Where Did Earth's Water Come From?

 Our  planet  is  wet.  Seventy-one  percent of Earth's surface is covered  in water. Most of that water is in the  oceans, but another 3.5 percent is in  rivers and lakes, locked up in the ice caps, or floating in the atmosphere in the form of water vapor. More fresh and salty water hides beneath the surface,  and  scientists  have  even discovered  that  Earth's  mantle  is replete with the wet stuff. The watery nature of our home planet makes it unique. So where did all this water come from?

      

 At least some of that water was here at the moment of creation. Scientists estimate that 30 to 50 percent of the water on Earth today originates from ice from the dust cloud that eventually coalesced into the Sun and its planets. Thanks to Earth's mass, volcanism, and distance from the Sun,  our  climate  now has  the  right temperature and atmospheric pressure for that ancient ice to exist in a state of liquid water (whereas on other planets, it either froze or outgassed back into space).  

   

   But where did the rest come from? For years, the most obvious source was comets -miles-wide snowballs that roam the solar system and could have bombarded the planet in the first billion years. Recent spectrographic observations of comets that buzzed Earth, and the latest findings from the European Space Agency's space probe, Rosetta, point in another direction. The spectrographic signature of the water of these objects indicates higher levels of heavy water-water with deuterium rather than ordinary hydrogen-than is found on Earth. Other findings from Rosetta indicate the presence of a bluish hue on pan of one comet   known   as   67/Churyumov- Gerasimenko, which would suggest the  presence of frozen water beneath the surface of dust and rock. So, if not comets, what and where did our water come from?   

     

  The process of elimination leads us to asteroids or, more specifically, a class of meteorites   called   chondrites,   which originated from space rocks in the inner solar system. These potential candidates harbored water on their surface without releasing it, thanks to the younger and cooler Sun, depositing the moisture.