Did you know there are two forms of water?

This is how Mars may have looked four billion years ago:

From an article by the news staff at Science2.0:

Four billion years ago, a young Mars had enough water to bury its whole surface under 400 feet of ocean, but it is more likely that, as on Earth, it pooled. In the case of Mars, it probably formed an ocean occupying almost half of Mars’s northern hemisphere, reaching a mile deep in some places.

This new finding is based on detailed observations of two slightly different forms of water in Mars’s atmosphere. One is the familiar form of water, made with two hydrogen atoms and one oxygen, H2O. The other is HDO, or semi-heavy water, a naturally occurring variation in which one hydrogen atom is replaced by a heavier form, called deuterium.

Thanks to James Taranto