This week PVL MSc candidate, Hemani Kalucha interviews a professor from her undergraduate institution about a few of the factors affecting our search for life on Mars. Image above from: https://geosciences.princeton.edu/people/tullis-c-onstott .
by Hemani Kalucha
Increasingly, experts are coming to the conclusion that if life exists or ever existed on Mars, we will only find signs of it far beneath the surface. There’s a brand new effort at JPL to create missions to Mars that go beneath the surface and you can read all about it here. As someone who hopes to be involved in the development of such a mission, I thought it would be a good idea to talk to one of the world’s leading experts on life underground: Professor Tullis Onstott! Professot Onstott is a professor of Geomicrobiology at Princeton University and an incredible mentor to me! Read about all his exciting discoveries here, here, and here.
Professor, could you talk a little bit about the research going on in your lab currently?
Well, we’ve had a project that has been running for twenty years now. We have been exploring the subsurface life in South Africa by making use of their gold mines. It’s a great way to access the subsurface and we look at the life that’s present in the fluid-filled fractures in the rock that are exposed by the mining industry. One particular location, Moab Khotsong, is a gold mine that has the single greatest elevator drop on the planet. You go down 3 kilometres in one cage. You can see the CBS 60 minutes footage here. And we discovered in this mine, hypersaline water brine. And this brine is very similar to what you would expect in the deep subsurface of Mars. So it is likely to be extremely old, on the order of billions of years. And it represents an environment, because it is hypersaline and at high temperature (55-60 degrees Celsius) that is very challenging to life. It is at the limits of life as we know it. It not only provides us exploration of what life might be like on Mars today but it also provides us a window into the origin of life, if it can occur on the subsurface. And as we’re finding, life is very limited in this environment, which allows us to see what prebiotic compounds can form there, for example, what type of amino acids can form by water rock interactions at high salinities and temperatures. Do you get amino acids, do you get nucleic acids, or do you get nucleobases forming at all over a period of billions of years? This is something I’ve been searching for since we first went to South Africa, and we have over twenty years, finally found it!
Another project we’re working on takes place in Northern Chile, in the Andean mountains at very high elevations, 4000 m+ above sea level. So you see we go to 3000 metres below sea level and 4000 metres above! In Chile, we’re looking at hypersaline salt hot springs. And because of its location, it has a very high ultra violet flux, which is an environment pretty analogous to what we think Early Mars had. Early Mars would’ve had these hot springs because we see geological evidence of these hot springs on Mars. Back then, there would’ve been hot springs in a very thin atmosphere with a very high UV flux, and the question is, would this have been a place where life existed and was preserved? In the case of Chile, one of the things being looked at is, are the organisms in the soil around the hot springs existing off the gases being emitted by the hot springs? The gases emitted by the hot springs that existed on Early Mars should be present in the atmospheres of Mars today, and it’s possible such organisms may have evolved to take up those gases today.
Is there a location in the Earth’s subsurface that is most similar to the Early Martian subsurface?
There are many locations on Earth in the subsurface where one can find an environment that’s analogous to what’s existing today and existed on Early Mars. The comparison can be drawn from the rocks found in Early Mars, seen in the exposed rock around Nili Fossae. These are exposures out of the subsurface of Mars: you can see volcanic rock and ultramafic rocks exposed there. On Earth, these types of rocks occurred in the Archean Eon. We find these types of rock environments in South Africa and the subsurface of the Canadian Shield and in the Fennoscandian Shield where there is evidence of water-rock interactions occurring. These interactions produce a lot of hydrogen gas and methane. And the hydrogen is a great fuel for subsurface microorganisms.
Based on that, is there an organism that is found in these places on Earth that is most likely to be found in the Martian Subsurface?
If you look at the evolutionary history of prokaryotes, bacteria, and archea, as you look into the deepest most primitive organisms that are present based upon phylogenetic analyses, there’s a common metabolism you see there, one of which hydrogen is oxidized and methane or acetane is produced: methane for archea, acetane for bacteria. And you can find both of those organisms present in the subsurface as well. So phylogenetically, the most primitive organisms are the ones found in the subsurface. Based on this, we can imagine that similar types of metabolisms could have evolved early on in Mars because we find they appear on Earth in the subsurface as a primitive evolutionary form as well. These organisms on Mars, if they evolved DNA, the DNA would have evolved into methanogens, or acetogens that consume hydrogen and CO2. Both these gases are present now on Mars, and would’ve been present early on Mars at an even higher concentration.
Could we expect them at similar depths below the surface as compared to Earth?
Back on early Mars, they would’ve been on the surface because the environment was much warmer. But because today Mars is so cold, you have to go quite deep – several km or deeper. You have to go down to a place where you have liquid water present. Extant life like we see in the subsurface today of Earth is very challenging. It’s not something that is easily possible. To find extant life today that is still cycling methane, they would have to be organisms that are able to be functional at extremely high salinities. And so that’s another project we’ve been looking at with our colleague Andrew Schuerger down at the University of Florida: to see whether or not these terrestrial organisms like methanogens can survive at high perchlorate concentrations and still produce methane at cold temperatures.
And so far, does it seem like they can survive?
So far, it’s looking good!
One last question, I read about how the methane composition on Mars could be analyzed to see if was changing and so was being consumed or produced by bacteria. Is that something SAM on MSL can do?
The TSL-SAM doesn’t have the sensitivity required to do the isotopic analyses. But we do have a cavity ring down spectrometer, which has the sensitivity do those types of analyses but it hasn’t been flown yet. So that would be an example of an instrument that could go to Mars to analyze the isotopic composition of methane. And it’s sitting in our lab right now!
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