Bright basal reflectors in radargram from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) of the Martian south polar layered deposits (SPLD) have been interpreted to be evidence of subglacial lakes. However, this interpretation is difficult to reconcile with the low Martian geothermal heat flow and the frigid surface temperature at the south pole. We conduct a comprehensive thermophysical evolution modeling of the SPLD and show that subglacial lakes may only form under exceptional circumstances. Subglacial lakes may form if the SPLD contains more than 60 % dust volumetrically or extremely porous ice (>30 %), which is unlikely. A thick (>100 m) layer of dirty ice (>90% dust) at the base of the SPLD may also enable basal melting, resembling a sludge instead of a lake. Other scenarios enabling subglacial lakes in the SPLD are equally unlikely, such as recent magmatic intrusions at shallow depths.

Martian meteoritic petrology and regional chemistry of Hesperian-Amazonian volcanism support secularly decreasing degrees of partial melting and thickening crust underlain by simple mantle convection. However, the applicability of this interior evolution model and resurfacing trends to the Noachian remains unknown. Using regional gamma spectroscopy and geophysical analysis, we find that supereruptions characterized Noachian volcanism in NW Arabia with co-enriched K, Th, and Si. Geophysical analysis reveals elastic thickness values below 20 km, indicating a heat flux exceeding many Hesperian volcanoes. Collectively, our results support large ion lithophile loss from low degrees of partial melting of the Noachian mantle, signifying an early stage of interior evolution that contrasts with the Hesperian-Amazonian model. Regional chemistry further suggests climate-altering supereruptive exhalations of ~109 kg S-phases.

Presence of a remnant crustal magnetic field and its spatial relationship to large impact basins indicates that Mars had a global dynamo during the Noachian eon (>4 Ga) (1-4). The geological processes responsible for the magnetization of the Martian crust, however, remain enigmatic. A plethora of morphological and compositional evidence suggests that high-temperature water-rock reaction was pervasive during the Noachian eon. Here we show that chemical rema-nent magnetization associated with serpentinization was possibly a key contributor to Mars’ crustal magnetic field. The conditions in the Martian subsurface during the Noachian eon were conducive to serpentinization, as we show through numerical models of hy-drothermal circulation. Geological features on Mars that implicate water-rock reaction statistically significantly align with areas showing a notably higher crustal magnetic field intensity than the average Noachian terrain. The spatial association of highest crustal magnetic field anomalies with areas of elevated heat-producing element concentrations further bolsters the likelihood of hydrothermal circulation sustained over geologic time. Such Noachian conditions would not only enable pervasive serpentinization of the mafic Martian crust but also release climate-transforming potent greenhouse gases such as H 2 and CH 4 while supporting a subsurface habitable environment.