Water, water everywhere: Comet not the source

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  • Published: Dec 15, 2014
  • Author: David Bradley
  • Channels: Chemometrics & Informatics
thumbnail image: Water, water everywhere: Comet not the source

Rational ratio

Mosaic of four images taken by Rosetta's navigation camera (NAVCAM) on 19 September 2014 at 28.6 km (17.8 mi) from the centre of comet 67P/Churyumov–Gerasimenko. Credit: http://en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimenko#mediaviewer/File:Comet_67P_on_19_September_2014_NavCam_mosaic.jpg (ISON thumbnail from: https://www.flickr.com/photos/hubble-heritage/9366384142/in/photolist-e2PGo8-dZ9QAX-CQDDz-gKYg5o-xk6si-fgFaFL-p2zJrS-qartA1-q8rraY-pWmNj3-6kEU2e-z2mZ8-pqdxn7-oDLPno-pd4HHB-oQBUMc-oB2ywb-p1vHL1-pEzYhS-gLiR9Z-oVMSVV-oxeVeB-pRRgiG-pHYiYv-owxWuq-p1uYrC-pmWPi8-oUUZ91-pM7Q32-paEMt2-pqarBe-okTVpy-ou9sXK-pn7gbf-oGobFV-pgWtYh-pgWtWo-pMDqP2-pKMNaE-pVaLou-oHGiBC-p2CBZu-p4CArJ-yo2kg-oQnJXd-puCnFv-oKJ46M-pjSNf3-pyA7HT-pYcuTR)

An analysis of the deuterium to hydrogen ratio of water sampled by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis shows that the water vapour from comet 67P is very different from the water we find on Earth suggesting that comets were perhaps not the water bearers for our planet that some theories have suggested.

The Rosetta spacecraft sent by the European Space Agency (ESA) to rendezvous with the comet 67P/Churyumov-Gerasimenko has extracted much fascinating data from the vapour emitted by the comet.

Specifically, the double focusing mass spectrometer (DFMS) on ROSINA, the Rosetta Orbiter Sensor for Ion and Neutral Analysis instrument was used to record more than fifty spectra between the 8th of August and the 5th of September 2014 to give researchers an important ratio - the D/H ratio in heavy and normal water.

According to Rosina team leader Kathrin Altwegg of the University of Bern, Switzerland, and her colleagues the ratio is markedly different from that seen on Earth.

Awesome source

Geologists have for a long time assumed that any water present on the hot, primordial Earth would have boiled away into space so that there must have been a secondary source billions of years ago that filled the oceans. The most prominent theory suggested that water could have been delivered to our planet from myriad icy comets entering our atmosphere over millions of years once the planet had cooled sufficiently. For that theory to hold [water], however, the D/H ratio we observe today would have to match the ratio of that seen in water molecules present on the kind of comet that would have splash landed many, many times.

Comets are unique tools for probing the early Solar System given that they carry with them material that was essentially "left over" after the proto-planetary disc from which the planets were formed had done its work; indeed, the cometary composition should reflect the primordial composition of their orbiting position in that disc. Unfortunately, the Solar System is not some kind of clockwork machine of steady state brass orbs and cogwheels as Newton's successors may well have imagined. The long-period comets that come from the very distant Oort cloud at the far reaches of the Sun's domain were originally formed within the orbital region of the planets Uranus and Neptune far enough from the Sun that water ice could survive but they were later scattered to greater more eccentric orbits by the gravitational fields of those outer giant planets.

Come closer, comet

In contrast, Jupiter-family comets such as 67P are thought to have formed initially much further out, in the Kuiper Belt way beyond Neptune, but there orbits were disrupted and their close encounters with the biggest of the solar planets Jupiter pulled them into the gravitational domain of that planet's influence. Today, 67P now travels around the Sun between the orbits of Earth and Mars at its closest and just beyond Jupiter at its furthest, with a period of about 6.5 years.

Previous measurements of the D/H ratio in other comets have shown a wide range of values. Of the eleven comets for which astronomers have obtained measurements, it is only the Jupiter-family Comet 103P/Hartley 2 that was found to match the composition of Earth's water, in observations made by ESA’s Herschel mission in 2011. Intriguingly, water from meteorites that originated in the Asteroid Belt between Mars and Jupiter also match the composition of Earth's water. Adequate numbers of asteroid, as opposed to cometary, impacts may well have been sufficient to fill the oceans. The D/H ratio obtained by ROSINA is more than three times higher than for terrestrial water and for the Jupiter-family companion, Comet Hartley 2. Moreover, it is also higher than that seen for Oort cloud comets.

"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," explains Altwegg. "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." Rosetta will be shadowing the comet for the next year and keeping a close eye on how this other-worldly world evolves.

Related Links

Science, 2014, online: "67P/Churyumov-Gerasimenko, a Jupiter  family comet with a high D/H ratio"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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