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

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The north polar layered deposits (NPLD) of Mars represent a global climate record reaching back millions of years, potentially recorded in visible layers and radar reflectors. However, little is known of the specific link between those layers, reflectors, and the global climate. To test the hypothesis that reflectors are caused by thick and indurated layers known as “marker beds,” the reflectivity of three reflectors was measured, mapped, and compared to a reflectivity model. The measured reflectivities match the model and show a strong sensitivity to layer thickness, implying that radar reflectivity may be used as a proxy for short‐term accumulation patterns and that regional climate plays a strong role in layer thickness variations. Comparisons to an orbitally forced NPLD accumulation model show a strong correlation with predicted marker bed formation, but dust content is higher than expected, implying a stronger role for dust in Mars polar climate than previously thought.

The stratigraphy of the north polar layered deposits (NPLD) of Mars is believed to contain a climate record of the recent Amazonian period. However, full utilization of this record is difficult without detailed information regarding the physical properties of the constituent layers. Here we present a method for determining the fractional dust content of individual layers using a combination of orbital radar reflectivity measurements and physical modeling. We apply this method to the upper 500 m of the NPLD at 10 study sites and compare the results to a cap‐wide radar‐mapped surface. Our results show that reflectivity can vary drastically both geographically and with depth, a result we attribute to changing dust content, though the impact of variable layer thickness cannot be totally discounted. These findings imply large‐scale regional patterns in ice and dust accumulation do not remain consistent through time. We also find that current models of Mars's dust cycle and polar ice accumulation consistently underpredict the dust content of layers, indicating that our understanding of dust transport, dust sequestration, or dust preservation remains incomplete. Comparisons of study sites on the NPLD also show that some locations contain fewer radar reflectors than others, meaning they may contain a less complete record of the planet's recent paleoclimate, and any future efforts to use the polar layered deposits as a climate proxy, including in situ measurements, should take this into account by choosing study sites wisely.