Full-Polarimetric capabilities evaluation of the ExoMars 2028 GPR on a 3D dataset acquired in Svalbard

 Introduction

The ExoMars 2028 Rosalind Franklin rover mission [1] is carrying onboard the Full Polarimetric Ground Penetrating Radar (FP-GPR) WISDOM (Water Ice Subsurface Deposits Observation of Mars) [2]. WISDOM will be the second FP-GPR to operate on the Martian surface, after the RoPeR/Tianwen-1 GPR [3] which has operated all along the Zhurong rover track in 2021.

WISDOM has shown unprecedented penetration depth in dry frozen environment during a field-test campaign in Svalbard; more than 15 meters in glacier ice, and more than 7 meters deep in lithic sedimentary material [4,5,6], which is truly promising for geological investigation of Oxia Planum, the ExoMars 2028 landing site. The dataset collected above a well-documented ice cave in Longyearbreen glacier [7] provides a valuable resource for testing WISDOM polarimetric capabilities in natural environment with varied polarimetric signatures.

Methodology

Polarimetric data representation

Both the transmitting (Tx) and receiving (Rx) antennas of WISDOM are made of two perpendicular radiating elements [8] labeled 0 and 1 on Fig. 1a. WISDOM is able to perform measurements in four linear polarimetric configurations involving one TX antenna and one RX antenna (two co-polarization and two cross-polarization) labeled 00, 11, 01 and 10.

To visualize the contributions of all polarizations simultaneously, we encoded them using Red (configuration 00), Green (configuration 11), and Blue (average cross-polarization configuration) channels and combined through additive synthesis (RGB additive synthesis - Fig. 1a) to produce false-color images. The resulting color are associated with specific scattering mechanisms detailed in Fig. 1b. The colored data product resulting from this process are called RGB-radargrams.

Polarimetric data representation

Both the transmitting (Tx) and receiving (Rx) antennas of WISDOM are made of two perpendicular radiating elements [8] labeled 0 and 1 on Fig. 1a. WISDOM is able to perform measurements in four linear polarimetric configurations involving one TX antenna and one RX antenna (two co-polarization and two cross-polarization) labeled 00, 11, 01 and 10.

To visualize the contributions of all polarizations simultaneously, we encoded them using Red (configuration 00), Green (configuration 11), and Blue (average cross-polarization configuration) channels and combined through additive synthesis (RGB additive synthesis - Fig. 1a) to produce false-color images. The resulting color are associated with specific scattering mechanisms detailed in Fig. 1b. The colored data product resulting from this process are called RGB-radargrams.

Figure 1: a) WISDOM polarimetric antenna onboard Rosalind Franklin rover. The two orthogonal radiating elements are labeled "0" and "1". Combination of these two linear polarimetric configuration allows to perform measurement in 4 polarization configuration labeled “00”, “11”, “01” and “10”, which are associated to color red, green and blue and merged in the same image following additive synthesis principle. Extracted and modified from [10] b) The resulting gamut of colors after additive synthesis reveals different scattering mechanisms.

Ice cave dataset

The Longyearbreen Glacier is located 5 km southwest of the University Center in Svalbard (UNIS) of Longyearbyen city. The investigated ice cave is part of a large englacial drainage system located near the steep western slope of the mountain (Fig. 3a). It was explored in-depth in spring of 2015 combining GPR (25 MHz and 100 MHz) and speleological survey [7]. We also explored the cave in detail at the location of the WISDOM survey (Fig. 2) in March 2022. The ice cave presents different feature ; (i) the meanders are various is size and shape, (ii) sedimentary rocks are embedded in the ice (Fig. 2c), (iii) water ice stalactites are growing from the ceiling (Fig. 2d) and (iv) Fresh pure water ice, recently frozen, covers floors made of sedimentary materials (Fig. 2b). Liquid water is also suspected to be present under the cave floor. We also determined the location of the meander of the cave, providing an extensive 3D ground truth to be compared with the interpretation of WISDOM radargrams.

The WISDOM survey consisted in a grid made of four lines represented on Fig. 2a and 3b-c, several meters away from the cave entrance (Fig. 2a and 3). WISDOM profiles are performed with a 10 cm step between consecutive soundings.

 

Figure 2: a) Location of the WISDOM survey above the ice cave of Longyearbreen glacier, and cave entrance. b) speleological survey of the investigated area. c) Sediments embedded in the glacier. d) Water ice stalactite and fresh water ice covering sedimentary rocks.

Results

The RGB-radargrams corresponding to the four WISDOM profiles (Fig. 3b) are shown on Fig. 3c. The different colors reveal varied scattering mechanism that are explored in-depth by [6]. The snow layering and snow/ice interface appear in yellow (Fig. 3c), suggesting large smooth interfaces. The spatially extended reflections associated to the ice cave itself appear in many different colors, including depolarization (blue, cyan, magenta, white), interpreted as multiple reflections in the large cavity (Fig. 3c). In some places, yellow reflection is associated to simple reflections of smooth sections of the ceiling of the cave. We also use the specificity of WISDOM antenna radiation patterns (Fig.1a) and colored RGB-radargrams to determine whether a scatterer detected in a radargram is located on the left (red-green), on the right (green-red), or below the rover's track (yellow). This is compared to actual 3D position of the scatterer and show promising results.

 

Figure 3: a) Location of the cave entrance on the Longyearbreen glacier. b) The four WISDOM profiles and the estimated meanders of the ice cave in the subsurface. Background map credit: Norwegian Polar Institute. c) 3D representation of WISDOM RGB-radargrams corresponding to profiles 1-3.

 

References

[1] Vago et al., 2017, Astrobiology

[2] Ciarletti et al., 2017, Astrobiology

[3] Zhou et al., 2020, Earth and Planetary Physics

[4] Brighi, 2024, PhD thesis

[5] Brighi et al., 2025, IEEE

[6] Brighi et al., under review, Geophysics

[7] Hansen et al., 2020, Journal of Glaciology

[8] Benedix et al., 2024, Planetary and Space Science

[9] Plettemeier et al., 2017, European Planetary Science Congress 2017

[10] Harrar et al., 2026, European Planetary Science Congress 2026