Rosetta Reignites Debate on Earth's Oceans
 
Dec. 14, 2014: Where did our planet get its oceans? Among planetary
scientists, this is one of the most important and perplexing questions about
the origins of Earth.
 
One popular theory holds that water was brought to Earth by the ancient
impacts of comets and asteroids. However, new data from the European Space
Agency's Rosetta spacecraft indicate that terrestrial water did not come from
comets like 67P/Churyumov-Gerasimenko. The findings were published Dec. 10th
in the journal Science.
 
http://www.nasa.gov/jpl/rosetta/pia18899/
 
This composite is a mosaic comprising four individual NAVCAM images taken from
19 miles (31 kilometers) from the center of comet 67P/Churyumov-Gerasimenko on
Nov. 20, 2014. The image resolution is 10 feet (3 meters) per pixel. Image
Credit: ESA/Rosetta/NAVCAM
 
Researchers agree that water must have been delivered to Earth by small bodies
at a later stage of the planet's evolution. It is, however, not clear which
family of small bodies is responsible. There are three possibilities:
asteroid-like small bodies from the region of Jupiter; Oort cloud comets
formed inside of Neptune's orbit; and Kuiper Belt comets formed outside of
Neptune's orbit.
 
The key to determining where the water originated is in its isotopic "flavor."
That is, by measuring the level of deuterium - a heavier form of hydrogen. By
comparing the ratio of deuterium to hydrogen in different objects, scientists
can identify where in the solar system that object originated. And by
comparing the D/H ratio, in Earth's oceans with that in other bodies,
scientists can aim to identify the origin of our water.
 
The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA)
instrument has found that the composition of comet 67P/Churyumov-Gerasimenko's
water vapor is significantly different from that found on Earth.
 
The value for the D/H ratio on the comet is more than three times the
terrestrial value. This is among the highest-ever-measured values in the solar
system. That means it is very unlikely that comets like 67P/Chur
umov-Gerasimenko are responsible for the terrestrial water.
 
The D/H ratio is the ratio of a heavier hydrogen isotope, called deuterium, to
the most common hydrogen isotope. It can provide a signature for comparison
across different stages of a planet's history.
 
"We knew that Rosetta's in situ analysis of this comet was always going to
throw us surprises," said Matt Taylor, Rosetta's project scientist from the
European Space Research and Technology Center, Noordwijk, the Netherlands.
"The bigger picture of solar-system science, and this outstanding observation,
certainly fuel the debate as to where Earth got its water."
 
Almost 30 years ago (1986) the mass spectrometers on board the European Giotto
mission to comet Halley could, for the first time, determine D/H ratio in a
comet. It turned out to be twice the terrestrial ratio. The conclusion at that
time was that Oort cloud comets, of which Halley is a member, cannot be the
responsible reservoir for our water. Several other Oort cloud comets were
measured in the next 20 years, all displaying very similar D/H values compared
to Halley. Subsequently, models that had comets as the origin of the
terrestrial water became less popular.
 
This changed when, thanks to the European Space Agency's Herschel spacecraft,
the D/H ratio was determined in comet Hartley 2, which is believed to be a
Kuiper Belt comet. The D/H ratio found was very close to our terrestrial value
-- which was not really expected. Most models on the early solar system claim
that Kuiper Belt comets should have an even higher D/H ratio than Oort cloud
comets because Kuiper Belt objects formed in a colder region than Oort cloud
comets.
 
The new findings of the Rosetta mission make it more likely that Earth got its
water from asteroid-like bodies closer to our orbit and/or that Earth could
actually preserve at least some of its original water in minerals and at the
poles.
 
"Our finding also disqualifies the idea that Jupiter family comets contain
solely Earth ocean-like water," said Kathrin Altwegg, principal investigator
for the ROSINA instrument from the University of Bern, Switzerland, and lead
author of the Science paper. "It supports models that include asteroids as the
main delivery mechanism for Earth's oceans."
 
Comets are time capsules containing primitive material left over from the
epoch when the sun and its planets formed. Rosetta's lander obtained the first
images taken from a comet's surface and will provide analysis of the comet's
possible primordial composition. Rosetta will be the first spacecraft to
witness at close proximity how a comet changes as it is subjected to the
increasing intensity of the sun's radiation. Observations will help scientists
learn more about the origin and evolution of our solar system and the role
comets may have played in seeding Earth with water, and perhaps even life.
 
Credits:
Production editor: Dr. Tony Phillips | Credit: Science@NASA
 
Rosetta is an ESA mission with contributions from its member states and NASA.
The Jet  Propulsion Laboratory, Pasadena, California, a division of the
California Institute of Technology in Pasadena, manages the U.S. contribution
of the Rosetta mission for NASA's Science Mission Directorate in Washington.
JPL also built the MIRO instrument and hosts its principal investigator,
Samuel Gulkis. The Southwest Research Institute (San Antonio and Boulder)
developed the Rosetta orbiter's IES and Alice instruments, and hosts their
principal investigators, James Burch (IES) and Alan Stern (Alice).
 
For more information on the U.S. instruments aboard Rosetta, visit:
 
http://rosetta.jpl.nasa.gov
 
More information about Rosetta is available at:
 
http://www.esa.int/rosetta
 
 
Regards,
 
Roger

--- D'Bridge 3.99
 * Origin: NCS BBS - Houma, LoUiSiAna (1:3828/7)