Mars has been the subject of speculation regarding the presence of liquid water for well over a century (Lowell, 1895;Lowell & Lockyer, 1906), and this hypothesis persisted until the Mariner missions visited the planet for the first time in the 1960s and found the planet to be dry, rocky, and mostly covered with impact-induced craters (McCauley et al., 1972). Numerous outflow channels were observed, but with higher resolution and better coverage from the Viking missions, they turned out to be ancient and dry (Baker, 1982). As a result of these robotic investigations, discussions of current liquid water on Mars moved into the subsurface, where temperature and pressure regimes could plausibly lead to melting of ice (Clifford, 1993), potentially at the poles (Clifford, 1987). It was primarily for this reason that the Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument (MARSIS) was sent to Mars, as specifically noted in a paper describing the instrument: "The primary scientific objective of the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS), which will be on board Mars Express mission scheduled for launch in 2003, is to map the distribution and depth of the liquid water/ice interface in the upper kilometres of the crust of Mars." (Picardi et al., 2004).In 2018, after more than a decade of acquiring observations and examining subsurface reflections (or the lack thereof), a candidate liquid detection was found beneath a portion of the south polar layered deposits (SPLD, Orosei et al., 2018). Orosei et al. (2018) reported anomalously bright MARSIS radar reflections near a region around 81°S, 193°E (Figure 1), in which the subsurface reflection was brighter than the surface

The cavi unit at the north pole of Mars is a deposit of aeolian sand and water ice underlying the Late Amazonian north polar layered deposits. Its strata of Middle to Late Amazonian age record wind patterns and past climate. The Mars Reconnaissance Orbiter Shallow Radar (SHARAD) reveals extensive internal and basal layering within the cavi unit, allowing us to determine its general structure and relative permittivity. Assuming a basalt composition for the sand (ε′ = 8.8), results indicate that cavi contains an average ice fraction between 62% in Olympia Planum and 88% in its northern reaches beneath the north polar layered deposits and thus represents one of the largest water reservoirs on the planet. Internal reflectors indicate vertical variability in composition, likely in the form of alternating ice and sand layers. The ice layers may be remnants of former polar caps and thus represent a unique record of climate cycles predating the north polar layered deposits.

The long-term geological evolution of a planet is dependent on the bulk concentration of the long-lived heat-producing element (HPE; 238 U, 235 U, 232 Th, and 40 K) and their distribution between the crust and the mantle. High enrichment of HPE in the crust depletes the mantle of heat production and lowers the mantle potential temperature. In contrast, crust with lower enrichment of HPE thermally insulates the mantle increasing the mantle potential temperature (e.g., O'Neill et al., 2005). The thermal state of a planet's mantle modulates its convection, which influences volcanism, crustal tectonics, and geomagnetism (e.g., Stevenson, 2007). These geological processes directly impact hydrospheric and atmospheric processes (e.g.,

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The polar ice caps are the largest reservoir of water ice on Mars. The north polar ice cap is composed of the ice‐rich north polar layered deposit (NPLD) and a lower‐lying, silicate‐rich basal unit (BU). Together, these units represent a record of Martian climate history throughout the Amazonian period. Here we place a bulk compositional constraint on the BU by modeling its gravity signature in both spatial and spectral domains using two independent approaches. We find the density of the BU to be 2007+493−445 kg/m3, suggesting that it may contain 55 + 25% water ice. We estimate that the BU contains ~1.5‐m global equivalent layer of water making it one of the largest reservoir of water‐ice on Mars. Our compositional constraint suggests that the north pole of Mars was not only a cold trap for ices but also an aeolian trap for silicates during the Amazonian period.

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