Mars Mars is the fourth planet from the Sun and the outermost of the four terrestrial worlds in the Solar System. It lies outside Earth s orbit. Mars s orbital eccentricity is 0.093, much larger than that of most other planets only the innermost and the outermost planets, Mercury and Pluto, have more elongated orbits. Because of this, Mars s perihelion distance from the Sun 1.38 A.U. (207 million km) is substantially smaller than its aphelion distance 1.67 A.U. (249 million km) resulting in a large variation in the amount of sunlight striking the planet over the course of its year. The intensity of sunlight on the Martian surface is almost 45 percent greater when the planet is at perihelion than when it is at aphelion. This has a substantial effect on the Martian climate. Physical Properties of Mars Unlike Mercury and Venus, Mars has two small moons in orbit around it: Phobos and Deimos. The larger of the two, Phobos, orbits at a distance of just 9378 km from the center of the planet once every 459 minutes. Applying the modified version of Kepler s third law (the square of a moon s orbital period is proportional to the cube of its orbital semimajor axis divided by the mass of the planet it orbits), we find that the mass of Mars is 6.4 1023 kg, or 0.11 times that of Earth. The orbit of Deimos yields the same result. From the mass and radius, we find that the average density of Mars is 3900 kg/m3, only slightly greater than that of the Moon. If we assume that the Martian surface rocks are similar to those on the other terrestrial planets, this average density suggests the existence of a substantial core of higher than average density within the planet. Planetary scientists now believe that this core is composed largely of iron sulfide (a compound about twice as dense as surface rock) and has a diameter of about 2500 km. Mars rotates once on its axis every 24.6 hours. One Martian day is thus very similar in length to one Earth day. The planet s equator is inclined to the orbit plane at an angle of 24.0, again very similar to Earth s inclination of 23.5. Thus, as Mars orbits the Sun, we find both daily and seasonal cycles, just as on Earth. In the case of Mars, however, the seasons are complicated somewhat by variations in solar heating due to the planet s eccentric orbit southern summer occurs around the time of Martian perihelion and so is significantly warmer than summer in the north. Viewed from Earth, the most obvious Martian surface features are the bright polar caps. They grow or diminish according to the seasons, almost disappearing at the time of Martian summer. The caps are mostly
frozen carbon dioxide (that is, dry ice), not water ice, as at Earth s North and South poles. The polar caps do contain water, but it remains permanently frozen. The dark markings are actually highly cratered and eroded areas around which surface dust occasionally blows. Repeated covering and uncovering of these landmarks gives the impression (from a distance) of surface variability, but it s only the thin dust cover that changes. The Martian surface dust is borne aloft by strong winds that often reach hurricane proportions (hundreds of kilometers per hour). When the U.S. Mariner 9 spacecraft went into orbit around Mars in 1971, a planetwide dust storm obscured the entire landscape. The Surface of Mars Maps of the surface of Mars returned by orbiting spacecraft show a wide range of geological features. Mars has huge volcanoes, deep canyons, vast dune fields, and many other geological wonders. About 5000 km across, the Tharsis region bulges out from the planet s equatorial zone, rising to a height of about 10 km. The large volcanoes on the left mark the approximate peak of the bulge. One of the plains flanking the Tharsis bulge, Chryse Planitia, is toward the right. Dominating the center of the field of view is a vast canyon known as Valles Marineris. The northern hemisphere is made up largely of rolling volcanic plains, not unlike the lunar maria. These extensive lava plains much larger than those found on Earth or the Moon were formed by eruptions involving enormous volumes of lava. They are strewn with blocks of volcanic rock as well as with boulders blasted out of impact areas by infalling meteoroids (the Martian atmosphere is too thin to offer much resistance to incoming debris). The southern hemisphere consists of heavily cratered highlands lying some 5 kilometers above the level of the lowland north. Most of the dark regions visible from Earth are mountainous regions in the south. The northern plains are much less cratered than the southern highlands. This smoother surface suggests that the northern surface is younger. Its age is perhaps 3 billion years, compared with 4 billion in the south. Most scientists assume that the southern terrain is the original crust of the planet. How most of the northern hemisphere could have been lowered in elevation and subsequently flooded with lava remains a mystery. Olympus Mons is the largest volcano known on Mars or anywhere else in the Solar System. Nearly three times taller than Mount Everest on Earth, this Martian mountain measures about 700 km across the base and 25 km high at the peak. It seems currently inactive and may have been extinct for at least several hundred million years.
Another feature associated with the Tharsis bulge is a great canyon known as Valles Marineris (the Mariner Valley). Planetary astronomers believe that it was formed by the same crustal forces that caused the entire Tharsis region to bulge outward, making the surface split and crack. These cracks, called tectonic fractures, are found all around the Tharsis bulge. The Valles Marineris is the largest of them. Cratering studies suggest that the cracks are at least 2 billion years old. Valles Marineris runs for almost 4000 km along the Martian equator, about one-fifth of the way around the planet. At its widest, it is some 120 km across, and it is as deep as 7 km in places. The Grand Canyon in Arizona would easily fit into one of its side tributary cracks. Valles Marineris is so large that it can even be seen from Earth. Prior to the arrival of Mars Global Surveyor, astronomers believed that all the water below the Martian surface existed in the form of ice. However, in 2000, Surveyor mission scientists reported the discovery of numerous small-scale gullies in Martian cliffs and crater walls that apparently were carved by running water in the relatively recent past. The runoff channels are found in the southern highlands. They are extensive systems sometimes hundreds of kilometers in total length of interconnecting, twisting channels that seem to merge into larger, wider channels. They bear a strong resemblance to river systems on Earth, and it is believed by geologists that this is just what they are the dried-up beds of long-gone rivers that once carried rainfall on Mars from the mountains down into the valleys. These runoff channels speak of a time 4 billion years ago (the age of the Martian highlands), when the atmosphere was thicker, the surface warmer, and liquid water widespread. The outflow channels are probably relics of catastrophic flooding on Mars long ago. They appear only in equatorial regions and generally do not form the extensive interconnected networks that characterize the runoff channels. Instead, they are probably the paths taken by huge volumes of water draining from the southern highlands into the northern plains. The onrushing water arising from these flash floods probably also formed the odd teardrop-shaped islands (resembling the miniature versions seen in the wet sand of our beaches at low tide) that have been found on the plains close to the ends of the outflow channels. Judging from the width and depth of the channels, the flow rates must have been truly enormous perhaps as much as a hundred times greater than the 105 tons per second carried by the Amazon river, the largest river system on Earth. Polar Caps The Martian polar caps are composed predominantly of carbon dioxide frost dry ice and show seasonal variations. Each cap in fact consists of two distinct parts the seasonal cap, which grows and shrinks each year the residual cap, which remains permanently frozen. At maximum size, in southern midwinter, the southern seasonal cap is some 4000 km across. Half a Martian year later, the northern cap is at its largest, reaching a diameter of roughly 3000 km.
The two seasonal polar caps do not have the same maximum size because of the eccentricity of Mars s orbit around the Sun. During southern winter, Mars is considerably farther from the Sun than half a year later, in northern winter. The southern winter season is longer and colder than that of the north, and the polar cap grows correspondingly larger. The seasonal caps are composed entirely of carbon dioxide. Their temperatures are never greater than about 150 K (-120 C), the point at which dry ice can form. During the Martian summer, when sunlight striking a cap is most intense, carbon dioxide evaporates into the atmosphere, and the cap shrinks. In the winter, atmospheric carbon dioxide refreezes, and the cap reforms. As the caps grow and shrink, they cause substantial variations (up to 30 percent) in the Martian atmospheric pressure a large fraction of the planet s atmosphere freezes out and evaporates again each year. From studies of these atmospheric fluctuations, scientists can estimate the amount of carbon dioxide in the seasonal polar caps. The maximum thickness of the seasonal caps is thought to be about 1 m. Why is there such a temperature difference (at least 50 K) between the two residual polar caps, and why is the northern cap warmer, despite the fact that the planet s northern hemisphere is generally cooler than the south? The reason is not fully understood, but it seems to be related to the giant dust storms that envelop the planet during southern summer. These storms, which last for a quarter of a Martian year (about six Earth months), tend to blow the dust from the warmer south into the cooler northern hemisphere. The northern ice cap becomes dusty and less reflective. As a result, it absorbs more sunlight and warms up. Atmosphere Long before the arrival of the Mariner and Viking spacecrafts, astronomers knew from Earth-based spectroscopy that the Martian atmosphere was quite thin and composed primarily of carbon dioxide. In 1964, Mariner 4 confirmed these results, finding that the atmospheric pressure is only about 1/150 the pressure of Earth s atmosphere at sea level and carbon dioxide makes up at least 95 percent of the total atmosphere.
Composition of the Martian atmosphere is now known to be 95.3 percent carbon dioxide, 2.7 percent nitrogen, 1.6 percent argon, 0.13 percent oxygen, about 0.07 percent carbon monoxide, and about 0.03 percent water vapor. The level of water vapor is quite variable. On average, surface temperatures on Mars are about 50 K cooler than on Earth. The low early-morning temperatures often produce water-ice fog in the Martian canyons. Higher in the atmosphere, in the stratosphere, temperatures are low enough for carbon dioxide to solidify, giving rise to a high-level layer of carbon dioxide clouds and haze. For most of the year, there is little day-to-day variation in the Martian weather: The Sun rises, the surface warms up, and light winds blow until sunset, when the temperature drops again. Only in the southern summer does the daily routine change. Strong surface winds (without rain or snow) sweep up the dry dust, carry it high into the stratosphere, and eventually deposit it elsewhere on the planet. At its greatest fury, a Martian storm floods the atmosphere with dust, making the worst storm we could imagine on Earth s Sahara Desert seem inconsequential by comparison. The dust can remain airborne for months at a time. In order for the dust to stay aloft for so long, it must be made up of very fine-grained particles and rise to great heights at least 20 30 km. Strong solar heating of the ground during southern summer probably produces strong updrafts, injecting the dust into the lower stratosphere. When the blown dust finally settles back to the surface, it forms systems of sand dunes similar in appearance to those found on Earth.