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

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 explore the environmental effects of very large impacts 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; (ii) a low amount of hydrological cycling of water; (iii) deluge-style precipitation; and (iv) precipitation patterns that are uncorrelated with the observed regions of valley networks. We show that the impact-induced stable runaway greenhouse state predicted by Segura et al. 2012 is physically inconsistent. We confirm the results of Segura et al. 2008 and Urata & Toon 2013 that water ice clouds can significantly extend the duration of the post-impact warm period, and even for cloud coverage lower than predicted in Ramirez & 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 impacts can raise the near-surface internal heat flux up to several hundreds of mW/m$^2$ (i.e. up to $\sim$ 10 times the ambient flux) for several millions years at the edges of the impact crater. However, such internal heat flux is insufficient to keep the martian surface above the melting point of water. Our numerical results support the prediction of Palumbo & Head 2018 that very large impact-induced rainfall could have caused degradation of craters and formed smooth plains, potentially erasing much of the previously visible morphological surface history. Such hot rainfalls may have also led to the formation of aqueous alteration products on Noachian-aged terrains.

💡 Research Summary

The paper investigates the environmental consequences of the largest bolide impacts on early Mars by employing a hierarchy of numerical models: a three‑dimensional Global Climate Model (GCM), a one‑dimensional radiative‑convective model, and a two‑dimensional mantle dynamics code. The authors focus on impacts with impactor diameters greater than 100 km (crater diameters > 600 km), which are capable of vaporizing an amount of water comparable to the present Martian water inventory (~34 m global equivalent layer).

The 3‑D GCM simulations reveal that an impact instantly creates an ultra‑hot atmospheric plume (temperatures of several thousand kelvin) that expands globally. Silicate material condenses first and falls back, forming a thin, hot spherule layer, while water vapor remains aloft, raising atmospheric pressure to several hundred millibars and increasing the water mixing ratio dramatically. The atmosphere then cools rapidly; within a few tens of Earth years surface temperatures return to pre‑impact conditions.

The 1‑D radiative‑convective model is used to explore a wide parameter space (impact size, initial CO₂/H₂O ratios, cloud microphysics). It confirms that a short‑lived warm period (tens of years) is robust, but a stable runaway greenhouse state, as suggested by Segura et al. (2012), cannot be sustained when convection and water‑vapor condensation are properly accounted for. High‑altitude water‑ice clouds can extend the warm phase, yet only for a very narrow range of cloud particle sizes (≈1–3 µm) and supersaturation levels (10–20 %). Outside this narrow window, cloud radiative forcing is negligible and the warm period collapses within a few years.

Regarding the hydrological cycle, the models show that total precipitation is essentially limited by the initial post‑impact water‑vapor reservoir. The simulated precipitation rate corresponds to about 2.6 m global equivalent layer per Earth year—a deluge‑style rainfall that is globally distributed. However, the spatial pattern of this rainfall does not correlate with the observed Noachian valley networks, implying that impact‑induced rain was not the primary driver of valley formation. Instead, the intense, short‑duration rainfall would have preferentially eroded and smoothed the impact basins themselves, supporting the hypothesis of Palumbo & Head (2018) that large impacts erased small craters and created smooth plains.

The 2‑D mantle dynamics simulations indicate that the thermal anomaly generated by a giant impact can persist for tens of millions of years, raising the near‑surface heat flux at the crater rim to several hundred mW m⁻² (about ten times the ambient flux). Nevertheless, this elevated heat flow is insufficient to keep surface temperatures above the melting point of water, so long‑term geothermal heating does not sustain a warm climate.

Overall, the study concludes that (1) giant impacts produce a brief, intense warm episode and massive, globally distributed rainfall, but the warm episode cannot be prolonged without very specific cloud properties; (2) the precipitation pattern is inconsistent with the distribution of valley networks, making impacts unlikely as the direct cause of those fluvial features; (3) mantle heating from impacts does not provide enough surface warmth for long‑term liquid water stability; and (4) the most plausible geomorphic legacy of such impacts is basin‑scale erosion, crater degradation, and the formation of smooth, possibly clay‑rich plains. These findings refine earlier work by Segura et al., Urata & Toon, and Ramirez & Kasting, adding new constraints on cloud microphysics and mantle heat flow in the context of early Martian climate evolution.