Andrea Zorzi, Earth and Planetary Science PhD student, Stanford

Title: Impact induced formation of prebiotic molecules on terrestrial planets

Abstract: New chemical species can form from reactions induced by shock-heating upon formation of an impact vapor plume and its interaction with the background atmosphere of a rocky planet. Previous studies have assumed chemical equilibrium in computing the abundance of chemical species: such a simplified model can underestimate those concentrations by a factor of 10 in high-temperature shock conditions. A more accurate description of the plume/atmosphere interaction demands a coupled hydrodynamics and kinetics calculation. We present a new model extending the work of Ishimaru et al. (2010) by considering an atmosphere on the target planet. We assess the production of prebiotic molecules (HCN, CH4, NH3) for different impact scenarios, varying kinetic energy of the impactor, atmospheric surface density and composition. We find that prebiotic species are produced on Earth-like planets with a N2-CH4 atmosphere. The presence of oxygen in today’s inner solar system planets prevents the production of HCN. On Titan, impacts can constitute an additional sink for methane. For Archean Earth conditions, it is possible to form HCN and CH4. Our findings provide necessary but not sufficient conditions for prebiotic chemistry to start, to assess the astrobiological potential of impacts on terrestrial worlds.

Matt Reinhold, Earth and Planetary Science PhD Student, Stanford

Title: Ignan Earths:  Habitability of Terrestrial Planets with Extreme Internal Heating

Abstract: Is it possible for a rocky planet to have too much internal heating to maintain a habitable surface environment? In the Solar System, the best example of a world with high internal heating is Jupiter's moon Io, which has a heat flux of approximately 2 W/m^2 compared to the Earth's 90 mW/m^2 . The ultimate upper limit to internal heating rates is the Tidal Venus Limit, where the geothermal heat flux exceeds the Runaway Greenhouse Limit of 300 W/m^2 for an Earth-mass planet. Between Io and a Tidal Venus there is a wide range of internal heating rates whose effects on planetary habitability remain unexplored. We investigate the habitability of these worlds, referred to as Ignan Earth's. We demonstrate how the mantle will remain largely solid despite high internal heating, allowing for the formation of a convectively buoyant and stable crust. In addition, we model the long-term climate of Ignan Earth's by simulating the carbonate-silicate cycle in a vertical tectonic regime (known as heat-pipe tectonics, expected to dominate on such worlds) at varying amounts of internal heating. We find that Earth-mass planets with internal heating fluxes below 15 W/m^2  produce average surface temperatures that Earth has experienced in its past (below 30 C), and worlds with significantly higher heat fluxes still result in surface temperatures far below that of 100 C, indicating a wide range of internal heating rates may be conducive with habitability.

Lunch held at Mitchell Earth Sciences patio, 12:00pm