Life-forms resembling the bacteria found on Earth may have once existed on Mars. When the evidence for this was first announced, some people considered it the most important discovery in the history of science. The evidence came from a Martian meteorite that was found in Antarctica. Had the evidence been confirmed, it would have indicated that the evolution of life from simple molecules was not an isolated event on the Earth. If life originated on two planets in the same solar system, it is quite likely to have occurred many times on many other planets throughout the universe.

Mars is the planet most similar to Earth, in temperature and chemical composition, in the solar system. Jupiter and Saturn consist largely of cold liquids, while Venus is extremely hot. Some moons of Jupiter and Saturn are more likely to have conditions suitable for the origin of life, although they too are very cold. Scientists have long believed that, if life ever existed outside of the Earth, it would have been on Mars or on one of these moons.

In recent years, astronomers have presented evidence for several planets orbiting other stars in the Milky Way galaxy. The first photograph of such a planet was published in 2005. Because these planets are usually too far away to be directly observed, the evidence for their existence is usually the gravitational force that they exert upon the stars around which they revolve. From the movements of these stars, the mass and orbit of the unseen planets can be calculated. Most of these planets appear to be large and gaseous and very close to their stars ("hot Jupiters"). However, this does not mean that Earth-like planets are rare; small planets like Earth are much less likely to be detected by such methods. The presence of planets may also be inferred by periodic slight decreases in star luminosity, perhaps produced when the planet passes between the star and the human observer.

The U.S. National Aeronautics and Space Administration (NASA) sent two spacecraft that landed on Mars in 1976. These spacecraft sent back photographs and data, none of which indicated life on Mars. A burst of carbon dioxide production from soil was thought at first to indicate microbial life on Mars, but scientists later decided that the carbon dioxide could have been produced by an inorganic reaction. Other spacecraft sent to Mars in 1997 and 2004 have sent back data that reinforce the conclusion that Mars is lifeless at the present time. . .

Free term papers are not written to satisfy your specific instructions. You can use our professional writing services to buy a custom written research paper, term paper, or essay on Science at affordable price. CustomTermPapers is the best solution for those who seek help in writing term papers, essays, and research papers related to Science and other relevant topics.