Although Mars may have once resembled Earth, the rocky red planet now appears far from hospitable. Still, certain details – such as ice-bound water – pique researchers’ curiosity. Much like on Earth, ice contains valuable records of past climate that could say whether Mars hosts, or has ever hosted, life.
Aside from the poles, where ice is sometimes exposed, most of Mars’ ice is buried beneath a dusty surface layer. Dust impacts climate by darkening ice, which changes how much sunlight is reflected into space. A University of Washington study published Sept. 8 in npj Space Exploration shows that the north pole of Mars contains less dust than scientists thought.
“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.”
Much of what we know about how Earth’s climate has evolved through millennia comes from sampling ice. Scientists drill deep into polar ice to draw out long cylinders that contain a record of the local atmosphere. Like a tree’s rings, these records can be dated to ancient periods of Earth’s history. But this is no easy task on Earth. On Mars, it’s even more complicated.
The National Aeronautics and Space Administration celebrated its first successful mission to Mars 50 years ago. In 2008, the Mars Phoenix mission successfully sampled ice near the north pole, a major triumph after the Polar Lander went missing near the south pole in 1999. The UW researchers used data from the Mars Phoenix mission, combined with observations from orbiting satellites, to complete this study.
Several years ago, Khuller discovered a discrepancy in the way researchers were analyzing the physical properties of ice on Mars. The leading approach was developed for studying soil on the Moon, but when Khuller checked its accuracy on Earth, the results seemed off.
In this study, Pari Mohan, who recently graduated from the UW with a degree in geoscience, worked with Khuller to redo the calculations using a different method. Theirs is based on an approach developed by Steve Warren, UW professor emeritus of Earth and space science, and an expert in analyzing snow and ice.
“His methods had been used successfully to study snow and ice on Earth for decades. So I thought it would be interesting to adapt these Earth-tested methods to Mars,” Khuller said.
The results suggest that the north pole is stacked “like an ice-cream sandwich,” with layers of dustier ice between slabs of cleaner ice. A dusty layer of frost forms over the ice every winter and disappears in the summer, revealing older, cleaner ice. Previous estimates suggested that the top layer of polar ice contained as much as 25% dust by mass, but this study says it’s closer to 3%.
“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. In the Martian summer it goes away, exposing cleaner, older ice,” Khuller said.
These layers contain key details about the climate of Mars thousands of years ago, when the ice is thought to have formed from snowfall. Mars experiences massive ice ages that have deposited shallow ice on roughly one-third of the planet.
Earth is stabilized by the gravitational pull of its moon. Mars, having only two small moons, “oscillates wildly,” Khuller said, which causes these dramatic ice ages.
In a previous study, Khuller and colleagues suggested that layers of dust and ice could create conditions for life on Mars. The dark layers could help trap sunlight and form pockets of meltwater within the ice. These pockets, enriched with nutrients from the dust, could potentially host bacteria and primitive life forms.
Similar pockets of shallow, dusty meltwater found in ice on Earth are often teeming with life in the summer. In the winter, the liquid water freezes and the microbes become dormant until the next summer.
“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?”
Uncovering details about the climate moves the needle, but answering this question may take years. Khuller hopes to expand on this work by applying these improved methods to other regions of the red planet.
