http://solarsystem.nasa.gov/planets/profile.cfm?Object=Mars

Mars

Astronomers have known of the existence of Mars for thousands of years, but it is only recently that scientist have been able to study this planet in detail through the use of satellite imagery and rovers. NASA's Mars Exploration Program is attempting to discover if Mars is, was, or can be made habitable. Today, Mars is a cold planet, and water is found only in ice as a solid and in a thin atmosphere as a gas. However, there are still several similarities between Mars and Earth; for example, Mars has polar ice caps, canyons, and volcanos. In studying the geography and evolution of Mars, most scientists now agree that large sources of water did exist in the history of Mars.

Missions

http://www.universetoday.com/97638/a-river-ran-through-it-how-do-they-know-there-was-once-water-on-mars/

Mars has a rich history of exploration, the richest of all planets in our solar system. The first missions were simply flybys, where the early Mariner spacecraft would take as many pictures as possible as they flew past Mars and transmit this data back to Earth. Subsequent missions, such as Mariner 9 and the Mars Global Surveyor, took the process a step further: orbiting the planet and allowing long-term, global studies for the first time.

In 1975 the first rovers destined for Mars were launched, Viking I and II, and became the first US missions to safely land on the surface of Mars and transmit images back to earth. For the first time, scientists were able to study detailed images of Martian soil, which resulted in the controversy of Martian microfossils. Further missions, including PathfinderSpirit and Opportunity, and Phoenix, have all been enormously successful, outliving their expected lifetimes and even finding evidence that the Martian soil was at one time wet and likely capable of supporting life.

The Mars Science Laboratory, also called the Curiosity rover, is the most recent and most sophisticated lander to reach Mars. Its mission is to study the Martian soil and determine if Mars ever had an environment capable of supporting microbial life. Recent evidence, such as the smooth river rock-type pebbles shown in the image (above left), provide further indications that Mars did at one point contain flowing water.

Mars as an Exoplanet

Scientists are fortunate to be able to study Mars with orbiters and landers. However, what could be learned from Mars if it were much farther away? One technique that scientists use to study exoplanets is spectroscopy. In 2004, the discovery of methane by Mars Express's spectrometer was announced. Although the discovery was made by an orbiter, transiting exoplanetary spectra can be observed and studied in the same way using space based telescopes. This discovery was significant since methane cannot exist long in the Martian atmosphere, and therefore there must be a source supplying methane to the atmoshpere. The source could be some unknown geologic or volcanic process, or perhaps even the result of metabolism in microbial life. These questions remain open, however we can be certain that Mars will continue to be a focal point in humanity's exploration of space.

http://www.nasa.gov/mission_pages/mars/news/marsmethane.html