Mars, the fourth planet from the Sun, is located between Earth and Jupiter, and is named after the Roman god of war. From Earth, Mars appears reddish; this apparent tint is why it's often called the Red Planet. Mars appears quite bright in the night sky, and when it is near opposition (its closest approach to the Sun), it is one of the brightest objects.

Mars has been an object of speculation ever since early telescopes revealed seemingly linear features on its surface. Scientists and science fiction writers alike imagined them to be canals, perhaps the results of a vast irrigation system constructed by a race of Martians whose climate was becoming too cold and dry. Variations in the color patterns on the surface of Mars were interpreted as vegetation, perhaps crops or other plants that changed with the seasons.

Unfortunately, closer examination of Mars through spacecraft observation showed that the linear features seen by astronomers were probably just optical illusions. Interestingly enough, however, the most reliably seen canal corresponds to Valles Marineris, which is the largest canyon system on Mars. The changes in color patterns turned out to be windblown sand from the huge sandstorms that take place on Mars.

Similarities to Earth

Spacecraft observations have shown that Mars is a fascinating planet, and also quite Earthlike. If Venus is too hot for life, Mars is too cold—its average surface temperature is about −55 degrees Centigrade (−67 degrees Fahrenheit). Mars is farther from the Sun than Earth is, but its atmosphere is also very thin. The average surface pressure is only 7 millibars, which is less than 1 percent of the atmospheric pressure at sea level on Earth. The atmosphere is mostly made up of carbon dioxide, with a little nitrogen, argon, oxygen, and water.

Mars has polar ice caps similar to those on Earth, but the Martian ice caps are primarily carbon dioxide ice, or dry ice. In the summer, the ice cap in Mars's northern hemisphere heats up and the solid carbon dioxide turns into gas, leaving behind just a thin layer of water ice. This process raises the atmospheric pressure substantially, up to 25 percent, but the atmosphere is still much too thin to breathe (and has too little oxygen). Mars also has two small moons, Phobos and Deimos, which are thought to be captured asteroids.

FIGURE 6-5:The “Face on Mars” is just a strangely shaped mountain(refer to page 278 for more information)

Mars Exploration

Many spacecraft, both orbiters and landers, have observed Mars. The first observations of Mars came from the U.S. Mariner 4 orbiter spacecraft in 1965. The first spacecraft to land on Mars was the Soviet Mars 2 spacecraft, which made a crash landing in 1971. The two U.S. Viking spacecraft made soft landings on Mars in 1976. This very successful mission also included two orbiting spacecraft, and thoroughly characterized Mars. The landers returned 4,500 pictures and the orbiters returned over 50,000 pictures, mapping 97 percent of the surface of Mars.

The surface of Mars appears red due to the iron oxides that cover most of the surface. Even the sky is tinted pink (rather than blue) due to the different scattering properties of dust particles in the Martian atmosphere. The surface of Mars is covered with a variety of geological features. Some look quite familiar from Earth, such as large fields of sand dunes produced by the wind. The winds on Mars also cause huge sand storms, which can block the view of the surface for weeks or months.

Mars contains some spectacular mountains, including the giant volcano Olympus Mons. This volcano is actually the largest mountain in the solar system. It is 24 kilometers high (78,000 feet), well over twice the height of Mt. Everest!

Mars also has a huge canyon that puts Earth's Grand Canyon to shame: Valles Marineris is a system of canyons 4,000 kilometers long and 2 to 7 kilometers deep. Valles Marineris resulted from the stretching and cracking of Mars's crust during the formation of the Tharsis bulge, the volcanic rise where Olympus Mons is located.

Other canyons on Mars, however, appear to have been formed by some sort of fluid, in the way river valleys are carved by water on Earth. Water is the most obvious suggestion for what has eroded the surface of Mars, but there are alternate possibilities, including carbon dioxide flows and other, more exotic, suggestions.

There is also evidence of giant outflow channels caused by immense floods. Some scientists even believe that the entire low northern hemisphere of Mars was once filled with a giant ocean of water. This flooding could have happened during a warmer, wetter period on Mars, when the atmosphere was thick enough to support liquid water on the surface. At that point in its proposed early history, Mars would have been much more Earth-like than it is now.

Over time, the carbon dioxide keeping the atmosphere thick and warming the surface through the greenhouse effect (as on Venus) would have been turned into carbonate rocks. This scenario occurred on Earth as well, but remember that Earth has plate tectonics, which recycle these rocks back into the crust and release the trapped carbon dioxide. Unfortunately, Mars has no plate tectonics, so it would not have been able to recycle its carbon dioxide. Eventually the atmosphere became thinner, and the surface colder.

Life on Mars

Scientists who believe in a warm, wet period in Mars's history think that much of this water could still be present there, possibly buried beneath its surface. This idea has important implications for the possibility of life on Mars, and also its future colonization by humans. If Mars once had much more water than it does now, it is quite possible that life could have evolved on Mars as it did on Earth.

FIGURE 6-6:The surface of Mars

We're still not quite sure how life originated here on Earth, but life in general seems to require three things: liquid water, a sufficient source of energy, and certain chemical compounds including carbon and other elements. It is possible that early on, the Red Planet possessed all three of these requirements. If so, life could have originated on Mars completely separate from Earth, or it's possible that early life was transferred back and forth from Mars to Earth and from Earth to Mars through meteorites.

Scientists have discovered that rocks can be ejected into space in an impact event, and that it's possible that the rocks could protect small spores or amino acids in their interiors, which then could be transferred between planets. So it's even possible that life originated on Mars first, and then was transferred to Earth!

Some of this excitement about life on Mars comes from the discovery in 1996 of certain compounds in a Mars meteorite that seemed to be the result of life. Since we don't have any direct samples taken from the surface of Mars yet, scientists have to study the handful of Mars meteorites which have landed on Earth and been identified. One such meteorite, named ALH84001, was studied carefully, and scientists thought it contained a variety of chemical compounds and physical structures that appeared to be microscopic remains of biological activity. Perhaps the most exciting of these discoveries was the presence of so-called microfossils, structures in the rock that looked a lot like tiny, microscopic, fossilized organisms!

Other scientists have studied the rock, however, and the evidence looks a lot less conclusive than it once did. Nevertheless, this discovery, followed by the landing of Mars Pathfinder on Mars in 1997, reignited public excitement about the possibility of life on Mars. When we also consider that there could be a current reservoir of liquid water beneath the surface of Mars, there is not only the possibility of fossilized life, but also perhaps current life surviving underground!