Rosetta comet challenges notions on source of water in Earth's oceans
Meteorologist/Science Writer
Friday, December 12, 2014, 10:52 AM - Where did the water in Earth's oceans ultimately originate from? The ESA's Rosetta spacecraft has provided new clues about this mystery, but comets may be proving to be a red herring.
When the European Space Agency launched a spacecraft out to rendezvous with a comet, in addition to getting a chance to examine one of these enigmatic objects close-up, it also had an important mission to perform regarding the Earth.
Our planet has an abundance of water on its surface, but the question of where that water came from still has yet to be answered. A certain amount would have been released from minerals as the planet formed and cooled, but this primordial water wouldn't have lasted long in the still-boiling-hot, volcanic 'cauldron' that was Earth's surface at the time. Therefore, the water we have today must have come from somewhere off-world, and scientists have identified two potential sources - comets and asteroids.
Both types of objects contain water. Scientists know this from sampling meteorites and from observing comets as they fly through the inner solar system. Earth has certainly been impacted by enough of both, so either could be the primary source of our water. Proving which, though, comes down to one very specific detail about the water itself.
Your standard everyday water is H2O, with two hydrogen atoms joined together with an oxygen atom. However, simply by natural occurrence, a small number of these water molecules will have one or both of those hydrogen atoms replaced by an atom of what's called deuterium.
DID YOU KNOW? While an atom of hydrogen contains one electron orbiting around a proton, deuterium (aka heavy hydrogen) has one electron orbiting a proton and a neutron. Water (normally H2O) with deuterium in place of both hydrogen atoms (thus D2O) is called heavy water, while water with one hydrogen atom and one deuterium atom (HDO) is called semi-heavy water.
If you examine a sample of the water here on Earth, you would find that for every 6,400 hydrogen atoms you find, there will be one deuterium atom. That D/H ratio of 1 to 6400 is a very specific 'signature' or 'fingerprint' for Earth's water, and scientists refer to this when they examine the water in meteorites, asteroids and comets, to see if they can find a match. Earth's D/H ratio could have been reached due to a combination of different sources mixing together, but the idea is that the primary source of the water here will likely have the same, or at least a very similar, fingerprint.
First measurements of comet’s water ratio. Credit: ESA/ATG medialab; ESA/Rosetta/NavCam; Altwegg et al.
When the Rosetta spacecraft arrived at Comet 67P/Churyumov–Gerasimenko in August 2014, the spacecraft began examining the jets of water vapour streaming off the surface of the comet. It was specifically checking the D/H ratio for this fingerprint. It didn't find it.
Instead, Comet 67P's water has a D/H ratio of around 1 to 1900 - over three times the amount of deuterium found in Earth's water. That rules out comets like 67P as sources of Earth's water.
In fact, as the graph below shows (a blow-up from the image above), most comets have D/H ratios that are higher than Earth's.
Deuterium-to-hydrogen in the Solar System. Credit: Altwegg et al. 2014.
The only objects that do have D/H ratios close to Earth's are asteroids, many of which are almost spot-on, and at least two 'Jupiter family' comets that we know of Comet 103P/Hartley 2 is very close and Comet 45P/Honda–Mrkos–Pajdušáková (H-M-P) isn't too far off. Comet 67P is also considered a Jupiter-family comet - one who's orbit is controlled by Jupiter's gravity, but it has turned out to be even richer in deuterium than the nine Oort Cloud comets that have been examined via telescope.
So, although some studies have been pointing towards comets as the source of Earth's water, these results promise to stir up the debate again, and providing more insights into comets and where they form.
"This surprising finding could indicate a diverse origin for the Jupiter-family comets - perhaps they formed over a wider range of distances in the young Solar System than we previously thought," says Kathrin Altwegg, principal investigator for ROSINA , according to an ESA press release. "Our finding also rules out the idea that Jupiter-family comets contain solely Earth ocean-like water, and adds weight to models that place more emphasis on asteroids as the main delivery mechanism for Earth’s oceans."
Matt Taylor, the ESA's Rosetta project scientist added: "We knew that Rosetta’s in situ analysis of this comet was always going to throw up surprises for the bigger picture of Solar System science, and this outstanding observation certainly adds fuel to the debate about the origin of Earth’s water. As Rosetta continues to follow the comet on its orbit around the Sun throughout next year, we’ll be keeping a close watch on how it evolves and behaves, which will give us unique insight into the mysterious world of comets and their contribution to our understanding of the evolution of the Solar System."
