Life on Mars

Environmental conditions in the past may have suited the origin of life on Mars. One 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 could help elucidate the origin of life on Earth and, possibly, elsewhere in the universe. If not detected, the question of whether terrestrial life is unique would remain open.

Overview

On the basis of their observations from telescopes, photographs, remote sensors from orbiters, and from 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, the fourth planet from the Sun and Earth’s immediate outer neighbor. Indeed, no conclusive evidence of life of any kind has been found. However, circumstantial evidence that life might have existed in the past and could persist even in the twenty-first century has steadily accumulated ever since 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 changing environmental conditions. Most agree that this entails the ability to process energy and nutrients, grow, and reproduce. Accordingly, the search has been on for environments on Mars that could foster these activities. However, it is a harsh world. The Viking 1 and Viking 2 landers of the mid-1970s 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 experienced huge floods billions of years ago, 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. In 2015, the National Aeronautics and Space Administration (NASA)'s Mars Reconnaissance Orbiter (MRO) detected hydrated minerals under dark streaks in the mountains called recurring slope lineae (RSL). There is seemingly some degree of flowing water beneath the RSLs. Additionally, using the radar instrument (the Mars Advanced Radar for Subsurface and Ionosphere Sounding) on the Mars Express spacecraft, in 2018, scientists reported the discovery of what appeared to be the existence of a subglacial lake approximately one mile below the ice on the planet's surface in the south polar layered deposits (SPLD). It was theorized, based on analysis of the feature's composition and pending further supportive observations, that this reservoir of liquid water would represent the first and largest (it is believed to be about twenty kilometers, or twelve miles, wide) known stable body of liquid water on the planet, and that this discovery would continue to fuel the debate about life on Mars.

If organisms exist in Mars's 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 (negative twenty degrees Celsius to -121 degrees Celsius), 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 by-products 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. 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 thirty-five centimeters (1.1 foot) below the surface to as much as ten 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 NASA announced that evidence for life was found in a Martian meteorite known as ALH 84001, 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 have been raised by scientists. Just as ALH 84001 was blasted off Mars by a meteorite and made its long journey to Earth, chunks of Earth have probably reached Mars; they could have potentially carried Earth organisms there, seeding Mars 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 do not mean that Earth organisms have survived in the harsh Martian environment, but their presence could nevertheless confuse efforts to find native Martian life. Similarly, if Earth organisms have survived and 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). The Sojourner (1997), Opportunity (2004–2018), and Spirit (2004–10) rovers confirmed that finding at other locations. These findings did not absolutely rule out life, however. Viking experiments may have missed a biomarker that they were not designed to detect, as a reexamination of their data in 2012 indicated—there may have been evidence of microbial life in the soil samples tested in 1976—or the rocket exhaust from their landing may have killed organisms within their reach.

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

When it became probable that no life inhabits the Martian surface, scientists began investigating the possibility of organisms living below the surface. The research depends on an analogy to similar Earth habitats, and this has inspired scientists to search for life in unexplored realms on our planet. The results have greatly expanded knowledge about life on Earth: terrestrial extremophiles near volcanic vents deep in the oceans, in gelid water, in porous underground rock, or in salty, acidic, or alkaline conditions. Paleobiologists have uncovered fossilized organisms from much farther in the past than previously suspected.

In late 2008, analyses of data from the MRO indicated that Mars in its distant past must have had sufficient water flowing across the surface such that clay-rich (carbonate) minerals could form. MRO picked up the signatures of those clay minerals. That Mars may have been wetter and favorable for primitive life to develop is evidenced by the fact that these clay minerals have survived to the present. As Mars began losing its water, becoming a drier planet, the remaining water would have become acidic. Carbonate clay minerals are relatively easily dissolved in acidic water. Where the clay minerals survived on the surface to the present era would have been less hostile to life as the planet continued to become drier and drier. Although highly suggestive, this new information still did not provide direct evidence that life may ever have existed on Mars.

Another discovery, this one made by ground-based telescopes atop Mauna Kea, Hawaii, was reported in an early January 2009 Science Update aired on NASA Television. Reporting scientists explained that the infrared signature of methane gas had been detected in significant amounts in the Martian atmosphere. This was seen as potential indirect evidence that primitive life might exist on Mars in the present era because as solar ultraviolet radiation penetrates through the thin Martian atmosphere, methane molecules dissociate, and over the eons of geologic time, the methane should have dissipated without replenishment. Replenishment is possible by either one of two mechanisms: a geological mechanism involving the conversion of iron oxide to serpentine minerals in the presence of water, carbon dioxide, and a heat source or a biological mechanism involving digestion occurring in primitive microorganisms. More direct examination of methane vents on the planet’s surface is needed to determine if methane production is of geological or biological origin.

In early 2013, the Curiosity rover analyzed powdered rock samples containing some of the most important chemical "ingredients" for life: nitrogen, sulfur, hydrogen, carbon, and phosphorus. This is yet another indicator that living microbes may have existed in Mars's past. In addition, more clay was found by the Opportunity rover. In May 2013, the rover found in a fractured rock called Esperance ("hope") evidence supportive of the belief that Mars had a wet ancient environment. The clay minerals found in this ancient rock, one of the oldest the rover examined, could have formed from water running over volcanic rock. Opportunity found a number of signs indicative of flowing water in Mars's ancient past. This water would have been greatly acidic; in other words, Opportunity has discovered evidence of sulfuric acid. Esperance, however, likely formed in more neutral water—water safe enough to drink.

In 2018, NASA announced that Curiosity had found additional evidence of both atmospheric methane and organic molecules. the search for signs of life was also a central part of the Perseverance rover mission, which began operating on Mars in 2021. It aimed to explore environments that may have harbored ancient life and to collect samples that might provide evidence of microbial life. The earliest photographs from the rover confirmed that its landing site was, as suspected, the remnant of an ancient river delta and, therefore, a strong potential site for evidence of microorganisms.

Negative findings are also important, as much to human culture as to science. It is clear that Mars supports no civilization and, in all probability, no 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 have discounted the possibility, popular in science fiction, that Martian life poses a threat to Earthlings.

In April 2023, NASA’s Curiosity rover discovered a pattern of mud cracks on Mars, indicating the planet experienced wet and dry cycles that were conducive to microbial life. Also in 2023, the Perseverance collected ten test tubes of samples of Martian rock, which could hold the key to answers about life on Mars. These samples are slated to be brought to Earth in the ambitious Mars Sample Return Project.

Context

The absence of Martians as imagined by science-fiction authors such as 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 above 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.

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