Earth’s water source a mystery as comets discounted
WASHINGTON // Early results from the Rosetta spacecraft have challenged a long-held theory that comets delivered water to Earth.
In August, the European Space Agency’s probe became the first spacecraft to put itself in orbit around a comet.
Since then, Rosetta has been closely examining 67P/Churyumov-Gerasimenko, a comet of the variety that some scientists thought could have brought water to our planet four billion years ago.
But while the probe did find water on 67P, a study released on Wednesday said the water was too heavy.
Chemical analysis of the water shows it contains three times more deuterium — a variation of hydrogen — than water on Earth, the study said.
“The question is, who brought this water: was it comets or was it something else?” said the study’s lead author, Kathrin Altwegg of the University of Bern in Switzerland.
Ms Altwegg believes it was something else, most probably asteroids. But others disagree.
Many scientists have long believed that Earth had water when it first formed, but that it later boiled off. According to this theory, the water on our planet now had to have come from an outside source.
Rosetta’s findings complicate not just the mystery of the source of Earth’s water, but our understanding of comets also.
Until now, scientists had more or less 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 analysed 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 found it was a perfect match. So the comet theory was back, stronger than ever.
The comet examined by Rosetta is a Kuiper Belt comet, but its water was even heavier than Halley’s. This shows that Kuiper Belt comets aren’t as uniform as scientists believed, and once again complicates the issue of Earth’s water.
The measurements from 67P are so much higher that even if only a few comets of its type smashed into Earth, our planet’s water deuterium ratio would not be what it is today, Ms Altwegg said.
“That probably rules out Kuiper Belt comets from bringing water to Earth.”
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 programme manager Donald Yeomans agrees with Ms Altwegg, however, and thinks the research probably does rule out comets.
Asteroids are now a likely suspect for bringing water to Earth, Ms Altwegg said. The asteroids that bombarded Earth four billion years ago resembled mini-planets and probably held more water than the dry, rocky bodies circling the sun beyond Mars today. Another possibility is that Earth kept some original water in its crust or in ice at the poles.
Also on Wednesday, scientists said the search for Rosetta’s companion probe, Philae, continues.
Philae made an unprecedented descent to the surface of 67P on November 12, bounced twice and settled in what appears to be a crater. It ran through two-and-a-half days of pre-programmed science experiments before its battery died.
Results of Philae’s studies, which include chemical analysis of samples drilled out from the comet’s body, have not yet been released.
Rosetta will continue to accompany 67P for about another year.
* Associated Press and Reuters
Published: December 11, 2014 04:00 AM