The Case For Life On Mars

Mars is the fourth planet from the Sun in the Solar System. The planet is named after Mars, the Roman god of war. It is also referred to as the "Red Planet" because of its reddish appearance, due to iron oxide prevalent on its surface.

Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the volcanoes, valleys, deserts and polar ice caps of Earth. It is the site of Olympus Mons, the highest known mountain in the Solar System, and of Valles Marineris, the largest canyon. Furthermore, in June 2008 three articles published in Nature presented evidence of an enormous impact crater in Mars's northern hemisphere, 10,600 km long by 8,500 km wide, or roughly four times larger than the largest impact crater yet discovered, the Moon's South Pole-Aitken basin. In addition to its geographical features, Mars' rotational period and seasonal cycles are likewise similar to those of Earth.

Until the first flyby of Mars by Mariner 4 in 1965, many speculated that there might be liquid water on the planet's surface. This was based on observations of periodic variations in light and dark patches, particularly in the polar latitudes, which looked like seas and continents, while long, dark striations were interpreted by some observers as irrigation channels for liquid water. These straight line features were later proven not to exist and were instead explained as optical illusions. Still, of all the planets in the Solar System other than Earth, Mars is the most likely to harbor liquid water, and perhaps life. Radar data from Mars Express and the Mars Reconnaissance Orbiter have revealed the presence of large quantities of water ice both at the poles (July 2005) and at mid-latitudes (November 2008). The Phoenix Mars Lander directly sampled water ice in shallow martian soil on July 31, 2008.

Mars is currently host to three functional orbiting spacecraft: Mars Odyssey, Mars Express, and the Mars Reconnaissance Orbiter. With the exception of Earth, this is more than any planet in the Solar System. The surface is also home to the two Mars Exploration Rovers (Spirit and Opportunity) and several inert landers and rovers, both successful and unsuccessful. The Phoenix lander recently completed its mission on the surface. Geological evidence gathered by these and preceding missions suggests that Mars previously had large-scale water coverage, while observations also indicate that small geyser-like water flows have occurred during the past decade.

Observations by NASA's Mars Global Surveyor show evidence that parts of the southern polar ice cap have been receding.

Mars has two moons, Phobos and Deimos, which are small and irregularly shaped. These may be captured asteroids, similar to 5261 Eureka, a Martian Trojan asteroid. Mars can be seen from Earth with the naked eye. Its apparent magnitude reaches ?2.9, a brightness surpassed only by Venus, the Moon, and the Sun, although most of the time Jupiter will appear brighter to the naked eye than Mars. Mars has an average opposition distance of 78 million km but can come as close as 55.7 million km during a close approach, such as occurred in 2003.

Extraterrestrial life is defined as life which does not originate from planet Earth. The existence of such life is theoretical and all assertions about it remain disputed.

Hypotheses regarding the origin(s) of extraterrestrial life, if it exists, are as follows: one proposes that it may have emerged, independently, from different places in the universe. An alternative hypothesis is panspermia or exogenesis, which holds that life emerges from one location, then spreads between habitable planets. These two hypotheses are not mutually exclusive. The study and theorization of extraterrestrial life is known as astrobiology, exobiology or xenobiology. Speculated forms of extraterrestrial life range from life with the simplicity of bacteria to sapient or sentient beings.

Suggested locations which might have once developed, or presently continue to host life similar to our own, include the planets Venus and Mars, moons of Jupiter and Saturn (e.g. Europa, Enceladus and Titan) and Gliese 581 c and d, recently discovered to be near Earth-mass extrasolar planets apparently located in their star's habitable zone, and with the potential to have liquid water.

To date, no credible evidence of extraterrestrial life has been discovered which has been generally accepted by the mainstream scientific community.

All other proposals, including beliefs that some UFOs are of extraterrestrial origin (see extraterrestrial hypothesis) and claims of alien abduction, are considered hypothetical by most scientists. UFO sightings are sightings of unidentified flying objects that may or may not be connected with extraterrestrial intelligent life. Most of these sightings can be dismissed as sightings of Earth based aircraft or known astronomical objects, or perpetration of hoaxes. Some sightings have remained unexplained, in some cases having been reported by trained professionals.

In 2006, New Scientist published a list of ten controversial pieces of evidence that extraterrestrial life exists,http://www.newscientist.com/article/dn9943-top-10-controversial-pieces-of-evidence-for-extraterrestrial-life.html but scientists do not consider them credible since no direct observational evidence has been encountered. Many scientists, such as Carl Sagan, believe that it is nearly impossible for no other intelligent life to exist in the universe.

An extremophile (from Latin ' meaning "extreme" and Greek ' () meaning "love") is an organism that thrives in and even may require physically or geochemically extreme conditions that are detrimental to the majority of life on Earth. In contrast, organisms from moderate temperature or neutral pH environments may be termed mesophiles or neutrophiles.

In the 1980s and 1990s, biologists found that microbial life has an amazing flexibility for surviving in extreme environments - niches that are extraordinarily hot, or cold, or dry, or under immense pressures - that would be completely inhospitable to complex organisms. Some scientists even concluded that life may have begun on Earth in hydrothermal vents far under the ocean's surface.

Most known extremophiles are microbes. The domain Archaea contains renowned examples, but extremophiles are present in numerous and diverse genetic lineages of both bacteria and archaeans. Furthermore, it is erroneous to use the term extremophile to encompass all archaeans, as some are mesophilic. Neither are all extremophiles unicellular; protostome animals found in similar environments include the Pompeii worm, the psychrophilic Grylloblattodea (insects), Antarctic krill (a crustacean), and the "water bear".