Mars
The fourth planet from the Sun is Mars. Easily visible from Earth with the naked eye, it has intrigued stargazers since ancient times. It often appears quite bright and reddish in the night sky. Babylonians mentioned Mars in records from about 3,000 years ago, associating the blood-red planet with their god of death and disease. The name Mars is that of the ancient Roman god of war.
Mars is also a nearly ideal subject for observing with a telescope from Earth. Venus approaches more closely to Earth in its orbit. However, Mars also passes relatively near Earth and, unlike Venus, its surface is generally not obscured by thick clouds. Mars is farther from the Sun than Earth is, so the planet often appears high in the sky. Venus, on the other hand, can never be seen far from the glare of the Sun.
Basic Planetary Data
Size, Mass, and Density Mars is the second smallest planet in the solar system, after Mercury. Its diameter at the equator is 4,221 miles (6,792 kilometers), which is only slightly more than half the size of Earth's. Mars's lower density makes the planet only about a 10th as massive as Earth. In fact, Mars's density—which is about four times that of water—is closer to that of Earth's Moon than to that of the three other inner planets.
Orbit and Spin Like all the planets, Mars travels around the Sun in an elliptical, or oval-shaped, orbit. Its average distance from the Sun is almost 142 million miles (228 million kilometers), which is roughly 1.5 times greater than Earth's. Mars's orbit is more eccentric, or elongated, than Earth's, however, so its distance from the Sun varies more. Mars is about 128 million miles (207 million kilometers) from the Sun at its perihelion, or the closest point in its orbit to the Sun. At its aphelion, its farthest point from the Sun, it is some 155 million miles (249 million kilometers) away.
The planet completes one revolution around the Sun in about 687 Earth days. In other words, a year on Mars is about 687 Earth days long. That is almost twice the time it takes Earth to complete its orbit of about 365 days.
Mars's distance from 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 Earth so that it appears both bright and large. The planet is easiest to observe when it is at opposition, or when it is on the opposite side of Earth from the Sun. Mars then appears high in Earth's sky, and its full face is lighted. Oppositions of Mars occur about every 26 months. About every 15 years Mars is both close to Earth and in opposition, an arrangement that provides optimal viewing conditions.
Atmosphere, Surface, and Interior
Atmosphere
The lower Martian atmosphere is roughly as cold as the air on Earth above Antarctica in the winter. It is typically about −100° F (−70° C). Clouds, haze, and fog are common in the air near the ground, especially over valleys, craters, volcanoes, and other areas of low or high surface elevation. Most of these low-lying clouds are formed of water ice. In addition, very thin clouds, perhaps of carbon dioxide, are found quite high up in the atmosphere.
At the surface the winds are generally light. The average wind speeds are typically less than 4.5 miles (7.2 kilometers) per hour, though gusts sometimes blow as fast as 90 miles (144 kilometers) per hour. In winter Mars has strong jet streams like Earth's, which blow westward high in the atmosphere. These winds are much lighter during the Martian spring and summer. Unlike on Earth, the air circulation on Mars also has a fairly strong north-south pattern, which transfers air from pole to pole.
The two caps differ somewhat in size and composition. The southern cap is larger. It extends to about 50° S. at its greatest extent, compared with 55° N. for the northern ice cap. The north pole has a small permanent cap of water ice. It temporarily accumulates larger areas of carbon dioxide ice (or “dry ice”) in fall and winter. The south pole's cap seems to contain some carbon dioxide ice and some water ice year-round (which grow and shrink seasonally).
Surface
Water
Interior Scientists do not have direct information about the Martian interior. Instead, they develop models of the interior based on the planet's known characteristics, such as its size, mass, rotation rate, volcanic activity, magnetic properties, and gravity signature. In addition, more than 30 meteorites that have fallen to Earth are known to have come from Mars. The chemical composition of the meteorites indicates that Mars has separated into three main layers like Earth. Earth and Mars both have a metal-rich core at the center; a large, rocky middle layer called the mantle; and an outer crust.
Mars's core is probably rich in iron and sulfur. Scientists estimate that the core has a diameter of about 1,600–2,400 miles (2,600–4,000 kilometers). Unlike most other planets, Mars has no global magnetic field. Scientists believe that fluid motions in a planet's core help generate a magnetic field, so having such a field would indicate flow in the core. It is not known whether Mars's core is currently solid or liquid. However, ancient, highly magnetized rocks in its southern hemisphere suggest that in the past Mars had a strong magnetic field, which disappeared as the planet cooled. Scientists believe that Mars is still volcanically active, so its mantle is probably still warm and in some places is undergoing melting.
Measurements of the planet's gravity indicate that the crust is thinner and denser in the northern hemisphere than in the south. The thickness of the crust is thought to vary from about only 2 miles (3 kilometers) in places just north of the equator to more than 60 miles (90 kilometers) in southern Tharsis.
Moons
The surface of Phobos is very heavily cratered and grooved. One of its craters, named Stickney, is about half as wide as Phobos itself. The surface of Deimos appears smoother because its craters are almost buried in a layer of fine rubble. The moons reflect very little light. They are probably similar in composition to carbonaceous chondrite meteorites. Phobos and Deimos may once have been asteroidlike objects that came too close to Mars and were captured by its gravity.
Observation and Exploration
Many unmanned spacecraft have been sent to study Mars, including craft that have flown by, orbited, and landed on the planet. Wheeled robotic roving vehicles called rovers have also investigated the surface. No manned crews have yet been sent to the planet, but it is likely that Mars will be the first world other than Earth and Earth's Moon that humans visit.
Four of the Mariner series of unmanned space probes launched by the United States 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 by in July 1965. Its images showed heavily cratered surfaces that resemble Earth's Moon. Mariners 6 and 7 analyzed the atmosphere and captured images as they flew by Mars in July–August 1969. The first spacecraft to orbit a planet other than Earth was Mariner 9. It photographed the Martian surface for nearly a year in 1971–72, revealing widespread volcanic activity and features carved by water in the remote past.
The Soviet Union also sent a series of unmanned space probes to Mars in the 1960s and '70s. Its Mars 3 lander was the first craft to successfully soft-land on the planet, in December 1971. Unfortunately, it touched down during a global dust storm, which caused its communications systems to fail after about 20 seconds.
The Soviet Union sent two probes, Phobos 1 and 2, to study the Martian moon Phobos in July 1988. Mission scientists lost contact with the first craft before it reached its target, but Phobos 2 successfully reached the moon in 1989. It collected data on Phobos and Mars for several days before it, too, malfunctioned.
A fairly high percentage of missions to Mars have failed, including three United States missions in the 1990s: Mars Observer, Mars Climate Orbiter, and Mars Polar Lander. Nozomi, a Japanese orbiter launched in 1998, reached Mars but then malfunctioned and could not be placed into orbit.
Several craft began exploring Mars in the early 21st century. NASA's global mapping orbiter named 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 just below the surface. Another mapping orbiter, NASA's Mars Reconnaissance Orbiter, arrived at the planet in March 2006 to study the climate and surface features associated with liquid water.
The European Space Agency (ESA) sent its first mission to Mars in June 2003. After arriving at the planet in December of that year, the orbiter Mars Express mapped a variety of properties in the Martian atmosphere, surface, and subsurface and photographed surface features using a high-resolution stereoscopic camera. It found what appears to be a large frozen sea just under the Martian surface near the equator. Among the orbiter's other findings were the detection of auroras, tiny amounts of methane in the atmosphere, and both water ice and carbon dioxide ice in the southern polar ice cap. It found that the layered deposits in the south polar region contain huge amounts of water ice and are a couple of miles deep in some places. The orbiter carried on board a British lander named Beagle 2. After being released at Mars, however, the lander failed to return communications signals and was declared lost.
Mars is also a nearly ideal subject for observing with a telescope from Earth. Venus approaches more closely to Earth in its orbit. However, Mars also passes relatively near Earth and, unlike Venus, its surface is generally not obscured by thick clouds. Mars is farther from the Sun than Earth is, so the planet often appears high in the sky. Venus, on the other hand, can never be seen far from the glare of the Sun.
- Telescopic observers have noted distinctive bright and dark features on Mars for hundreds of years. …
- The first color photograph of the Martian surface shows the boulder-strewn plain of Chryse …
- An artist's rendering shows one of NASA's two Mars Exploration Rovers on the surface of the red …
Basic Planetary Data
- Images and simulations provide an overview of Mars, the fourth planet from the sun. Mars is the …
Size, Mass, and Density Mars is the second smallest planet in the solar system, after Mercury. Its diameter at the equator is 4,221 miles (6,792 kilometers), which is only slightly more than half the size of Earth's. Mars's lower density makes the planet only about a 10th as massive as Earth. In fact, Mars's density—which is about four times that of water—is closer to that of Earth's Moon than to that of the three other inner planets.
Orbit and Spin Like all the planets, Mars travels around the Sun in an elliptical, or oval-shaped, orbit. Its average distance from the Sun is almost 142 million miles (228 million kilometers), which is roughly 1.5 times greater than Earth's. Mars's orbit is more eccentric, or elongated, than Earth's, however, so its distance from the Sun varies more. Mars is about 128 million miles (207 million kilometers) from the Sun at its perihelion, or the closest point in its orbit to the Sun. At its aphelion, its farthest point from the Sun, it is some 155 million miles (249 million kilometers) away.
The planet completes one revolution around the Sun in about 687 Earth days. In other words, a year on Mars is about 687 Earth days long. That is almost twice the time it takes Earth to complete its orbit of about 365 days.
Mars's distance from 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 Earth so that it appears both bright and large. The planet is easiest to observe when it is at opposition, or when it is on the opposite side of Earth from the Sun. Mars then appears high in Earth's sky, and its full face is lighted. Oppositions of Mars occur about every 26 months. About every 15 years Mars is both close to Earth and in opposition, an arrangement that provides optimal viewing conditions.
- Mars's spin axis is tilted about 24.9° relative to the plane in which it orbits. As the planet …
Atmosphere, Surface, and Interior
- Small metallic spheres, dubbed “blueberries,” on Mars suggest the past presence of …
- A microscopic view taken by the Mars Exploration Rover Opportunity shows spheres of hematite amid …
Atmosphere
- Mars Global Surveyor photographed a large storm system high above Mars's north polar region. The …
The lower Martian atmosphere is roughly as cold as the air on Earth above Antarctica in the winter. It is typically about −100° F (−70° C). Clouds, haze, and fog are common in the air near the ground, especially over valleys, craters, volcanoes, and other areas of low or high surface elevation. Most of these low-lying clouds are formed of water ice. In addition, very thin clouds, perhaps of carbon dioxide, are found quite high up in the atmosphere.
At the surface the winds are generally light. The average wind speeds are typically less than 4.5 miles (7.2 kilometers) per hour, though gusts sometimes blow as fast as 90 miles (144 kilometers) per hour. In winter Mars has strong jet streams like Earth's, which blow westward high in the atmosphere. These winds are much lighter during the Martian spring and summer. Unlike on Earth, the air circulation on Mars also has a fairly strong north-south pattern, which transfers air from pole to pole.
- A whirlwind of dust, called a dust devil, appears in an image captured by Mars Global Surveyor. Its …
- The permanent cap of water ice at the Martian north pole appears in two images taken by Mars Global …
The two caps differ somewhat in size and composition. The southern cap is larger. It extends to about 50° S. at its greatest extent, compared with 55° N. for the northern ice cap. The north pole has a small permanent cap of water ice. It temporarily accumulates larger areas of carbon dioxide ice (or “dry ice”) in fall and winter. The south pole's cap seems to contain some carbon dioxide ice and some water ice year-round (which grow and shrink seasonally).
Surface
- The “McMurdo” panorama taken by the Mars Exploration Rover Spirit shows the area around …
- A large Martian crater called Victoria, shown in a Mars Reconnaissance Orbiter image, has a …
- A simulated flight through a large Martian canyon was generated by computer from spacecraft data. …
- The Martian volcano Olympus Mons is the largest known volcano in the solar system. It appears in a …
- A map of the topography of Mars shows the contrast between the heavily cratered highlands in the …
- A composite of several images taken by the Mars Pathfinder lander shows the boulder-strewn surface …
- A mosaic of more than 100 photographs of Mars taken by the Viking orbiters shows the large …
- Olympus Mons, Mars's tallest volcano, appears in an image taken by the Mars Global Surveyor …
- Valles Marineris is the largest canyon system on Mars. A composite of images taken by the Viking 1 …
- An image taken by Mars Global Surveyor shows outcroppings of sedimentary rock layers in the …
Water
- Many surface features on Mars seem to have been formed by liquid water. Three outflow channels …
- The steep-walled channel Nanedi Valles, photographed by the Mars Express spacecraft, may have …
- An image reveals what might be a fractured sea of frozen water lying just below the Martian surface …
- Just south of the Martian north pole, at about 70.5° N., the Mars Express spacecraft found a …
- Gullies line the steep wall of a crater in Newton Basin in Sirenum Terra on Mars in an image taken …
Interior Scientists do not have direct information about the Martian interior. Instead, they develop models of the interior based on the planet's known characteristics, such as its size, mass, rotation rate, volcanic activity, magnetic properties, and gravity signature. In addition, more than 30 meteorites that have fallen to Earth are known to have come from Mars. The chemical composition of the meteorites indicates that Mars has separated into three main layers like Earth. Earth and Mars both have a metal-rich core at the center; a large, rocky middle layer called the mantle; and an outer crust.
Mars's core is probably rich in iron and sulfur. Scientists estimate that the core has a diameter of about 1,600–2,400 miles (2,600–4,000 kilometers). Unlike most other planets, Mars has no global magnetic field. Scientists believe that fluid motions in a planet's core help generate a magnetic field, so having such a field would indicate flow in the core. It is not known whether Mars's core is currently solid or liquid. However, ancient, highly magnetized rocks in its southern hemisphere suggest that in the past Mars had a strong magnetic field, which disappeared as the planet cooled. Scientists believe that Mars is still volcanically active, so its mantle is probably still warm and in some places is undergoing melting.
Measurements of the planet's gravity indicate that the crust is thinner and denser in the northern hemisphere than in the south. The thickness of the crust is thought to vary from about only 2 miles (3 kilometers) in places just north of the equator to more than 60 miles (90 kilometers) in southern Tharsis.
Moons
- The Martian moons Phobos (left) and Deimos (right) are shown in photographs by the Viking orbiters. …
- The Martian moon Phobos appears in an image taken by Mars Express. The large Stickney crater is …
The surface of Phobos is very heavily cratered and grooved. One of its craters, named Stickney, is about half as wide as Phobos itself. The surface of Deimos appears smoother because its craters are almost buried in a layer of fine rubble. The moons reflect very little light. They are probably similar in composition to carbonaceous chondrite meteorites. Phobos and Deimos may once have been asteroidlike objects that came too close to Mars and were captured by its gravity.
Observation and Exploration
- The unmanned U.S. Mariner 4 spacecraft was launched on Nov. 28, 1964, and passed within about 6,117 …
Many unmanned spacecraft have been sent to study Mars, including craft that have flown by, orbited, and landed on the planet. Wheeled robotic roving vehicles called rovers have also investigated the surface. No manned crews have yet been sent to the planet, but it is likely that Mars will be the first world other than Earth and Earth's Moon that humans visit.
Four of the Mariner series of unmanned space probes launched by the United States 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 by in July 1965. Its images showed heavily cratered surfaces that resemble Earth's Moon. Mariners 6 and 7 analyzed the atmosphere and captured images as they flew by Mars in July–August 1969. The first spacecraft to orbit a planet other than Earth was Mariner 9. It photographed the Martian surface for nearly a year in 1971–72, revealing widespread volcanic activity and features carved by water in the remote past.
The Soviet Union also sent a series of unmanned space probes to Mars in the 1960s and '70s. Its Mars 3 lander was the first craft to successfully soft-land on the planet, in December 1971. Unfortunately, it touched down during a global dust storm, which caused its communications systems to fail after about 20 seconds.
- The Viking 2 lander (foreground) was photographed on Mars in 1976 by one of the spacecraft's own …
The Soviet Union sent two probes, Phobos 1 and 2, to study the Martian moon Phobos in July 1988. Mission scientists lost contact with the first craft before it reached its target, but Phobos 2 successfully reached the moon in 1989. It collected data on Phobos and Mars for several days before it, too, malfunctioned.
A fairly high percentage of missions to Mars have failed, including three United States missions in the 1990s: Mars Observer, Mars Climate Orbiter, and Mars Polar Lander. Nozomi, a Japanese orbiter launched in 1998, reached Mars but then malfunctioned and could not be placed into orbit.
- A microscopic image of the Martian meteorite ALH84001 shows an elongated structure at center that …
- The robotic rover Sojourner rolls onto the surface of Mars. The Pathfinder spacecraft captured the …
- Artwork depicts the Mars Global Surveyor spacecraft orbiting Mars above the volcano named Olympus …
Several craft began exploring Mars in the early 21st century. NASA's global mapping orbiter named 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 just below the surface. Another mapping orbiter, NASA's Mars Reconnaissance Orbiter, arrived at the planet in March 2006 to study the climate and surface features associated with liquid water.
The European Space Agency (ESA) sent its first mission to Mars in June 2003. After arriving at the planet in December of that year, the orbiter Mars Express mapped a variety of properties in the Martian atmosphere, surface, and subsurface and photographed surface features using a high-resolution stereoscopic camera. It found what appears to be a large frozen sea just under the Martian surface near the equator. Among the orbiter's other findings were the detection of auroras, tiny amounts of methane in the atmosphere, and both water ice and carbon dioxide ice in the southern polar ice cap. It found that the layered deposits in the south polar region contain huge amounts of water ice and are a couple of miles deep in some places. The orbiter carried on board a British lander named Beagle 2. After being released at Mars, however, the lander failed to return communications signals and was declared lost.
- Engineers at NASA's Jet Propulsion Laboratory experimented with a test rover on Earth to explore …