September 28, 2015
Water on Mars: Astrobiology Implications Must be Taken with a Grain of Salt
What do Mars, Antarctica, and the Jordan Rift Valley between Jordan, Israel, and Palestine have in common? A lot according to a new study to be published today in the journal Nature Geoscience. The answer is saltwater.
Since the late 19th century, when astronomer Percival Lowell thought that he saw Martian canals through his telescope, the idea of water on the Martian surface has been a major topic in planetary science. In the 1970s, NASA probes started arriving and photographing Mars from up close, revealing that Lowell had been completely wrong. Mars is extremely dry, at least on the surface. Nevertheless, missions by NASA and other space agencies to study the Red Planet have made the search for water a major focus.
Because the atmospheric pressure at the Martian surface is extremely low, just 7-10 millibars, pure water can exist as liquid only transiently. At certain times of the day during the Martian summer, the temperature gets high enough for droplets to form, from water vapor condensing, or from ice melting. But usually, water on Mars doesn’t even have a liquid state and instead sublimates between vapor and ice. That’s not great news for those of us interested in finding microorganisms on Mars, but fortunately there’s a loophole: salinity.
Salt changes the equation. The saltier water gets, the the more stable liquid water becomes, even at very low atmospheric pressures like the pressure on Mars. Thus, lakes, ponds, seas, rivers, and streams of water are possible on the surface of Mars, but only if they are brines, like the Dead Sea, which has a salinity of 33.7 percent. That’s so salty that you float without a life preserver and the existence of similar super briny water on Mars is supported by multiple lines of evidence.
Firstly, imaging data from orbit have revealed dark slope streaks (often called “recurring slope lineae” [RSL]) on the Martian surface that look as if carved by flowing liquid. The second line of evidence comes from Mars meteorites, pieces of the Martian crust that were catapulted to Earth by asteroid impacts. One meteorite that was ejected from Mars roughly 10.75 million years ago crashed in Nakhla, Egypt in 1911, where it ended its voyage and hit a dog, contains chemical evidence that about 600 million years ago, while still on Mars, it was bathed in water full of chloride and sulfate salts. Another Mars meteorite, EETA79001, contains nitrate salts, and salts of an ion called perchlorate (ClO4-).
Perchlorate may be key to the Mars puzzle. As a third line of evidence, perchlorate also has been discovered in the present Martian environment. Earlier data from NASA instruments have shown perchlorate in different locations on the Martian surface, and so perchlorate salts are ubiquitous to the planet. The study published today presents an analysis of spectral data from NASA’s Mars Reconnaissance Orbiter showing perchlorate in association with the dark streaks themselves. In other words, precisely in the areas that appear carved by liquid is a chemical compound that can make water a stable brine, rather than a transient entity that would quickly freeze or evaporate.
To harbor life, a planet needs water, and from the perspective of astrobiology, even extremely salty water sounds like good news, at least initially. Despite its name in English and some other languages, the Dead Sea actually has some microorganisms. There is a species of archaea called Haloarcula marismortui, a type of halophile (salt-lover), an organism that thrives in high salinity. Given the prospect that long-standing surface water on Mars has to be a brine, H. marismortui has been studied as a Mars analogue organism, a microbe that lives on Earth that has features that would be useful to an organism that’s native to Mars. As part of The Planetary Society’s Living Interplanetary Flight Experiment (LIFE), several investigators, including this writer, sent samples of H. marismortui into space on the space shuttle Endeavour in 2011 in preparation for sending it on a future mission through interplanetary space. The main reason is to determine how easily life ejected from ancient Mars could have crossed the ocean of space and possibly seeded Earth.
The findings published in the new paper –whose first author, Lujendra Ojha, is a planetary science graduate student at Georgia Tech– solve the mystery of the dark streaks on Mars. The streaks really are due to brines, which means that water on the surface is not something that happened only billions of years ago. Instead, bodies of salty water also exist from time under the current Martian climate. Recently, there could have been lakes, ponds, and rivers. There could even be small bodies of surface brine right now that we have yet to discover. Also, the fact that it’s perchlorate brine could be significant for astrobiology, because perchlorate could provide microbes a source of energy, which is another requirement for life.
But when it comes to finding life forms in Martian surface brine, there’s a caveat.
The concentration of perchlorate is actually too high for life. H. marismortui is an example of how an organism can thrive in the 33.7 percent salinity of the Dead Sea and our understanding of biochemistry dovetails with this. Rather than being like the Dead Sea, the brine that we envision now on Mars would be similar to a briny spot in Antarctica, called Don Juan Pond, where the salinity is a whopping 44 percent, due to high levels of calcium chloride. Even H. marismortui, and unfortunately, any similar Mars organism that we could imagine, would not survive in that.
The new study is still good news though, for a couple of reasons. It may take many years and numerous missions, but possibly we could follow the water down beneath the surface, perhaps to caves or other locations where the salinity is lower, and those places would be excellent for drawing samples to test for the presence of life.
Another reason to remain hopeful relates not to the search for Martian water, but to another essential ingredient for life: organic matter. Since the 1970s, a great mystery has loomed over Mars exploration. Chemistry tests performed on Martian surface samples by two landing NASA probes called the Viking Landers 1 and 2 failed to detect organic compounds. That was strange, because scientists knew that the planet was pounded continuously with pieces of asteroids and comet carrying organic matter from space. But the story has a twist. Testing by the Viking chemistry instrument, called a gas chromatograph-mass spectrometer (GCMS), actually did detect some organic compounds, specifically chloromethane and dichloromethane. This happened at both landing sites, called Chryse Panitia and Utopia Planitia, which are separated by thousands of kilometers. But chloromethane and dichloromethane are cleaning agents that the Viking chemistry team decided must have contaminated the vehicle before it left Earth. Otherwise, how could they be the only organic compounds found at the landing sites?
The Viking team didn’t know it, but it turns out that there is a way. When you take a host of organic compounds and expose them to perchlorate, guess what the mixture produces – chloromethane and dichloromethane. Perchlorate on Mars was overlooked during the Viking mission, but it was found later by subsequent Mars probes. This prompted scientists to take a new look at Viking GCMS inorganic data and that reanalysis revealed perchlorate, enough of it to account for the chloromethane and dichloromethane found in the organic analysis.
This means that any organic compounds, whether from comets, or even microorganisms, don’t last very long. They are converted into cleaning agents. And so, while the new discovery must be taken with a grain, or more likely a ton, of salt, when it comes to astrobiology, the discovery also clinches the case that perchlorate is ubiquitous on Mars. This means, in turn, that initial reports of the lack of organic material on Mars were greatly exaggerated. And that can be vital to the quest for Martian life.
Written by David Warmflash
David is an astrobiologist and science writer. He received his M.D. from Tel Aviv University Sackler School of Medicine, and has done post doctoral work at Brandeis University, the University of Pennsylvania, and the Johnson Space Center, where he was part of the NASA's first cohort of astrobiology training fellows. He has been involved in science outreach for more than a decade and since 2002 has collaborated with The Planetary Society on studying the effects of the space environment on small organisms.