Salem Press

Solar System
Mars: Possible Life

Categories: Life in the Solar System; Mars; Planets and Planetology

Environmental conditions in the past suited the origin of life on Mars. A primary motive for sending orbiters and landers to the planet has been to detect Martian life; it would be a principal goal for a crewed mission as well. If detected, such life would help elucidate the origin of life on Earth and, possibly, elsewhere in the universe. If not detected, the question would remain open whether terrestrial life is unique.

Overview
On the basis of their observations from telescopes, photographs and remote sensors from orbiters, and direct inspection by means of experiments performed by landers and rovers on the planet's surface, scientists have ruled out the presence of intelligent life on Mars, fourth planet from the Sun and Earth's immediate outer neighbor. No direct evidence of life of any kind has been found. However, circumstantial evidence that life might have existed in the past, and could persist even today, steadily accumulated after the first probe photographed Mars in 1964. In all cases, it is evidence based on comparisons with the conditions that support life on Earth.

Mars specialists base their search on the broad biological definition of life as a chemical system capable of Darwinian evolution to accommodate to changing environmental conditions. Most agree that this entails the ability to process energy and nutrients, grow, and reproduce. Accordingly, the search is on for environments on Mars that could foster these activities. Unfortunately, it is a harsh world. The Viking landers of the mid-1970's confirmed what scientists had suspected, that iron oxide, poisonous to life on Earth, permeates the planet's surface. The atmosphere is thin, frigid, dry, and very low in oxygen. The absence of a planetary magnetic field permits intense ultraviolet radiation, also deadly to life, to reach the surface. These facts suggest that the existence of life above ground is highly unlikely.

Water is an essential element for life, although its presence does not guarantee life. According to evidence from orbiters and landers, Mars had oceans, lakes, rivers, and a thicker atmosphere in the past, and may still see occasional outbursts of underground water on the surface. However, most water now exists as ice in polar ice fields and as permafrost beneath the surface, a region known as the cryosphere. Below these there may be a hydrosphere, a band of water-permeated rock. Most scientists believe that life could thrive at the boundary between the cryosphere and hydrosphere. There is some evidence to encourage that belief. Probes have detected methane in Mars's atmosphere; methane is a waste gas from biological processes, and it is known as a biomarker or biosignature. Because methane only lasts in the atmosphere for a few hundred years, it must be replenished. That source could be Martian organisms now alive, although theoretical inorganic chemical processes also have been proposed as the source.

If organisms exist in the cryosphere-hydrosphere boundary region or as spores near ancient water bodies, they are probably not large. There could be multicellular organisms like the tube worms that feed from hydrothermal vents in the cold dark waters of the deep oceans on Earth, but most scientists foresee finding only single-cell organisms. Terrestrial organisms known as extremophiles live in conditions ranging from 253 to 394 Kelvins (^-4 to 250 degrees Fahrenheit), in a wide range of acidity, in very salty water, without light, or kilometers underground. Some extract energy and nutrients from hydrothermal vents (hyperthermophiles), such as hot springs. Others feed from inorganic chemicals in rocks (chemolithoautotrophs)—for instance, ingesting sulfide minerals and excreting sulfuric acid—and in slushy water or salty water colder than the normal freezing point (psychrophiles). It is Martian equivalents of such extremophiles that scientists hope to find.

In a subsurface ecosystem Martian organisms would show variety in form and function. The basic structure ought to be a cell with a semipermeable membrane, such as the sack of lipoproteins defining most terrestrial cells, and internal structures that split apart chemicals in order to use the byproducts in their metabolism. Most would be grazers, feeding off the ambient nutrient source, but there are likely to be predators that consume the grazers. Either may have a means of locomotion, such as cilia or the ability to expand and contract. In order to evolve, they would need some type of chemical record of their mechanism, as exists in the ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) of Earth organisms, to pass on during reproduction. Because the temperature and air pressure on Mars are low, the metabolism, reproduction, and movement of organisms may well be sluggish and sparse in comparison to those functions in Earth organisms. They could live from as little as 35 centimeters (1.1 foot) below the surface to as much as 10 kilometers (6.2 miles), scientists speculate.

Martian life may have once thrived and then declined and finally vanished as the ancient surface water dried up. In that case, fossils might remain. Paleobiologists have found fossilized forms of Earth's single-cell organisms, as in stromatolites, that date back at least 3.5 billion years before the present. Should such fossils survive on Mars, they would establish that life rose there and provide clues about which environmental conditions supported an ecosystem. In 1996 scientists from the National Aeronautics and Space Administration (NASA) announced finding evidence for life in a Martian meteorite known as ALH84001, discovered in Antarctica. Among the evidence was what appeared to be a fossilized cellular structure. Although the majority of scientists later discounted the evidence and judged the microscopic structure to be the result of nonbiological processes, research demonstrates that the conditions for preserving delicate structures, such as fossils, exist on Mars. Additionally, it is possible that early conditions were favorable for life but then, because of meteorite bombardments 3.9 billion years ago, changed before life actually got started. In that case, scientists might still detect the precursors to life in complex organic chemicals.

Two further possibilities for life on Mars worry scientists. Just as ALH84001 was blasted off Mars by a meteorite and made its long journey to Earth, a chunks of Earth probably reached Mars, and they could have carried Earth organisms there, seeding the Red Planet with terrestrial life. It is probable, moreover, that the landers sent to Mars by American, Russian, and European space agencies carried organisms with them despite decontamination protocols. Such "forward contamination" would be even more likely from a crewed mission to the planet. Seeding and contamination does not mean that Earth organisms have survived in the harsh Martian environment. On the other hand, if Earth organisms have adapted, scientists may have difficulty establishing their origin.

Knowledge Gained
Experiments performed aboard the Viking landers (1976) established that Mars's soil contains chemicals inimical to life, particularly oxidants such as iron oxide (rust), and the Sojourner, Opportunity, and Spirit rovers confirmed the finding at other locations. These findings do not absolutely rule out life. Viking experiments may have missed a biomarker that they were not designed to detect, or the rocket exhaust from their landing may have killed organisms within their reach. In any case, the search for life taught scientists much about the chemical nature of the soil and the distributions of oxidants.

In the 1990's NASA set a policy for its Mars probes: "Follow the water." The resulting search from orbit and on the surface found evidence of erosion and chemical deposits, such as hematite, that on Earth derive from flowing water. Sensors in orbiters detected underground ice as well. These discoveries not only encourage scientists to look further for life but also suggest that sufficient water exists to support human habitation on the planet. At the same time, the various landers and rovers, all of which far exceeded their expected performance, proved the versatility and hardiness of technology in the Martian environment.

When it became probable that no life inhabited the Martian surface, scientists investigated the possibility of organisms living below the surface. The research depended on analogy to similar Earth habitats, and scientists were inspired to search for life in heretofore unexplored realms. The result was to expand greatly the knowledge about terrestrial extremophiles near volcanic vents deep in the oceans, in gelid water, in porous underground rock, or in salty, acidic, or alkaline conditions. Paleobiologists uncovered fossilized organisms from much further in the past than previously suspected.

Negative findings are also important, as much to human culture as to science: Mars supports no civilization or, in all probability, animal life. The alteration in surface color through the Martian seasons that fascinated astronomers like Percival Lowell comes from wind storms, not vegetation. Thereby, scientists discounted the possibility, so popular in science fiction, that Martian life poses a threat to Earthlings.

Context
The absence of Martians as imagined by H. G. Wells, Robert A. Heinlein, or Ray Bradbury has not dampened popular enthusiasm for the search for life on Mars. It endures because it promises to answer a question that has long made humanity look to the heavens and wonder: "Are we alone?"

The discovery of life on Mars would have profound implications. If proven to be entirely independent of life on Earth, that finding would mark a shift in understanding the universe as great as that of the Copernican Revolution. Philosophy would be tasked to reconsider the human moral obligation to other organisms. Religions would have to cope with the fact that life on Earth is not a unique creation. Science would be encouraged to look for life on still more worlds, such as the satellites of Jupiter, and for biomarkers in the light from other planetary systems. At the same time, NASA and other space agencies would need to take measures to protect Martian life from Earth organisms hitchhiking aboard planetary probes, or, in the case of a sample-return mission or crewed mission, to protect Earth's ecosystem from Martian organisms.

If Martian organisms were found to be related to those on Earth, the knowledge would also be fundamentally important. It would establish the great durability of life in spreading from one planet to another and leave open the possibility, as proposed in the Panspermia hypothesis, that both Mars and Earth were seeded long ago with life that originated elsewhere.

Proof that life never existed on Mars would be significant as well. The question of Earth's uniqueness would remain unsettled, yet scientists would learn an important fact: The types of chemical and geophysical conditions found on Mars are not conducive to life. Why that should be true would pose a major question for further research.

Roger Smith

Further Reading
Elkins-Tanton, Linda T. Mars. New York: Chelsea House, 2006. In a style suitable for high school students, the author, a geophysicist, provides a technically specific overview of Mars's environment and geologic history, including a short, reasonable summary of the case for life. With glossary, appendixes on basic relevant science, and photos.

Forget, Françoise, Françoise Costard, and Philippe Lognonné. Planet Mars: Story of Another World. Chichester, England: Praxis, 2008. The lucid text, suitable for high school students, explains current knowledge about the geology, hydrology, and meteorology of Mars. Three short chapters discuss possible life, and a fourth summarizes knowledge gained by space probes. With abundant photographs and graphics.

Hartmann, William K. A Traveler's Guide to Mars: The Mysterious Landscapes of the Red Planet. New York: Workman, 2003. This entertaining overview of the Red Planet includes a section about the physical conditions on Mars that could support life, and the text is a model of popular exposition. With many photographs and illustrations by the author, a planetary scientist, writer, and artist.

Kargel, Jeffrey S. Mars: A Warmer, Wetter Planet. Chichester, England: Praxis, 2004. A planetary scientist, Kargel provides a comprehensive, sophisticated survey of the geology and climate of Mars throughout its history. The importance of water is a central theme, and he discusses the possibilities for life. With many graphics and photographs.

Kiang, Nancy Y. "The Color of Plants on Other Worlds." Scientific American 298 (April, 2008): 48-55. Although not specifically addressing possible Mars organisms, this article explains the nature and type of chemical evidence that scientists think will signal the presence of life in alien environments.

Walter, Malcolm. The Search for Life on Mars. Cambridge, Mass.: Perseus Books, 1999. Charming, concise, and equitable, this volume is especially valuable for Walter's explanations of how terrestrial paleobiology provides clues of what life on Mars might be like and the controversies and personalities behind the research.

See Also
Earth's Origin; Extraterrestrial Life in the Solar System; Habitable Zones; Life's Origins; Search for Extraterrestrial Intelligence (SETI).