Jakarta Globe – AFP, Dec 11, 2014
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| This NASA photo obtained on Dec. 10, 2014 shows from the International Space Station, a photo taken by Expedition 42 Commander Barry Wilmore of the Great Lakes and central US on Dec. 7, 2014. (AFP Photo/NASA/Barry Wilmore) |
Washington.
Water on Earth is more likely to have come from asteroids that hit our planet
billions of years ago than comets, European researchers said on Wednesday.
Mankind’s
first-ever probe of a comet came last month when the European Space Agency’s
Philae lander touched down on the duck-shaped 67P/Churyumov-Gerasimenko comet,
but the latest report in the journal Science comes from an instrument aboard
the Rosetta spacecraft that has been studying the comet’s interior since
August.
“We have to
conclude … the terrestrial water was brought by asteroids more likely than
comets,” said Kathrin Altwegg, principal investigator on the ROSINA mass
spectrometer that has been examining the chemical fingerprint of water and
other gases in the comet.
The report
in the peer-reviewed US journal is based on the atomic signature of water
molecules from the comet, showing that they are vastly different from water on Earth.
Scientists
measure the ratio between deuterium, a hydrogen isotope, and hydrogen, which
forms water when combined with oxygen.
The comet
has shown “probably the highest level of deuterium to hydrogen ratio, the most
heavy of any of the solar system’s bodies,” said Altwegg, a professor at the
University of Bern in Switzerland.
The ratio
is between 30 and 120 percent higher than that formed in water molecules found
in Halley’s comet, which belongs to the same comet family, Jupiter, formed in
the Kuiper Belt.
Such a high
ratio of deuterium to hydrogen “probably means that it was formed at very low
temperatures and that it could be… most probably the original material from the
very, very early time of our solar system, so it’s a real treasure chest to
explore how our solar system looks like 4.6 billion years ago,” she said.
On the
other hand, water in asteroids has a lower deuterium/hydrogen ratio and is more
similar to water on Earth.
Comets are
rich in water, which is not the case for asteroids — some of which have none,
said Francis Rocard, a Rosetta scientist at the France’s National Center for
Space Studies (CNES).
Also, many
more asteroids have been found to date (650,000) than comets (4,000), he told
AFP.
“To me,
these findings do not shake things up but render them a bit more complex than
we previously believed, all while reinforcing the hypothesis that asteroids are
the source of Earth’s water,” he told AFP.
“The ratio
of deuterium to hydrogen in water is variable from one comet to another, much
more so it appears than in asteroids and for the moment we are not sure why.”
Comets are
the most primitive objects in the solar system and are rich in carbon, and by
smashing into the Earth scientists say they may have brought elements that
allowed life to exist.
The Rosetta
mission, approved in 1993, aims at exploring the composition of comets,
believed to be primordial clusters of ice and dust left from the building of
the solar system 4.6 billion years ago.
Agence France-Presse
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