Scientists may have solved mystery of disappearing CO2 on Mars

Scientists may have solved the mystery of the carbon dioxide ice disappearance early in the Northern Martian springs followed by its sudden reappearance, revealing a very active water cycle on the planet.

Every Martian year, alternatively during northern and southern winter, a significant part of the atmosphere condenses on the surface in the form of frost and snow.

These seasonal ice deposits are mainly composed of carbon dioxide with minor amounts of water and dust.

During spring, the deposits sublimate (vaporise), becoming a substantial source of water vapour, in particular in the northern hemisphere of the planet.

Dr Bernard Schmitt, Mr. Thomas Appere and Dr. Sylvain Doute at the Laboratoire de Planetologie de Grenoble, France conducted the study.

"But we couldn't resolve their exact composition and how they were distributed on the planet. Near-infrared observations, such as the OMEGA data, are much better for detecting strong signatures of water and carbon dioxide ice," said Appere.

"During spring the ice signature disappeared from our data, but the surface temperature was still cold enough to sustain plenty of CO2 ice. We concluded that a thick layer of something else, either dust or water ice, was overlaid.

"If it was dust then it would also hide water ice and the surface of the planet would become darker," said Schmitt.

"None of these happened so we concluded that a layer of water ice was hiding the CO2 ice. We had to wait until the weather gets warm enough on Mars for the water to vaporise as well, and then the carbon dioxide signatures re-appeared in our data," he added.

The solar radiation after spring warms enough the CO2 ice lying on the top layer to cause it to vaporise. But the water ice needs higher temperatures to sublimate, so a fine-grained layer of water ice gradually forms hiding the carbon dioxide ice still lying beneath it.

"This hide-and-seek game didn't make much sense to us. It wasn't cold enough for CO2 ice to condense again, neither warm enough for water ice to sublimate," explained Schmitt.

"We concluded that somehow the water ice layer was removed. The topography of the North permanent Martian cap is well-suited to entail the formation of strong katabatic (downhill) winds," said Appere.

"We are confident that our results will make a significant contribution in this direction," Schmitt concluded.