Had you searched the sky with a telescope just a few hundred thousand years ago, you would have struggled to find a red planet. Instead, you would have seen a gleaming-white ice ball where Mars should be. A team of astronomers led by Isaac Smith, an astrophysicist at the Southwest Research Institute in Boulder, Colorado, has collected the first concrete evidence that Mars has just exited an extreme ice age, one so intense it would have put Earth's recent frosty foray to shame.

Using cameras and a radar-pinging device on board NASA's Mars Reconnaissance Orbiter, Smith's team deduced this history by dating the miles-deep layers of snow and ice packed onto the Red Planet's northern pole. They found that only a mere 370,000 years ago, "Mars would have actually looked more white than red," says Smith. The Mars research is outlined today in the journal Science.

Peering Into Ice

When scientists wish to peer back into the history of Earth's changing climate, they can dig down into our polar caps and extract long tubular ice cores. As snow and ice slowly accumulates on our poles, subtle snapshots of Earth's past climate get buried. This information is locked into gas bubbles or sediments in the ice; dissecting sections of the ice can reveal a detailed history of our planet.

"Mars would have actually looked more white than red"

However, no current Martian rover has the range or capability to go digging into Mars's snow and uncover the martian climate history locked away inside it. Instead, Smith's team had to be clever. The scientists took advantage of a ground-penetrating radar device on NASA's Mars Reconnaissance Orbiter, called the Shallow Subsurface Radar.

SHARAD pings down toward Mars more than 700 times per second. As the Mars Reconnaissance Orbiter floated over the northern pole, that radar "allowed us to see down to the bottom of the ice, about 2 km deep," Smith says, and helped the team slowly to create a 2D cross-section of the icecap—"an image a bit like how when you cut a cake with layers in it, you can see those layers," he says. Smith's team was able to compare those ice layers to reveal changes in ice accumulation at various points in Mars's history.

Terraforming Promise

Mars's past ice ages and its ones yet to come are wild and intense, thanks to some peculiarities in the planet's spin and orbit. The axis upon which the Earth spins varies by about 2 degrees over time; on Mars, it can deviate by up to 60 degrees. As Mars wobbles over the millennia, "you have points where you get almost direct, full sun on the south pole all summer," says Smith, keeping the Northern pole in an endless, wintry night. Mars's orbit around the Sun is also unique, bringing it "at times up to 12 percent farther from the sun, which has a huge effect on the sunlight that reaches the planet."

The takeaway is that Mars is currently in a lull between glacial periods. "Right now we're stabilized... but about 150,000 years in the future we can expect another ice age to happen," Smith says. That's far enough in the future that it likely won't effect even the longest-term plans for human colonization on Mars. But this new information could help us plan future terraforming of the Red Planet.

"The radar mapping of the poles shows that there is a lot of H2O and CO2 trapped there," says Chris McKay, an astrophysicist at NASA who was not involved in Smith's research. McKay finds this promising: "Terraforming is predicated on our ability to release these gases and liquids into the atmosphere."

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