Modern Mars is dry and dusty, but scientists have long suspected that water once flowed on the surface.

The question has been: how much water? NASA scientists estimate that Mars used to have at least 5 million cubic miles of water. That's more than the volume of water found within Earth's Arctic Ocean. If that much water were present on Mars today, it would cover 19 percent of the surface and reach a maximum depth of a mile. These results are based on detailed observations of two slightly different forms of water in Mars' atmosphere. By comparing the findings for water on Mars today with water trapped in an ancient Mars meteorite, the scientists determined how much water escaped from the planet into space and, thus, how much water Mars likely had.