This September, tech mogul Elon Musk unveiled his updated plans for colonizing Mars. By 2024, he said, his aerospace company SpaceX plans to deliver people to our neighboring planet in massive rocket ships, which he hopes to start constructing within the next year. Although perhaps the boldest declaration yet (outside of science fiction) of intent to actually spearhead extraterrestrial habitation, Musk’s ambition reflects an age-old curiosity: Can the Red Planet support life? Has it ever before?
In 1976, NASA’s Viking 1 and 2 set down on Mars with the primary mission of answering those questions. While the two landers discovered no clear signs of living microorganisms on the planet’s barren surface, photographs taken from orbit revealed geological features that suggested a once-watery environment—dry valleys that resembled those created by rivers on Earth. “If you assume that liquid water is all life needs, then this could count as the first evidence that life might have been possible on Mars in the past,” says Alfonso Davila, a research scientist at NASA Ames Research Center in California.
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.”)
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.
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.”
Recently, while examining data collected by the rover Curiosity, a group of researchers discovered boron, a chemical element that can stabilize the sugars used to make RNA (Geophys Res Lett, 44:8739-48, 2017). Some scientists believe that this element may have even contributed to the origin of life on Earth. “Boron, when it’s dissolved in water, has very special properties—it can react with organic molecules to form other types of organic molecules,” says Patrick Gasda, a postdoc at Los Alamos National Laboratory. “We found boron in this area [that used to have] lots of water; if there were organics there, that could actually mean that you could do these types of reactions on Mars.”
Scientists currently only have speculative estimates about when the Red Planet was last amenable to life. For example, NASA researcher Alfonso Davila and his colleagues have proposed that parts of Mars may have been habitable as recently as 5 million to 10 million Earth years ago (Astrobiology, 13:334-53, 2013). They estimate that during that period, the planet was tilted at an angle that may have provided polar regions with enough solar energy to melt the subsurface ice. After completing additional analyses, the researchers also posited that the water composition in the atmosphere during these periods was similar to that seen in the driest parts of the Atacama Desert in Chile, where microbes have been found living in extremely arid soil (Astrobiology, 16:159-68, 2016).
“While this does not necessarily mean that Mars was as habitable as the Atacama during those periods, it does suggest that the habitability window near the surface might have closed not billions of years ago, but perhaps tens of millions to several hundred million years ago,” Davila says. And the current conditions on the planet, while probably not conducive to modern microbial activity, are promising for researchers searching for signs of living organisms in the planet’s history, he adds. “Those same conditions, extreme dryness and extreme cold, that prevent life from being active in the environment are also very good at preserving evidence of life.”
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,
“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.”
NASA hopes to send humans to Mars by the 2030s, and private companies, such as SpaceX, Mars One, and Lockheed Martin, have grand plans to establish human settlements on the planet. But big questions remain about the plausibility and safety of such missions.
People who land on the Red Planet will face harsh conditions, such as frigid temperatures, low pressure, and an atmosphere with precious little oxygen. Micron-size dust particles may also be a major factor, as they could cause respiratory problems and contain toxic materials. In addition, Martian soil contains abundant amounts of perchlorates, a type of salt that can impair the functioning of the human thyroid, which could be hazardous to scientists digging in the dirt.
On the other hand, perchlorates might actually be extremely useful during a mission to the Red Planet. Not only are they a component of rocket fuel, the compounds could also be a source of oxygen for human consumption: many microbes metabolize perchlorates, generating this element as a by-product, and some scientists have proposed prototypes of portable emergency systems that exploit these microbial pathways to generate breathable air (Int J Astrobiol, 12:321-25, 2013).
A much more serious concern about living on Mars is radiation. Without a protective magnetic field like that surrounding the Earth, the surface of the Red Planet is constantly bombarded with galactic cosmic rays—high-energy particles from space that can lead to a variety of health problems. At the doses of cosmic radiation that humans would receive on a trip to the Red Planet, one of the primary problems they will face is cancer. According to analyses by Francis Cucinotta, a radiation biologist at the University of Nevada, Las Vegas, astronauts on the International Space Station can exceed their lifetime limits of radiation, based on NASA’s radiation standards, in just 18 months for women and two years for men (PLOS ONE, 9:e96099, 2014). And radiation levels would likely be even higher on a trip to Mars, which is far beyond the Earth’s protective magnetosphere. (The cancer risk is slightly higher in women because they have the added concerns of breast and ovarian cancer plus a greater risk of developing lung cancer, although the latter association is not well understood, Cucinotta says.)
Rodent experiments have revealed that exposure to radiation akin to that experienced on Mars can lead to an increased risk of cancer in “bystander” cells close to those damaged by radiation, which can release “oncogenic signals” (Sci Rep, 7:1832, 2017). Radiation exposure can also alter the tumor microenvironment in ways that promote cancer. Using mouse models of breast cancer, Mary Helen Barcellos-Hoff, a radiation oncologist at the University of California, San Francisco, and her colleagues discovered that when healthy epithelial cells were transplanted into an animal that had been exposed to Mars-like radiation, tumors developed from those unirradiated cells (Cancer Cell, 19:640-51, 2011). “You create the seed of the cancer with mutations, but they still have to be in the appropriate soil for the cancer to actually develop,” Barcellos-Hoff says. “[We’ve found that] the kind of radiation found in space likely perturbs [the tumor microenvironment] in a more profound way than radiation that’s found on Earth.”
More recently, scientists have amassed evidence suggesting that cosmic radiation may have worrisome effects on the brain. Specifically, Charles Limoli of the University of California, Irvine, and colleagues have shown in animal experiments, mostly with rodents, that these galactic particles can cause deficits in learning and memory, reduce the complexity and density of dendritic spines, and lead to persistent neuroinflammation (Sci Adv, 1:e1400256, 2015; Sci Rep, 6:34774, 2016). “The data suggests that the irradiated brain is never normal,” says Limoli. “Now, how precisely these cognitive deficits will manifest and impact astronaut performance is another important question that’s very difficult to pinpoint.”
While radiation risks are concerning, they are not deal breakers for future Mars travel, Limoli says, and researchers are now working on ways to mitigate these issues. For example, NASA is exploring ways to protect astronauts from radiation with compounds that repair damaged DNA. One such compound is nicotinamide mononucleotide, which scientists recently reported could reverse aging in mice by activating processes involved in DNA repair (Science, 355:1312-17, 2017).
In addition, Limoli and his colleagues are developing drugs that could help alleviate radiation effects in the brain. “We’re working on a variety of pharmacologic interventions,” Limoli says. “[And] we can always hope that our engineering colleagues come up with better and better shielding.”
Diana Kwon is a freelance science journalist living in Berlin, Germany.
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