Mars' North Pole Ice Holds Less Than 3% Dust, Far Cleaner Than Believed, UW Study Finds
A University of Washington team reworked the calculations using methods built for Earth's snow and found that earlier estimates of up to 25% dust were far too high.
The exposed water ice at Mars' north pole is much cleaner than scientists thought, according to a University of Washington study that puts its dust content at less than 3 percent by mass. Earlier estimates had run as high as 25 percent.
The paper was published Sept. 8 in the journal npj Space Exploration. It matters because dust changes how ice behaves. "We know there is water ice in the area surrounding the north pole of Mars, but there has been widespread disagreement as to how dusty that ice is," said Aditya Khuller, a senior research scientist at the UW's Applied Physics Laboratory. "If it is dustier, the ice will be darker. Just like a dark T-shirt in the sun makes you warmer, dusty ice gets warmer and vaporizes faster on Mars."
Khuller noticed several years ago that the leading way of analyzing ice on Mars did not hold up. The method was developed to study soil on the Moon, and when he tested its accuracy on Earth, the results seemed off. Working with Pari Mohan, who recently graduated from the UW with a geoscience degree, he redid the calculations with an approach based on the work of Steve Warren, a UW professor emeritus of Earth and space science who studies snow and ice. Warren's methods have been used on Earth for decades.
The team analyzed how the ice reflects light, using observations from orbiters including the European Space Agency's Mars Express and NASA's Mars Reconnaissance Orbiter, together with data from NASA's Phoenix lander, which sampled ice near the north pole in 2008. They looked at six north polar locations: two on the residual polar cap, Korolev and Louth craters, Chasma Boreale and the Phoenix landing site.
The results suggest the north pole is built "like an ice-cream sandwich," with dustier layers between slabs of cleaner ice. A dusty frost forms over the ice every winter and disappears in the summer, exposing older, cleaner ice beneath. At one site, the study reported a dust content of 0.04 percent on Sol 28, a Martian day, which rose to 0.45 percent 58 sols later as dusty frost built up.
"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 layers hold clues to Mars' climate history. The ice is thought to have formed from snowfall thousands of years ago. Mars goes through massive ice ages that have deposited shallow ice across roughly one-third of the planet, because the planet, with only two small moons, "oscillates wildly," Khuller said, unlike Earth, which is stabilized by the Moon's gravity.
The finding also bears on the question of habitability. In an earlier study, Khuller and colleagues suggested that layers of dust and ice could trap sunlight and create pockets of meltwater inside the ice, conditions that might support life. A cleaner ice cap changes how those calculations work, and it gives future missions a better picture of what drilling into the polar layers could reveal.
