Glacial paleoenvironments on Mars revealed by the paucity of hydrated silicates in the Noachian crust of the Northern Lowlands

Hydrated silicates occur widespread in thousands of locations in the Martian Noachian-aged southern highlands. If an ocean existed on the northern lowlands of Mars during the Noachian, phyllosilicates are likewise expected to be present in the primeval crust of the Martian plains, since on Earth they are common products of continental fluvial transportation and oceanic sedimentation. Here we analyze data from spaceborne imaging spectrometers searching for Noachian Mg/Al-phyllosilicates in 198 impact craters in the northern lowlands of Mars, as impact-excavated materials have already been demonstrated as qualifying samples of preexisting subsurface deposits. Our results indicate that only 15 impact craters in the Martian lowlands show evidence of excavated Mg/Al-phyllosilicates (<10%, consistent with previous estimates). We have also analyzed 88 craters in the highlands region located between −1000 and −3000 m, and in this case the number of craters showing excavated Mg/Al-phyllosilicates was 64 (>70%). Therefore, hydrated silicates form a mineralogical dichotomy noticeable in the ancient Noachian crust on a global scale. This is indicative of little or no significant fluvial transportation of phyllosilicate-rich, southern highland materials into the northern basins. But the Noachian phyllosilicate-rich terrains in the southern highlands are heavily dissected by ∼80,000 valley networks, revealing sustained surface runoff expected to be efficient at remobilizing sediments into the northern plains. The presence of widespread ice masses rimming cold glacial Noachian oceans is a plausible explanation for the lack of large-scale phyllosilicate minerals in the primeval basement of the lowlands of Mars.