Lower atmospheric processes, such as convection, dust storms and gravity waves, are vital to understanding how Mars lost its water, writes Erdal Yiğit in a Perspective. It’s widely thought that most of Mars’ water was slowly lost to space as hydrogen over several billion years. According to standard models, water molecules, broken apart by sunlight in the lower Martian atmosphere, form free hydrogen, which further dissipates into the upper atmosphere and floats off into space. But many details concerning this atmospheric hydrogen escape remain uncertain. 

Recent in situ observations have revealed that water can be transported directly into the upper atmosphere and converted into hydrogen by atmospheric ions. However, in these assessments, the role of lower atmospheric weather and variability produced by atmospheric gravity waves in this process are insufficiently explored. Yiğit argues that the processes that led to the loss of water on Mars are complex and require a whole-atmosphere perspective to understand. Here, the author describes how lower atmospheric processes influence the loss of Martian water to space, particularly the important mechanism of vertical coupling, or upward-propagating lower atmospheric gravity waves, which can potentially drive water transport to the thermosphere through wave-induced fluctuations of temperature in density. 

“Future coincident coordinated observations are required to constrain models and wave activity, and to help characterize the whole-atmosphere distribution of water and its constituents,” writes Yiğit. “Using several current observational capabilities such as ExoMars TGO, MAVEN, and Mars Reconnaissance Orbiter in complimentary ways may help accomplish this lofty goal.”