A simple model shows that a rocky planet close to its star may solidify so slowly that its water is lost to space and the planet becomes desiccated, whereas a planet farther out may solidify quickly and retain its water. See Letter p.607
Earth and Venus were probably built from similar rocky materials, having been formed by similar mass-accretion processes. The giant impacts that characterize these processes are thought to melt the growing planets to some depth, producing one or more magma-ocean stages during which the silicate portion of the planet is melted before solidifying. Thus, there has been no reason to suspect that Venus and Earth differed through the first tens and probably hundreds of millions of years of the Solar System, and Venus is commonly thought to have lost its water through some later divergence from Earth-like evolution. On page 607 of this issue, Hamano et al.1 present a simple model that might explain why rocky planets that have similar compositions but orbit at different distances from their host stars can end their magma-ocean stages with either an Earth-like wetness or a Venus-like dryness. This model does not require any later divergence to explain the differences between the planets.
Almost 30 years ago, researchers showed how a dense steam atmosphere can be generated on a young, hot planet by the solidification of an impact-generated magma ocean2. After the upper troposphere (the lowest portion of the atmosphere) of the young planet has become saturated with steam, that atmospheric layer imposes a strict upper limit on outgoing radiation from the magma ocean — about 300 watts per square metre. Therefore, as soon as the magma ocean produces a steam-saturated troposphere, the cooling rate of the planet is controlled by this one simple limit.
Previously, several groups had calculated that a magma ocean should solidify in just millions of years3,4,5. These calculations assumed that the planet had lower incoming heat flux from the star than outgoing heat flux from the magma ocean. The crucial feature of Hamano and colleagues' model is that some planets are close enough to their star for the incoming heat flux to be higher than the 300 W m−2 outgoing radiation limit, and thus the planet would be prevented from cooling at all until water was lost from the steam-saturated atmosphere.
For planets close to their star, solidification and cooling together may take orders of magnitude longer than for more distant planets, perhaps as long as hundreds of millions of years. After solidification, cooling proceeds only as water is stripped from the hot, inflated atmosphere by hydrodynamic escape. The longer that solidification and cooling take, the more water is lost to space, and the drier the planet becomes. Thus, the distance of a terrestrial planet from its host star might produce an evolutionary dichotomy (Fig. 1).
The authors further suggest that Earth solidified far enough from the Sun to have a net loss of planetary heat from the beginning, allowing it to solidify quickly. Earth's initial water inventory influenced the volume of only its initial oceans. Venus, however, may have had net heat flux into the planet, and its current dryness might be related to this early slow solidification and attendant atmospheric water loss, before cooling allowed the water in the steam atmosphere to cool and condense into liquid oceans.
Recent work on geochemical tracers has indicated that Earth's mantle solidified and differentiated from a magma ocean more than 4.45 billion years ago6, probably around 4.52 billion years ago7, which agrees with rapid solidification. For Venus, however, there are insufficient geochemical data to perform this test. Measurements of deuterium and hydrogen in the Venusian atmosphere indicate that the planet has lost a substantial amount of water over time8,9, but whether that loss occurred at the time of solidification or more recently is a matter of argument.
The authors' model underscores the importance of the earliest accretion and solidification steps in determining the future evolution of the rocky planets. However, several crucial caveats need to be considered in applying this model. First, in extrapolating back in time, the faint young star's radiation level needs to be considered. Second, initial atmospheres might not all be water-rich; the rocky building blocks for some planets might have produced atmospheres rich in methane and hydrogen, instead of steam10. In the absence of a steam atmosphere, there would be no outgoing radiation limit to slow solidification and cooling. Third, forming an initial atmosphere above a magma ocean is not a simple process. The removal of volatile gases from magma might require a significant degree of supersaturation and might not occur until late in solidification. If this is so, then solidification would proceed to a high degree before a steam atmosphere formed and occluded heat flux.
Although proximity to a star affects planetary water content, this is not the only parameter that dictates the habitability of a rocky planet — the planet's composition also has a strong influence on all aspects of habitability, such as bulk atmospheric composition, susceptibility to plate tectonics and formation of a shielding magnetic field. A challenge for the coming decades will be to make measurements of exoplanets that allow the testing of models for habitability, and these tests need to include composition. How do atmospheric species other than water affect the solidification rates of magma oceans? What atmospheric compositions would be expected in the wake of a slow solidification with substantial water loss? The habitability of Earth and the inhospitability of Venus may be the inevitable result of our planetary sibling order next to the Sun rather than later evolutionary bifurcations. If so, similar patterns of habitability are likely to be found in exoplanets.
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