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 has two moons, Phobos and Deimos, which are small and irregularly shaped. Scientists have observed Mars through telescopes based on Earth and in space.
Space probes have carried telescopes and other instruments to Mars. Early probes were designed to observe the planet as they flew past it. Later, spacecraft orbited Mars and even landed there. But no human being has ever set foot on Mars.
The Martian surface has many spectacular features, including a canyon system that is much deeper and much longer than the Grand Canyon in the United States. Mars also has mountains that are much higher than Mount Everest, Earth's highest peak.
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. Observations by NASA's Mars Global Surveyor show evidence that parts of the southern polar ice cap have been receding.
Phoenix will be followed by the Mars Science Laboratory in 2011, a bigger, faster, and smarter version of the Mars Exploration Rovers. In 2013, NASA plans to launch MAVEN, a robotic mission to provide information about Mars' Atmosphere. The ESA plans to launch its first Rover to Mars in 2013 as well.
Characteristics of Mars
Like Earth, Mars rotates on its axis from west to east. The Martian solar day is 24 hours 39 minutes 35 seconds long. This is the length of time that Mars takes to turn around once with respect to the sun. The Earth day of 24 hours is also a solar day.
Mars has many of the kinds of surface features that are common on Earth. These include plains, canyons, volcanoes, valleys, gullies, and polar ice. But craters occur throughout the surface of Mars, while they are rare on Earth. In addition, fine-grained reddish dust covers almost all the Martian surface.
Many regions of Mars consist of flat, low-lying plains. Most of these areas are in the northern hemisphere. The lowest of the northern regions are among the flattest, smoothest places in the solar system. They may be so smooth because they were built up from deposits of sediment (tiny particles that settle to the bottom of a liquid). There is ample evidence that water once flowed across the Martian surface. The water would have tended to collect in the lowest spots on the planet and thus would have deposited sediments there.
Along the equator lies one of the most striking features on the planet, a system of canyons known as the Valles Marineris. The canyons run roughly east-west for about 2,500 miles (4,000 kilometers), which is close to the width of Australia or the distance from Philadelphia to San Diego. Scientists believe that the Valles Marineris formed mostly by rifting, a splitting of the crust due to being stretched. Individual canyons of the Valles Marineris are as much as 60 miles wide. The canyons merge in the central part of the system, in a region that is as much as 370 miles wide. The depth of the canyons is enormous, reaching 5 to 6 miles in some places.
Mars has the largest volcanoes in the solar system. The tallest one, Olympus Mons (Latin for Mount Olympus), rises 17 miles (27 kilometers) above the surrounding plains. It is about 370 miles (600 kilometers) in diameter. Three other large volcanoes, called Arsia Mons, Ascraeus Mons, and Pavonis Mons, sit atop a broad uplifted region called Tharsis. All these volcanoes have slopes that rise gradually, much like the slopes of Hawaiian volcanoes. Both the Martian and Hawaiian volcanoes are shield volcanoes. They formed from eruptions of lavas that can flow for long distances before solidifying. Mars also has many other types of volcanic landforms. These range from small, steep-sided cones to enormous plains covered in solidified lava.
Many meteoroids have struck Mars over its history, producing impact craters. Martian craters are similar to craters on Earth's moon, the planet Mercury, and other objects in the solar systems. The craters have deep, bowl-shaped floors and raised rims. Large craters can also have central peaks that form when the crater floor rebounds upward after an impact. Mars has a few large impact craters. The largest is Hellas Planitia in the southern hemisphere, also known as the Hellas impact basin. The crater has a diameter of about 1,400 miles (2,300 kilometers).
Channels, valleys, and gullies occur in many regions of Mars, apparently as a result of water erosion. The most striking of these features are known as outflow channels. Many of the channels do not look like river systems on Earth, with the main river formed from smaller rivers and streams. Rather, those Martian channels arise fully formed from low-lying areas. Other regions of Mars have much smaller features called valley networks. These networks look more like river systems on Earth. The gullies are smaller still. Most of them lie at high latitudes.
Some of the most spectacular weather occurs on Mars when dust blows in the wind. Small, swirling winds can lift dust off the surface for brief intervals. These winds create dust devils, tiny storms that look like tornadoes. At larger scales, dust storms can blanket areas from more than 200 miles (320 kilometers) to a few thousand miles or kilometers across. The largest storms can cover the entire surface of Mars. Storms of that size are unusual, but they can last for months. The strongest storms can block almost the entire surface from view. Such storms occurred in 1971 and 2001. Dust storms are most common when Mars is closest to the Sun. More storms occur then because that is when the sun heats the atmosphere the most.
Mars has two tiny moons, Phobos and Deimos. the two satellites have many craters that formed when meteoroids struck them. The surface of Phobos also has a complicated pattern of grooves. These may be cracks that developed when an impact created the satellite's largest crater. The color of both satellites is a dark gray that is similar to the color of some kinds of asteroids.
Observation by Spacecraft
Robotic spacecraft began detailed observation of Mars in the 1960's. The United States launched Mariner 4 to Mars in 1964, Mariners 6 and 7 in 1969, and in 1971 Mariner 9 went into orbit around Mars. Mariner 9 made the first discoveries of the planet's canyons and volcanoes, and found what appear to be dry riverbeds.
The next major mission to Mars was Viking in 1975. Viking consisted of two orbiters and two landers. Its main goal was to search for life. The landers took the first close-up pictures of the Martian surface, and they sampled the soil. They found no strong evidence for life.
The next two successful probes were Mars Pathfinder, which was a lander, and Mars Global Surveyor, an orbiter. The main objective of Pathfinder was to demonstrate a new landing system. Inflated air bags cushioned the probe's landing in July 1997. Pathfinder also carried a small roving vehicle called Sojourner. The rover rolled down a ramp to the surface, and then moved from rock to rock. Pathfinder sent spectacular photos back to Earth, and Sojourner analyzed rocks and soil. In April 2001, the United States launched the Mars Odyssey probe.
In December 2003, Mars Express went into orbit around the planet and released its lander, Beagle 2. In early January 2004, the U.S. rover Spirit landed safely in an area called Gusev Crater. The rover Opportunity landed later that month in an area called Meridiani Planum. The rovers transmitted detailed photographs of Martian ground features and began analyzing rocks and soil for evidence that large amounts of liquid water once existed on the planet's surface.