Ideas that add up #168

Questions like where did the oceans come from? The Earth was born in fire, and all its water was boiled off into space from its molten surface. Scientists have speculated that the water that fills our oceans was delivered at a later time by vast numbers of impacting comets, just like comet 67P/Churyumov-Gerasimenko. Already, there is evidence that the “isotopic fingerprint” of water on one closely observed comet is similar to water in Earth’s oceans. Rosetta could prove the case once and for all.

Marcus Chown, ‘Life: the cosmic connection’ in New Humanist, Winter 2014

The result, announced later than publication of the above, was that the isotopic fingerprint of water on comet 67P/C-G, at least of water at its surface where the Rosetta’s lander took measurements before powering down, is not compatible with water on Earth. Too deuterium-rich, too “heavy”. So the theory of comet origins for ocean water has been tweaked to suggest maybe 10% from comets and the rest from asteroids, which are less icy but far bigger.  

This all comes as mind-boggling news to me. About a year ago a then-five-year-old of my acquaintance asked me where the oceans came from. I poked around on the Web and couldn’t come up with a respectable answer. Something as big and close as the oceans, and science hasn’t figured out their origins yet! Told him in the end – because it seemed plausible to me – that the oceans were formed of water squeezed from the core as the young Earth cooled. Didn’t mention the comet business because it sounded just too space-cadet.

But now a rethink. The planet’s internal water was allegedly “boiled off” (and the vapour was not held back by gravity or atmospheric pressure?). Meanwhile, there was a phase of mass bombardment in the Earth’s formative period, involving all kinds of debris left over from the assembly of the solar system. Now, comets really are made of water: misshapen clumps of ice and grit as light and porous as pumice stone, and coated in a black, gooey dust of carbon-rich compounds, hence the “dirty snowball” nickname. Supposedly, there could have been enough comets hitting the Earth’s surface, after the crust had cooled, to leave a sea of puddles. (This still sounds barmy – how many Central-Park-sized snowballs like 67P/C-G does it take to fill the Pacific?) Add to this the possibility, speculated on by some, that comets ferried in the planet’s first few micro-organisms, mixed in among the H2O and hydrocarbons – which could explain why living cells appear to have popped up with such ease as soon as the planet had cooled enough for them.

The Water of Life indeed!

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