BioEd Online

The tide of an ongoing debate about whether comets or asteroids supplied most of Earth's water has turned back to comets with the discovery that the Hartley 2 comet has a similar ratio of heavy water to ordinary water as Earth.

Paul Hartogh of the Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany, and his team measured the comet's ratio of deuterium to hydrogen using the European Space Agency's Herschel Space Observatory.

"With our one data point, we showed that comets can have Earth-like D/H ratios and therefore a larger amount of water may have been delivered by comets, perhaps all," says Hartogh. The findings are published today in Nature¹, and will also be reported by Hartogh tomorrow at a joint meeting in Nantes, France, of the European Planetary Science Congress and the American Astronomical Society's Division for Planetary Sciences².

A match made in space

Previous observations of six other comets had revealed ratios almost double that of Earth's oceans. By contrast, measurements of a class of meteorites thought to be fragments of carbon-rich asteroids in the outer part of the asteroid belt, just inside Jupiter's orbit, were a close match with terrestrial water.

This led most planetary scientists to conclude that asteroids were the main source of Earth's water, and that comets had contributed a measly 10% at best.

However, all six of those comets — which include Halley and Hale-Bopp — are thought to originate from the Oort cloud, a reservoir of ice at the very edge of the Solar System. Meanwhile Hartley 2, which passed close to Earth — and so Herschel — last November, comes from the much closer Kuiper belt reservoir, which lies just beyond the orbit of Neptune, notes Hartogh.

Although the study measures the D/H ratio in just one Kuiper-belt comet, "at the time of the Solar System formation there may have been a large reservoir of such comets with the correct ratio that bombarded the Earth," says Hartogh. Most scientists think it likely that both water and organic compounds arrived on Earth around 4 billion years ago, during an era known as the Late Heavy Bombardment, when both comets and asteroids frequently collided with inner Solar System bodies.

Over the summer, Hartogh's team used the Herschel Observatory to measure the D/H ratio in another Kuiper-belt comet, 45P/Honda-Mrkos-Pajdusakova. That data is now being analysed, Hartogh says.

The work "forces a reopening of the question of whether comets delivered a substantial amount of water to Earth, and I think that's very exciting", says comet observer Michael Mumma of NASA's Goddard Space Flight Center in Greenbelt, Maryland, who was not part of the study. He says the report is so important that he delayed publication of a review article he co-authored on the chemical composition of comets so he could cite the new measurement.

Water on another world

Hartogh's wasn't the only exiting comet-related news at the Nantes conference; yesterday another team of planetary scientists reported that they may have caught a comet delivering water to the habitable zone of another solar system³.

Carey Lisse of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, and his colleagues used NASA's Spitzer Space Telescope to examine the Sun-like star Eta Corvi, which is just one billion years old. That's not much older than the Sun was during the Late Heavy Bombardment.

Eta Corvi was already known to have an outer, cold ring of dust, which researchers have proposed is evidence of objects in that star's Kuiper belt colliding with each other, generating dust in the process. The new Spitzer observations reveal an additional, inner ring of warmer dust, about twice Mars' average distance from the Sun, that is rich in both carbon and water.

Lisse says the most likely explanation is that one or more Kuiper-belt comets was recently flung inwards. The comet or comets then collided with a planet, creating the warm ring. Lisse is urging astronomers to immediately embark on a search for such a planet circling Eta Corvi.