Like Earth, Mars has experienced periods of extreme glaciation or ice sheet coverage, which are known as ice ages. As these ice ages come and go, glaciers expand and contract along the planet’s surface, grinding huge boulders down to smaller rocks. By examining the size of boulders and rocks at specific locations on Mars, we should be able to understand the history of the Martian ice ages.
A new study did just that.
There’s a question mark hanging over the issue of Mars’ ice ages. The planet’s surface is populated with debris-covered glacier deposits. Were those glaciers the result of multiple ice ages over the past 300 million to 800 million years? Or was there one continuous ice age that spawned them all? If scientists could answer that question, they’d fill in some large gaps in their understanding of Mars’ history.
Joe Levy, a planetary scientist and assistant professor of geology at Colgate University, wanted to answer that question. He and his colleagues examined 45 glaciers on Mars using high-resolution images from the Mars Reconnaissance Orbiter. Their ground-breaking work is giving us a new understanding of the history of Mars, showing that Mars went through multiple ice ages.
The new study is titled “Surface boulder banding indicates Martian debris-covered glaciers formed over multiple glaciations.” The co-authors include Colgate students and alumni, as well as researchers from NASA and several other universities. The study is published in the Proceedings of the National Academy of Sciences.
“There are really good models for Mars’ orbital parameters for the last 20 million years,” Levy said in a press release. “After that, the models tend to get chaotic.”
A planet’s obliquity, or axial tilt, creates seasons. Changes in obliquity trigger ice ages. While Earth’s obliquity is relatively stable, Mars’ isn’t. Due to the gravitational torque from other larger planets, Mars’ axial tilt can change a lot and chaotically. Scientists think that it can reach values as high as 60 degrees and as low as 10 degrees. Compare that to Earth’s 23 degrees, which is kept stable with help from our large Moon.
There’s a critical difference between studying ice ages on Earth vs. ice ages on Mars. The glaciers that formed during Earth’s last ice age, which peaked about 20,000 years ago, have receded to the poles and mountains. But back when they expanded, they pushed a lot of rock ahead of themselves. After they receded, they left all those rocks behind.
But on Mars, the glaciers never receded. Instead, they’re still there, covered with debris. So are the lobate debris aprons, or LDA. “All the rocks and sand carried on that ice have remained on the surface,” Levy said. “It’s like putting the ice in a cooler under all those sediments.”
Levy and his team knew that if they could tease out the history of the Martian ice ages, they’d also be able to piece together the history of Mars’ orbital obliquity and its climate. They could also learn what types of rocks and gases are trapped inside the ice. And if there are any microbes in there, they may be able to determine what types.
Levy came up with a way to study the history of the Martian ice ages. Since rocks get ground to smaller sizes over time, examining rocks could be key. If Levy and his team could find areas on Mars with a steady downhill progression from larger rocks to smaller rocks, it would be evidence of one single, long-lasting ice age.
But it all had to be done using orbital images from the Mars Reconnaissance Orbiter’s (MRO) High-Resolution Imaging Science Experiment (HiRISE). The team used images of 45 different glaciers on Mars and got busy counting the size and number of rocks. HiRISE boasts an image resolution of 25 cm (9.8 in) per pixel, about the same size as an average dinner plate.
Ideally, researchers could use AI to identify, count, and measure all those rocks on all those glaciers. But AI couldn’t handle it; the task required human eyes and minds. So Levy had 10 students work on the images over two summers. Altogether the students counted and measured about 60,000 large rocks. “We did a kind of virtual field work, walking up and down these glaciers and mapping the boulders,” Levy said.
The results were unexpected. There was no orderly progression but more of a random dispersal of boulders. That was a shock to the researchers, at first. “In fact, the boulders were telling us a different story,” Levy said. “It wasn’t their size that mattered; it was how they were grouped or clustered.”
That led the researchers to an important insight. Since the rocks weren’t ground down, that means they had to be moving inside the glacier rather than grinding together. The rocks were arranged in bands across the glaciers’ surface, and Levy and his colleagues concluded that the bands signified different ice ages, as Mars wobbled on its axis.
In their paper, they write, “Supraglacial rocky debris on LDAs is thought to result partially from the sublimation of LDA ice that releases fine sediments (<?25 cm) that were entrained in the LDA accumulation zone and partially from rafting of large (>1 m) rockfall that travels supraglacially, in cases, forming longitudinal rocky zones analogous to medial moraines.
The team’s work showed that Mars went through between six and 20 distinct ice ages in the last 300 to 800 million years. Since those ice ages were caused by changes in the planet’s axial tilt, the result is a record—an incomplete one, but still a record—of Mars’ orbit and obliquity and its climate. “Lobate debris aprons may preserve ice spanning multiple glacial/interglacial cycles, extending Mars climate records back hundreds of millions of years,” the authors write in their paper.
“This paper is the first geological evidence of what Martian orbit and obliquity might have been doing for hundreds of millions of years,” Levy said.
The finding is important for other reasons, too. These glaciers are a repository of evidence about Mars’ history.
“These glaciers are little time capsules, capturing snapshots of what was blowing around in the Martian atmosphere,” Levy said. “Now we know that we have access to hundreds of millions of years of Martian history without having to drill down deep through the crust — we can just take a hike along the surface.”
The glaciers may have something to tell us in our search for life on Mars, too. “If there are any biomarkers blowing around, those are going to be trapped in the ice too,” Levy said.
These results will also be beneficial to AI. The data can be used to train AI to examine more images of more glaciers. Levy and his team are mapping the rest of Mars’ glaciers and are hoping that AI will identify and measure the boulders. All of that data will strengthen our understanding of Mars’ climate history.
And Mars’ climate history is important to humanity’s over-arching question about Mars: Did it ever support life?
“There’s a lot of work to be done figuring out the details of Martian climate history,” said Levy, “including when and where it was warm enough and wet enough for there to be brines and liquid water.”
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