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/Churyumov-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{at}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