Earth has a climatic pacemaker driven by subtle patterns in its orbit. The shape of its orbit shifts slightly, the tilt of its axis bobs up and down, and that axis wobbles like a top. Add up the way all that movement affects the distribution of sunlight in the high latitude Northern Hemisphere, and you get a predictable succession of glacial and interglacial periods.

Mars has orbital patterns that affect its climate, too. In fact, Mars’ orbital cycles swing to greater extremes than Earth’s. For example, Mars’ current axial tilt is about 25°, but it has varied within a range of 18° to 48° over the past 10 million years. Those orbital changes have influenced its climate as well.

On Earth, the “ice ages” resulted in a transfer of water from the ocean into growing continental ice sheets. On Mars, the changes cause transfers of water ice from the polar caps to lower latitudes, where it forms thin layers and possibly even glaciers. When conditions tilt back the other way, ice disappears from lower latitudes and the polar caps thicken again.

Layers in the Antarctic and Greenland ice sheets record ice age histories on Earth, and the ice caps of Mars’ would tell us a great deal—except we can't really go drill cores from the darn things. It's possible to get a rougher idea of climate by digging into a snowbank, which can reveal a timeline of winter weather, with clean layers of snow accumulation occasionally interrupted by undulating boundaries marking periods of melt. Working with images around the edges of the Martian ice caps, researchers have been able to catch glimpses of such layering. But without understanding how those glimpses connect together, the information they can reveal is limited.

A team of scientists led by Isaac Smith and Nathaniel Putzig of the Planetary Research Institute used radar data from the Mars Reconnaissance Orbiter to stitch together an X-ray-like look at the layering within the polar ice.

Most of the action was found at the northern pole, where the ice is up to 2km thick. The researchers were able to pick out layers of accumulation as well as several major breaks marking periods of disappearing ice. Since the last break about 370,000 years ago, ice has been moving back to the poles—meaning Mars is currently in an “interglacial” period.

The layer of new accumulation at the northern pole was as much as 320 meters thick. That number is much higher than early images had hinted at, but it is close to predictions made using models. This layer is thinner and a bit more complicated at the southern pole, equaling about eight percent of the volume of the northern layer. In total, there is about 87,000 cubic kilometers of newly accumulated ice—enough to cover the surface of Mars with about 60cm of it.

Without many direct clues about the timing of the ice age breaks in the layering, we’re reliant on modeling of the orbital cycles to figure out when they should have occurred. There are four thick layers of accumulation that probably cover the last 4 million years (though not in equal, million-year-long intervals).

Before that, Mars was in a phase where polar ice couldn’t build up (all that ice was found at much lower latitudes instead) due to a very long-term trend in the cyclical swings of the tilt of Mars’ axis. The tilt wiggles back and forth over about 120,000 years, but the strength of those wiggles changes over time. During the last 370,000 years, for example, tilt has only varied subtly. But over the preceding 2 million years, the tilt cycle was swinging more wildly.

Even in periods with larger tilt cycles, where brief periods of ice loss repeatedly occurred, the total volume of polar ice ratcheted upward. But in quiet periods like the most recent one, the rate of polar ice accumulation gets a boost.

That’s where Mars is at right now—with a relatively constant tilt favoring polar ice, more and more ice is disappearing from lower latitudes to thicken the red planet’s frozen white hat.

Science, 2016. DOI: 10.1126/science.aad6968 (About DOIs).