For over 100 years, one of the greatest mysteries in Solar System Science has remained: Is there life on Mars? While several tantalizing results hint at the possibility of microbial life, no incontrovertible evidence exists. While we can't answer this profound question definitively, modern evidence is at least making it clear life could exist in the Martian environment. Most astrobiologists agree that for life to exist a liquid must be present to catalyze reactions. We now know that frozen water exists on the Martian polar caps, and we believe that in the past, surface flooding carved many of the chasms and canyons that cut across the Martian surface. If ancient Mars had liquid water, did life arise there? If not, why not? If it did, how advanced did it get? Where is it today? Did it become extinct? Or is it somewhere out of sight? The upcoming Mars Science Laboratory, Curiosity, will seek to find answers to some of these questions, and will build on what we've learned through past missions.
By the time the Viking landers were built in the 1970s, scientists suspected that Martian life might exist not as large organisms, but as microscopic bacteria or other simple organisms in the soil. To test this idea, both Viking landers contained equipment to make chemical tests of the soil. The first test looked for organic molecules in the soil. If there is life on Mars, then living or dead organisms should create a residue of organic molecules in the soil, as on Earth. None were found, to an accuracy of a few parts per billion. In other words, the Martian soil is sterile.
Such an emphatic result might seem to rule out any life on the planet. However, Viking scientists had another trick up their sleeves. They included three more experiments on each lander that scooped up soil, —fed— it nutrients, and watched for signs of metabolic activity (such as we see with plants growing and releasing carbon dioxide). All three experiments detected some unexpected chemical activity! However, careful analysis indicated it was probably not from living organisms, but from unusual chemical conditions in the soil that relate to an interesting fact: there is no ozone layer on Mars. This means that strong ultraviolet light from the Sun reaches the surface and destroys any exposed organic molecules. (Remember that the Earth is protected by its ozone layer.)
Ultraviolet radiation can thus explain the unusual results of both sets of experiments. On one hand, the UV would have destroyed any organics in the soils, potentially causing a false negative on experiments seeking evidence of life in the soil. On the other hand, this UV also creates unusual soil conditions that may help create the unusual oxides that caused the chemical reactions in the Viking soil experiments, thus creating a false positive. Put together, these results suggest that there is probably no life in the particular soils at the Viking sites, but this result is not entirely conclusive.
Some scientists have suggested that the soil within a meter or so of the surface might not be the best place to look for life on Mars. Annual storms stir up the surface dust and blow it around on Mars, effectively sterilizing the whole surface layer of the planet. A similar process occurs in Antarctica, where icy winds sterilize the soils. There, tiny life forms live not in the inhospitable soil, but in fractures inside rocks. The rocks offer an environment protected from the environmental extremes of the exposed surface. Thus, in the years after the Viking experiments, scientists speculated that life & mash or life’s remnants — might be found hidden in subsurface layers or other protected environments of Mars as well.
The story of the search for life on Mars took an exciting turn in 1996 when we got to look inside a rock. NASA scientists in Houston were studying one of the oldest Martian meteorites, a Martian rock more than 4 billion years old. This rock had deposits of carbonates inside fractures in the rock, where liquid water had percolated. The carbonate contained concentrations of organic molecules as well as certain minerals that are produced on Earth by microbes that thrive in oxygen-poor environments. Were these features produced by non-biological chemical processes on Mars, or could they have been caused by forms of life? The answers are still elusive.
The possible discovery of past life on Mars electrified the scientific community and it quickly became one of the biggest news stories of the year. Carl Sagan once said on the subject of life beyond Earth, "extraordinary claims require extraordinary evidence." Claims of UFOs as alien visitations are not supported by this level of evidence. How does the claim for ancient Martian life stack up? The bottom line is a verdict of "not proven."
One issue is the origin of the Mars rock, called AHL84001. Although the odyssey of this rock from Mars to the Antarctic icecap seems extraordinary, we know that the gas trapped within the rock does not match the atmosphere of our planet. There is almost no doubt that the rock is from Mars. Another concern is the origin of the organic compounds in the rock. Perhaps they reflect contamination of the rock by terrestrial chemicals, which seeped in after the meteorite arrived on Earth. The chemicals are not generally concentrated toward the surface of the rock, as usually results from outside contamination. However, some of the organic material may represent terrestrial contamination. So it remains likely that some of the organic material came from Mars. There is also concern about the experimental methods used to study the meteorite.
The chemical evidence for life is unconvincing to many planetary scientists. The NASA researchers also found curious microscopic structures, and suggested that these may be fossils of actual Martian microbes. In the view of these researchers, microbial life may have evolved on Mars billions of years ago and perhaps died out later or persisted in primitive form in hidden sites on Mars. Other teams have found organic concentrations in at least one other Martian meteorite. It sounds good, so why do many planetary scientists consider the evidence for ancient Martian life still controversial? The answer is that the chemical evidence does not point uniquely to life.
Part of the concern stems from the difficulty of interpreting the chemical evidence. There are many complex chemical reactions that do not involve living organisms. So it is difficult to prove that chemical traces were caused by a living metabolism. Another concern is the interpretation of the —fossil— structures. They are much smaller than normal terrestrial bacteria, although they are similar in size to bacteria that exist in underground or nutrient-poor environments on Earth. Yet the evidence is ambiguous. The picture does not show the telltale signs of cell walls. Moreover, some scientists have argued that traces like these —fossil— forms are left when a rock is subject to extreme forces and sudden heating — which must have occurred when the rock was blasted off the surface of Mars.
The issue of looking for life on Mars illustrates how science works. While trying to unravel the mystery of AHL84001, there are conclusions we can reach with near certainty (the fact that the rock is from Mars), and conclusions we can not reach with much certainty at all (the fact that microbial life existed in the rock). Most of the key evidence — the chemical traces, the "fossil" forms — has more than one possible explanation. Other future tests will involve better chemical and isotopic analysis of the composition of the organic molecules. These tests might show more conclusively whether or not the chemical traces were created by living matter. Many research groups are now working with fragments of the few Mars rocks we have. We may never be sure until we go back to Mars for more evidence.
As if to egg us on and encourage our return, Mars keeps offering us new and inconclusive data. In 2008, the Mars Phoenix lander found evidence of water ice on Mars, something needed for life as we understand it on Earth. Since then orbiter has detected vast hidden glaciers and some of these ice deopits could have liquid water underneath. In 2009, methane was found in the Martian atmosphere. In general, methane only comes from either volcanic/geologic activity (not known to exist on Mars) or from life (on Earth, cows are one of the biggest producers of methane!). Martian methane is at a low level, just a few parts per billion and hundreds of time less abundant than it is on Earth, so it may have a geological origin. But the methane concentration varies with time, and that may be difficult to explain purely with geological processes.
What has humanity learned about the question of life on Mars? First, the theories of a century ago have been ruled out. There are no Martian civilizations, nor are there any large animals or plants. Second, although biological evolution has produced incredible adaptations to a range of conditions on Earth — from the dark, cold sea floor to hot geothermal pools in Yellowstone Park — it apparently was not able to evolve advanced forms under the surface conditions on Mars. On the other hand, during the more moist early history of Mars, biochemical evolution may have produced microbial Martian life, which may have died out since then. The most exciting possibility is that life could still exist in a primitive form in water that exists under pressure just under the Martian surface. Most of the research on Mars over the next few decades will be aimed at confirming or refuting this hypothesis. If it is confirmed, we will have exciting proof that life can evolve on other worlds, and that life on Earth may not be unique.
In the late 1980s, President George Bush announced an ambitious goal for NASA to send humans to Mars in the next few decades. The Soviet Union, too, with its large space station, seemed to be inching toward human exploration of Mars. However, these plans were never well funded, and they languished with the collapse of the Soviet Union. In the 1990s, President Clinton determined that no major effort to land humans on Mars would be pursued for the next decade. Sending astronauts to Mars would be a formidable and dangerous adventure, costing more than a hundred billion dollars. Compared to the Moon voyages, it is hundreds of times further to travel and would take years instead of weeks. Robotic space probes might not capture the public imagination the way an astronaut does, but they are cheaper and safer.
Therefore, the U.S. is pursuing vigorous plans to send various unmanned probes to Mars in the next decade. These missions, with various international partners, will send cameras, spectrometers, and geo-chemical instruments to gain more data about the climate history of Mars. They will follow on from the spectacular success of the relatively cheap Mars Pathfinder, which sent a small vehicle roaming over the surface of Mars in 1997. NASA's program was hampered by several expensive failures in 1999. Russia also planned major Mars missions, but the launch failure of its international Mars probe in 1996 devastated the nation's financially strapped planetary program. Several more missions are proposed by the U.S., Europe, and Russia in the next decade to set up a network of weather and seismic instruments to monitor Martian conditions. An attempt to return rock and soil samples is still being discussed, with a Mars sample return mission scheduled by NASA for sometime around 2025.