Radar for Europa Assessment and Sounding: Ocean to Near-Surface (REASON)

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 combination of radar measurement techniques including altimetry, reflectometry, sounding, interferometry, 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 solar 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; Europa Clipper; Ice shell; Ice-penetrating radar.

Fig. 1

Radargram generated from ice-penetrating radar data collected over Totten Ice Shelf, Antarctica (TOT/JKB2d/X16a) modified from Lindzey (2015)

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)

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)

Fig. 5

Radargram generated from ice-penetrating radar data collected over Filchner-Ronne Ice Shelf, Antarctica illustrating how a marine–meteoric ice interface could be mistaken as the ice–ocean interface, modified from Thyssen (1988)

Fig. 6

Altimetry profile (top) and radargram (bottom) generated from ice-penetrating radar and laser altimetry 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)

Fig. 7

Comparison between radargrams generated from ice-penetrating radar data collected over ice on Earth at 60 MHz center frequency, 15 MHz bandwidth (top) and Mars at ∼1 MHz center frequency and bandwidth (bottom), modified from Picardi et al. (2005)

Fig. 8

Comparison between radargrams generated from ice-penetrating radar data collected over the Mars North Polar Layered Deposits (top) by SHARAD (middle) and MARSIS (bottom), illustrating the scientific value of complementary sounding frequencies, modified from material provided by Ali Safaeinili

Fig. 9

Summary of REASON ice shell domains and measurement techniques

Fig. 10

Modeled radar attenuation in Europa’s ice shell adopting the thermal model of Chyba et al. (1998), electrical parameters of Moore (2000), and the regime modes described in Table 4

Fig. 11

Illustration of the geometry involved in interferometry, including two representations of off-nadir surface features that could generate clutter, where “ribbons” represent rough, flat terrain and “curbs” represent smooth, angled terrain (EPM refers to Europa Point Model, Sect. 4.3)

Fig. 12

EPMs used to represent Europa for evaluating Measurement Quality Requirements

Fig. 13

NSLE

Fig. 14

Depiction of Europa Clipper spacecraft with REASON antenna assemblies

Fig. 15

REASON functional block diagram

Fig. 16

REASON implementation block diagram

Fig. 17

HF RF In-vault block diagram

Fig. 18

VHF RF In-vault block diagram

Fig. 19

REASON HF stowed (top) and deployed (bottom)

Fig. 20

REASON VHF antenna assembly stowed (top) and deployed (bottom)

Fig. 21

REASON out-of-vault hardware

Fig. 22

Block diagram of the EGSE used in the in-vault radar hardware I&T campaigns

Fig. 23

The sequence of events and timing for a typical REASON flyby

Fig. 24

Visualization of planned cruise activities for REASON ($+/-$ 1 month), the asterisk denotes a backup activity awaiting approval

Fig. 25

Modeled HF (left) and VHF (right) antenna beam patterns, referenced to their phase centers at the center of the spacecraft bus

Fig. 26

REASON PDS data collections