“Our study provides a solid estimate of how much water Mars once had, by determining how much water was lost to space,” said Geronimo Villanueva, an author of the paper and scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md.
The estimate is based on observations of two slightly different forms of water in Mars’ atmosphere. One is the familiar H2O, made with two hydrogen atoms and one oxygen atom. The other is HDO, a naturally occurring variation in which one hydrogen is replaced by a heavier form, called deuterium.