Recent findings by the Mars Science Laboratory (MSL) have confirmed the presence of nitrates near Gale Crater on Mars. In this work, we consider the formation and deposition of HNOx species in cold early Mars climates. We find that solar energetic particles could facilitate nitrogen fixation by photochemically generating pernitric and nitric acid, which then deposit onto icy particles that settle onto Mars’ surface. This study demonstrates that such deposition would be more efficient under higher atmospheric pressures, consistent with Mars’ ancient atmosphere, and could account for the nitrate levels detected by the MSL. We find a more rapid deposition rate for pernitric acid over nitric acid (in agreement with Smith et al., 2014), and a significant enhancement of deposition rates through consideration of deposition onto icy particles. This distinction could be crucial for interpreting the MSL data.

For decades the scientific community has been trying to reconcile abundant evidence for fluvial activity on Noachian and early Hesperian Mars with the faint young Sun and reasonable constraints on ancient atmospheric pressure and composition. Recently, the investigation of H2-CO2 collision induced absorption has opened up a new avenue to warm Noachian Mars. We use the ROCKE-3D global climate model to simulate plausible states of the ancient Martian climate with this absorptive warming and reasonable constraints on surface paleopressure. We find that 1.5-2 bar CO2-dominated atmospheres with 3% H2 can produce global mean surface temperatures above freezing, while also providing sufficient warming to avoid surface atmospheric CO2 condensation at 0°-45° obliquity. Simulations conducted with both modern topography and a paleotopography, before Tharsis formed, highlight the importance of Tharsis as a cold trap for water on the planet. Additionally, we find that low obliquity (modern and 0°) is more conducive to rainfall over valley network locations than high (45°) obliquity.