The fourth planet from the Sun is Mars. Easily visible from Earth with the naked eye, it has intrigued star-gazers 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.
Over the centuries, observers have noted various phenomena on the Martian surface, some of which they thought might be signs of life. For instance, dark markings cover about a third of the surface and change in a seasonal pattern in both extent and color. They were once thought to be vast seas or areas of vegetation. A few astronomers even thought they saw straight lines that could have been canals.
Mars, the most Earth-like of the planets, appears in a computer-generated image based on photographs taken by Mars Global Surveyor on a day in the northern hemisphere’s summer. At the top of the globe is the northern polar ice cap, while the huge gash just below the equator is the canyon system Valles Marineris. White clouds of water ice surround the peaks of the most prominent volcanoes. NASA/JPL/Malin Space Science Systems
The explanation of these and many other observations had to await the first exploratory space missions in the 1960s and ’70s. Meanwhile, the so-called red planet captured the popular imagination as a possible home of alien life—the armies of “little green men” of science fiction stories, movies, and radio and television programs. Scientists now know that there are no manufactured canals on Mars. The changes in the dark areas result largely from dust, which shifts along with the winds. There are no humanoids on the planet, nor even any animals or plants. No forms of life have yet been found on Mars.
Nevertheless, the search for life on Mars continues, in part because it shows signs of having been wetter in the past. Water is necessary for all known forms of life. It is possible that microscopic life once existed on Mars. Today the surface is too cold and the air is too thin for liquid water to exist there for long. It is also bombarded with ultraviolet radiation from the Sun, which is very harmful to living things. (Earth’s denser atmosphere protects it from most of this radiation.) If any life-form exists on Mars today, many scientists believe it would be tiny organisms in protected niches just below the surface. Whether or not Mars has ever had life, the planet remains an intriguing object of study.
An image taken by Mars Global Surveyor shows outcroppings of sedimentary rock layers in the southwestern Candor Chasma region of the Valles Marineris canyon system. Some scientists have interpreted these formations as evidence that lakes once partially filled the canyons. NASA/JPL/Malin Space Science Systems
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.
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 is the outermost of the four inner planets of the solar system. 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.
Mars’s spin axis is tilted about 24.9° relative to the plane in which it orbits. As the planet travels in its orbit, first the northern hemisphere, then the southern hemisphere is tipped toward to the Sun. As a result, there are four distinct seasons on Mars. The ice caps at the poles alternately grow and shrink as the seasons change. Encyclopædia Britannica, Inc.
Mars’s two moons, Phobos and Deimos, are small and rocky. They are named after sons of the ancient Greek war god Ares (the counterpart of the Roman god Mars). Phobos means “fear” in Greek, while Deimos means “terror.” They were discovered in 1877 by the astronomer Asaph Hall. Both moons are so small that their gravity is too weak to pull them into spherical shapes. Instead, they are shaped more or less like potatoes. Phobos is about 16.5 miles (27 kilometers) long at its longest point, while Deimos is only about 9.5 miles (15 kilometers) long.
Each moon takes the same amount of time to rotate once on its axis as it does to complete one orbit around Mars. This means that, like Earth’s Moon, they always point the same face toward their planet. Deimos takes nearly one and a half Earth days to circle Mars, while Phobos completes about three orbits around Mars in one Earth day. Phobos is very close to Mars—within roughly 5,825 miles (9,375 kilometers) of the planet’s center—and the planet’s gravity draws the moon ever so slightly closer with each orbit. Astronomers think that Phobos might crash into Mars sometime in the next 100 million years. Deimos orbits Mars at about 14,575 miles (23,460 kilometers) from the planet’s center and is gradually moving farther away.
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. Phobos and Deimos may once have been asteroidlike objects that came too close to Mars and were captured by its gravity.
The Martian moons Phobos (left) and Deimos (right) are shown in photographs by the Viking orbiters. Deimos’s smooth surface is contrasted with the grooved, pitted, and cratered surface of Phobos. The prominent cavity on the end of Phobos is the crater Stickney. The images are not to scale; Phobos is about 75 percent larger than its companion. National Aeronautics and Space Administration/Malin Space Science Systems
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 rotates on its axis at roughly the same rate as Earth. A Martian day, called a sol, lasts about 24 hours and 37 minutes, which is just a bit longer than an Earth day. Its spin axis is also tilted at an angle similar to Earth’s—there is only about one degree of difference between the spin axes of the two planets. As a result, Mars has distinct seasons like Earth does. First one hemisphere, then the other receives more sunlight during the planet’s orbit around the Sun. Because Mars has a more eccentric orbit than Earth’s, Martian seasons are less equal in length. The Martian summer, for example, is about 16 percent longer in the north than in the south.
