Understanding the Lack of Bright Basal Echoes in the SHARAD Data collected at Ultimi Scopuli, Mars

Introduction
Anomalously bright basal reflections, stronger than the local surface echoes, have been detected by MARSIS (the radar sounder onboard MEX spacecraft) at Ultimi Scopuli, in the Martian South Pole (Lauro et al., 2021; Orosei et al., 2018). Such subglacial strong echoes have been interpreted as an indication of the presence of basal briny water (Cosciotti et al., 2023; Lauro et al., 2021; Mattei et al., 2022; Orosei et al., 2018; Stillman et al., 2022). This interpretation ignited a strong scientific debate, and alternative interpretations have been proposed, including the presence of conductive materials such as clay and saline ice (Bierson et al., 2021; Smith et al., 2021) and constructive interference resulting from sub-resolution layers beneath the SPLD ice sheet (Lalich et al., 2024). Numerous simulations have been conducted to support these alternative hypotheses, the majority of which have been focused on the composition of the basal materials, while considering the South Polar Layer Deposits (SPLD) as a homogeneous dust-laden ice layer. However, the enhanced signal-to-noise ratio data collected by SHARAD in VLR (Very Large Roll angle) mode (DiCarlofelice et al., 2023), highlighted the complex structure of the SPLD, which consists of a series of reflecting interfaces indicative of different dielectric properties among the layers. As the role of such a layered sequence in reducing the signal penetration has never been deeply explored, we propose here a multiple-layer model to simulate the propagation of electromagnetic waves through SPLD and assess the influence of dust-rich layers on the bulk attenuation and basal echo strength.
Model and simulation
The multiple-layer model consists of a 1.5-km-thick dust-poor ice sheet containing embedded dust-rich layers, underlain by a semi-infinite liquid water layer. The model parameters mainly include the number, thickness, and dust content of dust-rich and dust-poor ice layers. The parameters are constrained using multiple-source data such as optical imagery, SHARAD radargrams or estimated loss tangent. The SHARAD VLR radargram reveals that there are tens of subsurface interfaces producing reflection echoes strong enough to be identified. However, a one-to-one correspondence between radar data and marker beds identified in the outcrops of SPLD is not possible, implying that additional layers with lower dust content exist within SPLD. To build the model, the thickness of dust-rich layers has been constrained using the images collected on the outcrops (Limaye et al., 2012), whereas dust content in the layers has been randomly generated using a normal distribution with a varying mean value. In addition, we introduce small variations in layer thickness and dust content to simulate a distribution of subsurface-echo/surface-echo power ratio (P_b/P_s ). The dust content of the ice sheet is constrained by the bulk loss tangent estimated from MARSIS (Lauro et al., 2022; Plaut et al., 2007).
The simulations are conducted at 3, 4, 5 MHz (MARSIS bands) and 20 MHz (SHARAD center frequency). For each frequency, the simulation yields a distribution of P_b/P_s, from which the median value is extracted. Finally, we compared simulated and real data collected by MARSIS to verify the consistency of the procedure.
Preliminary results
The simulation results indicate that median values of the ratio P_b/P_s are larger than 0 dB and decrease with frequency, in accordance with MARSIS observation. In the MARSIS dataset, the differences of the P_b/P_s median values between adjacent frequencies (3/4 MHz and 4/5 MHz) are approximately equal (~1 dB). The attenuation caused by the multi-layer structure of SPLD consists of a contribution due to the transmission loss (which is frequency-independent) and an absorption loss in dust-rich ice (which is frequency-dependent). The transmission loss depends on the dielectric contrast between layers and number of layers, whereas the absorption loss depends on the complex permittivity of each layer. In our simulation, a larger number of layers with lower dust content (Figure 1) and fewer layers with higher dust content (Figure 2) can reproduce the observed MARSIS data.
Conversely to MARSIS, so far SHARAD has not been able to detect the basal echo at ~1.5 km at Ultimi Scopuli, even in VLR configuration; this fact suggests that at such frequency the basal echo is obscured by the background noise, which is of the order of  ~-20 dB. In our simulations, the configuration consisting of a larger number of layers having a low dust content can produce a P_b/P_s of <-15 dB (Figure 1), and the one consisting of fewer dust-rich layers generates a P_b/P_s of ~-15 dB (Figure 2). Thus, the model with more low-dust-content layers might better represent the actual SPLD stratification at Ultimi Scopuli.

Figure 1 Distribution of simulated P_b/P_s ratio from the model with a larger number of layers with low dust content (10-15%) for different frequencies. The dashed lines represent the median of the P_b/P_s ratio (2.6, 1.2, 0.1 dB) at 3, 4, 5 MHz measured by MARSIS and the SHARAD background noise (right to left).

Figure 2 Distribution of simulated P_b/P_s ratio from the model with fewer layers having a larger dust content (20-25%) at different frequencies. The dashed lines represent the median of the P_b/P_s ratio (2.6, 1.2, 0.1 dB) at 3, 4, 5 MHz measured by MARSIS and the SHARAD background noise (right to left).

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