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)