EXPLAINING how Earth ended up with enough water to form oceans has always been a sticking point for planetary scientists. Now, two of the latest models of planetary formation show that water could have accumulated rather easily on terrestrial planets. In some scenarios, planets could have started out with so much water that it would be impossible to find even the scraps of dry land that dotted Earth in the 1995 movie Waterworld.
Most models of planetary formation have assumed that sunlight could penetrate the solar nebula, the disc of dust and gas from which the planets formed. This warmth would vaporise the ice on any planetesimals inside the 鈥渟now line鈥�, a threshold just outside the orbit of Mars. The solar wind would then blow the vapour away, leaving the inner solar system dry. So, what is the source of the water that Earth has and that Mars and Venus may once have had?
In one of the latest models, the answer pops out as a surprise consequence of rethinking how our solar system鈥檚 gas giants formed. Last May, Alessandro Morbidelli of the C么te d鈥橝zur Observatory in Nice, France, and his colleagues came up with a theory to explain some of the puzzling features of gas giants, such as their unusually elongated orbits and the tilt of Saturn鈥檚 orbital plane with respect to that of Jupiter. Their model showed that these features could be explained if Jupiter and Saturn initially formed in more circular and less tilted orbits and then migrated to their current positions (Nature, vol 435, p 459).
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It now turns out that the Nice model can also account for Earth鈥檚 water 鈥� and then some. David O鈥橞rien of the Planetary Science Institute in Tucson, Arizona, modelled the formation of terrestrial planets by including the migrations of the gas giants, and it made all the difference. O鈥橞rien found that they acted like a monumental whisk, mixing planetesimals in the inner solar system with those further out that had not lost their ices to the sun鈥檚 heat. Because of that interaction, each terrestrial planet ended up with nearly 30 Earth-oceans worth of water.
In the second new model, Earth-like planets can end up even more watery. Ryosuke Machida of the University of Tokyo, Japan, found that if there was enough dust in the solar nebula, the initial vaporisation of ices inside the snow line would quickly create a highly opaque layer of dust, shading the proto-planetary disc from further heating. As a result, a substantial percentage of any terrestrial planets that later formed would retain prodigious amounts of water 鈥� up to two-thirds of the planet鈥檚 total mass. Depending on the initial conditions, Machida says you could end up with 鈥渨ater-ball planets, wet-rock planets and dry-rock planets鈥�.
鈥淵ou could end up with water-ball planets, wet-rock planets and dry-rock planets depending on the initial conditions鈥�
Planet-formation theorist Jeff Cuzzi at the NASA Ames Research Center in Moffet Field, California, says that Machida鈥檚 model is plausible, even though it produces some planets far wetter than any we know of. 鈥淭here may be a lot of different ways to make a solar system,鈥� he says. 鈥淭his is one of those areas that we鈥檙e just starting to figure out.鈥�
