Astronomy Picture of the Day

Explanation: How much of planet Earth is made of water?Very little, actually.Althoughoceans of water cover about 70 percent of Earth's surface, these oceans areshallow compared to the Earth's radius.The featured illustration shows what would happen if all ofthe water on or near the surface of the Earth were bunched up into a ball.The radius of this ball would be only about 700 kilometers, less than half the radius of theEarth's Moon, and slightly smaller than Saturn's moonRhea which, like many moons in our outer Solar System, is mostly water ice.The next smallest ball depicts all of Earth's liquid fresh water, while the tiniest ball shows the volume of all of Earth's fresh-water lakes and rivers.How any of this water came to be onthe Earth and whether any significant amount istrapped farbeneath Earth's surface remain topics of research.