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The origin of the presence of geological and mineralogical evidence that liquid water flowed on the surface of early Mars is now a 50-year-old mystery. It has been proposed (Segura et al., 2002(Segura et al., , 2008(Segura et al., , 2012 that bolide impacts could have triggered a long-term climate change, producing precipitation and runoff that may have altered the surface of Mars in a way that could explain (at least part of) this evidence. Here we use a hierarchy of numerical models (a 3-D Global Climate Model, a 1-D radiative-convective model and a 2-D Mantle Dynamics model) to test that hypothesis and more generally explore the environmental effects of very large bolide impacts (D impactor > 100 km, or D crater > 600 km) on the atmosphere, surface and interior of early Mars.Using a combination of 1-D and 3-D climate simulations, we show that the environmental effects of the largest impact events recorded on Mars are characterized by: (i) a short impact-induced warm period (several tens of Earth years for the surface and atmosphere to be back to ambient conditions after very large impact events); (ii) a low amount of hydrological cycling of water (because there is no surface re-evaporation of precipitation). The total cumulated amount of precipitation (rainfall) can be reasonably well approximated by the initial post-impact atmospheric reservoir of water vapour (coming from the impactor, the impacted terrain and from the sublimation of permanent ice reservoirs heated by the hot ejecta layer); (iii) deluge-style precipitation (∼2.6 m Global Equivalent Layer of surface precipitation per Earth year for our reference simulation, quantitatively in agreement with previous 1-D cloud free climate calculations of Segura et al. 2002), and (iv) precipitation patterns that are uncorrelated with the observed regions of valley networks.However, we show that the impact-induced stable runaway greenhouse state predicted by Segura et al. (2012) is physically inconsistent. We nevertheless confirm the results of Segura et al. (2008) and Urata and Toon (2013) that water ice clouds could in theory significantly extend the duration of the post-impact warm period, and even for cloud coverage significantly lower than predicted in Ramirez and Kasting (2017). However, the range of cloud microphysical properties for which this scenario works is very narrow.Using 2-D Mantle Dynamics simulations we find that large bolide impacts can produce a strong thermal anomaly in the mantle of Mars that can survive and propagate for tens of millions of years. This thermal anomaly could raise the near-surface internal heat flux up to several hundreds of mW/m 2 (i.e. up to ∼ 10 times the ambient flux) for several millions years at the edges of the impact crater. However, such internal heat flux is largely insufficient to keep the martian surface above the melting point of water.In addition to the poor temporal correlation between the formation of the largest basins and valley networks (Fassett and Head, 2011), these arguments indicate that the largest ...

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Ancient valley networks (VNs) and related open‐ and closed‐basin lakes are testimony to the presence of flowing liquid water on the surface of Mars in the Late Noachian and Early Hesperian. Uncertain, however, has been the mechanism responsible for causing the necessary rainfall and runoff and/or snowfall and subsequent melting. Impact cratering has been proposed (e.g., Segura et al. 2002) as a process for temporarily raising temperatures and inducing conditions that would produce rainfall, snowmelt, runoff, and formation of the VNs and associated lacustrine features. We refer to the collective effects of this process as the ICASE model (impact cratering atmospheric/surface effects). In this contribution, we assess the proposed impact cratering mechanism in order to understand its climatic implications for early Mars: we outline the step‐by‐step events in the cratering process and explore the predictions for atmospheric and surface geological consequences. For large and basin‐scale impacts, rainfall should be globally and homogeneously distributed and characterized by very high temperatures. Rainfall rates are predicted to be high, ~2 m yr−1, similar to rates in tropical rainforests on Earth, and runoff rates are correspondingly very high. These predicted characteristics do not seem to be consistent with the observed VNs, which are mainly equatorial and not homogeneous in their distribution. Prior to the Late Noachian, however, we predict that basin‐scale impact effects are very likely to contribute significantly to degradation of crater rims and regional smoothing of terrain, implying vast resurfacing and resetting of crater ages following large crater and basin‐scale impacts. Furthermore, the high temperatures of impact‐induced rainwater and snowmelt and the pervasive penetration of heat into the regolith substrate are predicted to have a significant influence on the mineralogical alteration of the crust and its resulting physical properties. We conclude by describing a case example (Isidis basin) and describe how the ICASE model provides an alternative scenario for the interpretation of the layered phyllosilicates in the Nili Fossae and NE Sytris regions. We outline specific conclusions and recommendations designed to improve the ICASE model and to promote further understanding of its implications for the geological, mineralogical, and climate history of early Mars.

In Earth's deep continental subsurface, where groundwaters are often isolated for >10 6 to 10 9 years, energy released by radionuclides within rock produces oxidants and reductants that drive metabolisms of nonphotosynthetic microorganisms. Similar processes could support past and present life in the martian subsurface. Sulfate-reducing microorganisms are common in Earth's deep subsurface, often using hydrogen derived directly from radiolysis of pore water and sulfate derived from oxidation of rock-matrix-hosted sulfides by radiolytically derived oxidants. Radiolysis thus produces redox energy to support a deep biosphere in groundwaters isolated from surface substrate input for millions to billions of years on Earth. Here, we demonstrate that radiolysis by itself could produce sufficient redox energy to sustain a habitable environment in the subsurface of present-day Mars, one in which Earth-like microorganisms could survive wherever groundwater exists. We show that the source localities for many martian meteorites are capable of producing sufficient redox nutrients to sustain up to millions of sulfate-reducing microbial cells per kilogram rock via radiolysis alone, comparable to cell densities observed in many regions of Earth's deep subsurface. Additionally, we calculate variability in supportable sulfate-reducing cell densities between the martian meteorite source regions. Our results demonstrate that martian subsurface groundwaters, where present, would largely be habitable for sulfate-reducing bacteria from a redox energy perspective via radiolysis alone. We present evidence for crustal regions that could support especially high cell densities, including zones with high sulfide concentrations, which could be targeted by future subsurface exploration missions.

The formation of martian geologic features, including degraded impact craters, valley networks, and lakes, has been interpreted to require a continuously "warm and wet" Noachian climate, with above-freezing surface temperatures and rainfall. More specifically, it has been argued that a change in the nature of rainfall in the Noachian, from a diffusive rain splash-dominated erosional regime to an advective runoff-dominated erosional regime, is the best explanation for the observed temporal differences of erosion style: the degradation of craters has been interpreted to be due to rain splash throughout the Noachian, while the formation of valley networks and lakes has been interpreted to be due to more erosive and abundant fluvial activity at the Noachian/Hesperian transition. However, the presence of a long-lived "warm and wet" climate with rainfall is difficult to reconcile with climate models which instead suggest that the longlived climate was "cold and icy", with surface temperatures far below freezing, precipitation limited to snowfall, and most water trapped as ice in the highlands. In a "cold and icy" climate, fluvial and lacustrine activity would only be possible during transient warm periods, which could produce "warm and wet" conditions for a relatively short period of time. In this work, we (1) review the geomorphic evidence for Noachian rainfall and the various rainfall-related erosional regimes, (2) explore climate model predictions for a "cold and icy" climate and the potential for short-lived "warm and wet" excursions, and (3) attempt to characterize the transition from diffusive to advective erosional rainfall regimes through analysis of atmospheric pressure and rainfall dynamics with the goal of providing insight into the nature of the Noachian hydrological cycle and thus, the Noachian climate. We conclude that (1) if rainfall occurred on early Mars, raindrops would have been capable of transferring sufficient energy to initiate sediment transport regardless of atmospheric pressure, implying that rain splash would have been possible 3 throughout the Noachian, and (2) in contrast to previous findings, maximum possible raindrop size does not depend on atmospheric pressure and, as a result, simple parameterized relationships suggest that rainfall intensity (rainfall rate) does not depend on atmospheric pressure. Therefore, our results, based on the implementation of a simple parameterized relationship for rainfall intensity, predict that there would not have been a transition from rain splash-dominated erosion to runoff-dominated erosion related solely to decreasing atmospheric pressure in the Noachian;we suggest that future work should test this conclusion with more advanced methods of calculating rainfall intensity. This finding is not necessarily consistent with the hypothesis of Craddock and Lorenz (2017) that the long-lived Noachian climate was "warm and wet" with continuous rainfall and that rainfall intensity changed as a function of atmospheric pressure declining through time; our findings do...

Observations of Late Noachian-Early Hesperian-aged Martian surfaces reveal valley networks, lakes, degraded craters, and putative oceanic shorelines, often interpreted to require a persistent "warm and wet" climate, characterized by mean annual temperature >273 K and abundant rainfall. We simulate this "warm and wet" climate (global mean annual temperature~275 K) with a 3-D global climate model to determine whether these features could have formed in this climate through rainfall activity. We find that rainfall is limited in abundance and areal distribution, precipitation is dominated by snowfall, and highlands temperatures are <273 K for the majority of the year. We conclude that, in this simulated climate scenario, (1) Late Noachian-Early Hesperian valley networks and lakes could not have formed through rainfall-related erosion, (2) crater degradation by rainsplash and runoff is not predicted, (3) global clay formation through long-lived rainfall, fluvial activity, and warm temperatures is unlikely, and (4) the presence of a rainfall-and overland flow-fed northern ocean is improbable. Plain Language Summary Observations and analyses of Martian surface features, including fluvialand lacustrine features, imply that liquid water was abundant~3.7 Ga. The characteristics of these features has led researchers to conclude that the early climate was likely to have been "warm and wet", characterized by abundant rainfall and surface runoff. Here we implement a three-dimensional climate model and simulate the conditions of a "warm and wet" climate scenario, with globally averaged surface temperature~275 K, just above the melting point of water, to determine whether these surface features could have actually formed in this climate scenario through rainfall-related activity. Contrary to previous predictions, we find that rainfall is extremely limited in this climate scenario, precipitation is dominated by snowfall, and temperatures are below freezing for the majority of the year in regions where the fluvial and lacustrine features are abundant. We suggest that snow accumulation, melting, and surface runoff may offer a more plausible explanation for the formation of these features.

Widespread Amazonian‐aged fluvial channels have been mapped proximal to Lyot crater, a ~225 km diameter impact basin in the northern lowlands of Mars. Comparable in area to some Noachian/Hesperian fluvial systems, their morphology differs, being dominated by broad, shallow braided channels. Using new developments in the study of cratering, water inventory, and climate history, we assess eight different models for their origin. Dewatering of excavated ice‐rich Lyot ejecta and contact melting from hot Lyot ejecta superposed on surface ice deposits are the most plausible channel origins. The existence of this extensive Amazonian fluvial system is attributed to: (1) the large size of Lyot, and its consequent hot ejecta, and (2) the presence of surface ice at the time of impact, attributed to obliquity changes redistributing polar ice to the mid‐latitudes, a relatively common occurrence in Martian geologic history.