Mars north pole ice is 3% dust, not 25%, study says

The water ice at Mars's north pole holds far less dust than planetary scientists once estimated, a new study finds, and that changes the story of the Red Planet's climate and its chances of hosting life.

Previous estimates put the dust content of the north polar cap at as much as 25% by mass. But Aditya Khuller of the University of Washington and graduate student Pari Mohan now conclude it is only about 3% dust, according to a study published Sept. 8 in the journal npj Space Exploration.

The distinction matters because dust in ice acts like a dark shirt on a sunny day: it absorbs sunlight, warms the ice, and makes it vaporize faster. "Just like a dark t-shirt in the sun makes you warmer, dusty ice gets warmer and vaporizes faster on Mars," Khuller said in a statement.

Cleaner ice reflects more sunlight back into space, which cools the planet, and it absorbs less heat, so less ice turns to vapor. A planet's reflectivity is called its albedo, and Mars, sitting at the outer edge of the sun's habitable zone, needs all the warmth it can get.

The new figure comes from a different method than past estimates. The old technique was built on the properties of lunar dirt, but when Khuller applied it to ice on Earth, the results did not hold up. So he and Mohan turned to a method developed by Steve Warren, professor emeritus at the University of Washington, which had been used for decades to study snow and ice on Earth. "I thought it would be interesting to adapt these Earth-tested methods to Mars," Khuller said.

The north polar cap is not a single block of ice. It sits atop mile-thick layers of water-ice deposits, some dustier than others, together called the North Polar Layered Deposits. Khuller calls it an "ice-cream sandwich": dirtier layers alternate with cleaner ones, and whichever layer is exposed at the surface affects Mars's climate at that time. The current top layer, the North Polar Residual Cap, appears nearly dust-free, helping keep Mars cold.

Seasonal change also matters. In winter, a shallow layer of carbon dioxide frost, about a meter deep at the north pole, covers the water ice. In summer it sublimates, exposing the older water ice below. Satellite observations from Europe's Mars Express and NASA's Mars Reconnaissance Orbiter and Mars Odyssey have watched the caps brighten and dim with the seasons. "By looking at how the brightness changed over time, we figured out that there is a frost that forms in the winter and it's more dusty," Khuller said. "In the Martian summer it goes away, exposing cleaner, older ice."

The finding also touches on whether Mars ever harbored life. In the past, when dustier layers of the sandwich were exposed, they would have absorbed enough sunlight to melt the ice at the bottom of that layer, creating shallow pools of nutrient-rich liquid water. On Earth, such dusty meltwater pools often teem with microbes that go dormant when the water freezes and revive when it melts. Mars's polar caps could have offered a similar habitat, Space.com reports, though no life has been found on the Red Planet.

"The fact that Mars and Earth both have these similar layers of water-ice and dust is interesting," Khuller said. "Why does one planet have life and the other doesn't?"

• Aditya Khuller
• habitability
• Mars
• npj Space Exploration
• polar ice
• University of Washington