Description

Mars is the fourth planet from the Sun and the second-smallestplanet in the Solar System, after Mercury.Often referred to as the"Red Planet"[13][14] because the iron oxide prevalent on itssurface gives it a reddish appearance.[15] Mars is a terrestrialplanet with a thin atmosphere, having surface features reminiscentboth of the impact craters of the Moon and the valleys, deserts,and polar ice caps of Earth. The rotational period and seasonalcycles of Mars are likewise similar to those of Earth, as is thetilt that produces the seasons. Mars is the site of Olympus Mons,the largest volcano and second-highest known mountain in the SolarSystem, and of Valles Marineris, one of the largest canyons in theSolar System. The smooth Borealis basin in the northern hemispherecovers 40% of the planet and may be a giant impact feature.[16][17]Mars has two moons, Phobos and Deimos, which are small andirregularly shaped. These may be captured asteroids,[18][19]similar to 5261 Eureka, a Mars trojan. Until the first successfulMars flyby in 1965 by Mariner 4, many speculated about the presenceof liquid water on the planet's surface. This was based on observedperiodic variations in light and dark patches, particularly in thepolar latitudes, which appeared to be seas and continents; long,dark striations were interpreted by some as irrigation channels forliquid water. These straight line features were later explained asoptical illusions, though geological evidence gathered by uncrewedmissions suggests that Mars once had large-scale water coverage onits surface at some earlier stage of its existence.[20] In 2005,radar data revealed the presence of large quantities of water iceat the poles[21] and at mid-latitudes.[22][23] The Mars roverSpirit sampled chemical compounds containing water molecules inMarch 2007. The Phoenix lander directly sampled water ice inshallow Martian soil on July 31, 2008.[24] On September 28, 2015,NASA announced the presence of briny flowing salt water on theMartian surface.[25]