Astronomers have learned a great many things about Mars since the first probes landed there nearly four decades ago.
We know that liquid water once flowed across its surface and that Mars and Earth were similar in their early history. When life on Earth arose, some 3.5 billion years ago, Mars was warmer than it is today and had liquid oceans, an active magnetic field and a thicker atmosphere. Given the similarity between the two planets, it seems reasonable to think that whatever steps led to life on Earth could also have occurred on Mars.
In fact, for all we know, microscopic life might still exist on the Red Planet. Every mission to our neighbor in the past 35 years has examined its geology, not its biology. Only the twin Viking 1 and 2 spacecraft, which touched down in 1976, conducted the first and thus far only search for life on another world. Each spacecraft carried four experiments relevant to the search, and each experiment returned ambiguous data. The Viking missions gave us puzzles, not answers. Yet we now know that Viking's methods would not have been able to find life on Mars even if it were there—which means the question of whether the planet harbors life remains open.
Fortunately, in the intervening decades microbiologists have developed a cornucopia of tools for detecting microorganisms. These tools are now unexceptional here on Earth. But if applied by one of the several missions soon expected to head for Mars, they could deliver a first: a definitive answer as to whether our closest neighbor also pulses with life.
The First Search
The Viking experiments looked for life using standard search techniques of the time. In the initial experiment, the lander took a scoop of Martian soil and added carbon compounds as food for any microorganisms that might be in the soil. If microbes did exist in the soil, we might expect them to consume the food and release carbon dioxide.
In fact, the Viking missions did see this behavior. On its own, the test would seem to indicate that microorganisms were present in the Martian soil. When combined with the results of the other experiments, however, researchers could not be sure.
The second experiment looked for evidence of photosynthesis but returned inconclusive results. A third experiment added water into a soil sample. If life were present, the moist soil might have produced carbon dioxide. Instead it produced oxygen. This was very strange, as no known soil on Earth does this. Scientists concluded that the oxygen came from a chemical reaction.
In the final experiment, the landers searched for organic compounds in the soil. Organics are carbon-containing compounds that form the building blocks of life. If any life existed on Mars, we would expect to find these compounds. Yet organics alone would not provide definitive evidence of life, because we also expect meteorites to continuously deposit organic compounds on Mars. Puzzlingly, the experiment found no evidence of organics whatsoever.
Taken together, the findings left investigators stumped. Most scientists believed that chemical reactions were responsible for the results in the last two experiments, but chemistry could not quite explain the first. A small but vocal minority of Mars scientists held that the first experiment did, in fact, find evidence of life. But most everyone else concluded that Mars was barren.
In 2008, 32 years after Viking landed, the solution to these puzzles began to emerge when NASA's Phoenix lander touched down in the northern polar region of Mars. To everyone's surprise, Phoenix detected perchlorate, a rare molecule on Earth that features four oxygen atoms connected to a chlorine ion, which are connected to a magnesium or calcium ion. When perchlorate salts reach 350 degrees Celsius, they decompose, releasing reactive oxygen and chlorine. Perchlorates are so reactive that they are used in many rocket fuels.
This finding made investigators see that the perchlorates could well have obliterated signs of life in the soil. Viking's organic search experiment first heated the soil sample to 500 degrees C so that it might vaporize any organic molecules and detect them in gaseous form. But in 2010 a team led by Rafael Navarro-González of the National Autonomous University of Mexico, which included one of us (McKay), showed that perchlorate would have completely destroyed any carbon compounds in the soil during the heating process.
Perchlorate also illuminates the puzzles of the first and third experiments. In the first experiment, adding food to the soil generated carbon dioxide. But perchlorate produces bleachlike compounds when exposed to cosmic rays. These compounds can decompose organic molecules (such as those found in the added food), producing carbon dioxide in the process. In the third experiment, oxygen emerged from moistened soil. The perchlorate bleach production also forms oxygen, yet the oxygen remains initially trapped in the soil. It is released only later, once the soil is wetted, as happened on Viking. Two mysteries solved.
Yet hope for the discovery of life is not lost. The Curiosity rover landed on Mars in 2012 and has been taking samples of the soil ever since. In early 2014 the Sample Analysis at Mars (SAM) instrument team (which includes McKay), led by Paul Mahaffy of NASA's Goddard Space Flight Center, reported that the experiment found carbon compounds in ancient mudstone sediments on the bottom of Gale Crater, even in the presence of perchlorate. Later, in 2015, the SAM team reported the presence of chlorobenzene in one of the Martian samples. Complex organics exist on Mars—Viking was just unable to find them. Could the same be true for life itself?
In the 40 years since the Viking landers were built, microbiology technology has changed dramatically. The Viking missions used culture-based methods, in which microorganisms grow in petri dishes. But these are no longer considered definitive, and we now know that only a small fraction of soil microbes can be cultured. Scientists have developed vastly more sensitive techniques that directly detect the biomolecules in microbial life-forms. These new methods provide the basis for a novel way to search for evidence of life on Mars.
The most widely known method is DNA detection and sequencing. No longer is it necessary to culture an organism so that it will replicate sufficiently to provide enough DNA for sequencing. Several teams are working on ways to incorporate DNA-extraction technologies into instruments suitable for upcoming Mars missions.
One drawback of relying on DNA detection to reveal life on Mars is that although DNA is common to all life on Earth, it may not occur in alien life. Or if it is present, it may be so different that DNA detectors built to find Earth biology will miss it.
Fortunately, Mars could harbor other signs of life. Among these biomarkers are proteins and polysaccharides. Proteins are long, linear chains composed of various mixtures of the 20 different kinds of amino acids used by life. Amino acids are present in meteorites and are likely to have been a common component of the prebiotic environment on any world. Polysaccharides are long chains of sugars constructed by enzymes (biological catalysts), which themselves are proteins.
Detecting molecules as complex as a protein or a polysaccharide would be strong evidence of life, widely defined: a biological system that encodes information and uses this information to build complex molecules. These complex molecules would stand out against any background of simple prebiotic molecules like a skyscraper would stand out against a field of boulders.
One of us (Parro García) has been developing an instrument for detecting such complex molecules on Mars. It is based on a technique—immunoassay testing—already in use for simultaneously detecting hundreds of different types of proteins, polysaccharides and other biomolecules (including DNA itself).
Immunoassay tests employ antibodies—Y-shaped proteins—each of which binds to just one type of biomolecule [see graphic above]. In an immunoassay test, a solution that might contain substances of interest is poured over a large array of antibodies, each one designed to bind to a specific target. If the sample solution contains a biomolecule that links to an antibody in the array, the antibody will capture and, by binding, identify it.
One nice feature of immunoassays is that antibodies can detect molecules that are smaller and less complex than full proteins are. The test can thus search for molecules that are life-related but of lesser complexity, such as fragments of proteins that have broken into bits. Finding these bits would also imply that life exists.
All of Earth's organisms collectively contain many millions of different proteins. With so many to choose from, how do we pick the few hundred that a single immunoassay test could search for? The short answer is that we cannot know for sure. But we can make educated guesses based on two strategies: First, we could search for proteins that would be useful or essential to survival on Mars. For example, we might search for enzymes that consume perchlorate, cold-adapted enzymes that would allow a microorganism to survive Mars's frigid temperatures or enzymes that would repair damage to DNA caused by Mars's strong ionizing radiation. Second, we could target molecules that are ubiquitous throughout the microbial world, such as peptidoglycan, which is a universal component of all bacterial cell walls, or adenosine triphosphate (ATP), which is used by all living organisms on Earth to transport chemical energy for metabolic activity.
Even if Mars's harsh environment has destroyed large molecules such as DNA and proteins, we might still find evidence for life in the debris. Many types of molecules are chemically equivalent to one another but may have opposite “chirality”—their bonds may twist to the left or the right. Life on Earth is dominated by left-handed amino acids. If an experiment detects amino acids and finds a particular set that has a dominant left- or right-handed chirality, this would be compelling evidence for the presence of life. Interestingly, if that chirality were right-handed—the opposite of Earth proteins—it would be evidence that the life-forms on Mars evolved independently from life on Earth.
Viking carried three biology experiments; we might imagine a mission to Mars that also carries three biomarker search instruments—perhaps a DNA detector, an immunoassay microchip, and an instrument to detect and characterize amino acids. The technology is nearly ready. The next task is to pick a target—the location that holds the best chance of harboring biomarkers.
Ice and salt are friends to biomarkers, protecting them from damage and decay. The enemies? Ionizing radiation and heat. Fortunately, the low temperatures on Mars make thermal decay negligible even over the age of the planet. Ionizing radiation, however, could completely destroy biomarkers that are within the first meter or so of the surface over a few billion years. The promising targets, then, are icy sites that may have harbored recent life—such as the Phoenix landing site near Mars's north pole—or sites where erosion has recently exposed the ancient material. In either case, one would want to drill down to extract samples from a meter or more below the surface.
The missions to Mars that are now being planned could conduct this search. The European ExoMars mission, slated for 2020, should be able to carry a drill. NASA is working on a mission that will launch another copy of the Curiosity rover in 2020. ExoMars and the new Curiosity could search the dry equatorial regions of Mars for biomarkers in salt and sedimentary deposits. (Neither rover can function in the polar regions.)
As for a polar search, NASA is studying an inexpensive lander called Icebreaker that could do the job. Equipped with a one-meter drill and an immunoassay instrument, it could search the water-rich northern permafrost of Mars for biomarkers in the ice-cemented ground.
Any one of these missions would be a worthy candidate to lead the next era of Mars exploration. The past few decades of research have left no doubt that Mars once harbored liquid water. The time has now come to test whether that once watery planet provided a home to any life-forms. If we find biological molecules on Mars—and especially if those molecules indicate that Martian life arose independently of Earth life—we will gain a profound insight into life beyond our home. Just as we know that there are many stars and many planets, we will know that there are many biologies. We will know that the universe is alive with diversity.