Desiccation of the TRAPPIST-1 Planets During Their Magma Ocean Phase

Abstract

<pre class="moz-quote-pre">The potentially habitable planets in the TRAPPIST-1 system (e,f,g) may have experienced a prolonged magma ocean phase during which volatiles were partitioned between the molten interior and the atmosphere. The duration of the magma ocean phase is determined by 1) the incident stellar radiation, 2) atmospheric heating due to the greenhouse effect, 3) water photolysis and hydrogen escape, 4) tidal heating, 5) radiogenic heating, and 6) the magma ocean’s initial temperature. We simulate these phenomena simultaneously with the VPLanet software package, including a new module called MagmOc that treats the thermal and geochemical evolution (water, O2, and CO2) of the magma ocean. We find the TRAPPIST-1 planets’ evolution depends on initial water content and distance from the host star. In a “dry” scenario (initial water content < 5TO, for planet g), the atmosphere after magma ocean solidification is desiccated and devoid of abiotically generated O2. In an “intermediate” scenario (initial water content between 5 and 50TO), the post magma ocean atmosphere still contains water. XUV photolysis of this water leads to abiotic O2 build-up. For “extremely wet” cases (initial water content > 50 TO) or extreme internal heating, the magma ocean lifetime can be extended and quench oxygen build up. The currently inferred high water content of the planets favors the extremely wet scenario for TRAPPIST-1 g and f, i.e. they likely ended their magma ocean state with large amounts of water vapor in their atmospheres but potentially avoid the build-up of large amounts of oxygen. TRAPPIST-1 e, on the other hand, could have experienced the intermediate scenario and is therefore even less likely to possess large amounts of abiotically created atmospheric O2.</pre>