Scientists examining rocky remnants in Mars’ northern polar region believe they have found evidence for a type of glacier unknown on Earth – one where the ice is made of frozen carbon dioxide rather than water.

Mikhail Kreslavsky (University of California, Santa Cruz) and James Head (Brown University) write in Icarus about finding sets of overlapping ridges in three locations in the Martian high arctic. They interpret these as drop-moraines: ridges of accumulated dust and debris left by glaciers whose bottom layer was frozen to the ground.

Considering the ridges’ location and past climate regimes on Mars, Kreslavsky and Head conclude that the most likely material for the glacier was CO2 ice, not water ice.

A dry-ice glacier would behave differently from a water ice one, they explain. At temperatures cold enough to keep both CO2 and water frozen, CO2 ice is softer and more plastic, and it tends to flow farther and more quickly. One outcome of these qualities is that CO2 glaciers would be prone to develop finger-like lobes to a much greater extent than a water-ice glacier. This fits the shape, size, and outline of the ridges.

A cold-based glacier has its bottom layer frozen to the surface; such glaciers do not slide across the ground, plowing up rocks and debris, as does a typical water-ice glacier on Earth. Instead, its leading edge would advance by rolling over the ground much the way that a tongue of cake batter flows into a pan.

When the glacier reaches its greatest extent – where the ice melts (or sublimates) as fast as it flows – the ice flow will continue within the glacier but won’t advance its snout. This internal conveyor belt carries forward debris accumulating on the glacier, and dumps it at the snout, building the ridge as a drop-moraine.

How long ago was the glacier active? Today’s climate on Mars would destroy a CO2 glacier in the same location. But cyclical changes in the tilt of the Martian axis can produce both warmer and colder climates in polar areas. Calculations of the tilts in past eras lead the scientists to narrow down when a CO2 glacier might have formed and been active.

As they explain, “The climate projections provide a rather certain age estimate of 600,000 to 800,000 years for the youngest glacier, and the oldest may be as old as 3.4 million years — but it also could be about 1 million years old.”