Scientists have long believed that this is due to the slow, stable process of dissociating water molecules in the lower layers of the atmosphere.
Their studies revealed a seasonal pattern – there was more water in the upper atmosphere when Mars was very close to the sun or when there was a big dust storm.
Atmospheric warming caused by those events allows more water to float in the air of the Red Planet.
“It really is a smile on the face,” said University of Arizona researcher Shane Stone.
“The global dust storm stands alone in data like nothing else.”
During global dust storms, up to 20 times as much water can be carried into the upper atmosphere.
Over the past billion years, researchers have calculated that the upper atmosphere of Mars may have expelled enough water to cover the planet’s surface in a liquid layer at a depth of 61cm.
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“Mars must have lost the equivalent of a global ocean tooth hundreds of meters deep in its entire history,” Dr. Stone said.