Free Astronomy Magazine November-December 2024

NOVEMBER-DECEMBER 2024 Mars’ water loss is linked to its distance from the Sun by NASA Ann Jenkins & Ray Villard M ars was once a very wet planet as is evident in its surface geological features. Scientists know that over the last 3 billion years, at least some water went deep underground, but what happened to the rest? Now, NASA’s Hubble Space Telescope and MAVEN (Mars Atmosphere and Volatile Evo- lution) mission are helping unlock that mystery. “There are only two places water can go. It can freeze into the ground, or the water molecule can break into atoms, and the atoms can escape from the top of the atmos- phere into space,” explained study leader John Clarke of the Center for Space Physics at Boston University in Massachusetts. “To understand how much water there was and what happened to it, we need to under- stand how the atoms escape into space.” Clarke and his team combined data from Hubble and MAVEN to meas- ure the number and current escape rate of the hydrogen atoms escap- ing into space. This information al- lowed them to extrapolate the es- cape rate backwards through time to understand the history of water on the Red Planet. Water molecules in the Martian at- mosphere are broken apart by sun- light into hydrogen and oxygen atoms. Specifically, the team meas- ured hydrogen and deuterium, which is a hydrogen atom with a neutron in its nucleus. This neutron gives deuterium twice the mass of hydrogen. Because its mass is higher, deuterium escapes into space much more slowly than regular hydrogen. Over time, as more hydrogen was lost than deuterium, the ratio of deuterium to hydrogen built up in the atmosphere. Measuring the ratio today gives scientists a clue to how much water was present during the warm, wet period on Mars. By studying how these atoms currently