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From the beginnings of telescopic observations of Mars, people have speculated about whether life could have started on the planet and what that life might be like. Early observers were concerned mostly with intelligent life, but the focus now is on life’s origin, microbial communities, and limits to their survival.
Views on the prospects for life on Mars have varied greatly in recent decades. In the 1960s the possibility that changes seen at the telescope could have a biological cause led to Mariner 9’s attempts to monitor surface changes in 1972 and to the launching of the Viking landers to Mars in 1975. The Viking spacecraft had an array of sophisticated experiments to detect metabolism and organic molecules. The negative results from these experiments resulted in considerable pessimism that continued through the 1980s for the prospects for life.
However, several factors subsequently contributed to a more optimistic view. The first is recognition that life can survive in a far wider range of conditions than was formerly thought possible, including near deep-sea vents at temperatures well over 1,000 °C (1,800 °F), in basaltic rocks deep below the surface, and in very saline and acid environments. The second is the discovery that on Earth life started very quickly, possibly before the end of heavy bombardment, which possibly indicates that the origin of life is not an extremely low-probability event but rather will follow if the right conditions are present. The third is mounting evidence that conditions on early Mars, when life arose on Earth, were Earth-like. A fourth factor is recognition that Earth and Mars exchange materials. As indicated above, more than 30 pieces of Mars have been found on Earth, despite the difficulty of distinguishing Mars rocks from Earth rocks. It is more difficult to get Earth rocks to Mars. Nevertheless, during the period of heavy bombardment, when life may have already started on Earth and conditions on Mars were Earth-like, pieces of Earth may have been transported to Mars. Thus, life may have originated independently on Mars or been seeded from Earth.
In 1996 the scientific world was shocked when a group of scientists announced that they had found evidence of life in a Martian meteorite. In support of their conclusion, they listed (1) bacteria-like objects in electron microscope imagery, (2) detection of hydrocarbons, (3) mineral assemblages that were not produced in chemical equilibrium, and (4) magnetic particles similar to those produced by some terrestrial bacteria. The announcement triggered a vigorous scientific debate into the validity of the claims. The scientific consensus now is that there are plausible abiological explanations for all the observations and that the claims are likely invalid.
Despite this setback, the main driver of the Mars exploration program is still the search for life. Because liquid water is so essential for life, the initial focus has been on the search for evidence of warm conditions that would enable the persistence of liquid water. The evidence for such conditions at least on early Mars is now compelling, and there is some evidence that liquid water sometimes flows on the surface in a few places. The exploration thrust will likely shift to search for more-direct evidence such as organic remains and isotopic signatures. It could be argued that the best strategy is to look for fossil remains from the early period in Mars’s history when conditions were more Earth-like. But the Martian meteorite debate and disagreements about early terrestrial life point to the difficulty of finding compelling evidence of microbial fossil life. Alternatively, it could be argued that the best strategy is to look for present-day life in niches, such as warm volcanic regions or the intermittent flows of what may be briny water, in the hope that life, if it ever started on Mars, would survive where conditions were hospitable.
Human exploration
Human exploration is still decades away despite optimism when the Apollo program ended in the early 1970s that Mars exploration would soon follow. The technical difficulties of getting people to Mars and back, while challenging, are not overwhelming. The main difficulty has been in coming up with a compelling rationale that would justify the tremendous costs and risks. Advocates have argued that exploring Mars and extending human reach beyond Earth-Moon space needs no practical rationale; to explore is an essential part of being human. Others have argued that practical benefits such as economic stimulus, scientific discovery, and technology feedback would result. Nevertheless, despite numerous strong advocates, human exploration of Mars now seems farther from reality than in the 1970s.
Several studies have been undertaken to determine how human missions to Mars might be implemented, and there have even been full-scale simulations on Earth of Mars missions. Mars500 was a joint project of the European Space Agency and the Russian Institute for Biomedical Problems in which six “astronauts” performed a simulated 520-day Mars mission from June 2010 to November 2011. The six participants had only voice contact with other people, and even that was done on a 20-minute delay, liket that of radio communications between Earth and Mars. In 2023, NASA began a series of year-long Mars mission simulations called CHAPEA (Crew Health and Performance Exploration Analog), in which four crew members simulated one year on the Martian surface.
There are two basic classes of missions that follow from the orbital motions of Mars and Earth. In opposition-class missions, the round-trip time is 500–600 days, with 30 days spent on Mars. For conjunction-class missions, the round-trip time is about 900 days, and the time spent on Mars may be as much as 550 days. Most studies conclude that the short stay time for opposition-class missions could not justify the cost and effort of getting there and that, despite the greater resources needed, a conjunction-class mission is preferred.
There are many variants on how such a mission might be implemented. In some scenarios a cargo ship is sent ahead of the humans to establish a robotically operated base, and the humans follow, possibly years later. In other variants all the resources needed accompany the crew. Another issue is the extent to which resources at Mars could be used for supporting people during their stay and for making oxidants and propellants for the return trip. In situ resource units (ISRUs) could be sent to Mars well ahead of the people to extract and store both oxygen from the atmosphere and hydrogen from water, using solar or nuclear power. People would not be launched from Earth until adequate resources for mission success had accumulated at Mars. To save fuel, the atmosphere at both Mars and Earth could be used to decelerate (aerobrake) the outgoing and incoming spacecraft, respectively; an additional option is direct entry. Another issue is what the astronauts would do at Mars. Should they stay close to a base and operate remote robots at various locations around the planet, or should they go on extended trips from the base and explore, thereby incurring the risk of being stranded?
One issue that is not so readily susceptible to quantitative engineering analysis is crew health. Long times in space have various physiological effects. The circulatory and vestibular systems are affected, and bones tend to demineralize. Countermeasures to minimize degradation in human performance would have to be devised. The crew would also experience psychological pressures from several hundred days in a confined space far from Earth with no other company. Radiation effects would also be significant, particularly if solar storms occurred during the two–three year trip.
Planetary protection is another issue. The first concern is scientific: Mars should not be contaminated with terrestrial materials before the potential for indigenous life has been adequately assessed. Robotic spacecraft sent to Mars are subject to rigorous cleanliness requirements, and parts that touch the surface are sterilized to minimize the possibility of interfering with the detection of indigenous life. Clearly, similar protective measures cannot be followed for human missions. The second issue concerns back contamination. International protocols require that materials returned from Mars be quarantined until they have been proved to be safe. The astronauts may similarly have to be quarantined. Fortunately, most of the planetary-protection issues should be resolvable well ahead of human missions by robotic return of Martian samples.
Michael J.S. Belton Michael C. Malin Michael H. Carr