Like Earth, Mars has experienced warming and cooling cycles. And a new study of ice at the Martian poles indicates the Red Planet is in an interglacial period dating back almost 400,000 Earth years. Confusingly, however, that means its poles are cooler, not warmer than when the ice was most extensive.

Before humans upset the planetary thermostat, the primary driver of climate change on Earth was orbital changes known as Milankovitch cycles. Although the average distance between the Sun and Earth barely changes, the polar axial tilt alters and the orbit shifts between a more rounded and an elongated shape.

On Earth, this is further complicated by the fact that one hemisphere is almost entirely water, while the other has plenty of land. Changes that affect how much sunlight falls on each at different times of the year can affect how much heat the planet retains. This isn't a factor on Mars, but the orbital variation is larger, greatly affecting the planet's climate. 

“The north pole layered deposits exchange substantial amounts of water with the mid-latitudes over the course of an obliquity cycle, sometimes leading to periods when Mars has abundant ice coverage over a wide range of latitudes,” the paper in Science reports.

^{The Phoenix Mars Lander detected frost at 68°N in this photograph, but during the the last Martian Ice Age the polar ice cap would have extended to this point and indeed much further south. NASA/JPL-Caltech/University of Arizona/Texas A&M University}

To terrestrial observers, there is a surprising twist in this tale. “These Martian ice ages occur when the poles are warmer than average,” the paper reports. This occurs because when the Martian poles warm, the ice there is eroded and transported as water vapor to latitudes as low as 30° to 35°, where it freezes. The total amount of ice is the same, but is spread over a much wider area, rather than being piled deep at the poles. 

First author Dr. Isaac Smith, of the Planetary Science Institute, told IFLScience that the total sunlight hitting Mars is the same during an ice age and an interglacial age, but during an ice age "the Sun hits the poles more". However, Smith said, this does not mean the planetary average temperature would be exactly the same. "The high albedo [reflectivity] of ice will reflect more of the sunlight when that ice is spread out over a greater area." This could cool the planet during an ice age, but the increased water vapor at the time could warm it. Smith said that the net effect on the average planetary temperature was "a good idea for another study."

The last Martian Ice Age is thought to have extended from 2.1 million to 400,000 years ago, but details have been vague. The paper dates the last ice age's ending to 370,000 years ago, and calculates the volume of polar ice deposited since then at 87,000 cubic kilometers (21,000 cubic miles), enough to bury the entire planet 60 centimeters (2 feet) deep.

Cycles of cold and (relative) warmth leave signatures in the polar ice caps. The paper reports that radar maps made by the Mars Reconnaissance Orbiter of the north pole show evidence for where ice has been built up as it migraited back from lower latitudes.

The recent, in geological terms, history of the north polar cap is dominated by what the authors call an “unconformity” – known as the "widespread, recent accumulation package (WRAP)” – which is a region of ice an average of 82 meters (269 feet) deep. This represents the ice laid down since the end of the last Martian ice age, as ice retreated in the mid-latitudes and was deposited back at the poles.

^{The thickness of the ice built up at Mars north (A) and south (B) poles since the end of the last planetary ice age. Smith et al/Science.}

Smith found that the WRAP forms a single continuous layer confirming that it was laid down at the same time. However, that does not mean it is equally thick. At the center it is 320 meters (1,050 feet) thick, four times the average.

Studies at the south pole were less productive, because the deposits there are discontinuous, leaving open the possibility they were laid down at different times. They were also less than a tenth the size of the north, reflecting the enormous geological difference between Mars' hemispheres.

The next Ice Age is set to start, Smith told IFLScience, in about 150,000 years. "However the next ice age maximum will be over 500,000 years from now," he added.