World news – Life As We Don’t Know It: Astrobiology and the Mars 2020 Mission


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February 17, 2021

by Lori Dajose, California Institute of Technology

Life as we know it was found nowhere else in our solar system or universe than on Earth. But that doesn’t necessarily mean it’s not out there.

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The Mars 2020 mission is the first NASA mission with an explicit astrobiological component. Mars 2020 and related missions are said to be carried out in several parts over decades and are said to be the first to return samples from another planet to examine for signs of life.

But what do scientists hope to find? How will you know if or when you found it? What does it mean for life on earth if something is found and what does it mean if it is not found?

To get an insight into these questions, we spoke to Woody Fischer, Professor of Geobiology and Deputy Director of the Caltech Center for Autonomous Systems and Technologies. Fischer examines ancient rocks on Earth for signs of ancient life.

What are the main features in which Mars differs from Earth in terms of potential life?

There are many things that make Mars different do, but one big thing is that it is not tectonically active. In some ways, Mars may even be better suited than Earth for preserving early life signatures. The earth is tectonically active, so any rock from the planet’s early history, roughly three to four billion years ago, has now been buried at high temperatures and pressures, and these conditions can obliterate any evidence of the life it contains. The surface of Mars, on the other hand, remains relatively unchanged due to surface processes such as tectonics or the erosion force of water. If Mars hosted life three to four billion years ago, it could be evidence of the earliest life in the solar system.

We know that Mars used to be much more humid, and we think water was a pretty important component for it the development and prosperity of life is. In fact, the Jezero Crater landing site was chosen for Mars 2020 because it once contained a lake the size of Lake Tahoe. Water acts on rocks, changes them, changes them after they are deposited. But now Mars is dry. And the lack of water means fewer changes have occurred. Mars might actually give us a better chance than Earth to answer questions about what the solar system was like three or four billion years ago.

Suppose the complex process of sample return brings parts of the Martian surface back to Earth. What would you look for in these samples?

Finding a skeletal fossil such as can be seen in natural history museums on Earth would of course be a complete paradigm shift. However, we assume that life on Mars would have been microbial and microbes don’t often leave skeletal fossils behind. Rare records of microbes in the earth’s rock record are filaments and cocoids that have been infested with minerals.

We know some factors that tend to capture biological activity well: Basically, new minerals such as carbonate salts are forming everywhere. These minerals can collect and encapsulate parts of their environment during their formation and encapsulation of microbial fossil information on Earth. It is believed that carbonate salts are located in the Jezero crater. Therefore, we are keen to collect samples of carbonates that we find in order to determine whether they retain structural evidence of life. We would also look for organic biomarker compounds, which are sets of molecules that are extremely inconvenient to produce without the help of biology. Examples are molecules like cholesterol in our own cells.

There are a lot of suggested biosignatures. Many are not unique to life or diagnostic of life, but they definitely suggest it because we can’t think of any other way they could be made without a cell.

Take stromatolites: those lumpy rock matter layered like a flaky cookie. On earth, we think they are the fossil debris from sticky networks of bacteria called microbial mats. Microbial mats were incredibly important biological communities even before animals evolved on our planet. For most of life history, stromatolites are the record. However, there are gaps in our knowledge of how they are made – and there can even be entire classes of stromatolites that result from biologically independent mineral growth processes. What if a stromatolite structure was found on Mars and there were no additional organic signatures in it? The search for life on Mars could help us to fill the gaps in our understanding of the processes on our own planet.

On the other hand, we know of some examples from the rock record on Earth that could fool you. If I were to show you a thin section of certain types of ancient rock under a microscope, you would look at it and say, « Oh, that is certainly a cell. It has to be. » And I would say, « No, actually I know this came from a metamorphic rock. This was created in conditions of temperature and pressure that life could not reach. »

I find it exciting that researchers recently suggested looking for signatures for prebiotic processes – the physical and chemical processes that are the prerequisites for life to arise. From our current data it is absolutely clear that Mars was home to habitable environments, but whether or not they were actually inhabited is the next question. Another way to ask this question is whether there were processes taking place in these environments that might have been the precursors of something that might have brought about life. There is an opportunity to say, maybe Mars never lived, but maybe it took that route. Maybe it started with this kind of prebiotic chemistry. Perhaps there is some exciting chemistry that happened or is happening in surface environments on Mars, but it never quite brought it to life.

There are a number of parallel ideas about what types of environments and processes are used to create the Were important in life. But we certainly know very little. And you cannot answer the question today in modern Earth environments because life is ubiquitous and can surpass such prebiotic processes. But maybe you could see evidence of these processes in an ancient Martian environment. Imagine we land in Jezero on Mars and discover that there is a lot of organic matter and that we are able to characterize some of this organic matter. And maybe it looks like some things are being made under laboratory conditions under some experimental setup, hydrothermal system, hot spring environment, or alkaline lake.

There is this real opportunity for us to witness something amazing from Mars to discover, but also to discover something on Mars that we take for granted on Earth. Perhaps there are abiotic ways to create some of the materials and textures that we believe are created by life on earth.

How would the scientific community reach consensus to explain, « This is a sign of the past life?  » Is there an official definition of « life »?

Imagine we find a stromatolite. There will be people on and off the team who are ready to declare victory. « That’s it, we’ve found life. » There will also be a lot of people saying, « Well, wait a second. How do we know we are so sure? »

This back and forth will happen, I think, when we discover these materials. That kind of dialogue is going to be really important in this process.

The science is so subtle. It’s always a discussion. With every new observation and discovery comes the ability to ask a more detailed, more precise question. When we talk about recognizing life right now, there are always reservations, and if so, and something like that. We don’t always translate that into the public incredibly well. But I think it would be a big deal to be able to say, « See, we’re seeing materials here that we associate with life, and we’re going to have our work cut out to test this in a rigorous way, but that’s closer than ever before to understand whether or not life is on a planet other than ours. « 

How would the presence of life on Mars change our view of life on Earth?

Imagine being able to find signs of ancient life on Mars. The next thing you want to see is whether there is life that continues on the planet to this day. Imagine, we think so too. The next thing you want to know is whether it is related to life on earth. How much does it remind us of ourselves or not? Is it a completely different experiment? It’s really interesting whether or not it evolved independently on Mars instead of being part of panspermia – the idea that life on meteorites or the like was somehow common throughout the solar system.

We want to know which ones There are similarities between life on Mars and Earth. How does life reproduce on Mars? What are the rules for being alive?

Imagine finding something like a cell. That could tell you that life has to be locked in a membrane. How do these cells harvest energy? Only about three types of energy have been used for cells on Earth so far – those in phosphoanhydride bonds like ATP (the molecule that provides energy to power processes in living things), redox reactions, and chemical gradients of the membrane – although this may have been more in the past that we don’t know about. Did that also apply to Mars? Has life there found ways to tap into other sources of energy that life on earth does not have?

The questions that arise from this are getting very exciting, very fast. And even if panspermia is ultimately responsible for life on Earth and on Mars, when did it happen? Such things would be absolutely fascinating to be able to study. Perhaps they could be studied in connection with sending humans to Mars because we are maybe a decade away. Despite the tremendous technological advances involved in sending a rover like Perseverance to Jezero Crater, the kind of research you can do with one person is still beyond what we can do with a rover.

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