Emergence of a Habitable Planet

Abstract

We address the first several hundred million years of Earth's history. The Moonforming impact left Earth enveloped in a hot silicate atmosphere that cooled and condensed over ∼1,000 yrs. As it cooled the Earth degassed its volatiles into the atmosphere. It took another ∼2 Myrs for the magma ocean to freeze at the surface. The cooling rate was determined by atmospheric thermal blanketing. Tidal heating by the new Moon was a major energy source to the magma ocean. After the mantle solidified geothermal heat became climatologically insignificant, which allowed the steam atmosphere to condense, and left behind a ∼100 bar, ∼500 K CO 2 atmosphere. Thereafter cooling was governed by how quickly CO 2 was removed from the atmosphere. If subduction were efficient this could have K. Zahnle ( ) K. Zahnle et al.taken as little as 10 million years. In this case the faint young Sun suggests that a lifeless Earth should have been cold and its oceans white with ice. But if carbonate subduction were inefficient the CO 2 would have mostly stayed in the atmosphere, which would have kept the surface near ∼500 K for many tens of millions of years. Hydrous minerals are harder to subduct than carbonates and there is a good chance that the Hadean mantle was dry. Hadean heat flow was locally high enough to ensure that any ice cover would have been thin (<5 m) in places. Moreover hundreds or thousands of asteroid impacts would have been big enough to melt the ice triggering brief impact summers. We suggest that plate tectonics as it works now was inadequate to handle typical Hadean heat flows of 0.2-0.5 W/m 2 . In its place we hypothesize a convecting mantle capped by a ∼100 km deep basaltic mush that was relatively permeable to heat flow. Recycling and distillation of hydrous basalts produced granitic rocks very early, which is consistent with preserved >4 Ga detrital zircons. If carbonates in oceanic crust subducted as quickly as they formed, Earth could have been habitable as early as 10-20 Myrs after the Moon-forming impact.

Insights into Planet Formation from Debris Disks

“…However, if the collision is energetic enough, the surface would be melted in the collision resulting in a magma ocean at 1000-3000 K for rocky planets. This molten surface would only be temporary since the heat would be radiated away (e.g., Earth's magma ocean may have survived 2 Myr; Zahnle et al 2007), but could have a dramatic effect on the emission spectrum of the planet. Whether the hot surface itself is observable depends on the planet's atmospheric properties, which would in turn be modified by the impact; e.g., the early nebula atmosphere could be stripped by impacts Inamdar and Schlichting 2015), and the atmosphere could also be replenished by magma ocean outgassing.…”

Terrestrial aftermath of the Moon-forming impact

“…Our discussion starts with this stage as the initial condition. Water and CO 2 are only modestly soluble within molten rock of mantle composition at hundreds of bars pressure [17,18], and thus much of the global inventory of these materials entered the atmosphere [5,14,15,19]. This inventory would have been comparable to the modern inventory, since outer Solar System objects (comets) are unlikely to have added globally significant masses of water after the Moon-forming impact (e.g.…”

“…The connection between tidal heating and the upper bound on radiative cooling created a stable climate buffer [4,5]. Dissipation by oscillating tidal stresses for hot silicate interiors can be characterized by the Maxwell time η/Γ (where η is viscosity and Γ is shear modulus).…”

“…The existence of another form of non-standard plate tectonics in the Hadean is conceivable given the Archaean to recent analogues and the geological record [5,44] (figure 4). We have already noted that at modern fast ridge axes mush can cumulate beneath a thin melt lens.…”

Constraints on the Habitability of Extrasolar Moons

“…Tidal surface heating, assumed to be distributed uniformly over the surface, is 0.017 W/m 2 in both panels. For reference, the Earth's outward heat flow is 0.065 W/m 2 through the continents and 0.1 W/m 2 through the ocean crust (Zahnle et al 2007). Parametrization of the star-planet system follows Borucki et al (2012).…”