding data are often
visualized using radargrams (Fig.
1
), which display the power of the return as a function
of travel time (fast time) on the vertical axis and along-track distance (slow time) on the
horizontal axis.
On Earth, radar altimetry profiles collected over ice have been an essential foundation
for understanding the evolution of our glaciers, ice sheets, and ice shelves (Rémy and
Parouty
2009
; Fricker and Padman
2012
). These profiles, obtained by identifying the surface
echo and the associated delay, have long been used to quantify the basal stress regime for
grounded ice, which is a function of ice surface slope (Cooper et al.
1982
; Bentley
1987
).
Knowledge of the state of stress provides essential context for discriminating geologic fea-
tures within the ice and the hypothesized processes responsible for their formation (e.g.,
Blankenship et al.
2001
). For floating ice, the detection of liquid water can be confirmed by
comparing the projected ice thickness, derived from surface elevations measured by altime-
try, to the measured ice thickness, obtained directly by sounding (Jenkins and Doake
1991
;
Fricker et al.
2001
). An example of particular relevance to Europa are floating icebergs (Pe-
ters et al.
2007b
). As illustrated in Fig.
2
, floating icebergs will protrude above the surface
Radar for Europa Assessment and Sounding: Ocean to Near-Surface...
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51
Fig. 2
Radargram generated
from ice-penetrating radar data
collected over iceberg B15,
Antarctica
(MCM/SJB2/BERG04c;
Blankenship, personal
communication)
Fig. 3
Near-surface properties of McMurdo Ice Shelf, Antarctica derived from ice-penetrating radar data,
modified from Grima et al. (
2016
)
by
∼
10% of the total ice thickness due to the
∼
10% difference in densities of water and ice
(
∼
1000 and 920 kg/m
3
, respectively).
In addition, assessment of near-surface properties has become increasingly important for
Earth’s cryospheric systems as the thermal/mass exchange between the ice and our atmo-
sphere is increasingly perturbed (Humphrey et al.
2012
; Machguth et al.
2016
; Mortimer
et al.
2016
). The embodiment and implications of these changes can contrast substantially
with the grounded and floating ice systems of Earth providing a broad diversity of con-
straints on ice accumulation and melting for its ice sheets and ice shelves. To understand
these complex and heterogeneous ice/water systems, statistical analyses of surface echo am-
plitude have been used to separate the reflected and scattered components of radar signals
of multiple frequencies to simultaneously obtain surface roughness and bulk permittivity
of the near-surface for both floating and grounded ice systems at multiple scales (Fig.
3
)
(Grima et al.
2014a
,
b
,
2016
,
2017
,
2019
). It has also been possible to combine these multi-
ple frequency approaches with a careful assessment of frequency segments across the band
51
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D.D. Blankenship et al.
Fig. 4
Radargram generated from ice-penetrating radar data collected over iceberg B15, Antarctica showing
extensive surface and basal crevassing, modified from Peters et al. (
2007b
)
of a particular radar to constrain both the lateral and vertical heterogeneity of these bulk
near-surface properties (Chan et al.
2023b
).
A great deal of the early airborne radar assessment and sounding of Earth’s cryosphere
was focused on its ice shelves (Fig.
1
) (Vaughan et al.
1995
). It was hypothesized over a
half century ago that instabilities in these ice shelves would be the primary trigger for rapid
draining of the substantial interior grounded ice reservoir for large segments of the Antarctic
ice sheet, causing sea level rise (Weertman
1974
; Mercer
1978
). Mapping of both surface and
basal crevassing (Fig.
4
) was an important element of this work with parallel studies of ice
shelf thickness. These analyses, which initially focused on qualitatively identifying regions
of clutter (Jezek et al.
1979
; Jezek and Bentley
1983
; Jezek
1984
), have recently evolved
to more quantitative assessments of crevassing processes, including the discrimination of
water versus ice filled basal crevassing (Peters et al.
2007b
). We anticipate that the arc of
radar studies of the icy worlds of the outer planets will follow a similar pattern in the shallow
subsurface, where brittle fracture might dominate.
An offshoot of the early radar sounding of Earth’s ice shelves, perhaps even more rele-
vant to icy ocean worlds, was the misinterpretation of the sub-ice ocean interface from radar
sounding over Filchner-Ronne Ice Shelf (Robin et al.
1983
; Crabtree and Doake
1986
).
When more precise ice surface altimetry became available, it was discovered that the “mea-
sured” thickness from radar sounding could not be reconciled with the new measurements of
surface elevation using Archimedes’ principle (Thyssen
1988
). Subsequent drilling showed
unequivocally that the basal feature that was interpreted as the ice–ocean interface was in-
stead a boundary between the glacier-derived floating “meteoric ice” and saltier “marine
ice” frozen from the ocean below (Engelhardt and Determann
1987
; Oerter et al.
1992
),
as illustrated in Fig.
5
. From that point on the radar altimetry and sounding observations
were combined to establish and map the thickness of the underplated marine ice to test vari-
Radar for Europa Assessment and Sounding: Ocean to Near-Surface...
Page 7 of 91
51
Fig. 5
Radargram generated from ice-penetrating radar data collected over Filchner-Ronne Ice Shelf, Antarc-
tica illustrating how a marine–meteoric ice interface could be mistaken as the ice–ocean interface, modified
from Thyssen (
1988
)
ous working hypotheses for the distribution of melting and freezing associated with sub-ice
ocean circulation (Fricker et al.
2001
; Joughin and Vaughan
2004
; Lambrecht et al.
2007
).
For grounded ice sheets on Earth, radar sounding has also led to the discovery of a vast
inventory of subglacial lakes in both the Arctic and Antarctic (Livingstone et al.
2022
).
Traditionally, this is accomplished by detecting regions that appear “brighter” than the sur-
rounding terrain (Carter et al.
2007
; Ilisei et al.
2019
). Again, surface altimetry serves as
an essential complementary data set to identify these subglacial lakes (Fig.
6
). Ice thickness
estimates coupled with surface elevation measurements enable hydrostatic analysis to de-
termine whether ice over putative lakes is in hydrostatic equilibrium (Vaughan et al.
2007
;
Ewert et al.
2012
). Variations in ice surface topography through time can indicate drainage
and recharge of active subglacial lakes (Wingham et al.
2006
; Siegfried and Fricker
2021
).
Confounding interpretation, some subglacial lakes identified through altimetry do not ap-
pear radar-bright (Carter et al.
2007
; Humbert et al.
2018
; Lindzey et al.
2020
). Ultimately
these anomalous subglacial lakes can only be unambiguously confirmed through subglacial
drilling (Talalay
2012
; Tulaczyk et al.
2014
;Priscuetal.
2021
)orseismicsurveys(Peters
et al.
2008
; Woodward et al.
2010
;Horganetal.
2012
).
The pace of radar studies of Earth’s cryosphere is increasing rapidly, not only because of
the threat of its role in rapid sea level rise on Earth, but also because of the recognition of
the importance of potential ice-covered habitats hosted by both rock and water systems on
Earth for understanding habitability across the solar system. Great advances have recently
51
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D.D. Blankenship et al.
Fig. 6
Altimetry profile (top) and radargram (bottom) generated from ice-penetrating radar and laser altime-
try data collected over Lake Vostok, Antarctica illustrating how complementary altimetry and radar sounding
data can support the identification of subglacial lakes, modified from Blankenship et al. (
2009
)
been made in quantifying radar reflectivity and scattering in pursuit of understanding ex-
change processes for the surface/near-surface and full depth/water interface studies on Earth.
Reflectivity has often been considered synonymous with the Fresnel reflection coefficient,
governed by the contrast in dielectric properties across an interface (Ulaby and Long
2015
;
Christianson et al.
2016
). However, the apparent reflectivity is also influenced by scattering
associated with the wavelength scale roughness of the surface and/or interface (Peters et al.
2005b
; Campbell
2009
;Rippinetal.
2014
) and properties of the subsurface ice, including
the presence of volume scatterers (Chu et al.
2018
; Culberg and Schroeder
2020
). Another
factor influencing reflectivity is the electrical conductivity, which is governed by the ther-
mal profile within the ice column as well as the concentration of lattice soluble impurities
(Fujita et al.
2000
; Moore
2000
; MacGregor et al.
2007
; Matsuoka et al.
2012
; MacGregor
et al.
2015
). Uncertainty in these factors challenges interpretation of radar data (Matsuoka
2011
). Methods have been developed to deconvolve these factors, which include estimating
and correcting attenuation through the ice column (Ashmore et al.
2014
; Hills et al.
2020
),
leveraging the statistics of echo power over a region (Grima et al.
2014a
), or examining the
properties of the return signal itself (Schroeder et al.
2014
; Jordan et al.
2017
). These novel
analysis techniques have enabled the mapping of ice shelf brines (Grima et al.
2016
), im-
proved identification of frozen vs. thawed beds (Jordan et al.
2018
;Frankeetal.
2021
), and,
most notably, the discovery of new subglacial aqueous systems as candidates for habitable
environments (Schroeder et al.
2013
; Rutishauser et al.
2018
,
2022
).
Radar for Europa Assessment and Sounding: Ocean to Near-Surface...
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51
1.4 From the Moon to Mars and Back
One of the earliest applications of radar sounding beyond Earth was the Apollo 17 Lunar
Sounder Experiment (ALSE), a multi-frequency imaging radar (5, 15, 150 MHz or 60, 20,
2 m) with a subsurface sounding requirement (Porcello et al.
1974
). The longer wavelengths
enabled deeper penetration into the lunar regolith, whereas the shorter wavelengths enabled
higher quality surface images and profiles. The primary science objective was the detection
of subsurface discontinuities (sounding) and three secondary science objectives were sur-
face imaging, surface profiling, and radio astronomy (i.e., measurement of galactic noise)
(Phillips et al.
1973
; Porcello et al.
1974
). Using the 5 MHz band, the ALSE produced el-
evation profiles of the Moon (Brown et al.
1974
) and ultimately identified two reflectors
beneath Mare Serenitatis at depths of 0.9 km and 1.6 km and one reflector beneath Mare
Crisium at a depth of 1.4 km (Peeples et al.
1978
).
After almost forty years, the ALSE was followed by the Mars Advanced Radar for Sub-
surface and Ionospheric Sounding (MARSIS) instrument flown on ESA’s Mars Express mis-
sion (Picardi et al.
2004
; Jordan et al.
2009
). MARSIS is a dual-channel radar sounder oper-
ating between frequencies of 1.3 and 5.5 MHz for subsurface sounding and 0.1 and 5.5 MHz
for ionospheric sounding. The primary science objectives were to map the subsurface distri-
bution of water ice and liquid water, probe subsurface geology, characterize the surface, and
sound the ionosphere. The principal results of the first decade of observation by MARSIS
are summarized in Orosei et al. (
2015
), including confirming the hypothesis of a predomi-
nantly icy composition for the Polar Layered Deposits (Plaut et al.
2007
). Recent analyses of
MARSIS data collected over the South Polar Layered Deposits (SPLD) suggest the presence
of subglacial hypersaline bodies of water (Orosei et al.
2018
; Lauro et al.
2021
,
2022
); how-
ever, this interpretation has been the subject of intense debate (Sect.
9.1.2
). MARSIS was
closely followed by the SHAllow RADar (SHARAD), a radar sounder flown on NASA’s
Mars Reconnaissance Orbiter (MRO) designed to detect shallow subsurface interfaces (Seu
et al.
2004
). SHARAD operates at a center frequency of 20 MHz and with a bandwidth
of 10 MHz. Major scientific findings derived from the first nine years of SHARAD are
summarizedinCrocietal.(
2011
), including the discovery that many mid-latitude lobate
features are ice-rich, debris-covered glaciers (Holt et al.
2008
). At the poles, SHARAD data
revealed a highly complex and temporally-varying pattern of deposition and ablation/subli-
mation (Putzig et al.
2009
; Smith et al.
2016
; Campbell and Morgan
2018
), and made the
revolutionary discovery of CO
2
ice deposits at the south pole trapped beneath a surface lag
and capable of doubling the current atmospheric pressure if released (Phillips et al.
2011
).
Following MARSIS and SHARAD, Mars became the most globally well-characterized
body by ground-penetrating radar, and the target for new radar sounding data transitioned
back to the Moon. The goals of the Lunar Radar Sounder (LRS) flown on JAXA’s SELeno-
logical and ENgineering Explorer (SELENE) were to map the subsurface structure of the
Moon to 5 km depth with a vertical resolution of 100 m by operating between 4 and 6 MHz
and to make passive observations of natural radio and plasma waves between 10 kHz and
30 MHz (Sasaki et al.
2003
). Some key results from LRS are presented in Ono et al. (
2010
)
and include the discovery of numerous subsurface reflectors several hundred meters deep in
the nearside maria interpreted to be regolith interbedded with basaltic lava flows.
1.5 Heritage from Existing Ice-Penetrating Radars
Technology has advanced significantly since the first radar soundings of glaciers (Stern
1930
; Steenson
1951
; Waite and Schmidt
1962
; Turchetti et al.
2008
). These early ob-
servations prompted the engineering of radar sounding systems dedicated to studying the