This month, Mars is closer to Earth than it's been in years. On a clear night, even without a telescope, "it looks like a big red jewel in the night sky," said planetary scientist Isaac Smith.
But if we were to gaze up at the neighboring planet 370,000 years ago, "it would be a big pink jewel," Smith said. "There'd be a lot of white mixing in with the red."
The source of that unfamiliar color: snow.
A thousand centuries ago, Mars was in the midst of an ice age, Smith reported in the journal Science on Thursday, and now it's coming out of that snow-covered spell. According to Smith, the find could help scientists better understand the Martian climate — and our own.
On Earth, fluctuating atmospheric gases and shifting tectonic plates can influence how temperatures rise and fall. Organisms may vociferously consume carbon, exacerbating an ice age, or profligately release it, prompting the planet to warm.
"These things all give feedback into a complex climatic system," Smith said. "But if we were to strip those away we'd get a situation like Mars: just ice, heat and sunlight — the simple things that make up a climate."
On Mars, where the atmosphere is too thin to trap much heat, plate tectonics are fairly primitive, and the air is almost always very, very cold, the distribution of ice is determined by the planet's tilt.
"If you have zero tilt, it gets really cold at the poles," Smith said, because the top and bottom of the planet get hardly any direct sunlight. But when the planet tilts, the poles are warmer, and they release some of the water that's been locked up in ice. It then falls as snow around the planet's middle latitudes. At the peak of its most recent ice age, Mars would have been capped by large swaths of white.
A record of these fluctuations is inscribed in the layers of ice and dust at the Red Planet's poles. But scientists can't go grab an ice core, the way they would on Earth.
Instead, Smith and his colleagues at the Southwest Research Institute (he is now based at the Tucson Planetary Science Institute) used data from instruments on board NASA's Mars Reconnaissance Orbiter, which launched in 2005 with a mission to understand the history of water on Mars. By bouncing radar off the many layers of ice in Mars's polar caps, Smith was able to reconstruct millennia of climatic change.
The geology of the upper layers shows that ice has been accumulating relatively rapidly at the poles, indicating that Mars is emerging from its ice age.
"Now we can go backwards in time," Smith said. "It lets us know where ice used to be, where it still could be."
That will be vital for the day that humans hope to land on Mars. But it also could have been critical in the planet's ancient past. If anything ever once lived on Mars, then climate records will tell us when and where and how.
"NASA is spending a lot of money trying to find out if Mars was ever habitable and where," Smith pointed out. "Understanding the climate cycle is a really big step toward getting there."