Following the discovery of the Hiawatha impact crater beneath the northwest margin of the Greenland Ice Sheet, we explored satellite and aerogeophysical data in search of additional such craters. Here we report the discovery of a possible second subglacial impact crater that is 36.5‐km wide and 183 km southeast of the Hiawatha impact crater. Although buried by 2 km of ice, the structure's rim induces a conspicuously circular surface expression, it possesses a central uplift, and it causes a negative gravity anomaly. The existence of two closely spaced and similarly sized complex craters raises the possibility that they formed during related impact events. However, the second structure's morphology is shallower, its overlying ice is conformal and older, and such an event can be explained by chance. We conclude that the identified structure is very likely an impact crater, but it is unlikely to be a twin of the Hiawatha impact crater.

The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument on Mars Express has detected strong subsurface radar reflections in the region of Ultimi Scopuli (81°S, 193°E), within the South Polar Layered Deposits (SPLD) (Lauro et al., 2021;Orosei et al., 2018). These subsurface reflections are ∼10 dB greater in power than the surrounding reflections and ∼3 dB greater than the reflections from the surface (Orosei et al., 2018). The reflectors are located at the base of the SPLD, approximately 1.5 km below the topographic surface. Because data acquired by MARSIS do not separate the real (ε′) and imaginary (ε″) parts of the complex permittivity of reflectors, the apparent permittivity (ε a ), a single parameter accounting for both ε′ and ε″ (Mattei Abstract Strong 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.Plain Language Summary Previous research has detected strong radar reflections from the interface between Mars' southern ice cap and their underlying sediments over a region with an area of 20 × 30 km and 1.5 kms beneath the surface. Radar reflections are caused by changes in electrical properties. Here, we analyze electrical property laboratory measurements of materials under Mars-like conditions. We find that a small amount of brine in ice samples could create strong radar reflections similar to those that are observed. A greater concentration of salt is needed in sediment-ice mixtures. We su...

It has recently been suggested that clay minerals, which are widespread on the Martian surface, could be the possible source of the basal bright reflections detected by MARSIS at Ultimi Scopuli, instead of briny water. This hypothesis is based on dielectric measurements on a wet Ca‐Montorillonite (STx‐1b) sample conducted at 230 K, which reported permittivity values (apparent permittivity of 39 at 4 MHz) compatible with the median value of 33 retrieved by MARSIS 4 MHz data inversion in the high reflectivity area. These experimental results are, however, incompatible with well‐established dielectric theory and with laboratory measurements on clays, at MARSIS frequency and Martian temperatures, reported in the literature. Here, we replicate the experiment using a setup to precisely control the rate of cooling/warming and the temperature inside and outside the clay sample. We found that the rate of cooling, the position of the temperature sensor and, consequently, the thermal equilibrium between the sample and the sensor play a fundamental role in the reliability of the measurements. Our results indicate that even for a large water content in the clay sample, at 230 K and 4 MHz, the apparent permittivity is only 8.4, dropping to 4.1 at 200 K, ruling out clays as a possible source of the bright reflections detected by MARSIS at the base of the SPLD.

The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument on Mars Express has detected strong subsurface radar reflections in the region of Ultimi Scopuli (81°S, 193°E), within the South Polar Layered Deposits (SPLD) (Lauro et al., 2021;Orosei et al., 2018). These subsurface reflections are ∼10 dB greater in power than the surrounding reflections and ∼3 dB greater than the reflections from the surface (Orosei et al., 2018). The reflectors are located at the base of the SPLD, approximately 1.5 km below the topographic surface. Because data acquired by MARSIS do not separate the real (ε′) and imaginary (ε″) parts of the complex permittivity of reflectors, the apparent permittivity (ε a ), a single parameter accounting for both ε′ and ε″ (Mattei Abstract Strong 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.Plain Language Summary Previous research has detected strong radar reflections from the interface between Mars' southern ice cap and their underlying sediments over a region with an area of 20 × 30 km and 1.5 kms beneath the surface. Radar reflections are caused by changes in electrical properties. Here, we analyze electrical property laboratory measurements of materials under Mars-like conditions. We find that a small amount of brine in ice samples could create strong radar reflections similar to those that are observed. A greater concentration of salt is needed in sediment-ice mixtures. We su...

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