Mars Advanced Radar For Subsurface And Ionosphere Sounding Research Articles

Revising the Basal Permittivity of the South Polar Layered Deposits of Mars With a Surficial Dust Cover

AbstractBright basal reflections from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) have been proposed to be consistent with permittivities characteristic of a wet material beneath the south polar layered deposits (SPLD). The characterization of a recently formed impact crater highlight the existence of a several meters thick ice‐poor layer associated to a unit blanketing a large portion of the SPLD. We revise the radar propagation model used to invert the basal permittivity by including a surficial thin layer. We find that the inverted basal permittivity is highly sensitive to the properties of such a layer, with solutions ranging from common dry rocks to an unambiguously wet base. We advocate toward a better characterization of the surficial cover to assess the wet or dry nature for the base, and possibly reconcile most of the literature on the topic.

Radar Observations of Liquid Water in the South Polar Region of Mars: Indications from Astrobiology Perspectives

In recent decades, extensive research has led to the understanding that Mars once hosted substantial liquid-water reserves. While the current Martian landscape boasts significant water-ice deposits at its North and South poles, the elusive presence of liquid-water bodies has remained undetected. A breakthrough occurred with the identification of radar-echo reflections at the base of the Martian South Pole, using MARSIS (Mars Advanced Radar for Subsurface and Ionospheric Sounding) in 2018. These radar echoes strongly suggest the presence of a highly concentrated liquid-water body. However, a counter-narrative has emerged, contending that the subterranean conditions beneath the ice cap, encompassing factors like temperature and pressure, may be inhospitable to liquid water. Consequently, alternative hypotheses posit that the observed bright echoes could be attributed to conductive minerals or water-absorbing clay-like materials. The ongoing discourse regarding the presence of liquid water beneath the southern polar ice cap is a hot topic in the realm of Martian exploration. The primary focus of this paper is to provide a comprehensive overview of Martian radar detection, the recent controversies regarding liquid water’s existence in the Martian South Pole, and the implications regarding the potential existence of Martian life forms in the water on Mars. The revelation of liquid water on Mars fundamentally suggests an environment conducive to the viability of Martian life, consequently furnishing invaluable insights for future exploratory endeavors in the pursuit of Martian biospheres. In addition, this paper anticipates the forthcoming research dedicated to Martian liquid water and potential life forms, while also underscoring the profound significance of identifying liquid water on Mars in propelling the field of astrobiology forward.

Thermophysical arguments against basal melting in the south polar region of Mars

The south polar layered deposits (SPLD) on Mars is primarily composed of H2O ice, with smaller quantities of impurities such as CO2 ice and dust. The presence of these impurities can affect the thermal structure of the SPLD, increasing the basal temperature and potentially leading to basal melting. Recently, bright basal reflectors in radargrams from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) of the SPLD at Ultimi Scopuli have been interpreted as evidence of a briny subglacial liquid water layer. However, this interpretation is difficult to reconcile with the low Martian geothermal heat flow and the frigid surface temperature at the Martian south pole. Here, we conduct a comprehensive thermophysical evolution modeling analysis to investigate the conditions that could enable basal melting at Ultimi Scopuli. Using numerical models that describe time-dependent firn densification and the thermal properties of porous solid mixtures, we first constrain the thermophysical parameters of the SPLD. We then run high-resolution thermal models that account for heat transfer and phase change to ascertain if basal melting can occur. Even when considering SPLD at Ultimi Scopuli to consist of 30% dust, basal melting does not occur. Considering the average surface temperature of the SPLD to be between 160 and 170 K and a maximum surface heat flow of 30 mW m−2, we find that the bulk thermal conductivity of the SPLD needs to be <1.5 W m−1 K−1 for basal melting to occur at Ultimi Scopuli. The combination of H2O ice, CO2 ice, and dust that can yield kbulk <1.5 W m−1 K−1 is at odds with current constraints on the composition of the SPLD. We also model the average dimension of a magmatic intrusion that can enable basal melting at Ultimi Scopuli. We find that an intrusion must be <15 km from the surface and at least 5 km wide to create a 20–30 km wide subglacial liquid water layer. We also find that subglacial liquid water sustained by magmatic intrusion will eventually refreeze over 1–2 million years unless continually sustained by newer intrusions. Given the difficulty in forming and sustaining subglacial liquid water on Mars, we suggest that alternative interpretations of the bright radar reflections merit additional consideration.

Serverless Architecture for Data Processing and Detecting Anomalies with the Mars Express MARSIS Instrument

The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) on board Mars Express has been sampling the topside ionosphere of Mars since mid-2005. The analysis of the main reflection (nadir) of the ionosphere through the ionograms provided by the MARSIS instrument is typically performed manually due to the high noise level in the lower frequencies. This task, which involves pattern recognition, turns out to be unfeasible for the >2 million ionograms available at the European Planetary Science Archive. In the present contribution, we propose a modular architecture based on serverless computing (a paradigm that stands on the cloud) for optimal processing of these ionograms. In particular, we apply serverless computing to detect oblique echoes in the ionosphere, which are nonnadir reflections produced when MARSIS is sounding regions above or nearby crustal magnetic fields, where the ionosphere loses the spherical symmetry. Oblique echoes are typically observed at similar frequencies to the nadir reflections but at different times delays, sometimes even overlaying the nadir reflection. Oblique echoes are difficult to analyze with the standard technique due to their nonconstant and highly variable appearance, but they harbor essential information on the state of the ionosphere over magnetized regions. In this work we compare the proposed serverless architecture with two local alternatives while processing a representative data subset and finally provide a study by means of cost and performance.

A new approach to retrieve total electron content based on dual-band echoes of MARSIS radar sounder

This paper proposes a new dual-band difference (DBD) method to retrieve total electron content (TEC) in Martian ionosphere with the dual-band echoes of the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS). It establishes the relationship between the group delay difference of two-band echoes and the plasmas frequency. Then the Martian ionospheric parameters are solved with this equation. Then the TEC is calculated by integrating the vertical distribution model of Mars electron density. This method is used to calculate the TEC of the typical ionosphere in the southern hemisphere of Mars, and the comparison with previous results validates this method. The experiments prove that this method can not only calculate the TEC of dayside and nightside in different periods, but also effectively eliminate the influence of ionospheric dispersion on radar echoes. Moreover, the time variation characteristics of the ionosphere are analyzed. According to the characteristics of this algorithm, the error sources are analyzed. By calculating the influence of different parameters on the results, it is proved that the maximum error of the algorithm is less than 0.1 TECU.

Topographical analysis of a candidate subglacial water region in Ultimi Scopuli, Mars

Radar signals of the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument at the base of the South Polar Layered Deposits (SPLD) in the region known as Ultimi Scopuli, on Mars, have been interpreted as reflections from putative subglacial standing bodies of liquid water. The interpretation of the MARSIS radar signals is not unique and different explanations have been proposed for the anomalous bright radar reflection observed in Ultimi Scopuli, including the presence of CO2 deposits within the ice, or clays and metal-bearing minerals. We analysed the surface topography of Ultimi Scopuli with Digital Elevations Models (DEMs) derived from MOLA (Mars Orbiter Laser Altimeter), photogrammetric processing of the High-Resolution Stereo Camera (HRSC) images and SHARAD radargrams. In each case, we found an anomalous flat and smooth area in correspondence to the potential subglacial water zone, indicating a condition of hydrostatic equilibrium of the ice, similar to the surface observations found above large terrestrial subglacial lakes. We found that the surface topography, in addition to MARSIS radar sounder signals, are compatible with the presence of subglacial standing bodies of liquid water at the base of Ultimi Scopuli.

Partially‐Saturated Brines Within Basal Ice or Sediments Can Explain the Bright Basal Reflections in the South Polar Layered Deposits

AbstractStrong radar reflections have been previously mapped at the base of the Martian South Polar Layered Deposits. Here, we analyze laboratory measurements of dry and briny samples to determine the cause of this radar return. We find that liquid vein networks consisting of brines at the grain boundaries of ice crystals can greatly enhance the electrical conductivity, thereby causing strong radar reflections. A brine concentration of 2.7–6.0 vol% in ice is sufficient to match the electrical properties of the basal reflection as observed by Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS). When brine is mixed with sediments, the brine‐ice mixture in the pores must be 2–5 times more concentrated in salt, increasing the brine concentration to 6.3–29 vol%. Our best fit of the median observed MARSIS value suggests a salt‐bulk sample concentration of ∼6 wt%. Thus, salt enhancement mechanisms on the order of a magnitude greater than the Phoenix landing site are needed. To form brine, the basal reflector must reach a temperature greater than the eutectic temperature of calcium perchlorate of 197.3 ± 0.2 K, which may be possible if more complex thermal modeling is assumed. Colder metastable brines are possible, but stability over millions of years remains unclear. Conversely, gray hematite with a concentration of 33.2–59.0 vol% possess electrical properties that could cause the observed radar returns, but require concentrations 2–3 times larger than anywhere currently detected. We also argue that brines mixed with high‐surface‐area sediments, or dry red hematite, jarosite, and ilmenite cannot create the observed radar returns at low temperatures.

Using MARSIS signal attenuation to assess the presence of South Polar Layered Deposit subglacial brines

Knowledge of the physical and thermal properties of the South Polar Layer Deposits (SPLD) is key to constrain the source of bright basal reflections at Ultimi Scopuli detected by the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) radar sounder. Here we present a detailed analysis of attenuation, based on data acquired by MARSIS at 3, 4, and 5 MHz. We show that attenuation is frequency dependent, and that its behavior is consistent throughout the entire region. This suggests that the SPLD are compositionally homogeneous at Ultimi Scopuli, and our results are consistent with dust contents of 5 to 12%. Using these values as input, and plausible estimates of surface temperature and heat flux, we inferred basal temperatures around 200 K: these are consistent with perchlorate brines within liquid vein networks as the source of the reflections. Furthermore, extrapolation of the attenuation to higher frequencies explains why SHARAD (Shallow Radar) has thus far not detected basal reflections within the SPLD at Ultimi Scopuli.

New Insights Into Subsurface Stratigraphy Northwest of Ascraeus Mons, Mars, Using the SHARAD and MARSIS Radar Sounders

AbstractThe Tharsis Montes volcanoes on Mars are the source of laterally extensive lava flows and other volcanic deposits generating a complex stratigraphy throughout the Tharsis Volcanic Province. We use SHAllow RADar (SHARAD) and Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) observations in a region northwest of Ascraeus Mons to determine the composition, density, thickness, and spatial distribution of these emplaced volcanic materials. We identified subsurface reflectors along 43 SHARAD and five MARSIS observations. Reflectors in the volcanic plains are interpreted to be sequences of basaltic lava flows with interspersed pyroclastic material, dust, or regolith during a hiatus in activity. Others correspond to the base of three major flow fields. Several plain reflectors were detected by both MARSIS and SHARAD. Other notable reflectors were identified near Ascraeus' flank where lava buried glacially derived sediment. We derived thickness and other material properties for flows using their distinct topographic boundary visible in the radar images. Permittivity ranged from 7.0 to 11.2 corresponding to lava flow densities of 3.20–3.52 g/cm3. Flow thicknesses ranged from 19.8 to 60.2 m. Loss tangents were low for the flow fields ranging from 0.024 to 0.043. Loss tangents in the plains ranged from 0.010 to 0.076. Higher loss tangents correspond to lossier regions that may have higher concentrations of radar absorbing minerals like hematite. Surface roughness controls where reflectors are detected. SHARAD detects the base of three out of the four flow fields in this region with muted surface roughness from dust mantling and erosion.

Geologic Context of the Bright MARSIS Reflectors in Ultimi Scopuli, South Polar Layered Deposits, Mars

AbstractRadar‐bright basal reflectors have been detected below the South Polar Layered Deposits (SPLD), using Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) data and have an exciting but controversial interpretation: liquid water from subglacial lakes. We mapped the surface of the SPLD immediately above and surrounding the putative lakes (1:2M map scale) in order to provide a geologic context for the interpretation of bright basal reflectors. We use Thermal Emission Imaging System daytime IR (100 m/pixel), Context Camera (6 m/pixel), and High Resolution Imaging Science Experiment (25 cm/pixel) data to characterize geologic units and typical surface roughness. We find evidence for multiple geologic units with features due to CO2 and aeolian‐related processes. We do not find evidence for surface modification linked to postulated lake locations. This is not consistent with the interpretation of the MARSIS basal radar reflector as subglacial lakes.

A New Method for Retrieving Electron Density Profiles from the MARSIS Ionograms

The Martian ionosphere was actively detected by Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) aboard the Mars Express. The detected echo signal of the MARSIS at an epoch is presented as a function of frequency and time delay to form an ionogram. Some MARSIS ionograms have been processed to obtain the electron density profiles of the Martian topside ionosphere. Unfortunately, more than half of the records cannot be processed with current methods due to the lack of local plasma density information at spacecraft altitude. In this work, we employ a piece-wise exponent to describe the electron density profile of the Martian topside ionosphere. The piece-wise exponent used in our method can reasonably capture the altitude structure of the Martian topside ionosphere, which has been validated with the MGS and MAVEN data. In an altitude regime of lower than 200 km, the average absolute height error of the same electron density between MGS data and fitted profiles is 0.006 km, and the average relative error is 0.008%. In an altitude regime of higher than 200 km, the average absolute height error of the same electron density between MGS data and fitted profiles is 0.55 km, and the average relative error is −0.1%. Based on the altitude structure knowledge of the Martian topside ionosphere, we put forward a new method to invert electron density profiles from MARSIS ionograms with/without local plasma density information. Compared with the previous results, the average absolute difference in the peak height of the retrieved profile is 7.38 km, within the margin of the MARSIS height resolution of 13.8 km. The average relative difference is only 3%. The application of the new method can greatly improve the utilization rate of MARSIS ionogram records.

The Basal Detectability of an Ice‐Covered Mars by MARSIS

AbstractThe detection of anomalously strong relative basal reflectivity beneath the Martian South Polar Layered Deposits (SPLD) from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) has led to hypotheses suggesting the presence of basal materials such as liquid water. Here, we propose a forward approach to assess whether such a high signal could be produced by a Martian terrain currently exposed at the surface without liquid water. We convert existing MARSIS surface reflectivity measurements into a basal reflectivity as if it were overlaid by an SPLD‐like ice deposit. 0.3%–2% of the surface could produce basal reflections of magnitude similar to the SPLD measurements in the assumption of a 10% impure ice. An ice loss tangent 0.01 is required to prevent any of the current Martian surface from producing a bright SPLD‐like basal reflection. The detected bright terrains are gathered within volcanic constructs of diverse geologic epoch.

UWB Processing Applied to Multifrequency Radar Sounders: The Case of MARSIS and Comparison With SHARAD

We re-adapt an UltraWide-Band (UWB) processing to enhance range resolution of the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) up to a factor six (25m). The technique provides for the estimation of radar signature over a wider spectrum via the application of well-known super-resolution (SR) techniques to adjoining sub-bands. The measured spectra are first interpolated and then extrapolated outside the original bands. The revised algorithm includes the estimation and removal of ionospheric effects impacting the two signals. Because the processing requires the realignment of the echoes at different frequencies, we derived the maximum tolerable retracking error to obtain reliable super-resolved range profiles. This condition is fulfilled by low-roughness areas compared to MARSIS wavelength, that proves to be suitable for the application of our processing. Examples of super-resolved experimental products over different geological scenarios shows the detection of shallow dielectric interfaces not visible from original MARSIS products. Our results are validated by comparison with the Shallow Radar (SHARAD) data acquired at the crossovers, demonstrating the potential of the method to provide enhanced imaging capabilities.

A Novel Approach to the Detection and Imaging of Candidate Martian Subglacial Water Bodies by Radar Sounder Data

Research based on Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) data detected unusual radar bright basal reflections located at about 1.5 km depth in a Mars region denoted as Ultimi Scopuli. These reflections were interpreted as a signature of subglacial liquid water even though this interpretation is still being debated in the literature. In this article, we propose a novel approach to the detection and imaging of candidate subglacial liquid water from radar sounding data. The approach combines the radar echo power traces with a suitable digital elevation model to provide a bidimensional representation of the surface. Even if the imaging method reconstructs a representation of the surface, we prove that it can be used to identify subsurface bright reflections in icy regions. Imaging is feasible even if the basal interface is not directly included in the processed data for image generation. To support this experimental evidence, we show that a relationship exists between the value of the reflected echo power originating from the englacial layers and the basal-to-surface-echo-power ratio. The observed relationship holds on both Ultimi Scopuli radar sounding data acquired on Mars and Lake Vostok data acquired on Earth. Our results show that the 2-D imaging provides an alternative way for locating candidate subglacial liquid water bodies on Mars over large areas also where the basal interface is not directly measured. The proposed approach complements previous research for further evaluation of the actual presence of liquid water on Mars.

A Dual-Band Difference Ionosphere Compensation Algorithm for Mars Orbiter Subsurface Investigation Radar

When detecting the underground structure of Mars using an orbital subsurface investigation radar, it is essential to compensate for the ionosphere distortion of the received echo. The existing algorithms, such as the contrast method (CM) and the phase-gradient-autofocus (PGA) algorithm, can mainly compensate for the defocus of the echo, but it is difficult to compensate for the group delay at the same time, resulting in the deviation between the echo position and the terrain profile. This article analyzes the interference of Martian ionosphere to the echo of subsurface investigation radar and proposes a dual-band difference (DBD) ionosphere compensation algorithm. First, the group delay model of radar echo under ionospheric interference is established. Then, the equivalent plasma frequency of the ionosphere is calculated according to the group delay difference of echoes in two bands, and the Taylor coefficients of the phase error are calculated. Finally, the echoes are accurately compensated with the Taylor series model of phase shift. This algorithm can correct the defocus and sidelobe interference and simultaneously compensate for the group delay, which is greatly significant to accurately locate the subsurface structure of Mars. The experiments on the simulation data and the echo of the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) show the feasibility of this method. Compared with the traditional way, this algorithm is suitable for both high- and low-frequency situations, and the high computational efficiency without iteration makes on-orbit real-time ionosphere compensation possible. Moreover, with the effective correction of topographic offset, the total electron content (TEC) can also be accurately retrieved.

A Fully Automatic Algorithm for Reflector Detection in Radargrams Based on Continuous Wavelet Transform and Minimum Spanning Tree

Spaceborne radar sounding is a powerful technique in planetary subsurface exploration due to its large penetration ability into loose desiccated materials. Three radar sounders, Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS), shallow radar (SHARAD), and Mars Orbiter Subsurface Investigation Radar (MOSIR), are orbiting Mars and have obtained a large quantity of radargrams. These direct observations provide valuable information for deciphering subsurface structure and composition of Mars, which are critical for understanding internal stratigraphy, climate change, and orbital variation in the planet. For this reason, automatic, effective, and precise extraction of subsurface reflectors in radargrams is a prerequisite and remains as a challenge though recent studies have made great progresses. In this study, we propose a fully automatic method for reflector detection in radargrams using transform domain techniques and graph theory. In the preprocessing step, the spatial spectrum information is used to automatically extract region with reflectors in a radargram, which can significantly reduce the computational cost. Then, continuous wavelet transform (CWT) with double threshold is used to detect peaks in each frame, and minimum spanning tree (MST) is further used to connect the peaks based on global information. Our proposed method can quickly reconstruct the reflectors in radargrams with a relatively low tracking error rate, which is better than previous methods based on local information. To testify the effectiveness of the algorithm, we applied our algorithm to SHARAD radargrams in the two polar regions. The results show that more than 90% (80%) of the reflectors can be well-detected in the strong (weak) echo regions, and their subsurface information can be extracted efficiently. Compared with previous methods, our method has a higher degree of automation and a wider applicability. The proposed algorithm can be applied to analyze radargrams over large regions of Mars and other bodies.

Ionosphere of Mars during the consecutive solar minima 23/24 and 24/25 as seen by MARSIS-Mars Express

The Mars' ionospheric behavior during two consecutive solar minima (23/24 and 24/25) is investigated with the same dataset. In particular, we use the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on board Mars Express to investigate the total electron content behavior of the whole atmosphere in relation to the solar irradiance (EUV and X-ray fluxes), the solar zenith angle and the heliocentric distance. The topside variability of the electron density profiles is also investigated through variations in the peak density and neutral scale height. Moreover, the equations of the NeMars empirical model of the Martian ionosphere for low solar activity are tested for both minima. We have found that the topside ionosphere of Mars behaved similarly at both solar minima. However, when considering the bottomside, a pronounced reduction in ionization in particular cases is suggested. In addition, larger TEC values are found during the solar minimum 24/25 in the nightside sector that may indicate possible larger plasma transport than during the minimum 23/24. Finally, this study confirms that the ionospheric empirical NeMars model equations derived by Sanchez-Cano et al. (2016) for the low solar activity period during the solar minimum 23/24 are also valid and accurate for the solar minimum 24/25. The long duration of Mars Express is a critical factor for determining the long-term Martian ionospheric variability, which in turn, is essential for understanding the global evolution of the planet's atmosphere.

Strong MARSIS Radar Reflections From the Base of Martian South Polar Cap May Be Due to Conductive Ice or Minerals

AbstractRecent results from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument have been interpreted as evidence of subsurface brine pooled beneath 1.3 km‐thick South Polar Layered Deposit (SPLD). This interpretation is based on the assumption that the regionally high strength of MARSIS radar reflections from the base of the ice cap is due to a strong contrast in dielectric permittivity across the basal interface. Here, we demonstrate that the high‐power reflections could instead be the result of a contrast in electric conductivity. While not explicitly excluding a liquid brine, our results open new potential explanations for the observed strong radar reflections, some of which do not require liquid brine beneath SPLD. Potential basal materials with suitably high conductivity include clays, metal‐bearing minerals, or saline ice.

Characteristics of the Basal Interface of the Martian South Polar Layered Deposits

AbstractWe expand on previous studies of the South Polar Layered Deposits' (SPLD) basal interface using data acquired by the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) to obtain detailed maps of elevation, topography, and reflected radar power. Using these maps, we derive the thickness (ranging from 0 to 3.7 km) and volume of the SPLD (∼1.60 × 106 km3). While most basal interfaces reflect less power than the average SPLD surface, areas with basal echo power exceeding that of the surface are widespread throughout the SPLD, including at the location of potential subglacial water bodies in Ultimi Scopuli. The occurrence of these high basal echo power signatures appears to be largely frequency independent in MARSIS data. While the cause of the relatively high basal echo power values is uncertain, our observations suggest that this behavior is widespread, and not unique to Ultimi Scopuli.

The Vertical Extent of Enhanced Densities in Cusp‐Like Regions of the Ionosphere of Mars

AbstractUnusually large values of peak electron density in the ionosphere of Mars are occasionally observed in cusp‐like regions of crustal magnetic field. These features are observed as atypical examples of oblique echoes in ionograms. However, the vertical extent over which electron density values are enhanced has not yet been determined. Furthermore, the physical processes responsible for these striking features are not yet understood. Here we analyze ionograms from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument on Mars Express to better constrain the spatial distribution of electron density in and around these rare ionospheric features. Data from orbits 2359, 2491, 4469, and 4513 are examined. When peak electron density is unusually large, electron density values from the peak to 165 km are increased by 30% ± 10% and the electron density at the spacecraft (∼300–400 km) is diminished. We investigate whether the central portion of an oblique echo is equivalent to a nadir echo, which would permit the reliable determination of an electron density profile in instances of unusually large peak electron densities. We find that uncertainties in the location of the center of an oblique echo render this approach unfeasible. We also report that 86.5 μs should be subtracted from travel time values reported in current MARSIS data products released prior to 2019.