A dust storm on Mars
© SA/Roscosmos/CaSSIS, CC BY-SA 3.0 IGO
A dust storm on Mars photographed by the European Space Agency’s Mars Express
Dust storms on Mars aren't all about dust - they're also full of water. A satellite orbiting Mars has taken the most detailed measurements yet of how these rare events trap water at lower altitudes, which may help reveal what happened to the water that used to be abundant on the Red Planet.

In 2018, the largest recorded dust storm circled the entire Martian globe, so thick that it hid the surface from the sun and killed the Opportunity rover. The ExoMars Trace Gas Orbiter watched this cataclysmic storm from orbit. Just before sunset and just after sunrise on Mars, it examined the atmosphere to determine how the dust storm absorbed sunlight.

Ann Carine Vandaele at the Royal Belgian Institute for Space Aeronomy and her colleagues used this data to determine how water was behaving in the storm. They found that just before the storm, there were water ice clouds in the atmosphere, but no water vapour more than 40 kilometres above the surface. This changed a few days later when water vapour appeared at altitudes of 40 and 80 kilometres, seemingly replacing the water ice clouds.

This probably occurs because the dust absorbs heat, warming up the atmosphere and making it circulate more strongly, which prevents the formation of ice clouds, says Vandaele.

The team believes that by observing the dust storms as they form, we may be able to figure out what causes them. "We hope to see something happening right at the beginning of the dust storms that could explain why they happen," says Vandaele.

The dust storms puff up the atmosphere, making it easier for gases including water vapour to escape into space, so this could help explain how Mars became so inhospitable.

"We are certain that in the past Mars was wetter and warmer, but we have to understand where the water went," says Vandaele. Understanding where it goes during atmospheric events now is just the first step.

Journal reference: Nature, DOI: 10.1038/s41586-019-1097-3