Water and the possibility of life on Mars

[Music] Welcome to Science Sessions, the podcasts of the proceedings of the National Academy of Sciences, where we connect you with Academy members, researchers, and policymakers. Join us as we explore the stories behind the science. The possibility that water and life exists or once existed on Mars has been a topic of speculation and scientific interest for centuries. Samples of Martian rocks, an atmosphere collected by NASA's Presbyterance Rover, and cached in Jezeroel Crater, represent a unique opportunity to potentially resolve these questions. Should the samples be successfully retrieved and returned to Earth? I'm Matthew Hardcastle, and in this special feature episode, we will hear from physicists, chemists, and geologists on what we know about the history of water on Mars and the missing information that might be answered through analysis of returned Martian samples. Mark Femans, a physical chemist at the University of California at San Diego, is the organizer of two recent PNES special features on water on the Moon and Mars and the scientific value of Mars sample return. Femans explains the importance of bringing samples from Mars back to Earth to help answer fundamental questions about the presence of water and the potential for life on Mars. A lot of it has to be done by bringing samples back, and that's because the precision of measuring things involve instruments that can measure much better but they're too big to fly and it requires giant magnets and giant backing systems or something like that. So you got to bring it back, especially for isotope measurements, because that tells you the past history. It's like on Earth. We all talk about climate change, but we wouldn't be talking about climate change if we didn't know the oxygen isotope record of water to pulse. That's the only way you can get the temperature over time, especially if you want to go back to the last ice age. We'll learn a lot that way. As there are plate tectonics and power of the different minerals formed, are there carbonates and sulfates and perchlorates that are formed that indicate water activity on Mars. You need the most sensitive, clean, organic measurement possible that you can look for prebiotic or biology. It may be done at the parts petroleum level. You can do that in the lab, but you're off by a factor of a thousand to being able to do things like that in the field. The things you really want to know, was there life on Mars? Is there life on Mars? You got to bring it back. While Martian samples have not yet been returned to Earth, fragments of the Martian surface have been discovered on Earth in the form of Martian meteorites. Arria Udry, a planetary scientist at the University of Nevada Las Vegas, is the author of a recent P&A's article in the Mars sample return special feature. Udry explains the current gaps in limitations in the theological record provided by Martian meteorites. There's a little bit more than 350 Martian meteorites, which is actually considered very rare. The limitation is that Martian meteorites are very young. More than 50% of the surface of Mars is very old. They're 3.7 billion years old. But most of the Martian meteorites are younger than 1.3 billion years old. We missing half of the Martian Geological History in those samples, which is a big deal. In addition, we also only have magnetic rocks. If we had potential for life and the right condition for life, we cannot really see that in magnetic rocks. We need sedimentary rocks to better understand that. The big thing also is that we don't have field contexts. Those Martian meteorites are very valuable. This is the only samples that we have from this planet, but there's a lot of gap in the knowledge, especially when we talk about potential for life. Water is essential for life as we know it. Constraining the history of water on Mars is therefore key to answering questions about the potential for life. Bruce Jekowski, a planetary geologist at the University of Colorado Boulder, is the author of a PNAS article in the Water on the Moon in Mars special feature. Jekowski explains what we currently know about the presence and history of water on Mars. We start at the present. We actually know a lot about water today. We can detect water vapor in the atmosphere. We see water ice in the polar caps, and we've identified water ice in the crust in the top few centimeters of the regolith. We know there has been liquid water at the surface. We see evidence of erosion by water of ancient craters. We see branching river tributary systems that look like they were carved by liquid water. Our current understanding says there was abundant liquid water at the surface early in Mars history, and then it disappeared. The places that water from the atmosphere or at the surface could have gone include loss to space, forming minerals that include water in their basic structure. The water could have percolated deep into the crust, and some of it is left behind in the polar ice we see today. We want to understand how the climate has changed over time, and that's a difficult problem. Returbed Martian samples could help resolve uncertainties about the history of water on the planet. Monica Grady, a planetary scientist at the Open University of the United Kingdom, is the author of another article in the Mars Sample Returns Special feature. Grady explains how analyzing volatile species in Martian samples could provide insight into the planet's past climate. Water is one example of a volatile species, as is carbon dioxide, methane, carbon monoxide, oxygen, things like that. If you heat water, what happens is the lighter isotope bubbles away first, leaving the residue enriched in the heavier isotope. By looking at the isotopic composition of volatile species in different components in a reservoir, for instance, you can say whether these isotopes have been fractionated, if the heavier have been removed from the lighter. If you've got samples that have got well-known ages, then you can say, right, there is a fluid of that composition running at such and such a temperature, five million years, 50 million or a billion years ago, if you can look at a whole series of well-dated rocks. You can look and see how the fluid composition and how the temperature have changed with time. Until we've got a really well-dated series of samples, then it's difficult to infer too much about what's been happening in detail to the planet. Given the cold temperatures and thin atmosphere found on Mars today, liquid water is unlikely to be found abundantly on the planet's surface. Rachel Slingk, a planetary scientist at the Lunar and Planetary Institute in Houston, Texas, is the author of another article in the Water on the Moon and Mars Special Feature. Slingk describes the potential presence of liquid brines on Mars, which may be more stable than pure water. Pure water has a freezing point of zero degrees Celsius. When we want to lower that freezing point but still have liquid water, we can put salts into the mix. That's called a eutectic temperature. It's negative 75 degrees Celsius if you add in a salt like calcium per chlorate. The average temperature of Mars is about negative 50 degrees Celsius. The only way to really have stable liquid water is to have salt mixed in. The problem is, as you have a warm but very dry Mars, you don't have enough water to have the salt absorb for a brine to occur. So you have to go more towards the poles where you can get it colder so you could have that humidity, but then it's cold enough that most of the salts aren't reaching the eutectic. Life as we know it, which is only the life we have on Earth, needs water. It also needs temperatures above negative 20 degrees Celsius to survive. As we know right now on the surface of Mars, there is not a brine that can provide both of these conditions simultaneously. The Martian crust may contain some of the water that was once abundant on the surface, potentially providing a habitat for microorganisms. In a recent PNAS article, Bashan writes, a geophysicist at the University of California San Diego, and his colleagues used recordings from a seismometer onboard NASA's inside lander to estimate the volume of liquid water that might be contained in the Martian mid crust, 11.5 to 20 kilometers beneath the surface. When a Mars quake occurs or an impact hits the planet, what happens is that it creates waves that travel down and through the entire planet and back up. These waves are recorded by the seismometer. We're able to use those data to determine how the speed of those waves change with depth in the Martian subsurface. The speed of sound waves change depending on what the rocks are made of. When we run the model for roughly 5 million iterations, more models come back favor in the condition where there is liquid water in the subsurface. The data that we analyze are most sensitive to within 50-ish kilometers surrounding the seismometer. If you were to extend that and say that this is representative of the Martian crust, Mars, which remains a question, and you take out all the liquid water and you place it on the planet, you would get roughly 1 to 2 kilometers of water. If life once existed on Mars, it may have left behind identifiable molecular signatures in the form of biomarkers. Mark Zephton, an organic geochemist at Imperial College London, is the author of yet another article in the Mars sample return special feature. Southin explains what biomarkers are and what kind you might expect to find in Martian samples. Biomarkers are molecular fossils. They indicates a particular organism that lived in a particular environment. If we are looking after bringing back a sample with relatively recent biology in it, then we'd see all of the compounds that you would expect to find in life. Possibly the genetic material, the carbohydrates, the amino acids, and membrane molecules, the sort of molecular structures that help to keep all of the biological machinery of a cell in place. Now, if life is something that happened in the distant past, then some of those molecules would have been degraded. But that's okay because we still have the carbon skeletons left behind often. The very resistant parts of biology that tend to persist in the geolotroreco. Although they are less sensitive than Earth-based labs, instrument suites carried by Martian landers and rovers have been used to search for biomarkers. In a recent PNES article, Carolyn Fresone, an astrochemist at the laboratory for atmospheres, observations, and space, and the French National Center for Scientific Research, and her colleagues use instruments on board NASA's Curiosity Rover to identify long-chain organic molecules within a sample of Martian mudstone. Fresone describes this discovery and the difficulties of conclusively identifying organic molecules as biomarkers. In the past, we have found some organic molecules that contained up to six carbons. In this study, we have found the heaviest, biggest molecules that we have ever identified on Mars. They are composed of 10, 11, and 12 carbons in a backbone of carbon and hydrogen. We cannot differentiate between a chemical origin of those molecules and biological origins. Both are compatible with our result. We can look at the ratio between the molecules that have even number of carbons and the molecules that have an even number of carbons. If there is a difference, that would be a hint that the molecules are formed from biology. We would also love to see amino acids. If there is an excess of one mirror form compared to the other, that would be another hint of biology. If we combine different hints that would give us more and more confidence to say that biology is one possible explanation for all these molecules that we find on Mars. To have a definitive proof, we have to take a picture and see a mormote coming out of the ground. That would be awesome, but that's not the type of life we're looking on Mars. Analysis of returned Martian samples could provide unprecedented insight into the history of water and climactic conditions on Mars. Help narrow down where and when life might have existed and even identify potential biomarkers. However, a recent NASA budget proposal includes the cancellation of the Mars sample return program. Mark Theeman's explains the potentially far-reaching value of continuing to support the mission. I think it's an important mission to do. When I understand that people a lot of times will say, "We have so many needs financially to do other things besides this." It's always important to do things that lead to something else that's useful for society. I looked around to see, "Can I find a time when we invest in an emission of some sort?" James Webb Telescope, Apollo of samples, the Mars rovers and whatnot, where there hasn't been some application of it that benefits day-to-day life. You just have been always new techniques to show up somewhere. Thanks for tuning in to Science Sessions. You can subscribe to Science Sessions on iTunes, Spotify, or wherever you get your podcasts. If you like this episode, please consider leaving a review helping us spread the word.