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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 (epsilon' = 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.

Plain Language Summary The north polar region of Mars includes the so-called cavi unit, a deposit of water ice and sand hundreds of million years old that lies beneath the current ice cap. The Shallow Radar on Mars Reconnaissance Orbiter can image through the cavi unit deposits, revealing their internal structure and composition. We find that these deposits are very rich in ice, which lies in horizontal slabs alternated with sand. The occurrence and volume of ice slabs increase toward the north pole. This ice may be the leftover of former ice caps that diminished during warm periods and therefore represent an important record of past Martian climate. The large volume of ice preserved within the cavi unit represents one of the largest water reservoirs on the planet.