Henry Dodds, GSA 2022 Science Communication Intern, The University of Texas at Dallas
We know a lot about Earth—more than any other planet. Though there are still many open questions and poorly understood processes, when compared with other bodies in the Solar System, what happens on our blue marble is extremely well studied. It’s easy enough to understand; if we want to learn something about our own planet, all we have to do is go outside and start looking; if we want to figure out exactly what’s going on with Mars, we can’t exactly show up and start running tests. But discoveries are indeed constantly being made about the other planets in our Solar System, so how exactly do we go about figuring these things out?
This past October, the Geological Society of America held its annual GSA Connects meeting in Denver, Colorado. There was a session of presentations named “Geomorphology and Landscape Evolution of Mars,” during which researchers gave summaries of their projects. I interviewed two presenting scientists investigating vastly different things going on at the surface of the red planet in order to figure out how this research is done and how we interpret it into knowledge to help unravel the mysteries of Mars.
Observing from Orbit
Many satellite missions have been launched toward Mars by space agencies around the globe. These satellites hold powerful instruments, which have sent a wealth of measurements of many kinds back to scientists on Earth. Grace Fanson, a student at the Georgia Institute of Technology, is using measurements from a couple of different satellites orbiting Mars to study powerful volcanic events that happened far in the past and shaped the surface today. She’s looking at landforms called pateras, in a region called Arabia Terra—a vast area about 2800 miles (4500 km) long and 1500 miles (2500 km) wide. The region is pock-marked with impact craters, a fact that researchers use to deduce that the crust there is very old. For a long time, it’s been assumed that all of the circular depressions surrounded by a ridge must be craters from meteor impacts, but that idea is being challenged in an important way. More and more evidence suggests that some of these were actually created by large, violent volcanic eruptions.
“A patera is a circular depression in the ground; it doesn’t speak to how it was formed; it could be a lake, an impact crater, or a caldera in this case,” explains Fanson.
On Earth, calderas are numerous and well-studied. They often form when magma beneath Earth’s surface comes into contact with groundwater, which then rapidly vaporizes from the heat and expands until it explodes violently, launching rock and lava potentially very far and sending steam and ash high into the atmosphere. This spectacular loss of material causes the center of the erupting volcano to collapse downward, typically forming a roughly round depression.
It might seem like these rather different processes would leave rather different surface expressions—in one, material crashes down from space at high speeds, and in the other material is shot upward and spews out across the surface. But Mars has strong winds and an abundance of dust to blow around, which weathers features and covers them in drapes of dirt, so that some diagnostic clues from the geometry (or “geomorphology”—the shape of surface features on a planet) can be difficult to pick out. Fanson explains, “In the past, the studies have been mostly morphological in nature, and I thought a more mineralogical examination was in order.”
An experienced eye can examine visual images taken by satellites, and when aided by studies and models of similar features on Earth, a researcher can produce theories about the origins of some features, which is what previous work has done with these pateras. But this approach alone neglects a lot of available data these satellites gather, and its results are far from conclusive. When asked what she’s using for the mineralogical approach, Fanson said she uses datasets from CRISM, HiRISE, and THEMIS primarily. There’s a lot going on in those acronyms, and a researcher in this field has to be knowledgeable not only about the features and processes they’re studying, but also must be mindful of the nature of the instruments and data they’re using as a lens to investigate. CRISM, HiRISE, and THEMIS are devices mounted on satellites orbiting Mars, which measure different qualities of martian surface materials. CRISM gives a spectrum of visible and infrared light emitted by the surface, which allows us to identify what types of minerals are around what features. According to Fanson, “Each mineral, each element has its own spectral signature.” Her research aims to test an existing hypothesis about a particular patera being an ancient volcano by finding minerals consistent with volcanic eruptions around it.
More than Just Looking
Just north of Arabia Terra, another scientist, An Li, is looking for something very different: large gouges in the surface that were carved by huge, moving masses of ice. We know many glaciers exist on the surface of Mars, but they are generally found pretty close to the planet’s poles (though layers of ice beneath the surface can be found in the mid-latitudes). The area Li, a student at the University of Washington, is investigating, is called Deuteronilus Mensae, and occupies a northern mid-latitude region of Mars. Deuteronilus Mensae contains many glacier-like forms, which are mixes of slowly flowing ice and rocky rubble. What we don’t know is how glaciers have been distributed throughout Mars’ past. “If these alcoves were in fact eroded by ice, then we should be able to figure something out about the climate,” says Li.
On Earth, we have a pretty complete record of recent glaciers, much farther from the poles than they currently span. Glaciers carve down through the bedrock they flow over, forming round, bowl-like scars called cirques, among many other unique landforms. Glaciers also leave behind a very specific trail of sedimentary rocks, allowing us to trace their extent very far into Earth’s past. For most of the surface of Mars, however, all we have are images and other electromagnetic spectrum data collected from satellites. Li is looking at around 2000 individual landforms, which her team has termed “alcoves,” in the region to investigate whether they were carved by glaciers.
“I actually don’t study existing glaciers, I study alcoves that we think may have been left behind by those glaciers,” she explains. Many of them look like Earthly cirques upon first inspection, but Li is using a much more rigorous math and data-based approach. “We’ve mostly used Context Camera (CTX), which comes from the Mars Reconnaissance Orbiter, and also HRSC, and that’s coming from Mars Express. Recently, we’ve also started using HiRISE to examine the landforms at a higher resolution.”
The Mars Reconnaissance Orbiter is a satellite launched by NASA in 2005, which also carries CRISM and HiRISE. Mars Express was launched in 2003 by the European Space Agency. The data from the CTX and HRSC can be used to make measurements of the shapes of landforms, which allows Li and her team to apply a method of study called “morphometrics.” They calculate ratios and other formulas using things like length, width, and depth of the alcoves and compare the values they get to values from a large number of cirques on Earth.
The analogous features on Earth are used to calculate empirical values for features we know are carved by glaciers, which can then be compared with martian values to see whether it’s likely they were formed by the same process.
This study can even reveal more specific information about the glaciers, a quality called the basal thermal regime: glaciers are broadly either cold-based or wet-based. “Cold-based is where you have the glacier frozen to the bedrock, as opposed to wet-based where you have a layer of water in between,” says Li. “For cold-based, that means there’s less liquid water available, and for wet-based that means there’s something allowing that liquid water to be there, and that has implications for biology.” In the same way we do on Earth, tracking the places glaciers were in the past helps us understand what the martian climate used to look like, which is massively important for the possibility of life in the history of the planet and which is in turn a huge goal of NASA’s investigations.
Studies like these require similar themes: landforms of interest are located via images and datasets from tools we’ve sent to study Mars. Then we look at similar features across Earth, which we have a strong understanding of and can use as comparative analogs to tell us what we should expect to see in the Mars data, if we are indeed looking at what we think we are. Results from isolated research can be put together to form a bigger picture of the history and evolution of the planet’s surface features and even its climate millions of years ago. These studies require interdisciplinary knowledge of physics, geology, and data science, and involve a strong understanding of what’s going on across the surface of both Earth and Mars.