There are two schools of thought as it concerns the early Martian surface. The first is that, about 3.5 billion of years ago, Mars used to be a cold and icy planet. The second contends that Mars had a more temperate climate, and rivers and lakes flowed over the land.
So, which theory is correct? The answer lies in an unlikely place: The bottom of a California lake.
Answers in zeolite
Zeolites are a class of minerals that form within a wide variety of rock types, both in marine and nonmarine environments – especially in areas where volcanic ash reacts with different sources of water. Scientists mainly find them in saline-alkaline lakes, in deep sea sediments, hydrothermal environments, and in certain soils and land surfaces on Earth. The general rule is that zeolites need some form of water in order to form their structure from precursor materials.
Therefore, Kodikara said, if scientists can infer that zeolites form on Earth in certain circumstances – a closed basin lake where volcaniclastic materials are present, for example – then it’s reasonable to deduce that zeolites will form, or did form, under the same circumstances on Mars.
“When we look at Mars, we have a lot of signatures of volcanoes. There are key minerals that indicate a hydrothermal process,” Kodikara pointed out. “Then we have rivers, that carved canyons on Mars. Then we have lakes, closed basin lakes with inlets and open basin lakes with inlets and outlets, and seas.”
In other words, there are plenty of places on the Martian surface where it’s likely that zeolites may have formed.
So, where are they?
“We have data mainly from four rovers on Mars: Spirit, Opportunity, Curiosity, and Perseverance. These rovers studied several locations on Mars, but none of them have found these zeolite minerals yet,” Kodikara said. “We have a lot of images from Mars covered by satellites, but from these satellite images only one (zeolite) mineral called analcime have been identified as a possible detection.”
That is where UWM graduate student Gayantha R. L. Kodikara (“Kodi”), who is working toward his PhD in geosciences, is examining lake sediments in hopes of better understanding a class of minerals called zeolites. In turn, those zeolites on Earth might give researchers a better understanding of Mars.
It’s possible that there just aren’t any zeolites, he acknowledged. Perhaps they existed at one point but were removed through a physical or chemical process. Or maybe they were just absent to begin with.
Or, Kodikara said, the zeolites really are there, but our current orbital remote sensing methods can’t detect them.
Answers in a lake bed
To figure it out, he’s using Earth analogs to answer Martian questions. Kodikara and his PhD advisor, geosciences professor Lindsay McHenry, are mainly focusing on Lake Tecopa in California. In 2018 and ’19, the pair spent a few weeks gathering sediment samples from the saline-alkaline paleolake and analyzing them for zeolite minerals. They also assessed the possibility of identifying the zeolites using orbital spectral image data analog to Mars orbital spectral data.
“We found that, in the lake sediments, zeolite can be covered by other kinds of base. It can be covered by the dust, and when it’s covered by dust, we can’t see it. Or it can be covered by other kinds of sediment, like rock material,” Kodikara said.
“The other thing is, (if) the thickness of the zeolite is very small, they can’t be detected by the remote sensing data. So, it might be there on Mars, but we can’t detect it for these reasons. Even on the Earth, with remote sensing data, it’s difficult to identify, and we know it’s there!”
Common zeolites in paleolake deposits include chabazite, clinoptilolite, erionite, and phillipsite, among others. It might also be that these zeolites are present, but their spectral signatures are so similar to each other and to polyhydrated sulfates, that it’s possible that the non-analcime zeolites can be identified as other minerals such when using orbital remote sensing data. Kodikara is adopting deep learning methods to see if he can distinguish them from each other.
He also collected the bulk mineral assemblages of over 2,360 sediments from previously published work and his studies from paleolake deposits from the U.S., Mexico, Greece, and Tanzania. Using machine learning methods to understand their chemical patterns, Kodikara gained a better understanding of how zeolites form on Earth.
Then, he needed to apply that knowledge to Mars. But there were a few snags.
“The problem is, Earth and Mars are completely different,” he laughed. “The second thing is, I’m not thinking about today’s Mars. I’m thinking about 3.5 billion-years-ago Mars. The only way to look at this is with thermodynamic modeling and chemical modeling.”
Mars minerals
In essence, if scientists assume that Mars used to be warm and wet, they can hazard a guess as to what its atmosphere must have been like, given current evidence and knowledge about the conditions of the solar system billions of years ago. Kodikara used those estimates to establish parameters about what Mars’ oxygen and carbon dioxide contents might have been like, the pressure of the atmosphere, and other factors that would affect zeolite formation.
“From numerical thermodynamic modeling, we found that it is possible to form the same mineral assemblages with the given environmental conditions on Mars, as we see on the Earth,” Kodikara said.
So Kodikara’s work continues. If he and other scientists can use what they know of Earth’s zeolites and how to detect them using orbital remote sensing, they may be able to refine their process of searching for zeolites on Mars.
A personal passion
Kodikara plans to graduate in fall after a summer spent interning with the Lunar and Planetary Institute (LPI). He hopes to find a postdoc position where he can continue researching zeolites and the Martian surface.
He loves his work because it’s exactly what he envisioned as a child.
“When I was a small kid, I had a dream to be a planetary scientist. I collected all the newspaper articles about space and planets,” he recalled. Kodikara attended college in his home country, Sri Lanka, before being accepted to a graduate programs in the Netherlands (ITC) and India (IIRS) before arriving at UWM as a PhD student.
As a geoscientist, Kodikara firmly believes the theory that Mars used to be a warmer, wet planet. The evidence is in the craters and canyons that carve up the planet’s surface, he argues, and scientists need a better understanding of what Mars used to look like.
“For me, it’s very personal. I am researching Mars to see what happened to Mars,” Kodikara said. “If, billions of years ago, there was a lot of water, what was the reason we lost it? Where did it go? Why did Mars become a dry planet?”
Once again, he’s thinking about analogs – this time from Mars to Earth.
“I’m not looking to go and colonize Mars because first of all, my thinking is, we need to save our Earth. To save our Earth, we planetary scientists can help. We can see what happened to other planets,” he said. “I am looking at Mars to see what happened to Mars. If there was life there, what happened to it?”
And what, he added, does that mean for Earth?
By Sarah Vickery, College of Letters & Science