Mars (planet)

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Future Exploration of Mars

The United States and other countries are in the midst of an ambitious, long-term program of Mars exploration. NASA’s Phoenix Mars Lander mission was launched in 2007 and scheduled to land in May 2008. The Phoenix Mars Lander mission is intended to land a spacecraft in an ice-rich area of the north polar region of Mars and to scoop up samples of soil for detailed analysis. It is part of the Mars Scout program, which is conceived as a series of low-cost missions to Mars. The Mars Science Laboratory in 2009 is intended to put a rover on Mars that can travel over a region at least 20 km (12.5 mi) wide. A possible sample-return mission from the surface of Mars is being studied for a potential 2018 or 2020 launch, although no specific plans have yet been announced for this mission.

ESA is considering an advanced Mars rover called ExoMars as part of its Aurora planetary exploration program. If approved for launch, ExoMars would search for signs of present or past life on Mars. That mission could launch in 2013 and land in 2014. A European-led sample-return mission from the Martian surface is also under study, possibly in collaboration with NASA and other countries. Russia and China have announced plans to study joint missions to return samples to Earth from the moon Phobos and possibly from the surface of Mars.

What would be the most ambitious Mars mission yet was announced in January 2004 by U.S. president George W. Bush, who called for the establishment of an astronaut base on the Moon that would provide a launching pad for a later human mission to Mars. NASA announced more details about this new round of human space exploration in 2006 in what is being called the Constellation Program. The primary vehicles would be the piloted Orion space capsule, which carries a crew of six, and the Ares booster rocket. Flights to Mars would require a much more powerful version of the booster called Ares V. Plans for this ambitious return of humans to the Moon and then on to Mars are still being intensely discussed and debated in the United States, and specific launch and mission schedules have not yet been established.

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Search for Life on Mars

Mars is the most Earth-like place in the solar system besides Earth itself, and so it is only natural to wonder if the similarities extend to the existence of life. People have speculated about the possibility of life on Mars for centuries, and one of the major justifications for sending spacecraft to Mars is the search for direct evidence of past or present life.

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Early Speculations

Astronomers have often fueled the speculation that life may exist on Mars. For example, the 19th century Italian astronomer Giovanni Schiaparelli reported that he saw long, straight markings on Mars that he called canali (Italian for “channels”). He and other astronomers of that era also reported seeing evidence for seasonal color changes on Mars that could be interpreted as evidence for vegetation. Some astronomers of the early 20th century, as well as American entrepreneur and amateur astronomer Percival Lowell, turned Schiaparelli’s canali into the now-famous “canals,” forever changing the public’s perception of the red planet. Lowell believed that the canals indicated the existence of an advanced civilization on Mars. He wrote several books and magazine and newspaper articles on the subject and lectured extensively about his theory around the country to sold-out audiences. He proposed that the canals were a planetary-scale irrigation project, carrying water from the wet polar regions to the dry equatorial deserts. As telescopes improved, however, and as it became possible to record photographs of Mars on film instead of relying on human vision alone, astronomers were unable to see repeatable evidence for Lowell’s canals. Close-up images of Mars from the Mariner spacecraft finally proved that the canals did not exist, although numerous channels carved naturally by flowing water were discovered.

Scientists now know that windblown dust causes the color changes and that the canals are no more than an optical illusion caused by the limitations of human eyesight at the telescope. But Lowell’s beliefs about civilization on Mars have had a powerful and lasting effect on human perception of the planet. British author H. G. Wells’s The War of the Worlds (1898) and American actor and director Orson Welles’s 1939 nationally-broadcast radio hoax based on that novel put a sinister face on our interplanetary neighbors. American author Edgar Rice Burroughs’s series of Mars books, starting with A Princess of Mars (1912), provided a more benevolent expansion of the influence of Lowell’s ideas and inspired a generation of would-be planetary explorers. For a while in the 1970s, some people even thought there were human faces and pyramid-like structures carved into landforms in places on Mars, until better images revealed these, too, to be optical illusions. Even today, science fiction stories, movies, and television shows about Mars and Martians continue to be popular around the world.

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Current Scientific Knowledge

A major focus of the Viking missions was to search for actual scientific evidence of life. Several instruments on the Viking landers were designed specifically to detect organic molecules in the soil, and to test soil samples for evidence of metabolism, growth, or photosynthesis of possible Martian life forms. Even though all of these experiments were sensitive enough to have been able to detect life even in the most arid, cold, or otherwise hostile environments on Earth, none of them showed any convincing evidence for the presence of life on Mars.

Most scientists today think that it is highly unlikely that there is any life on the surface of Mars. Conditions at the surface are extremely hostile to life as we know it. Temperatures are usually well below the freezing point of water, and the atmosphere is extremely thin and dry. Without a protective ozone layer like Earth’s, ultraviolet radiation bathes the surface and would destroy any organic molecules exposed there. Static electricity generated by dust storms and dust devils may also create hydrogen peroxide, which can break down organic chemicals.

However, a growing number of scientists believe that some form of life could possibly exist on Mars today in more protected environments such as underground or inside pores and cracks in rocks—places sheltered from the extreme conditions of the surface and where liquid water could exist even at very low temperatures. This new appreciation for the possibility of life on Mars has been driven by the discovery, only in the last decade or so, of simple life forms on Earth tenaciously surviving and in some cases even thriving in what used to be considered inhospitable conditions. On Earth, life has been found at great depths on the ocean floor, deep underground in volcanic rocks, in highly acidic cave waters, in near-boiling hot springs, and in almost permanently frozen tundra sediments. If life can maintain a foothold in even these extreme environments on Earth, then it may also be able to exist on Mars. See also Hydrothermal Vent.

A more basic question is whether conditions on Mars were ever Earthlike enough, and for a long enough time, for some form of life to have evolved. Liquid water has been essential for life as we know it to evolve and survive on Earth. The Spirit and Opportunity Rovers both found evidence that liquid water once existed on and below the surface of Mars billions of years ago, but the results seem to indicate that water would have been salty and extremely acidic, resembling dilute sulfuric acid. Although preexisting Martian microorganisms could possibly have adapted to to saltiness and high acidity, it is less clear that life itself could start in such a harsh environment.

Ancient terrains examined from orbit hint that conditions more favorable for the evolution of life may have existed at an earlier period on Mars before volcanic eruptions changed the surface chemistry with large amounts of sulfur. Detailed images from the Mars Reconnaissance Orbiter also show evidence that water or other fluids may have flowed underground between layers of rock, providing a possible environment for microorganisms.

The Odyssey orbiter may also have found large areas of salt on the surface that could have been left by bodies of water that evaporated. Some impact craters also show evidence that they once contained lakes. The discovery by the Spirit rover of silica deposits from apparent hot springs suggests that isolated spots on the surface may once have been more favorable to life. If life once existed on Mars but died out, the preserved chemical signatures of life, and possibly even actual fossils, may exist in ancient sediments.

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Meteorites from Mars

Rare types of meteorites found on Earth have a chemistry very similar to rocks on Mars. Scientists think these meteorites are chunks of Mars that were blasted into space by large impacts on the planet’s surface. The rocks went into orbit around the Sun and eventually fell to Earth as meteorites. Some of these meteorites have been studied for possible signs of Martian life.

In 1996 a group of NASA scientists announced that a meteorite thought to have come from Mars contained possible fossil evidence of bacteria-like life forms. The Allen Hills meteorite was found in Antarctica and had traveled through space to Earth millions of years ago after being blasted from the surface of Mars, probably by the impact of a large meteor. The scientists’ evidence was based on the presence of certain chemicals and minerals—as well as microscope pictures of bacteria-like features—within the meteorite. Intense scrutiny of this meteorite by other scientists has not provided support for this theory, however.

Later studies of volcanic rocks formed by eruptions in freezing conditions on Earth found tiny spheres of carbonate minerals very similar to those in the Martian meteorite. Nonetheless, the discovery that chemical reactions with rock could create organic material in association with minerals such as magnetite indicates that organic compounds could form under conditions that exist on Mars.

In 2006 a group of scientists suggested that microscopic etch marks found in another meteorite from Mars could be possible evidence that Martian bacteria had once fed on minerals in the rock. Underground bacteria found in Earth leave similar etch marks when they extract chemical energy from iron minerals in rock. The meteorite was a volcanic rock that is thought to have been exposed to water about 600,000 years ago on Mars and later blasted into space by a large impact. No evidence of DNA was found in the sample.

Even if these kinds of Martian meteorites do not preserve actual evidence of life on Mars, their most important message may be that Mars is one of the few places in the solar system where we know that the conditions were habitable and life was at least possible. The goal now is to figure out how and where to look for more convincing evidence such as traces of organic material. Robotic Mars missions such as the Mars Science Laboratory, ExoMars, and sample returns are the next logical steps.