The environmental effects of very large bolide impacts on early Mars explored with a hierarchy of numerical models

Abstract

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 ...

Impact Degassing of H2 on Early Mars and its Effect on the Climate System

“…Eventually, escape at the diffusion limit sets in, and H 2 abundances decline exponentially. The duration of above freezing temperatures is on the order of several hundred thousand years, which is several orders of magnitude larger than that expected from energy dissipation alone (Segura et al, 2002;Steakley et al, 2019;Turbet, Gillmann, et al, 2019). Thus, warm wet conditions persist for much longer periods following an impact event when hydrogen production is considered.…”

Climate Simulations of Early Mars With Estimated Precipitation, Runoff, and Erosion Rates

“…Furthermore, neither impact‐induced cirrus clouds nor runaway greenhouses generate the durations and water amounts necessary to form the valleys (Ramirez & Kasting, ). It has recently been argued that impact‐induced cirrus cloud warming could be more effective if such clouds could have formed in the stratosphere (Turbet et al, ). However, this scenario invokes convection in the CO 2 moist adiabat region, which is unphysical because water vapor concentrations are predicted to be very small there (e.g., Ramirez & Kasting, ; Wordsworth et al, ).…”

The Oldest Highlands of Mars May Be Massive Dust Fallout Deposits

“…The atmospheric fallout of the vast volumes of suspended sediments would have occurred as the winds lost speed and as the atmosphere cooled. A likely consequence of the subsequent cooling of a water vapor-rich atmosphere is the development of transient rainy paleoclimatic conditions [24][25][26][27][28] . We hypothesize that the rainfall could have effectively accelerated the removal rates of suspended sediments, resulting in catastrophic sedimentary fallouts in the form of muddy rains and the formation of voluminous muddy substrates over the Martian highlands.…”

“…Segura et al 25 considered possible www.nature.com/scientificreports www.nature.com/scientificreports/ transient wet periods lasting several centuries. However, other researchers hypothesize that the rainfall lasted several months to a few years [27][28] . Thus, it is conceivable that extensive Early Noachian glacio-fluvial and aeolian deposits covered large parts of the Martian megaregolith.…”

“…Thus, Mars retains extensive surface exposures of the materials that make up these extremely old highlands. The goal of this article is to determine whether the highland stratigraphy that lies buried beneath the Noachian intercrater plains contains a geologic record consistent with widespread sedimentation that happened as a consequence of the previously proposed impact-modified atmospheric and paleoclimatic conditions [24][25][26][27][28] . We find strong similarities between the sedimentary record preserved in NW Hellas and NW Isidis, suggesting a link between basin-forming impacts, consequential paleoclimate changes, and catastrophic sedimentary fallout and deposition.…”