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Comet Hartley, as imaged by NASA's EPOXI spacecraft. Image: NASA
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New evidence supports the theory that comets delivered a
significant portion of Earth's oceans, which scientists believe formed about 8
million years after the planet itself.
The findings, which involve a University of Michigan
astronomer, are published online in Nature.
"Life would not exist on Earth without liquid water, and
so the questions of how and when the oceans got here is a fundamental
one," says U-M astronomy professor Ted Bergin, "It's a big puzzle and
these new findings are an important piece."
Bergin is a coinvestigator on HiFi, the Heterodyne Instrument
for the Infrared on the Hershel Space Observatory. With measurements from HiFi,
the researchers found that the ice on a comet called Hartley 2 has the same
chemical composition as our oceans. Both have similar D/H ratios. The D/H ratio
is the proportion of deuterium, or heavy hydrogen, in the water. A deuterium
atom is hydrogen with an extra neutron in its nucleus.
This was the first time ocean-like water was detected in a
comet.
"We were all surprised," Bergin says.
click to enlarge
This illustration shows the locations of various classes of comets in the Solar System, relative to the orbits of the planets. The left panel shows the inner Solar System along with the orbit of Jupiter-Family comet Hartley 2. The central panel shows a larger portion of the Solar System beyond the orbit of Jupiter, as well as the Kuiper Belt, one of the two main reservoirs of comets in the solar system. The right panel shows the Oort Cloud, the other main reservoir of comets located well beyond the outer solar system. Image: ESA/AOES Medialab |
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Six other comets HiFi measured in recent years had a much
different D/H ratio than our oceans, meaning similar comets could not have been
responsible for more than 10% of Earth's water.
The astronomers hypothesize that Hartley 2 was born in a different
part of the solar system than the other six. Hartley most likely formed in the
Kuiper belt, which starts near Pluto at about 30 times farther from the sun
than the Earth is. The other six hail from the Oort Cloud more than 5,000 times
farther out.
The source of earth's oceans has been a subject for debate
among astronomers for decades. Until now, asteroids were thought to have
provided most of the water. Now, however, Herschel has shown that at least one
comet does have ocean-like water.
"The results show that the amount of material out there
that could have contributed to Earth's oceans is perhaps larger than we
thought," Bergin says.
Herschel, a European Space Agency mission with NASA
participation, is an orbiting telescope that allows astronomers to observe at
the far-infrared wavelengths where organic molecules and water emit their
chemical signatures.
The paper is called "Ocean-like water in the
Jupiter-family comet 103P/Hartley 2."
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