What happened to the water that once covered Mars? Scientists know that some went deep underground, but where is the rest? Evidence shows that some water molecules broke into atoms, which rise through the Martian atmosphere and escape into space. By combining data from Hubble and MAVEN, a team measured the number and current rate of escaping hydrogen atoms.

They discovered that the escape rates of hydrogen and "heavy hydrogen," called deuterium, change rapidly when Mars is close to the Sun. This upended the classical picture that scientists previously had, where these atoms were thought to slowly diffuse upward through the atmosphere to a height where they could escape. Extrapolating the escape rate backwards through time helped the team to understand the history of water on the Red Planet.