Of all the planets, Mars has fascinated human beings the most, its fiery orange-red hue makes it a distinctive sight in the night sky and has earned it the name of the Red Planet. The Romans named it after Mars, their god of war.
Although much smaller than the earth, Mars shows many similarities to our own planet. It has seasons, and its day is a little over 24 hours long. There is also an atmosphere, although a very slight one, and icecaps at the poles.
Mars has two satellites, discovered by Asaph Hall in 1877, and named by him Phobos and Deimos (Fear and Terror). The origin of these satellites is unknown, although their composition and small size—Phobos is about 14 miles (23 kilometers) across and Deimos 6 miles (10 kilometers)—have led to suggestions that originally they might have been asteroids that were “captured” by Mars.
In the late 1800’s, astronomers began observing Mars closely with ever more powerful telescopes. In 1877, the Italian astronomer Giovanni Schiaparelli reported seeing “channels” on the surface of the planet. Astronomers also noted the so-called “wave of darkening” that sweeps over the planet in spring. These observations suggested to many people that Mars must be inhabited by intelligent creatures. One of the most avid supporters of this theory in the 1890’s was the American astronomer Percival Lowell, who built an observatory at Flagstaff, Arizona, specifically to observe Mars and its surface features.
As time went on and more powerful instruments were trained on Mars, it became less and less likely that the planet could support life of any kind. When spacecraft began observing the planet close-up from 1965 onward, the prospects of life virtually vanished. In 1976, Viking spacecraft soft-land
d on Mars and sent back television pictures of a barren, water-less landscape. They also searched for signs of life by sampling the soil, but they searched in vain. They could find no traces of any kind of organic matter.
Rhythm of the seasons
Mars comes closer to Earth than any other planet, except Venus. At times of most favorable oppositions, it approaches to about 48,700,000 miles (78,390,000 kilometers) away from Earth and shines with a magnitude of —2.5, rivaling even Jupiter in brightness. But at most oppositions, which occur every 26 months, Mars is much farther away because of the eccentricity of its elliptical orbit around the sun.
This eccentricity is responsible for the unequal length of the seasons on Mars, which are caused by the inclination of the planet’s axis of rotation. It is also responsible for the variation in temperature between the Martian hemispheres from season to season. Thus, the southern summer is shorter than the northern summer, which explains the difference in behavior of the polar icecaps. The southern cap disappears almost completely at southern midsummer, whereas the northern cap remains quite large, even in the middle of the northern summer.
Temperatures—low by earth standards— vary widely over the planet. At noon in midsummer they may rise to —63° F. ( — 17° C) fora time; in winter at the poles, however, temperatures fall to below —225° F. (—143° C). There is also a wide variation in daily temperatures, and at night, temperatures fall rapidly because the atmosphere is too thin to retain much of the daytime heat.
The Martian atmosphere
A haze can often be seen over the icecaps, showing that the planet has at least some atmosphere; elsewhere on the planet, clouds can also occasionally be distinguished. Sometimes features on the planet’s surface are obscured by huge dust storms.
Measurements by spacecraft instruments have shown that the atmosphere is less than one one-hundredth as dense as the earth’s and is made up mainly of carbon dioxide. There are also traces of nitrogen, argon, carbon monoxide, and even some oxygen and water vapor; in addition, hydrogen has been detected in the upper atmosphere. The polar caps are made up of a mixture of frozen carbon dioxide (“dry ice”) and frozen water. The clouds in the tenuous atmosphere appear to be water-ice clouds.
From the earth, we can distinguish a variety of dark markings on Mars that rotate with the planet. Particularly prominent is the region called Syrtis Major near the Martian equator. The size and shape of the markings are not
particular, there are extensive cratered regions, reminiscent of a lunar highland landscape. In contrast, the northern hemisphere has vast, lightly-cratered plains, like the mare regions on the moon.
Within the cratered southern hemisphere, two huge basins stand out—Hellas, which is more than 1,000 miles (1,600 kilometers) across, and Argyre, which is about half the size of Hellas. They are circular areas of dusty desert containing a few craters and were formed as a result of the impact of huge meteorites eons ago. The floor of Hellas is some 2.5 miles (4 kilometers) below that of the surrounding landscape.
The most prominent feature of the northern hemisphere is a vast volcanic ridge called Tharsis, which has four massive volcanoes. The biggest, called Olympus Mons, rises to a height of about 16 miles (25 kilometers) from a base 375 miles (600 kilometers) in diameter. The other three volcanoes stand in a row to the southeast of Olympus Mons.
To the east of the Tharsis Ridge and just south of the Martian equator is a great gash in the surface, which has been called the Grand Canyon of Mars, but is more properly known as Valles Marineris (after Mariner9, the American spacecraft that discovered it). It extends for some 2,500 miles (4,020 kilometers), reaches a maximum width of about 250 miles (400 kilometers), and is as much as 4.5 miles (7 kilometers) deep.
In Valles Marineris—and in many other places on the planet—there are what appear to be dried-up water channels. Studies of the direction and flow patterns of the channels make it almost certain that they were in fact made by flowing water many millions of years ago. In some places, there is evidence of flash floods, which probably occurred when water-ice in the long-frozen ground was suddenly melted, either by meteorite impact or by volcanic activity. In such regions, the surface rocks have collapsed, leaving behind them what is termed chaotic terrain.
The water that once flowed on Mars was probably released from the interior of the planet during the volcanic eruptions that helped to shape the Martian landscape millions of years ago. (Water vapor and carbon dioxide are released in huge quantities during volcanic eruptions on Earth.)
There is no free water on Mars today, but there is probably subsurface water almost everywhere, locked in the ground as ice. And water-ice also makes up most of the permanent north polar ice-cap. This cap has a fascinating spiral pattern, shaped partly by the prevailing winds. When the cap melts and shrinks as summer approaches, interesting layered terrain is uncovered that shows how alternate layers of ice and dust are laid down and eroded year after year.
The Martian soil
The Viking landers, although they touched down at different locations (Chryse and Utopia), pictured a similar landscape and reported similar soil types at each site. The soil is soft and a vivid orange-red in color. Rocks of many shapes and sizes litter the landscape to the horizon. Most of the rocks are pitted and appear porous, probably as a result of outgassing during their formation or of wind erosion.
Automatic analysis of the soil at the landing sites showed that it contains mainly silicon and iron, together with magnesium, aluminum, cerium, calcium, and titanium. There is more sulfur than there is on average in the earth’s crust, but less potassium. The rusty color of the Martian soil is undoubtedly caused by the presence of iron oxides, probably much like terrestrial hematite or the magnetic magnetite.
The soil appears to be similar in many respects to an iron-rich clay found on earth, called nontronite. It is formed when volcanic basalt rock breaks down under the action of the weather, and scientists believe that a similar process, involving liquid water, must have been at work on Mars to produce this type of soil.