Mars is emerging from an ice age that ended about 400,000 years ago

Mars is emerging from an ice age, according to a new study. Studying the Martian climate and how it changes over time can help scientists better plan future missions to Mars and even understand climate change here on Earth, the study authors say.

Models had already predicted that Mars underwent several rounds of ice ages in the past, but little physical measurements ever confirmed those predictions. Today’s study,

published in the journal Science, is the first to map the ice deposits on the north and south pole and confirm that Mars is emerging from an ice age, in a retreat that began almost 400,000 years ago. The researchers also calculated just how much ice accumulated over the poles; the amount is so big that if it were spread throughout Mars, the entire planet would be covered by a 2-foot thick layer of ice.

Studying climate change on Mars is important for multiple reasons, says study co-author Isaac Smith, who studies sedimentary systems on Mars at Southwest Research Institute. By understanding ice ages, we can get a better understanding of how ice — and water — behaved through time on the Red Planet. It can help us figure out how Mars went from being a wet world to the barren, frigid land it is today. And it can tell us where ice deposits can be found. That’s key if we plan to send humans on Mars. "We want to know the history of water," Smith says. "At some point, we’re going to have some people there and we’d like to know where the water is. So there’s a big search for that."

The Martian climate can also inform scientists about climate change here on Earth, Smith says. Mars is the most similar planet to Earth in the Solar System and it provides a good testing ground for climate research, because there are no people burning fossil fuels and pumping global warming pollutants into the atmosphere. "Mars is a very good laboratory for what happens on Earth," Smith says. "Climate science actually has a very simple but perfect laboratory in Mars, where we can learn about the physics of climate change and then apply what we learn to Earth."

Ali Bramson, a planetary scientist and PhD candidate at the University of Arizona, who did not work on the study, agrees. "I think it’s a really great study and I think it’s very timely," she says. "I was really excited to see it. … Climate change is obviously a very salient topic on Earth, but understanding the distribution of water-ice on Mars is also something that’s of great interest because there’s a lot of interest in sending humans one day to Mars. So if we know where there are reservoirs of water-ice, that could potentially be useful for future human exploration."

Just like Earth, Mars undergoes cycles of climate change and ice ages. But unlike Earth, climate change on Mars is affected primarily by how "tilted" the planet is. Every planet has an axis around which the planet rotates. Earth’s axis is tilted 23.5 degrees and it’s pretty stable, varying only a couple of degrees over time. Mars’ axis is currently tilted 25 degrees, but it wobbles between from 10 to 40 degrees. That happens for two reasons: first, Mars doesn’t have a moon as big as ours to stabilize its orbit; second, it’s much closer to Jupiter, and Jupiter's gravity affects Mars’ rotation. When the Red Planet’s axis is more tilted, the poles receive more sunlight and get warm — so the ice to redistributes to the mid-latitudes, just above the tropic. That’s when Mars undergoes an ice age. "The impact is pretty dramatic," says Peter Read, a physics professor at the University of Oxford.

Today’s study was based on predictions that 400,000 years ago such a shift in the planet’s axis took place. The researchers used radar instruments onboard the Mars Reconnaissance Orbiter, a NASA spacecraft that’s orbiting Mars. They analyzed the radar images of the ice deposits within the planet’s polar ice caps, looking out for signs of erosion and other features, like so-called spiral troughs that are created by the wind. Tracing these features can reveal how ice accumulated and retreated through time. The researchers confirmed that around 400,000 years ago an ice age ended. Since the end of that ice age, about 87,000 cubic kilometers of ice accumulated at the poles, especially in the north pole. That’s exciting, because 400,000 years is pretty recent when talking about planets in the Solar System.

The study is "another bit of evidence that climate is still actively changing on Mars," says Stephen Lewis, a senior lecturer at the Open University, who didn’t work on the study. Mars "is not a dead, static world. Things are going on and changing."

Correction: A previous version of this story stated that an ice age began on Mars about 400,000 years ago. In fact, it ended 400,000 years ago. The story has been changed throughout to reflect this fact. We regret the error.