chasing martians
Could life exist on Mars today?
Certain conditions must hold true in order for chemically based life to develop and take root. The environment almost certainly must have liquid water and a source of energy. Mars has ancient, dried-up river valleys, deltas, and lakelike formations, and the Sun shines brightly on Mars. Check.
The atmosphere and soil must supply a handful of bio-essential elements: carbon, oxygen, nitrogen, hydrogen, phosphorous, and sulfur. Mars has an abundant supply of all of these elements. Check.
The environment must maintain these conditions for a long enough time for life to develop. Tantalizing evidence, summarized in 2016 by Ray Arvidson, the James S. McDonnell Distinguished University Professor of Earth and Planetary Sciences at Washington University in Saint Louis, reveals multiple time periods when Mars was warm enough and wet enough for long enough to support life.1 The Opportunity rover explored the ancient (4-billion-year-old) remnants of the 14-mile (22-kilometer) wide Endeavor Crater and found, he writes, that “the formation of this crater generated an extensive and long-lived hydrothermal system . . . that would have produced a relatively habitable subsurface environment, at least in terms of sustained availability of water.” Opportunity also explored the 3–4-billion-year-old Burns Formation Outcrops, which show multiple signatures (e.g., ripple patterns and sedimentary rocks) that are evidence for the long-term presence of surface and subsurface water. Arvidson explains that “the wet surface environments, if on Earth, would have been habitable, if only for microorganisms adapted to acidity and long periods of aridity. . . . The subsurface groundwater . . . would have had more clement conditions and if on Earth surely would have been habitable.” Arvidson explains that the Spirit rover explored a region of rolling hills surrounding an eroded, volcanic region known as Home Plate, a region dated to be no older than 3.7 billion years. “On Earth all of the aqueous environments around Home Plate would be habitable and may have been for Mars.” The Curiosity rover explored a region of deposits inside the 93-mile (150-kilometer) wide Gale Crater, including the region of sedimentary strata around Mount Sharp, the mountain at the center of this large crater where some of the rocks are as young as 2.9 billion years. Curiosity’s measurements reveal that Gale Crater was once an enormous lake where mudstones now overlay sandstones. The Gale Crater region deposits formed at a time with “significant surface runoff and associated erosion and transport of sediment.” The Curiosity data “point to environments conducive to preservation of organic molecules and ones that were habitable, at least in terms of sustained presence of water.” Given that “all of these environments would likely have been habitable, at least in terms of the availability of water,” we know that many regions on and under the surface of Mars were habitable, if not inhabited, for at least the first 1.5 billion years of Martian history. Check.
No definitive evidence for the presence of past or present life on Mars has yet been discovered. We are still waiting for that proof or for clear and strong evidence that says Mars is and has always been barren. But without any doubt, Mars was once hospitable to life; it was a planet on which Earthlike forms of life might have flourished. The combination of the rover discoveries with the possible (though not convincing) evidence of ancient, or even present, Martian life as seen in ALH 84001, and with the possible (though not convincing) measurements of fluctuating atmospheric methane abundances that might be at levels above that which could be produced without biological activity demonstrate that we absolutely cannot definitively deny that life might have existed and might still exist on Mars. We are still chasing Martians.
Indeed, the idea that Mars once had life is not far-fetched, and if Mars once hosted living things, those living things could have adapted to the changing Martian climate and found a way to survive all the way until the present day. We can very reasonably conclude that the idea that Mars might host colonies of subsurface microscopic life-forms is plausible. Therefore, the most important questions about Mars today are, Did the known habitable regions have the “bioavailable compounds” and sources of energy necessary to sustain life? and, Does any form of life exist on Mars now?
If we have Martian neighbors, they have been teasing and testing us for 400 years. Once the age of telescopes arrived, we discovered what some astronomers thought were clues to the presence of Martians in the form of the canals stretching across the surface. Yes, the evidence was indirect. No, we didn’t see any Martians, but the apparent evidence was hard to miss and even harder to ignore, at least to those who actually thought the lines on the surface of Mars were canals and believed in Martians. Of course, those surface markings were never as long or as straight as Giovanni Schiaparelli and Percival Lowell thought they were, and Mars never had any artificial canals. No evidence of life.
In the twentieth century, astronomers’ certainty as to exactly what life-forms they might find on Mars changed. They developed a strong consensus that Lowell was wrong and that Mars did not host intelligent, canal-building engineers, but perhaps Lowell and others were right about Mars having vast forests or large swaths of quick-growing vegetation. Some astronomers then found alleged evidence for vegetation on Mars via the presence of chlorophyll, or at least via the presumed effect of chlorophyll on light reflected from Mars. Disappointment returned again when other astronomers showed that Mars was never green and that the reflected light from Mars was inconsistent with the presence of chlorophyll. No chlorophyll; thus, no evidence of life.
A few years after astronomers lost their enthusiasm for chlorophyll, they claimed decisive spectroscopic evidence first for lichens and then for algae, both of which are more primitive life-forms than photosynthesizing plants. But the spectroscopic evidence was misunderstood and misinterpreted. No lichens. No algae. No evidence of life.
Mars today is a tough environment for life. It is cold, with liquid water on the surface only rarely, if ever. The atmosphere is thin and provides little protection from high-energy photons and particles. If any Martian life-forms that once thrived have survived, they must be hardy but also hard to spot. Modern Martians might all be microscopic; in addition, because the Martian atmosphere cannot protect them from the Sun’s deadly ultraviolet rays, they might need to hide under rocks or burrow deep underground in order to secure the warmth and water necessary for their survival and to protect themselves from dangerous radiation from space.
Even subsurface, microscopic Martians, however, need to inhale and exhale. The chemical waste products from their respiratory activities should build up in the soil and eventually, slowly but surely, seep into the atmosphere. Even at the level of one biological tracer molecule out of every billion other nonbiological molecules in the atmosphere of Mars, we should be able to sniff out the presence of those biologically produced molecules. Even at the detectability threshold of only a few parts in a trillion, we have developed the ability to ferret out the evidence of the effluence of microscopic Martians with the high-tech sniffers and probes we have sent and continue to send to Mars on our landers and rovers.
The atmospheric methane gas astronomers thought they had detected offered evidence to anyone who understood the details of that kind of data that Martian life-forms were affecting the chemical makeup of their atmosphere by inhaling and exhaling. Using cameras on our rockets to and landers on Mars, we have not seen any macroscopic creatures, Carl Sagan’s hypothesized Martian macrobes, wandering around on the surface of Mars, but a high level of atmospheric methane appears to demand microscopic biological activity as the source. If Mars does have substantial and changing levels of atmospheric methane, the Martians may have been found.
As it turns out, the first and second and third “discoveries” of methane were likely all wrong. Almost certainly, none of these were actual detections of methane. But, like a phoenix rising from the ashes, claims for definitive discoveries of methane in the atmosphere on Mars have returned again and again. Did Krasnopolsky and his collaborators detect low levels of methane in 1988? In 2003? In 2006? Maybe. Did Mumma and his collaborators detect low levels of methane in 2003 and 2006 and 2009? Maybe. Did the Mars Express team detect low levels of methane in 2004? Maybe.
Are the most recent measurements from the Curiosity rover that purportedly are evidence for a slightly enhanced level of methane in the Martian atmosphere over a brief, two-month period indisputably detections of methane? Yes. Are the arguments identifying methanogenic, subsurface bacteria as the source of this methane indisputable? No. Can this evidence be ignored? No way.
Is the actual steady-state level of methane in the Martian atmosphere, that being the amount found in between methane hiccups (if those hiccups are real), in the range of 0.1 to 1.0 parts per billion, as detected by Curiosity? Probably. Is this amount of atmospheric methane consistent with the continuous input of organics from meteorites, the inorganic production of methane through serpentinization, and with the several-centuries-long photochemical lifetime of methane in the Martian atmosphere? Yes. The Martians have not been found, and maybe they are not there to be found. We simply don’t know, yet.
Perhaps Mars used to have life. Some scientists are convinced that our Martian neighbors sent us evidence, in the form of a meteorite rocketed off the surface of Mars, that says, “we slept here.” After orbiting the Sun for about seventeen million years, that meteorite landed on Earth, in Antarctica. Thirteen thousand years later, we found it and sent it to one of our geochemical laboratories already equipped to study moon rocks and to find tiny traces of rare elements in meteorites. In this Martian meteorite, our Martian neighbors did not simply send us indirect evidence, from which we could infer their presence. No, they offered certainty by sending to us what might be self-portraits in the form of fossils. Incredibly, we now have fossil evidence of ancient life on Mars, provided you believe these are actually fossils. After two decades of passionate debate based entirely on new scientific evidence that continues to emerge, all of the fossil evidence has now been called into question. Almost all scientists think the rod-shaped fossils of “bacteria” are merely interestingly shaped, nonbiological minerals. Is all the evidence for life found in the meteorite known as ALH 84001 absolutely, indisputably wrong? No. Most of these claims have withered under intense scientific scrutiny, but a very small possibility exists that some of the evidence, in particular the presence of tiny magnetite grains, could be evidence of life. The chances that the clues in ALH 84001 point to ancient life on Mars are slim, but they are not nonexistent. Possibly ancient Martians have been found.
The Outer Space Treaty, adopted by the United Nations in 1967, includes the following principles, among others: “The exploration and use of outer space, including the moon and other celestial bodies, shall be carried out for the benefit and in the interests of all countries” and states shall “conduct exploration of them so as to avoid their harmful contamination.”2 We might ask, today, whether we know enough about Mars to be certain that current plans to send humans to Mars are consistent with these principles in the Outer Space Treaty. If the possibility exists that any life exists on Mars that is native to Mars, might it be in the interests of all countries to avoid further contamination of Mars until we can answer, with great certainty, the question, Did or does life exist on Mars?
Today, NASA’s Curiosity rover continues to probe the atmosphere of Mars, testing for Martian methane and identifying locations on Mars where life might once have flourished and might still exist. New robotic spacecraft are under construction or on drawing boards that could explore these locales and look for signs of life. Telescopes continue to peer down from Martian orbit, looking for more clues. Tomorrow, next week, next year, or next decade we may finally discover that we are not alone in the universe and that Mars, our close planetary neighbor, has secrets it can no longer keep.
Chasing Martians is very much a We-the-People activity. We the People believe in Martians. Sometimes We the People are the Martians, as was astronaut Mark Watney in the 2015 movie The Martian and as are the First Hundred and their descendants in the Red Mars, Green Mars, Blue Mars trilogy written in the 1990s by Kim Stanley Robinson. Sometimes we are descended from ancient alien Martians, as we all are in the 2000 movie Mission to Mars. Sometimes we find little Martian plants, as did the astronauts in the 2016 National Geographic mini-series Mars. Other times, when we first venture to Mars, we experience a dangerous encounter with alien Martians, as do the astronauts in Greg Bear’s short story Martian Ricorso (1976).
We the People want and expect scientists to find life on Mars. Some of us think we are looking to find a pathway toward our future, our destiny, because the existence of life on Mars, now or in the past, would make the possibility that humans will colonize and survive on Mars seem more likely. These are good motivators for further study of Mars. They also provide good reasons for humanity to voluntarily delay our collective ambitions to colonize Mars and give the scientific community enough time to learn, to a high degree of certainty, whether Mars is and always has been sterile. If Mars is barren, we would no longer have any reason to restrain ourselves in colonizing and attempting to terraform Mars; however, if Mars hosts life, then humanity’s future relationship with Mars becomes more complicated.
In the purest sense, we have already contaminated Mars. The Committee on Space Research of the International Council on Science (COSPAR),3 which was founded in 1958, established rules that required spacecraft headed to the surface of Mars to be sterilized. After the Viking missions revealed no apparent evidence of life and uncovered a Martian world that appeared inhospitable to life, COSPAR changed the rules for Mars. Today, Martian landers must achieve a high level of cleanliness, but they need not be sterilized. Yes, modern Martian landers are assembled in “clean” rooms; yes, Martian landers are likely nearly sterile. But they are not sterile. Some terrestrial bacteria have already hitched rides to Mars on the exposed, outer surfaces of spacecraft where most of them will die from exposure to solar ultraviolet radiation before arriving at Mars. Others, however, will reach Mars as stowaways inside spacecraft where they might survive for thousands of years. NASA’s Chris McKay estimated, in 2007, that the Exploration rovers, Spirit and Opportunity, each were host to as many as 100,000 “microscopic Earthlings.”4 McKay pointed out that the first Martian colonists from Earth arrived on July 4, 1997, when NASA’s Pathfinder rover touched down on Mars. McKay also argued that these terrestrial visitors are “incapable of growing and spreading. They cannot grow because there is no liquid water, and they cannot spread, because once released into the environment, they are rapidly killed by the Martian ultraviolet light.” If we are lucky, McKay is right. Even if McKay is wrong, we can still safely assume that virtually the entire Martian surface remains uncontaminated by terrestrial life-forms. Meanwhile, despite this minor level of contamination, we should allow the scientists in the laboratories studying meteorites, the astronomers on mountaintops collecting data with their telescopes, and the engineers in machine shops who are building new rovers and detectors to strap into rockets and send to Mars, to continue their work and find answers before we (further and irreparably) contaminate Mars.
Mars may be our destiny. But not yet. We can and should continue to study and explore Mars, but for now we do not need to contaminate Mars any more than necessary.
If any chance remains that Earth is not the only world in our solar system with active biology, that another world might have developed life independently of Earth, if life-forms of Mars may once have flourished but now lie low, we need to preserve our chances of confirming this possibility before it is too late.
By chasing Martians, we have already learned a great deal, even if we don’t yet know whether Martians exist. Percival Lowell, motivated by his study of the supposed Martian canals, built Lowell Observatory and that is where, in 1930, Clyde Tombaugh discovered Pluto. Infrared astronomy developed rapidly in the 1950s, at least in part thanks to the interests of the pioneers Gerard Kuiper and William Sinton, who wanted to push infrared astronomy forward so that they could study and better understand the colors of Mars. Dozens of scientists, inspired by the possibility of an extreme form of life in ALH 84001, have discovered a dizzying array of creatures known as extremophiles that live in extreme environments here on Earth, including thermophiles that live in very high-temperature environments, halophiles that live where salt concentrations are excessively high, and acidophiles that thrive in high-acid locations. Planetary scientists, motivated to understand whether life could exist in the Martian environment, have identified Europa, Enceladus, and Titan as places in the solar system where life might exist independent of sunlight and solar heating. Scientists don’t always discover what they are looking for when they design their experiments, but once an experiment is under way they almost always discover things worth knowing. Our pursuit of Martians is a century-long example of curiosity-driven science leading to important discoveries.
On the critical question of whether life exists on Mars, the jury is still out. We have to answer the questions, “Does Mars now or did Mars ever have life?” and “What is Martian life like?” before we destroy the evidence and before we decide whether colonizing and terraforming Mars is an activity we humans should undertake. Until and unless the preponderance of the evidence we are able to collect, using remote and robotic tools, indicates Mars is sterile, we should heed Carl Sagan’s admonition: “If there is life on Mars, . . . Mars then belongs to the Martians, even if the Martians are only Microbes.”5