Where Did theEarth's Water Come From?

Earth is locatedwithin the"snow line"of the solar system,the region closest to theSun where H2 0 is primarily in liquid or gaseous form,if at all.______(46)Thesnow line

phenomena isreflected in the water content of planets like Mercury,Venus,and Mars. Water isabsent on Mercury.On Venus,H 2 O only exists as a trace element in the

atmosphere.Mars onlyhas a thin veneer of ice in its polar regions.In general,water is rare withinthe snow line.

Why does Earth haveso much water relative to the other inner planets?71%of the surface is coveredin the oceans,more than half of which is deeper than 3,000 meters,with

an approximate totalvolume of 3 billion cu km.______(47)

There are varioustheories as to where all the Earth's water came from,but several theories havefared better than the others.______(48)When the Earth was in the process of

forming,with aradius just 40%smaller than at present,it would have had enough gravity to holdon to a tenuous atmosphere with water vapor. The first water vapor on the

planet would havecome from the planet's internals,where volatile(low weight)chemicals would havea tendency to float to the top,and heavy chemicals(iron and nickel)would

sink.

Though the first ofEarth's water came about through volcanism,this alone probably didn't produceenough to form stable pools on the surface.______(49)Comparing the

isotope ratios ofwater on Earth and water from comets and asteroids has revealed that themajority of the Earth's water comes from asteroids.

Throughout itshistory,Earth's water has increased in volume due to biological processes. Inthe early seas of Earth,hydrogen sulfide would have been in great

supply,which,whenreacted with carbon dioxide from the atmosphere during photosynthesis insulfide-reducing bacteria,would have produced hydrogen,sulfur,and

water.______(50)________（50）

A.More water wasadded to the planet during several hypothesized large impacts from asteroidsfrom the outer asteroid belt.

B.We know that theoceans existed as early as 100 million years after the formation of the Earth.

C.Still,the oceansonly make up 0.023%of the Earth's total mass.

D.Astronomers sayEarth s water may have come from space.

E.The snow line islocated in the outerasteroid belt,between the orbits of Mars and Jupiter.