Mars (planet)

I

Introduction

Mars (planet), fourth planet in distance from the Sun in the solar system. Mars is of special scientific interest because of its similarities to Earth. It has an atmosphere with seasons and changing weather, and its surface shows evidence of ancient water and volcanoes. The length of its day and the tilt of its axis are similar to those of Earth. Mars takes about two years to circle the Sun at an average distance of 228 million km (141.7 million mi). The possibility of life on Mars, now or in the distant past, is one of the major questions in astronomy. More space probes have been sent to Mars than to any other planet. Mars is named for the Roman god of war. It is sometimes called the red planet because it appears fiery red in Earth’s night sky, the result of rusty, iron-oxide mineral dust that covers its surface.

Mars is a relatively small planet, with a diameter of about 6,794 km (4,222 mi) or about half the diameter of Earth. Mars has about one-tenth Earth’s mass. The force of gravity on the surface of Mars is about three-eighths of that on Earth. Mars has twice the diameter and twice the surface gravity of Earth’s Moon. The surface area of Mars is almost exactly the same as the surface area of the dry land on Earth. Mars is believed to be about the same age as Earth, having formed from the same spinning, condensing cloud of gas and dust that formed the Sun and the other planets about 4.6 billion years ago.

Mars has two moons, Phobos and Deimos, which are named after the sons of the Roman god Mars. These tiny bodies are heavily cratered, dark chunks of rock and may be asteroids captured by the gravitational pull of Mars. Phobos orbits Mars once in less than one Martian day, so it appears to rise in the west and set in the east, usually twice each day. Deimos has the more ordinary habit of rising in the east and setting in the west.

II

Observation from Earth

Mars appears as a fairly bright, red, starlike object in Earth’s night sky. Because of the relative movements of Earth and Mars around the Sun, Mars appears to move backward in the sky for a short time around opposition, which is the time when the two planets are closest. As Mars and Earth orbit the Sun, the distance between them varies from about 56 million km (about 35 million mi) at their closest approaches to about 375 million km (about 233 million mi) when the planets are on opposite sides of the Sun. This change in distance causes the apparent size of Mars to vary by more than a factor of 5 and its brightness to vary by a factor of 25. Because the orbit of Mars is elliptical and not circular, Earth and Mars approach each other more closely during some orbits than others. For example, in late August 2003 Earth and Mars passed closer to each other than at any time since 1924. The two planets will not get that close again until the year 2287.

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When Mars is viewed through a telescope, it looks like a reddish-orange disk. When Mars is close to Earth, an observer with a telescope can usually see white ice caps at the north and south poles of Mars. These polar caps grow and shrink throughout the Martian year, just as the polar caps of Earth do. The darker areas of Mars’s surface may look greenish to the telescope observer, but this is an optical illusion caused by the contrast in color between the dark patches and the redder, brighter areas. Scientists believe that the dark areas are regions of relatively unweathered dark rocks and sand, while the bright areas are regions with deposits of dusty, fine-grained oxidized iron minerals. Scientists now believe that the “canals” people observed on Mars during the 19th century are actually another optical illusion, caused by the mind’s tendency to draw connections between irregular patches in a fuzzy image.

The Hubble Space Telescope (HST) provides the clearest Earth-based views of Mars, and astronomers use it to study the composition of the surface and to monitor the weather on the planet. HST has provided detailed images of local and global dust storms, enormous spiral-shaped water ice cloud systems, and changes in the bright and dark surface markings that have occurred since the first detailed images were taken during the 1970s. The telescope also has enabled spectroscopic measurements that provide comprehensive information on atmospheric chemistry and on the nature and variability of ices and minerals on the surface. Using HST images and other data, astronomers have determined that the atmosphere of Mars is generally cooler and clearer when the planet is farther from the Sun and warmer and dustier when it is closer. There also appear to be longer-term trends in the Martian climate, but as is the case for Earth’s climate, scientists are only now beginning to untangle the complexities required to understand and perhaps one day even predict climate changes on Mars. Orbiting spacecraft around Mars furnish constant data about the planet. However, they orbit so close to the planet and are in a fixed orientation relative to the Sun that they cannot see features in the early morning or late afternoon parts of the Martian day. As a result, astronomers still need telescopes like the HST to study Mars, particularly its early morning and late afternoon cloud formations.

III

Orbit and Rotation

Mars orbits the Sun at an average distance of about 228 million km (141.7 million mi), or 1.524 astronomical units (AU). An AU is equal to the average distance between the Earth and the Sun, or about 150 million km (93 million mi). However, Mars’s orbit is more elliptical than Earth’s—its nearest point to the Sun (perihelion) is about 42 million km (26 million mi) closer than its farthest point (aphelion), compared with only a 5 million km (3 million mi) difference between perihelion and aphelion for Earth. Mars’s year, or the time it takes to revolve once around the Sun, is about two Earth years long (687 Earth days). Mars receives less than half the amount of sunlight Earth does and is much colder.

Mars is tilted on its axis by about 25° (Earth is tilted at 23.5°). This tilt gives Mars seasons similar to Earth’s seasons. The elliptical orbit of Mars, however, causes the planet to have seasons of unequal lengths. For example, the southern hemisphere’s summer on Mars is about 25 days shorter than the northern summer. The intensity of sunlight also changes substantially during the Martian year: solar heating during the southern summer, when Mars is closer to the Sun, is 40 percent more intense than in the northern summer. During the warmer spring and summer period in the southern hemisphere, great dust storms have sometimes been observed through telescopes as bright yellow clouds. Sometimes white clouds of water vapor are visible, especially during the northern summer when Mars is near its farthest point from the Sun and its thin atmosphere is the coldest.

Like Earth, Mars turns counterclockwise on its axis (from west to east) when seen from its north pole and orbits the Sun in a counterclockwise direction. It takes Mars 24 hours and 37 minutes to rotate once on its axis (its sidereal day). Its solar day (the time between when the Sun next crosses the noon point in the sky) is about 24 hours and 39 minutes—its orbital motion around the Sun adds two minutes to its rotation period. (Earth’s solar day (24 hours) is four minutes longer than its rotation period.) The Martian solar day is sometimes called a sol.

IV

The Interior of Mars

The density of Mars is about 30 percent less than that of Earth (3.94 g/cm³ vs. 5.52 g/cm³). Based on spacecraft measurements of the Martian gravitational field, scientists believe that the planet’s interior consists of a crust, mantle, and core like Earth’s interior. While the relative sizes of these components are not known for certain, the planet’s lower density combined with spacecraft mapping of the structure of its gravity field suggest that the planet’s iron-rich core and mantle are a smaller fraction of its volume than in the case of Earth. Mars therefore probably has a relatively thick crust compared to Earth. Beneath the Tharsis bulge, an area of volcanic activity in the northern hemisphere, the crust may be as thick as 130 km (80 mi). But the crustal thickness appears to vary significantly. For example, beneath the landing site of the United States spacecraft Viking 2, it may be as thin as 15 km (9 mi).

The Martian core is probably much like Earth’s, consisting mostly of iron, with a small amount of nickel. If other light elements, particularly sulfur, exist there as well, the core may be larger than presently thought. From studying Earth’s magnetic field and core, scientists theorize that the motions of the liquid rock in Earth’s core generate its magnetic field. Mars does not have a significant magnetic field, so scientists believe that Mars’s core is probably solid. However, spacecraft data indicate that Mars probably did have a strong magnetic field early in its history, suggesting that the core of Mars may have been at least partially liquid at one time.

Tectonics on the Earth is dominated by the relative motions and collisions of a few dozen large, moving lithospheric plates. Earthlike plate tectonics does not appear to be active on Mars today. However, there is considerable debate over whether Mars may have had plate tectonics in the distant past, when the core may have been molten. Ancient magnetic field patterns preserved in the crust show some similarities to magnetic field patterns that arise from plate tectonic processes on Earth. Because Mars is so much smaller than Earth, however, its more rapid cooling and crustal thickening after formation may have favored the creation of a one-plate planet rather than Earthlike plate tectonics.

Heat that melted at least some of the Martian interior has sculpted parts of the planet’s surface. In some places molten rock broke through the crust to form volcanoes. In other places, large-scale motions of the partially molten mantle cracked the crust to form large rifts and canyon systems. Scientists do not know if the interior of Mars is still geologically active. No evidence for active volcanism or tectonic movement has been found on the planet. However, images from orbiting spacecraft suggest that some of the Tharsis volcanoes have been periodically active in the last 100 to 350 million years, and perhaps as recently as 2 million years ago. Smaller volcanic cones discovered around the north pole may have erupted as recently as 1 million years ago.

Additional details about the Martian interior may have to await a time when more sophisticated spacecraft or even astronauts bring instruments such as seismometers to the planet, providing information similar to that which scientists routinely obtain for Earth’s interior today.

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