Mars’ surface is covered with craters that formed when comets and/or asteroids impacted the red planet. These craters are especially numerous in the southern highlands of Mars (an indication of its older age relative to other surfaces of the planet with fewer craters) and the largest of these craters were produced by impactors that were more than 100 km in diameter. These impacts would have been capable of delivering large amounts of kinetic energy to Mars’ atmosphere, as well as water from the impactor itself and from subsurface ice excavated during crater formation. We are simulating the climate response of ancient Mars’ atmosphere to large impacts in order to test whether this could have jump-started greenhouse warming and lead to precipitation.
Segura et al. (2002) first suggested this impact-heating hypothesis as a potential mechanism to produce the warm and wet early Mars climate implied by geological evidence. We are testing this hypothesis with the early Mars Global Climate Model (eMGCM), a version of the MGCM with special physical treatments for ancient Mars that utilizes the legacy Arakawa c-grid dynamical core. Simulations include impacts of various diameters (30 km, 50 km, and 100km) and different surface pressure scenarios (150 mbar, 1 bar, and 2 bar). We find that although these impacts do induce periods of warm and wet conditions, ultimately these conditions are short-lived, on the order of 10s of sols (ie, Martian days) to a few years, and would not have supported the formation of river valley networks (Figure 1). The evolution of these post-impact climate conditions can be characterized in four phases: 1) a rapid radiative cooling phase, 2) a latent heat phase in which both cloud formation and the radiative effects of water vapor induce a temporary warm period with significant precipitation , 3) a transition phase in which cooling accelerates due to sublimation at the surface and the lack of available water in the atmosphere for greenhouse warming and in which water vapor begins to contribute less to surface warming than water clouds, and 4) a steady- state phase with mean annual surface temperatures below freezing and minimal precipitation. Scenarios with high surface pressures and radiatively active water clouds experience the longest periods of above-freezing post-impact temperatures and result in the highest mean annual temperatures during the fourth and final phase, highlighting the potential significance of water clouds in the early Martian climate and the importance of their careful physical treatment in models. Although we find that the water and energy injected into the atmosphere by an impact are insufficient to produce sustained warm and wet periods, impacts are also capable of delivering other reduced greenhouse gases such as H2 and CH4. This is another topic of investigation here at the MCMC.
