"Earth after the Moon-forming impact "

Kevin Zahnle
NASA Ames Research Center

The Hadean Earth is widely and enduringly pictured as a world of exuberant volcanism, exploding meteors, huge craters, infernal heat, and billowing sulfurous steams; i.e., a world of fire and brimstone punctuated with blows to the head. In the background the Moon looms gigantic in the sky. The popular image has given it a name that celebrates our mythic roots. A hot early Earth is an inevitable consequence of accretion. The Moon-forming impact ensured that Earth as we know it emerged from a fog of silicate vapor.

The impact separated the volatiles from the silicates. It took ~1000 years to condense and rain out the bulk of the vaporized silicates, although relatively volatile elements may have remained present in the atmosphere throughout the magma ocean stage. The magma ocean lasted ~2 Myr, its lifetime prolonged by tidal heating and thermal blanketing by a thick CO2-rich steam atmosphere. Tidal heating was concentrated bear the base of the mantle. A stable feedback between melting and tidal dissipation ceded control over lunar orbital evolution to the atmosphere. With thermal blanketing acting as a tether to the Moon, the Moon's orbit evolved orders of magnitude more slowly than it would otherwise. This enabled capture of the Moon into certain resonances that ultimately gave the lunar orbit its inclination.

Water oceans condensed quickly after the mantle solidified, but for some 10-100 Myr the surface would have stayed warm (~500 K), until the CO2 was removed by weathering reactions and subsequent subduction into the mantle. Thereafter the faint young Sun suggests that a lifeless Earth would always have been evolving toward a bitterly cold ice world. The cooling trend was frequently interrupted by volcanic or impact-induced thaws.