Subsequent missions to the planet started to paint a clearer picture of its potential biological history. For example, in the early 2000s, NASA rovers Spirit and Opportunity discovered sediments and minerals that couldn’t have formed without water, as well as materials, such as patches of silica, typically found in hot springs and steam vents, where extremophiles thrive on Earth. Most recently, the rover Curiosity, which landed on the planet in August 2012, has detected simple carbon-based organic compounds in the Gale Crater, a large cavity near the Martian equator.

Despite growing evidence that Mars might have been teeming with life eons ago, exploration of the planet has painted a bleak image of its contemporary environment.

Despite growing evidence that Mars might have been teeming with life eons ago, exploration of the planet has painted a bleak image of its contemporary environment. Because it lacks a thick atmosphere and a magnetic field, which are essential for making Earth a hospitable place to live, Mars is exposed to harmful ultraviolet (UV) light and ionizing radiation from cosmic rays. Those features, along with low temperature and pressure, “make the environment pretty hostile to life as we know it,” says Manish Patel, a senior lecturer in planetary sciences at the Open University in the U.K.

Nevertheless, scientists are uncovering aspects of the planet that indicate Mars could still be harboring isolated pockets of life. Although the chances may be small, these findings have major implications for continued missions to the Red Planet—and, of course, its potential future colonization by humans. (See “A Hostile Planet.”)

MARTIAN MALADIES: Humans may have grand dreams of colonizing Mars, but before that happens, scientists and engineers will need to devise ways to protect travelers from the planet’s hostile environment. Spacesuits can help protect against most environmental harms, such as frigid temperatures and low oxygen. However, high levels of space radiation, which is the biggest concern, will be the most difficult to avoid.
See full infographic: WEB | PDF© SYLVAIN SARRAILH

Water marks

Remnants of a wet Mars remain the clearest hint that the planet once could have harbored life. Data gathered by Curiosity point to the existence of a massive freshwater lake in the Gale Crater billions of years ago, and scientists’ analyses suggest this environment had habitable conditions: a relatively neutral pH, low salinity, and elements that make up the building blocks of life—carbon, oxygen, hydrogen, sulfur, nitrogen, and phosphorus.¹

Curiosity has also detected evidence of simple organic molecules in this region, including methane,² chlorobenzene,³ and hints of longer-chain molecules resembling fatty acids⁴—all of which have primarily biological origins on Earth. “The consensus is that Mars had a lot of water in its ancient past, and that life could have existed and grown then,” Patel says. (See “Ancient Microbes on Mars.”)

Nowadays, however, confirmed sources of Martian water exist solely as ice, primarily in the planet’s polar regions, with very recent evidence pointing to the possibility of ice patches at much lower latitudes, near the planet’s equator.⁵ And life—at least as we know it—needs liquid water to survive.

In 2000, scientists detected Martian gullies, channels traversing the landscape that appear similar to those created by flowing water on Earth.⁶ Images that the Mars Global Surveyor spacecraft captured along the sides of craters, pits, and valleys suggested that these formations are relatively young, as they lack geological features such as impact craters or dusty dunes. These images hinted at the possibility that liquid water might have existed in the planet’s recent past—and might still sometimes be present on the planet’s surface. More evidence for this idea emerged a few years later when researchers reported that new, light-colored streaks in the form of fingerlike branches had appeared in some of the gullies, further signaling recent activity.⁷

Subsequent analyses, however, revealed that the streaks could have been produced through other processes. In 2010, based on images from the Mars Reconnaissance Orbiter (MRO), scientists reported that the streaks appeared only during the Martian winters. During that time of year, water stays frozen and dry ice builds up on the planet’s surface, meaning that carbon dioxide, a gas that makes up more than 90 percent of the planet’s atmosphere, may have been the cause.⁸

Sure enough, when Patel and his colleagues tested this hypothesis last year, they found it to be a likely explanation. In the Open University’s Mars Chamber, which simulates the temperature, pressure, and atmospheric composition of the Red Planet, the researchers deposited carbon dioxide frost onto the surface of soil, then warmed the chamber with a heat lamp to mimic what happens when the sun rises. The resulting process of sublimation—where a solid transitions directly into gas—was enough to create very similar formations.⁹ And in another 2016 study, an independent group of researchers reported that data from MRO supplied no evidence of minerals associated with flowing water in those structures.¹⁰

Meanwhile, another feature of the steep Martian slopes, dubbed recurring slope lineae (RSLs), has provided more-tantalizing evidence that the planet could occasionally host liquid water. Unlike gullies, RSLs are dark streaks that appear during the warmest parts of the year, growing in the summer, when ice is most likely to melt, and fading in the winter.¹¹ And although scientists have never directly detected liquid water, it may not take as much of it as some researchers expect to generate these features. In another Mars Chamber experiment, published last year in Nature Geoscience, Patel and colleagues placed a block of ice in the simulated Martian environment and found that a small amount of water, which boiled at much lower temperatures due to low pressure, was able to kick up the soil to create streak-like features.¹² “That showed that if there is water, you need a lot less than originally [thought],” Patel says. Altogether, the presence of liquid H2O on the planet remains up for debate.

Two features of Mars’s surface suggest that water may, at times, flow on the planet. Channels known as gullies (top two images) that appear on steep slopes look comparable to formations created by flowing water on Earth, although recent analyses indicate that these were likely formed by other processes. More recently, researchers have identified recurring slope lineae (RSLs; lower two images), seasonal streaks also suggestive of flowing water. The primary theory, based on the identification of perchlorates, is that RSLs are formed by brine, or very salty water. Where the water would come from is still a mystery, and alternative theories challenge the idea that water is needed to form such structures. For example, some scientists have posited that dry sand avalanches could result in the same streaking pattern.NASA/JPL-CALTECH/UNIV OF ARIZONA

Salty surfaces

The case for contemporary water on Mars has been bolstered by signs of perchlorates, a type of salt, in the seasonal streaks.¹³ Perchlorates lower the freezing point and evaporation rate of water, which would allow H2O to exist as a liquid in Martian conditions. On Earth, perchlorates also act as an energy source for some microorganisms.

“People were getting really excited because they were thinking, well, bacteria can metabolize perchlorates, so perhaps these are potential habitats that we could maybe explore on future missions,” says Jennifer Wadsworth, a PhD student in astrobiology at the University of Edinburgh. “So we thought, okay, well let’s look at perchlorates and see [whether] bacteria could survive under Martian conditions.”

As it turned out, when bathed in UV light, these salts can actually be lethal. When Wadsworth and her advisor exposed the soil bacterium Bacillus subtilis to perchlorates while irradiating the cells with UV levels typical for the Martian surface, the microbes died within minutes.¹⁴ “Perchlorate seems to be quite abundant everywhere, and the radiation penetrates quite a few meters [beneath the planet’s surface], according to models,” Wadsworth says. “So it could mean that the top few meters of soil are in fact uninhabitable.” However, she adds, this finding does not rule out the possibility that there might be extremophiles that could survive these conditions, or that more-conventional microbes live farther underground.

Deep below the surface, UV and ionizing radiation are significantly reduced, while pressure and temperature begin to increase. “You can reach a point where you’re shielded from all the nasty things, and the temperature and pressure could be high enough to allow a habitable environment,” Patel says. “The evidence is piling up that if we want to find these signs of life on Mars, we really need to get down below the surface to get away from nasty oxidants and environmental influences.”

Experiments in the Open University’s Mars chamber, which simulates the environment on the planet, could help determine the conditions that form these geological structures.MANISH PATEL/OU

Curbing contamination

Of course, the most definitive way to confirm life on Mars would be to collect live or previously living specimens. ExoMars, a rover that the European Space Agency plans to send to Mars in 2020, will be equipped with a drill that can extract soil samples from depths down to two meters, the deepest of any Mars sampling to date. The robot’s onboard laboratory will carry out tests on collected specimens. Another upcoming rover expedition, NASA’s Mars 2020, plans to collect samples to set aside for future missions to ferry back to Earth.

Without knowing exactly what life-forms, if any, exist on our red, dusty neighbor, it is difficult to predict what people might encounter when they eventually get there. “How do you look for something that you don’t know [about]?” Patel asks. “It’s a real problem that we face. All we can do is look for what we do know—and even then, it’s incredibly difficult to measure everything.”

Directly probing for life on the Red Planet takes some finesse, as scientists must ensure that they do not accidently misidentify organisms that hitched a ride from Earth as Martian. Although it is not possible to reduce the risk of contamination to zero, researchers can take measures to lower the chances that they will introduce Earthly organisms into their experiments. Curiosity, for example, is barred from exploring the RSLs, due to concerns that the rover, which was not completely sterilized prior to launch, might contaminate the suspected water in those regions.

The evidence is piling up that if we want to find these signs of life on Mars, we really need to get down below the surface to get away from nasty oxidants and environmental influences.—Manish Patel,
 Open University

“Being able to clean [spacecraft] well enough to identify Mars microbes if they might be present and distinguish them from the residual contamination from Earth is an extremely challenging problem,” says Cassie Conley, NASA’s planetary protection officer. Future rovers will be subjected to various sterilization strategies before launch, including wiping down surfaces with sterilizing solutions, baking heat-resistant components at high temperatures, and using highly sensitive biosensors to identify the presence of microbes.

Researchers are also trying to ensure that the human explorers NASA plans to send to Mars by the 2030s do not contaminate the planet—a much more difficult task, as most of the methods used to clean spacecraft cannot be applied to people. “We can be confident about how much contamination we sent on [robots], because we can measure it before launch and be confident that it won’t increase,” Conley says. “Once humans start landing on Mars, there will be associated microbes that come along.”

Monitoring microbial migrants within astronaut communities is also important for managing human health. In a study published earlier this year, Kasthuri Venkateswaran, a senior research scientist at NASA’s Jet Propulsion Laboratory who is involved in the Planetary Protection Program, and colleagues found that after four people spent 30 days in an enclosed habitat that mirrored conditions on the International Space Station, the diversity of certain fungi—including those associated with allergies and asthma—in their surroundings increased.¹⁵ In another recent investigation, researchers reported that bacterial communities in a simulated spacecraft changed after hosting six crew members for 520 days.¹⁶ In this case, cleaning agents were able to keep the microbial populations under control, pointing to the importance of maintaining strict sterilization protocols in space.

Keeping any potential life-forms native to Mars from hitching a ride back to Earth is another concern. Scientists and policy makers want to ensure that samples brought back by rovers or human explorers—or living organisms that accidently hitch a ride—will not endanger species on Earth. Such Mars-to-Earth contamination, Conley says, presents “a much more complicated set of questions about public health and the potential for invasive species.” 

Diana Kwon is a freelance science journalist living in Berlin, Germany.

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