Scientists using radar data from NASA’s Mars Reconnaissance Orbiter (MRO) have found a record of the most recent Martian ice age recorded in the planet’s north polar ice cap.
“These radar results provide a quantitative confirmation that, just like the Earth, Mars enjoys glacial periods caused by variations in both the planet’s orbit around the Sun and the orientation of its rotational axis,” said Roger Phillips, professor emeritus in the Department of Earth and Planetary sciences in Arts & Sciences at Washington University in St. Louis. Similar but much smaller variations in Earth’s orbit are called Milankovitch cycles.
“During a glacial period, water vapor that would otherwise travel to the north polar cap instead snows out at lower latitudes, where ice then accumulates,” said Phillips, who was U.S. leader of the radar experiment, an activity that was initiated while he was on the faculty at Washington University. “During an interglacial period, this ice sublimates and the water vapor travels to the north pole, where ice piles up at a faster rate.”
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.
The research, published in the May 27 issue of the journal Science, helps refine models of the Red Planet’s past and future climate by allowing scientists to determine how ice moves between the poles and mid-latitudes, and in what volumes.
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 analyzed hundreds of such images to look for variations in the layers extent.
The researchers identified a boundary in the ice that extends across the entire north polar cap. Above the boundary (the end of the last ice age), 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 planetary scientist Isaac Smith of the Planetary Science Institute in Tucson, Ariz., and lead author of the study.
On Earth, ice ages take hold when the polar regions and high latitudes become cooler than average for thousands of years, causing glaciers to grow toward the mid-latitudes. In contrast, the Martian variety 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 vapor migrates toward 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. This retreat and regrowth of polar ice is exactly what the scientists see in the record revealed by the SHARAD radar images.
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, as the poles began to cool relative to the equator. Models suggest that since then, the polar deposits would have thickened by about 980 feet (300 meters).
The upper unit identified by Smith and colleagues reaches a maximum thickness of 1,050 feet (320 meters) across the polar cap, which is equivalent to a 2-foot (60-centimeter) thick global layer of ice. That is essentially the same as model predictions made by other researchers in 2003 and ’07.
“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,” Smith said. “Using these measurements, we can improve our understanding of how much water is moving between the poles and other latitudes, helping to improve our understanding of the Martian climate.”
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