Sunday Times 2
Earth’s water did not come from comets
The mystery of where Earth’s water came from has got murkier.
Results from the Rosetta space probe has proved that water did not come from a comet, as had been previously thought.
The Rosetta mission, which made history by landing on Comet 67P in November, shows the water on the icy mass is unlike that on our planet.
Over the past few months, the European Space Agency’s Rosetta space probe closely examined the type of comet that some scientists theorized could have brought water to our planet 4 billion years ago.
It found water, but the wrong kind – it was too heavy.
One of the first scientific studies from the Rosetta mission found that the comet’s water contains more of a hydrogen isotope called deuterium than water on Earth does.
‘The question is who brought this water: Was it comets or was it something else?’ said Kathrin Altwegg of the University of Bern in Switzerland, lead author of a study published in the journal Science.
Something else, probably asteroids, Altwegg concluded.
But others disagree.
Many scientists have long believed that Earth had water when it first formed, but that it boiled off, so that the water on the planet now had to have come from an outside source.
The findings from Rosetta’s mission to the duck-shaped comet 67P/Churyumov-Gerasimenko complicate not just the question of the origin of Earth’s water but our understanding of comets.
Until now, scientists sorted comets into two types: near and far.
The near ones, sometimes called the Jupiter family, originally come from the Kuiper Belt outside Neptune and Pluto.
The far ones hail from the Oort Cloud, which is much farther out.
In 1986, a spacecraft came within about 400 miles of Halley’s comet, an Oort Cloud comet, and analyzed its water.
It proved to be heavier than Earth’s water.
But three years ago, scientists examined the water in a Kuiper Belt comet, Hartley 2, and it was a perfect match for Earth’s water, so the comet theory was back, stronger than ever, Altwegg said.
The comet visited by Rosetta is a Kuiper Belt comet, but its water was even heavier than Halley’s, Altwegg said.
That shows that Kuiper Belt comets aren’t as uniform as thought, and it once again complicates the issue of Earth’s water.
‘That probably rules out Kuiper Belt comets from bringing water to Earth,’ she said.
University of Maryland astronomer Michael A’Hearn, who wasn’t part of the research, called the results startling but said they don’t eliminate comets altogether.
The water could have come from other types of Kuiper Belt comets, he said.
NASA Near Earth Object program manager Donald Yeomans, however, thinks it does pretty much rule out comets.
While asteroids are a good suspect – they probably had more water on them 4 billion years ago than they do now – another possibility is that Earth kept some of its original water in its crust or in ice at the poles, Altwegg said.
‘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 and lead author of the paper reporting the results in the journal Science this week.
‘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.’
‘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,’ says Matt Taylor, ESA’s Rosetta project scientist.
‘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.’ © Daily Mail, London