text: Did Ocean Water Originate From Comets?
  
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  A family of comets from the Kuiper Belt may have assisted
  asteroids with bringing water to early Earth, a key development
  in the planet's ability to host life.
  
  Scientists believe Earth’s water was delivered sometime after the
  planet formed, as its close proximity to the sun would have
  boiled off water inside rocks that were part of the original
  building materials.
  
  Previous studies of meteorites, which are bits of asteroids that
  land on Earth, show a water chemistry that is similar to Earth’s
  oceans. The analysis is based on the ratio of regular water --
  two hydrogen atoms bound with an oxygen atom, or H2O -- and
  so-called “heavy water,” which has an extra neutron in its
  nucleus.
  
  The target, Comet Hartley 2, is believed to have formed in the
  Kuiper Belt region, located beyond Neptune's orbit. Hartley 2 is
  now a member of the Jupiter family of short-period comets that
  swing around the sun in less than 20 years. The comet's water
  chemistry matches that of Earth's.
  
  "This is an important constraint on models of the formation of
  planets in the solar system, a field of science which is moving
  forward very rapidly at present," astronomer Miriam Rengel of the
  Max Planck Institute for Solar System Research in Germany, wrote
  in an email to Discovery News.
  
  Previous studies, which looked at comets originating from the
  Oort cloud, a spherical cluster of icy bodies in the outer solar
  system, led scientists to conclude that only about 10 percent of
  Earth's water could have come from comets.
  
  The finding significantly boosts the pool of resources available
  to carry water to Earth.
  
  "The reservoir of material that has the same isotopic signature
  as the Earth’s water is much larger in our solar system than we
  had previously assumed," astronomer Ted Bergin, with the
  University of Michigan, told Discovery News.
  
  The studies were made in October and November 2010 as Hartley 2
  passed closely by Earth with the European Space Agency's Herschel
  space telescope. The research is reported in this week's
  Nature.