Radar penetrates only the youngest geological units on Mars

[1] Signals from the Shallow Radar were intended to penetrate hundreds of meters or more into Mars, but subsurface reflections are abundant only in known or inferred ice‐rich units and young (middle to late Amazonian), apparently pristine, volcanic units. As volcanic units age, fewer subsurface reflections are detected. Also, no subsurface reflections are detected from any northern hemisphere units inferred to be altered by water. We suggest that the general lack of subsurface reflections on Mars is not likely an indication that the shallow interior is devoid of structure and stratigraphy but rather an indication that dielectric contrasts cannot be detected due to signal attenuation originating from scattering and/or absorption. We constrained the attenuation rate in regions with no subsurface reflections to 0.065–0.27 dB/m. This corresponds to scattering losses from meter‐scale fractures and/or lithologic density variations of 0.27–1.03 g/cm. Alternatively, our laboratory measurements have shown that three monolayers of adsorbed water on 2.2–14 vol % smectite clays can completely absorb radar energy and would be equivalent to a global water layer just ∼0.2–0.6 m thick. We suggest that the increased attenuation in volcanic units comes from an increase in fracture density. Attenuation in water‐altered units may be due to the greater heterogeneity in sedimentary units and/or chemical alteration that has formed high‐surface‐area smectites capable of holding the necessary amount of adsorbed water. Overall, the lack of widespread, deep subsurface reflections is due to the more Earth‐like radar properties of Mars, as compared to the Moon‐like properties that were anticipated.