Mars

As it is visible from Earth without the aid of a telescope, the planet Mars has intrigued stargazers since ancient times. Babylonians mentioned Mars in records from about 3,000 years ago, associating the red planet with their god of death. The modern name Mars is that of the Roman god of war; the planet's two moons, Phobos (Fear) and Deimos (Terror), are named after the sons of Ares, the Greek god of war.

Mars passes relatively close to the Earth in its orbit. In addition, it generally has no obscuring layer of clouds, unlike the Earth's other neighbor, Venus. Thus Mars is a nearly ideal subject for telescopic observation. Over the centuries observers have noted various phenomena on the planet's surface, including a seasonal growing and shrinking of the polar caps and seasonal changes in the appearance of dark markings. The explanation of many early observations had to await the exploratory space missions by the United States and Soviet Union during the 1960s and 1970s.

Planetary Data

The fourth planet from the sun, Mars is the outermost of the terrestrial, or Earth-like, planets, which are the dense, rocky worlds closest to the sun. Because Mars has an elliptical orbit, its distance from the sun varies, from about 129 million miles (207 million kilometers) at the closest point in its orbit, or perihelion, to some 155 million miles (249 million kilometers) at its farthest point, or aphelion. The planet completes one revolution around the sun in about 687 Earth days, almost twice the time it takes the Earth to complete its orbit of about 365 days. Its distance from the Earth varies considerably, from less than 35 million miles (56 million kilometers) to nearly 250 million miles (400 million kilometers). The best time to view Mars from Earth is when it is at its closest to both the sun and the Earth so that it appears both bright and large.
With a mean diameter of about 4,219 miles (6,790 kilometers), Mars is the third smallest planet in the solar system. It is about half the size of Earth and is much less dense. Its gravity is about a third of the Earth's, its surface area about a fourth, and its mass only about a 10th. Like the Earth, Mars is roughly spherical, with a slight bulging at its equator and flattening at its poles.
Mars rotates on its axis at roughly the same rate as Earth; a Martian day, called a sol, lasts about 24.7 hours. The red planet is also tilted on its rotational axis at an angle similar to that of the Earth. Consequently, like Earth, it is subject to seasonal variations in climate as first one hemisphere and then the other receives more sunlight during the planet's orbit around the sun. Because of its more elliptical orbit, the seasons on Mars are not as even as they are on Earth. The spring and summer in the north, for example, last about 382 days, or more than half the 687-day year. In the south the summer is shorter.

Atmosphere, Surface, and Interior

The Martian atmosphere is composed mostly of carbon dioxide and is very thin, exerting about ¹/₁₀₀ the surface pressure that the Earth's atmosphere exerts. The thin atmosphere on Mars does not insulate the planet as well as the thicker one does on Earth. The surface of Mars is thus colder than Earth's would be if the two planets were the same distance from the sun. The temperature at the Martian surface varies widely during the course of a day, from about −118 ° F (−83 ° C) just before dawn to about −28 ° F (−33 ° C) in the afternoon. The atmosphere also does not shield the surface from ultraviolet radiation from the sun. This intense radiation bombardment is one reason why scientists believe that no living things currently exist on the surface of Mars.The Martian surface is dry and dusty. The question of whether liquid water has existed on the surface of Mars is of particular interest to scientists trying to determine if life ever existed on the planet. Liquid water is a requirement for all known forms of life. (But of course water does not in itself indicate the presence of living things.) Water currently exists on Mars as ice deposited at the poles, as ice trapped below the surface, and as vapor in the atmosphere. Rivers, lakes, and seas may have been present on Mars in its remote past, when temperatures may have been warmer and the atmospheric pressure higher. Because of the current low temperatures and pressure, it has been thought that liquid water has not existed at or near the surface in modern times. Recent findings, however, have prompted a reassessment of this view. Photographs taken in 2000 by the orbiting spacecraft Mars Global Surveyor showed hundreds of gullies that seemed to have formed relatively recently. Some planetary scientists believe that the gullies were carved by flowing water. They theorize that, episodically, small amounts of liquid water have flowed on and just below the surface in geologically recent times and that liquid water may still be present in parts of the planet's subsurface. But this theory has been disputed.

 Also challenged by recent findings was the view of Mars as a geologically “dead” planet on which volcanic activity had last occurred a few billion years ago. Data from the Mars Global Surveyor indicates that volcanoes may have erupted some 40 million years ago, which is considered recent in geologic time. The planet has the largest volcano in the solar system, Olympus Mons. At a height of 17 miles (27 kilometers), the volcano is three times higher than Earth's Mount Everest and covers an area the size of the state of Arizona. It sits on the Tharsis Plateau, a broad, elevated plain dotted with large volcanoes and fractures. The largest fracture system is Valles Marineris, a huge valley about 2,500 miles (4,000 kilometers) long and varying from 2¹/₂ to 6 miles (4 to 10 kilometers) in depth. The Tharsis Plateau may have been formed by a rising plume of hot mantle material, possibly accompanied by plate tectonic activity.

Other regions on Mars include smooth plains, densely cratered areas, mesas, and rolling hills formed by various combinations of fracturing, volcanism, and atmospheric-related erosion and deposition. The planet's hemispheres show striking geologic differences, with ancient, heavily cratered highlands in the south and younger, flat lowlands in the north.
Dark markings have been observed on the Martian surface for hundreds of years. These markings cover about a third of the surface and change in a seasonal pattern in both extent and color. Once thought to be vast seas or vegetation, the dark areas are now known to result from the accumulation of dust, which shifts along with seasonal winds.
Dust storms occur frequently on the planet, especially in the southern hemisphere in spring and summer. About every two or three years, Mars is engulfed by global dust storms. Local temperature differences generate strong winds that lift the dust from the surface to form thick clouds. The clouds block the sunlight, gradually causing the surface temperatures to even out and the winds to subside. Some of the atmospheric dust is deposited in a snowfall of dust and ice in the polar regions.
Ice caps form over both the north and south poles according to seasonal changes. Each cap grows larger when its hemisphere experiences fall and shrinks during the spring. During summer, most of the ice cap melts. The south pole's ice cap is larger. Its permanent ice cap, the portion that survives the summer melting, is covered with mesas, holes, and troughs and appears to be formed of carbon dioxide ice. Smaller pits and cracks mark the flatter permanent cap at the north pole, which is made of water ice.Scientists do not have direct information about the Martian interior, but they have developed a model based on the planet's known characteristics, such as its size, mass, gravity signature, and surface elevations. Mars most likely has a metal-rich core with a diameter of about 2,100 miles (3,400 kilometers). Surrounding the core is a molten rocky mantle that is probably more dense than the Earth's mantle. The thickness of the Martian crust is thought to vary from about only 2 miles (3 kilometers) in some places to more than 60 miles (90 kilometers) in others.
Unlike most of the other planets (except for Venus and maybe Pluto), Mars has no global magnetic field, though there are indications that the planet once had a strong field. Ancient rocks in the crust of the southern hemisphere are highly magnetized in striped patterns that suggest that the planet's magnetic field may have reversed polarity several times, as has the Earth's. The rocks are about 10 times as magnetized as any on Earth.

Satellites

Mars has two small satellites, Phobos and Deimos, which may be captured asteroids. Both are so small that they do not have enough internal gravity to draw them into spherical shapes; instead, they are shaped more or less like potatoes. Phobos is about 17 miles (27 kilometers) long; Deimos is about 9¹/₂ miles (15 kilometers) long. Both have rotational periods equal to their orbital periods, so that they always point the same face toward Mars. The surface of Deimos appears smooth because its craters are almost buried in regolith, a layer of fine rubble generated by repeated impacts with other bodies. Phobos is also covered with regolith, but its surface is far more rugged and very heavily cratered.Phobos is very close to Mars, and its orbit is gradually decaying, so that it is drawing closer to the planet with each orbit. Astronomers estimate that Phobos may fall to the Martian surface sometime in the next 100 million years. Deimos is in a more distant orbit and is gradually moving away from the planet.
Both satellites are very dark and are probably made of a carbonaceous chondrite material. This is a primitive substance that includes many of the first materials to precipitate out of the solar nebula during the creation of the solar system. It is found on many satellites, asteroids, and meteorites.

Observation and Exploration

For centuries astronomers have considered the possibility that life might exist on Mars, the most Earth-like of the planets. In the 1600s astronomers began to observe the planet with the aid of newly developed telescopes. In 1877 the Italian astronomer Giovanni Schiaparelli described what he believed was a system of interconnecting, straight-edged channels on the planet. He called these features canali, meaning “channels,” or “canals.” U.S. astronomer Percival Lowell thought that these features were not naturally occurring waterways but structures that had been built by an advanced but dying Martian civilization. Most astronomers could see no canals, however, and many doubted their reality. The controversy was finally resolved only when pictures sent from the United States Mariner probes showed many craters but nothing resembling manufactured channels or canals.Four of the Mariner series of unmanned space probes launched by the National Aeronautics and Space Administration (NASA) investigated Mars. The first craft to successfully fly by Mars was Mariner 4, which photographed the planet as it passed it in 1965. It was followed by Mariners 6 and 7, which analyzed the atmosphere and captured images as they flew by in 1969. The first spacecraft to orbit a planet other than Earth was Mariner 9, which circled Mars for nearly a year in 1971–72. The Soviet Union also sent a series of unmanned space probes to Mars in the 1960s and 1970s. Its Mars 3 lander, the first craft to successfully soft-land on the planet, touched down in 1971 amid a global dust storm. Its communications systems failed after about 20 seconds.
The United States Viking probes, consisting of two orbiting spacecraft and two landers, were intended in part to search for evidence of past or present forms of life on Mars. The two landers touched down on the planet in 1976 and performed numerous experiments, including a detailed chemical analysis of the Martian atmosphere and soil. No trace of any organic material was found.The next United States probe, the Mars Observer, was launched in September 1992 and was programmed to land on Mars in August 1993. The 980-million-dollar spacecraft lost contact with Earth, however, and was presumed lost in September 1993.
A team of scientists announced in 1996 that a meteorite from Mars that fell to Earth 13,000 years ago contained organic molecules, minerals, and carbonate globules that are all associated with bacterial life. The team believed that this provided the first evidence of life on early Mars. Most scientists, however, were skeptical of this claim.
In 1996 the United States launched the Mars Global Surveyor, the first in a new series of unmanned spacecraft designed to explore the planet. The probe began to orbit Mars in September 1997. After a delay due to an equipment malfunction, in March 1999 the craft began mapping a variety of the planet's properties, including its gravity and magnetic fields and the topography and mineral composition of the surface. It also took more than 100,000 photographs of the surface.
In 1997 the next Martian mission—the Pathfinder—landed on Mars to study the geology and atmosphere of the planet. On board the Mars Pathfinder was a small roving vehicle called Sojourner that collected and analyzed samples of the Martian soil and beamed images back to Earth. Evidence collected during Pathfinder's 83 days of surface operations indicated that the surface of Mars bears signs of an ancient Earth-like atmosphere and geology. Water-worn rock conglomerates and other sand and surface features that could only have been created by flowing, liquid water mark the surface of the planet. A tremendous amount of differentiation—heating, cooling, and recycling of crust material—appeared to have taken place at some point in the planet's history.Mars exploration suffered two major setbacks in the late 1990s. NASA's unmanned Mars Climate Orbiter, launched in December 1998, was designed to transmit daily weather images and other atmospheric data for a full Martian year, or 687 days. However, confusion of English and metric units in key navigation figures sent the craft off course as it attempted to enter orbit around Mars in September 1999. Less than three months later, NASA lost contact with another craft, the Mars Polar Lander, as it approached touchdown on the Mars surface.
Another global mapping orbiter, NASA's 2001 Mars Odyssey, reached the planet in October 2001. In addition to mapping the chemical composition of the surface, the orbiter confirmed the presence of water ice in the subsurface and revealed its distribution. Along with the Mars Global Surveyor, Odyssey also acted as a communications relay for two robotic wheeled rovers, Spirit and Opportunity. NASA's twin rovers landed on Mars in January 2004 to collect geologic data to help determine whether the planet's environment was suitable for life in the past. Spirit landed in Gusev Crater, which some scientists believe may have been an ancient lake bed. Opportunity arrived at Meridiani Planum. In the gravel at its landing site, Opportunity confirmed the presence of gray hematite, a mineral that on Earth usually forms in association with water.
The European Space Agency (ESA) sent its first mission to Mars in 2003. The orbiter Mars Express, designed to map a variety of properties in the Martian atmosphere, surface, and subsurface and to photograph surface features using a high-resolution stereoscopic camera, arrived at the planet in December 2003. The orbiter carried on board the British lander Beagle 2. After being released at Mars, however, the lander failed to return communications signals and was declared lost.