Mars polar ice is cleaner than scientists thought. What could this mean for life?

Scientists have discovered that the water ice at Mars's north pole is much cleaner than they once believed. Researchers Aditya Khuller and Pari Mohan from the University of Washington found that the ice contains only about 3% dust — far less than the 25% earlier studies had estimated. This surprising finding could change how we understand Mars's climate, both now and in the distant past. It may even offer new clues about whether tiny forms of life could have once survived on the Red Planet.

For years, planetary scientists thought the ice near Mars's north pole was packed with dust. Dust in ice is a big deal because it acts like a dark blanket, soaking up sunlight and causing ice to melt or turn into vapor. Khuller explained the idea simply: 'Just like a dark t-shirt in the sun makes you warmer, dusty ice gets warmer and vaporizes faster on Mars.' If the ice really had 25% dust, it would have been losing a lot of its mass over time.

The new study used a better method to measure the properties of the ice. Khuller noticed that scientists had been using a technique based on dirt from Earth's Moon, but when he tested it on Earth's own ice, it gave wrong answers. So he turned to a method developed by Professor Steve Warren, also at the University of Washington, which had been successfully used to study snow and ice on Earth for decades. When Khuller and Mohan applied this improved method to Mars, they got a very different result — just 3% dust.

Cleaner ice reflects more sunlight back into space instead of absorbing it. Scientists call this reflectivity 'albedo,' and the higher it is, the less warmth a planet keeps. This matters a lot for Mars, which already sits on the cold outer edge of the zone in our solar system where liquid water could exist. So cleaner, brighter ice makes Mars even colder, which shapes the whole planet's climate.

Mars has ice caps at both its north and south poles, and in winter a thin layer of carbon dioxide ice covers the water ice beneath. At the north pole, this carbon dioxide layer is only about one meter deep, so it disappears faster in summer. When it melts away, it reveals what scientists call the North Polar Residual Cap, sitting on top of thick older layers of ice and dust. Khuller describes the whole structure as an 'ice-cream sandwich,' with cleaner and dirtier layers stacked on top of each other.

These different layers have affected Mars's climate in different ways over thousands of years. When a dustier layer was on top, it absorbed more sunlight and may have even melted some ice at its base, creating small pockets of liquid water. On Earth, similar pools of dusty meltwater in polar regions are often full of tiny microbes that go dormant in winter and wake back up in summer. Mars's ice layers could have worked the same way long ago.

Orbiting spacecraft like NASA's Mars Reconnaissance Orbiter and Mars Odyssey, and Europe's Mars Express, have watched the polar caps change with the seasons. Scientists noticed the caps getting brighter and darker as carbon dioxide ice built up and then disappeared. '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,' said Khuller. 'In the Martian summer it goes away, exposing cleaner, older ice.'

This research raises a big question about life on Mars. If dusty layers of ice once melted and formed pockets of liquid water, those spots might have been perfect homes for tiny living things. Both Earth and Mars have these layered mixtures of water ice and dust, yet only Earth has life as far as we know. 'Why does one planet have life and the other doesn't?' Khuller asked. The study was published on September 8 in the journal npj Space Exploration.