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Meridiani Planum is one of the most intensely studied regions on Mars, yet little is known about the physical properties of the deposits below those examined by the Opportunity rover. We report the detection of subsurface echoes within the Meridiani Planum deposits from data obtained by the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument. The delay time between the surface and subsurface returns is indicative of materials with a real dielectric constant of 3.6 +/- 0.6. The real dielectric constant is strongly modulated by bulk density. Newly derived compaction relationships for Mars indicate that the relatively low dielectric constant of the Meridiani Planum deposits is consistent with a thick layer of ice-free, porous, basaltic sand. The unique physiographic and hydrologic setting of Meridiani Planum may have provided an ideal sediment trap for eolian sands. The relatively low gravity and the cold, dry climate that has dominated Mars for billions of years may have allowed thick eolian sand deposits to remain porous and only weakly indurated. Minimally compacted sedimentary deposits may offer a possible explanation for other nonpolar region units with low apparent bulk dielectric constants.

Plain Language Summary In spite of being among the most intensely studied regions of Mars, little is known about the makeup of the Meridiani Planum deposits below those examined by the Opportunity rover. The MARSIS radar sounder has detected subsurface echoes in Meridiani Planum. MARSIS transmits low-frequency radio pulses that can penetrate into certain crustal materials and are reflected back by dielectric discontinuities related to physical or compositional changes. Subsurface reflectors that are significantly offset in time delay from surface returns are interpreted to be from the interface between the Meridiani Planum deposits and buried ancient cratered terrain. The time delay is consistent with a material having a relatively low dielectric constant. Deposits on Mars with low dielectric constants are generally interpreted to indicate the presence of substantial water ice. The dielectric constant is strongly influenced by bulk density, and a thick sedimentary deposit will undergo significant self-compaction without the presence of pore-filling ice, increasing in bulk density and dielectric constant. Newly derived compaction relationships for Mars indicate that the relatively low dielectric constant of the Meridiani Planum deposits is consistent with a thick sequence of ice-free, porous, eolian sand. Thus, low dielectric constants are possible in other nonpolar deposits without the presence of water ice.