As if November’s historic feat of landing the Philae probe on a comet wasn’t enough, now the Rosetta mission is sending back data that blows a hole in the hypothesis that comets delivered much of Earth’s water to our oceans. Or, if they did, then they weren’t comets like 67P/Churyumov-Gerasimenko!
This surprisingly monochrome lump of space rock (it looks grey even in colour images), is currently being orbited by the Rosetta probe, which is sniffing the chemical vapour that pours off the icy rock. Data sent back from the orbiter gathered by the Rosina instrument, consisting of two mass spectrometers, suggests that the water from the comet has a different signature from that found on earth.
How do we know? Scientists have been looking at the proportion of so-called “heavy” water found in the vapour. According to results that have been published in the journal Science, the water from the comet contains a distinctly different proportion of deuterium atoms – it’s heavier – from that found on Earth, which tends to have three deuterium atoms in every 10,000 water molecules.
Other studies have analysed water on different types of comets and some scientists are now ruling out these bodies as the source of the water in Earth’s oceans. Instead, a better candidate for the water delivery mechanism may be asteroids.
Rosina's principal investigator, Professor Kathrin Altwegg, from the University of Bern in Switzerland, argues that asteroids – dense, rocky objects that formed closer to the Sun than comets – are the source. “We know already something about the characteristic of asteroids by studying meteorites, which are pieces of asteroids, and the characteristics of asteroids are very much like our water,” she explained.
“They are also much closer to the Earth so it is more likely that they hit the Earth than the very distant comets, which are beyond Neptune.”
It also turns out that the ice in comet 67P/Churyumov-Gerasimenko is buried beneath a dusty, boulder-strewn surface. If there were any ice patches at the surface, they would show up in the latest colour images as patches of blue. The latest images released by the Max Planck Institute for Solar System Research indicate that the comet “looks dark grey, in reality almost as black as coal”, according to Dr Holger Sierks, the principal investigator on Rosetta’s Osiris camera.
Six facts about Rosetta and 67P/Churyumov-Gerasimenko
- Rosetta is the first space flight to orbit and land on a comet, making it one of the most complex and ambitious missions ever undertaken. Scientists had to plan in advance, in the greatest possible detail, a ten-year trip through the Solar System.
- Comet, 67P/Churyumov-Gerasimenko, is a relatively small object, about 4km in diameter, moving at a speed of up to 135,000 km/hour.
- The comet loops around the Sun between the orbits of Jupiter and Earth – between about 800 million and 186 million km from the Sun. However, the rendezvous with the comet required the spacecraft to travel a cumulative distance of over 6.4 billion km over ten years, using the gravity of Mars and Earth in a series of fly-bys to slingshot it into the right orbit.
- Rosetta will be the first spacecraft to witness, at close proximity, how a comet changes as it approaches the increasing intensity of the Sun’s radiation, when it develops the so-called ‘coma’ (essentially the comet’s atmosphere) and the two characteristic ion and dust tails.
- Rosetta’s Philae lander sent back to Earth the first images from a comet’s surface before it shut down due to lack of power.
- Previous research has found that water held in a class of comets that originated in the
Oort Cloud (see diagram) in the outer reaches Solar System also have a different signature from water on Earth. So far, only two comets from the Kuiper-belt, an area that extends to just beyond Neptune, have been analysed: comet 67P and comet Hartley 2.
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