NASA's announcement that liquid water flows on the surface of Mars has reignited enthusiasm over the possibility of alien life on our planetary neighbor. Liquid water is essential for life to exist here on Earth, and knowing that H20 is flowing on the Red Planet strengthens the odds that extraterrestrial life exists somewhere on Mars, as well. Now more than ever, NASA wants to send probes to the Martian surface to find out for sure.

Any alien life lurking on Mars will most likely be in the form of tiny microorganisms — not complex biological beings like the ones on our planet. That means finding these aliens is going to be tricky; they will probably be quite small and simple, hiding inside soil samples or hard to reach places. They could look like microbes here on Earth, or they may look like nothing we've ever seen before.

To confirm the existence of such creatures, NASA and other research institutions are developing numerous technologies to search for biosignatures on Mars. A biosignature is any substance with a biologic origin — whether that be a one-million-year-old fossil or a living microbe. Some of these bio-hunting instruments will be incorporated into future spacecraft, like NASA's Mars 2020 rover or the European Space Agency's ExoMars Mission. Others are still in development, looking to be incorporated into a future mission to the Red Planet. But despite this abundance of life detection technologies, it's still extremely difficult to determine if something is — or was — alive on another planet. There is no consensus on the best method for detecting life, and numerous hurdles stand in scientists' way.

Bacillus subtilis was one of the strains of bacteria to survive on Curiosity (Riraq25/Wikimedia Commons)

One of the main issues is that most detection methods run the risk of cross-contamination. Any robot that's sent to Mars is always going to have hitchhiking microbes from Earth along for the ride. NASA and other space agencies sterilize their hardware as much as possible before they're sent into space, but microorganisms are resilient. A swab of NASA's Curiosity rover revealed 377 strains of bacteria still on the spacecraft prior to launch. Further analysis showed that 11 percent of these strains can withstand UV exposure, freezing temperatures and pH extremes. So it's likely that these tiny Earthlings survived Curiosity's interstellar voyage and remain on the rover.

That's a problem, because many robotic instruments that look for chemical signatures of life — such as amino acids or carbon — usually require gathering a sample of Martian soil and analyzing it up close. It's very possible that Earth microbes still on a robot can contaminate these samples, making it unclear if detected biosignatures are really coming from Mars. "When we put a sample in there, we want to know all the chemical components have actually come from the thing we’ve extracted them from," said Mary Beth Wilhelm, a research scientist at NASA’s Ames Research Center.

Additionally, NASA and other agencies are limited in where they can search for life, due to something called planetary protection. This stems from the 1967 Outer Space Treaty, which forbids nations from "harmful contamination" of other planets with Earth biology. The treaty established the International Council for Science’s Committee on Space Research (COSPAR) to outline ways to avoid contamination. COSPAR specifically forbids researchers from exploring "Special Regions" on planets — areas that have a "high potential for the existence of extant Martian life." For example, places on Mars that might have flowing water are probably off limits for exploration; they are more likely to have microbial life, and the presence of water makes it a potential breeding ground for contamination by Earth microbes.

ESA's ExoMars Rover will search for signs of life on Mars. (ESA)

That's why many life detection instruments try to analyze a sample's chemical components from afar. One such technique is Raman spectroscopy; it involves shining a low laser light on a sample of soil that's more than 30 feet away. The laser excites the materials within the sample, causing their molecular bonds to vibrate. Organic bonds like those of carbon vibrate at different frequencies than other bonds, allowing researchers to determine if carbon is present in the sample. A Raman spectrometer is one of the many instruments that will be included on the Mars 2020 rover and the ExoMars Rover.

"The whole idea is to do this sort of stand-off analysis — drive near to something and then analyze it before it gets too close." Alison Olcott Marshall, an expert on Raman spectroscopy at the University of Kansas. "There's potential that winds could blow things off the rover, but it mostly removes the issue of contamination."

Raman spectroscopy is also adept at distinguishing between organic compounds that were once alive and those that are living now. Living organisms usually contain diverse compounds of pigment, which give off different spectral signatures when hit by the laser. However, it's harder for Raman spectroscopy to distinguish between fossils and carbon-based materials that were never alive. There's also the issue of sensitivity. When examining samples from far away, it's harder to guarantee the results than if the samples were examined up close. "I wouldn’t take a Raman as a definitive tool for life," said Craig Marshall, Alison's husband and research partner at the University of Kansas. "It's very good to look at mineral composition and good to look at contemporary microbes. But it’s more of a screening technique."

It makes gathering direct samples a more attractive — and more definitive — method. Gary Ruvkun, a professor of genetics at Harvard Medical School, maintains if we really want to find aliens on Mars we're going to have to get over our fears of contamination and explore the so-called special regions. "Every interesting place on Mars is completely protected," said Ruvkun."I say let's protect half of them, but let us have the other half of them to explore." Ruvkin and his teams at MIT and Harvard are working on the Search for Extra-Terrestrial Genomes (SETG). The goal of their technology is to search for the true building blocks of life: DNA. "Looking for DNA is almost the standard way to look for life in extreme environments on Earth. If you go to some Arctic lake and want to know what’s there, the standard is to filter material, extract DNA, and do deep genome sequencing."

The microchip inside the SETG device that will be used to analyze DNA (Gary Ruvkun)

The method involves bringing a microchip sequencer to Mars that would thread soil samples through a minuscule hole known as a nanopore. These pores are only big enough for DNA to pass through, so if any genetic material is on the Martian surface, the sequencer will pick it up. The instrument can then order any sequences of DNA it finds into a genetic tree to see if it resembles creatures on Earth. If not, then it's possible the genomes are coming from alien life.

SETG stems from the idea that there has been some sort of exchange of living matter between Mars and Earth. Many scientists believe that the building blocks of life were delivered to Earth by meteorites, creating a primordial soup for the first microbes to form. If that's the case, it's possible these same meteorites hit Mars, depositing similar organic materials, as well.

But if Martian life doesn't have similar biological origins, the SETG method — and other life detection techniques — will fail to work. "If there ever was life on Mars or if there is life on Mars, it may not look like terrestrial life as we know it," said NASA's Wilhelm. "All we can do is plan for the best and hope that it uses the same chemical signatures." So truly, no one knows the best way to look for life, because it could look like something we've never studied before.

Perhaps the best way to know what lies in the Martian soil is to do a Mars sample return. It's one of the top priorities laid out in the Planetary Science Decadal Survey, a report made by the United States National Research Council to help guide NASA's planetary exploration initiatives. Returning a sample of Mars soil to Earth would be monumental, allowing researchers to use all the laboratory tools at their disposal to uncover what's in the dirt. But just getting a tiny gram off Mars is a huge undertaking, requiring extra fuel and an additional rocket to transport the sample back to Earth. "Returning a gram of dirt from Mars won’t happen for at least 20 years," said Ruvkin.

So for now, we're stuck trying to find life on Mars remotely. That is, until we send astronauts to Mars in the 2030s to gather samples themselves — which may make things even trickier. Humans carry with them a diverse microbiome of more than 100 trillion microbes, opening up the potential for even more contamination. That means no matter how we explore Mars, it'll be hard to know whether we've found alien life or just the bacteria we brought with us.

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