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Case Study of Life on Mars

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Case Study of Life on Mars Educator Forums Help Feedback Bug Report Astropedia Search here SEARCH HOME TEXTBOOK WIKIPEDIA IMAGES VIDEOS PODCASTS COURSE ASTROBITES Glossary News Chapter 1 - How Science Works Case Study of Life on Mars 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 current Mars rover Mars This artist concept features Science Laboratory, nicknamed Curiosity, will seek to find answers to some of these questions, and will build NASA's Mars Science on what we've learned through past missions. Laboratory Curiosity rover, a mobile robot for investigating By the time the Viking landers were built in the 1970s, scientists suspected Mars' past or present ability to that Martian life might exist not as large organisms, but as microscopic sustain microbial life. bacteria or other simple organisms in the soil. To test this idea, both Viking Curiosity is being tested in landers contained equipment to make chemical tests of the soil. The first preparation for launch in the test looked for organic molecules in the soil. If there is life on Mars, then fall of 2011. In this picture, living or dead organisms should create a residue of organic molecules in the rover examines a rock on the soil, as on Earth. None were found, to an accuracy of a few parts per Mars with a set of tools at the billion. In other words, the Martian soil is sterile. end of the rover?s arm, which Carl Sagan with a model of Such an emphatic result might seem to rule out any life on the planet. extends about 2 meters (7 the Viking lander. Click here However, Viking scientists had another trick up their sleeves. They included feet). Two instruments on the for original source URL. three more experiments on each lander that scooped up soil, "fed" it arm can study rocks up close. nutrients, and watched for signs of metabolic activity (such as we see with Also, a drill can collect plants growing and releasing carbon dioxide). All three experiments detected some unexpected chemical sample material from inside of activity! However, careful analysis indicated it was probably not from living organisms, but from unusual rocks and a scoop can pick up chemical conditions in the soil that relate to an interesting fact: there is no ozone layer on Mars. This means samples of soil. The arm can that strong ultraviolet light from the Sun reaches the surface and destroys any exposed organic molecules. sieve the samples and deliver (Remember that the Earth is protected by its ozone layer.) fine powder to instruments inside the rover for thorough Ultraviolet radiation can thus explain the unusual results of both sets of experiments. On one hand, the UV analysis. The mast, or rover?s would have destroyed any organics in the soils, potentially causing a false negative on experiments seeking ?head,? rises to about 2.1 evidence of life in the soil. On the other hand, this UV also creates unusual soil conditions that may help meters (6.9 feet) above create the unusual oxides that caused the chemical reactions in the Viking soil experiments, thus creating a ground level, about as tall as false positive. Put together, these results suggest that there is probably no life in the particular soils at the a basketball player. This mast Viking sites, but this result is not entirely conclusive. supports two remote-sensing Some scientists have suggested that the soil within a meter or so of the surface might not be the best place instruments: the Mast to look for life on Mars. Annual storms stir up the surface dust and blow it around on Mars, effectively Camera, or ?eyes,? for stereo sterilizing the whole surface layer of the planet. A similar process occurs in Antarctica, where icy winds color viewing of surrounding sterilize the soils. There, tiny life forms live not in the inhospitable soil, but in fractures inside rocks. The terrain and material collected rocks offer an environment protected from the environmental extremes of the exposed surface. Thus, in the by the arm; and, the years after the Viking experiments, scientists speculated that life - or life's remnants - might be found hidden ChemCam instrument, which in subsurface layers or other protected environments of Mars as well. There is good indirect evidence for is a laser that vaporizes sub-surface aquifers on Mars, so liquid water may be present tens of meters under the surface, far deeper material from rocks up to than any current or planned rover can dig. about 9 meters (30 feet) away and determines what The story of the search for life on Mars took an exciting turn in 1996 when we got to look inside a rock. NASA elements the rocks are made scientists in Houston were studying one of the oldest Martian meteorites, a Martian rock more than 4 billion of. Click here for original years old. This rock had deposits of carbonates inside fractures in the rock, where liquid water had source URL. 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? Most planetary scientists are unconvinced. 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 but it is a perfect match for the gases sampled by the Viking landers. 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 Structures on ALH84001 about the experimental methods used to study the meteorite. meteorite. Click here for The chemical evidence for life is unconvincing to many planetary scientists. The NASA researchers also original source URL. 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 analyses 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.
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