Mars—the Red Planet, fourth rock from the Sun, representing the god of war in at least a dozen cultures—has fascinated humans throughout history. At first glance, it looks Earth-like. There are mountains, volcanoes, canyons, vast plains, polar ice caps, and a pale, sometimes cloudy sky that changes from pink or red at sunset and sunrise to a yellowish brown during the day. However, there are some things missing from Mars: surface water, a thick atmosphere, and life. Also, it has impact craters, and while Earth does have a few, Mars has many more. The red surface color, due largely to the presence of iron oxides (rust) in the soil, is another clue that Mars, no matter how familiar its landscapes may look, is not Earth. It’s a very different world, one with a past planetary scientists are working to understand. Its two moons, Phobos and Deimos, pose geological mysteries of their own. They were likely asteroids, captured by Mars’s orbit sometime in the distant past.
Mars formed 4.5 billion years ago, about the same time as Earth. The newborn planet cooled quickly and formed a thick crust, but no tectonic plates. The core also cooled, which stopped the dynamo action and left Mars with no way to generate a strong magnetic field. Early Mars had a warm, wet, and thick carbon dioxide atmosphere, but that changed about the time the magnetic field disappeared. Ancient floodplains, riverbeds, and shorelines of ancient lakes and shallow oceans are now seen everywhere. Along with layered sedimentary rock, they suggest the existence of liquid water in the past.
For a long time the lack of water on Mars has been a mystery. Where did it and the atmosphere go? Billions of years ago, the combination of Mars’s low gravity and lack of magnetic field simply allowed much of the atmosphere to escape into space. That caused the temperatures and atmospheric pressure to drop below the point where liquid water could exist. What’s left of Mars’s water is locked up in the polar caps and ice deposits below the dusty surface. On Earth when subsurface ice melts, it shapes some spectacular landscape changes. The same happens on Mars. In the north polar region, spacecraft images show what’s called patterned ground where ice has frozen, softened, and then remelted. In other places where subsurface ice melted, the overlying layers of rock settled in, forming what is called chaotic terrain. The water released during the collapse carved out large flow channels.
Mars also experienced tremendous volcanic activity. The largest of its volcanoes—Olympus Mons—rides high atop a thick piece of the crust called the Tharsis Bulge. As the volcano and the bulge built up over time, tectonic stresses cracked the surface. The famous Valles Marineris is one such crack. It stretches across a third of the planet. This system of canyons has also been carved by wind erosion, and there is some evidence that parts of it were eroded by flowing water.
Today, Mars is a dry and barren desert planet. Its thin atmosphere is made mostly of carbon dioxide with small amounts of nitrogen and traces of argon, oxygen, and water vapor. The atmospheric pressure on Mars is very low, about 6 percent of Earth’s surface pressure. The planet is much less massive than Earth, and that gives it a gravitational pull about 30 percent less than ours.
The combination of the tilt of its axis and its lengthy year (almost twice as long as Earth’s) gives Mars a seasonal climate. Each season is roughly twice as long as Earth’s. A balmy day on Mars in mid-summer at the equator might get up to 35°C (95°F) according to measurements taken by the NASA Mars Opportunity rover. Most of the time Mars is quite a bit colder, perhaps closer to 0° C. On a cold wintry night, the temperature can plummet to -150°C (-238°F). Images of Mars taken in the colder months show frost covering the ground, much as it does when the weather turns very cold here on Earth. Future Mars explorers will have to wear space suits that help keep them warm and pressurized and supply oxygen to breathe.
After Earth and Venus, Mars is one of the most-explored planets in the solar system. Telescopes around the world continually monitor the planet, and the Hubble Space Telescope keeps a watch from orbit. Space agencies from the United States, the former Soviet Union, Japan, and the European Space Agency have sent dozens of spacecraft to Mars since the early 1960s. At last count, seventy-five missions have gone, and NASA is funding a new Mars rover project to go to Mars in 2020. The Indian Space Research Organization is also working on the Mangalyaan Mars mission.
Recent successful missions include:
These advance scouts have sent back dazzling images, studied the dust and rocks searching for water and life residues, and made atmospheric measurements. In the future, the Mars Atmosphere and Volatile Evolution (MAVEN) mission will help scientists figure how and when Mars lost its atmosphere and surface water. In 2016 and 2018, the European Space Agency and the Russian Roscosmos agency will launch the ExoMars mission to search for traces of life called biosignatures.