Last ice age on Mars ended 400,000 years ago: study

Scientists have found a record of the most recent Martian ice age in the red planet's north polar ice cap that ended about 400,000 years ago, by using radar data from NASA's Mars Reconnaissance Orbiter (MRO).

The new results agree with previous models that indicate a glacial period ended about 400,000 years ago, as well as predictions about how much ice would have been accumulated at the poles since then.

A seasonal cover of carbon dioxide ice and snow is observed to advance and retreat over the poles during the Martian year.

During summertime in the planet's north, the remaining northern polar cap is all water ice; the southern cap is water ice as well, but remains covered by a relatively thin layer of carbon dioxide ice even in southern summertime.

Mars also undergoes variations in its tilt and the shape of its orbit over hundreds of thousands of years. These changes cause shifts in the planet's climate and ice ages.

Scientists used data from MRO's Shallow Subsurface Radar (SHARAD) to produce images called radargrams that are like vertical slices though the layers of ice and dust that comprise the Martian polar ice deposits.

For the new study, researchers analysed hundreds of such images to look for variations in the layer properties.

They identified a boundary in the ice that extends across the entire north polar cap. Above the boundary, the layers accumulated very quickly and uniformly, compared with the layers below them.

"The layers in the upper few hundred meters display features that indicate a period of erosion, followed by a period of rapid accumulation that is still occurring today," said Isaac Smith, who led the work while at Southwest Research Institute in the US.

Martian ice age occurs when - as a result of the planet's increased tilt - its poles become warmer than lower latitudes.

During these periods, the polar caps retreat and water vapour migrates towards the equator, forming ground ice and glaciers at mid-latitudes.

As the warm polar period ends, polar ice begins accumulating again, while ice is lost from mid-latitudes.

An increase in polar ice following a mid-latitude ice age is also expected from climate models that show how ice moves around based on Mars' orbital properties, especially its tilt.

These models predict the last Martian ice age ended about 400,000 years ago. Models suggest that since then, the polar deposits would have thickened by about 300 metres.

The upper unit reaches a maximum thickness of 320 metres across the polar cap. That is essentially the same as model predictions made by other researchers in 2003 and 2007.

"This suggests that we have indeed identified the record of the most recent Martian glacial period and the regrowth of the polar ice since then," said Smith, who is now at the Planetary Science Institute in the US.

The study was published in the journal Science.