The strange disappearance of carbon dioxide ice in early martian spring followed by its sudden reappearance is explained by the planet's very active water cycle, and strong winds, say scientists presenting their findings at the European Planetary Science Congress this week.

Every martian year, alternatively during northern and southern winter, part of the atmosphere condenses onto the surface as frost and snow in layers up to one metre thick. These deposits consist predominantly of carbon dioxide (CO₂) ice with small amounts of water ice and dust. As spring takes hold the deposits sublimate (that is, they turn directly from a solid into a vapour), supplying the atmosphere with water vapour.

But a strange phenomena had been noted based on changes in the ices' reflectivity and temperature: carbon dioxide ice that had disappeared in the early northern hemisphere spring, suddenly reappeared. Now, using data collected by the OMEGA instrument onboard ESA's Mars Express orbiter, which enables near-infrared observations well-suited for detections of water and carbon dioxide ice, scientists have studied two regions in detail – Gemina Lingula, a northern plateau, and a spiral trough structure in the north polar cap – and think they may have an explanation for the curious behaviour.

"During spring the ice signature disappeared from our data, but the surface temperature was still cold enough to sustain plenty of (CO₂) ice," describes Bernard Schmitt from the Laboratoire de Planétologie de Grenoble, regarding the Gemina Lingula plateau. "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. 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 vaporize as well, and then the carbon dioxide signatures re-appeared in our data."

As spring takes hold, sunlight falling onto the martian surface is enough to vaporize the top layer of carbon dioxide ice. Since water ice needs higher temperatures to sublimate, a thin layer gradually forms hiding the carbon dioxide ice still lying beneath it. "A layer only two tenths of a millimetre thick is enough to completely hide the (CO₂) ice," says team member Thomas Appéré. "Also some water that has been vaporized at lower, warmer, martian latitudes condenses as it moves northward and may be cold trapped on top of the (CO₂) ice."

In the permanent north polar cap, the carbon dioxide ice re-appeared very quickly after its initial disappearance. "This hide-and-seek game didn't make much sense to us," says Schmitt. "It wasn't cold enough for (CO₂) ice to condense again, neither warm enough for water ice to sublimate."

The team concluded that the water ice layer must somehow be removed. "The topography of the north permanent martian cap is well-suited to entail the formation of strong katabatic (downhill) winds," suggests Appéré. "Dr Aymeric Spiga used a model from the Laboratoire de Météorologie Dynamique du CNRS to simulate those winds and he indeed confirmed the sudden re-appearances of (CO₂) ice where strong katabatic winds blow."

The results show that the martian water cycle and the planet's topography and weather phenomena are intimately linked, and will help planetary scientists to decipher the dynamics of seasonal ice changes.

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