Scientists, too, have speculated about the possibility of life on Mars. During the 1800s, some leading astronomers thought that an advanced civilization inhabited the planet -- a civilization that built vast "canals" to sustain a dying world. But investigations with increasingly sophisticated ground-based telescopes, followed by the Mariner and Viking missions of the 1960s and '70s, crushed the dreams of grand Martian cities. Mars is too cold and too dry, and its air is too thin, to sustain large organisms.
The farthest of the terrestrial planets from the Sun, Mars orbits between Earth and the asteroid belt. It takes Mars almost twice as long as Earth to orbit the Sun, but a day on Mars lasts almost exactly as long as a day on Earth -- slightly more than 24 hours.
Mars is only about half as big as Earth, but it is enveloped by a thin, cold atmosphere of carbon dioxide and traces of other gases. Temperatures seldom climb to the freezing mark, and can plummet below –200 degrees Fahrenheit (–120 C).
The planet's surface is red because iron in its soil long ago reacted with the tiny amount of oxygen left on Mars to produce rust. Dry riverbeds criss-cross the surface, and several giant volcanoes rise high into the Martian sky. The largest, Olympus Mons, towers 17 miles (27 km) high and covers an area as big as the state of Missouri. Mars also boasts a giant network of valleys that would stretch from New York to Los Angeles. Known as Valles Marineris, it dwarfs the Grand Canyon.
Ice caps cover the Martian poles. When sunlight warms the polar ice caps, water and carbon dioxide vaporize and rush into the air, stirring up tiny grains of dust. The Martian winds carry them around the planet, picking up even more dust. That stirs up dust storms that can last for days or weeks and cover almost all of the planet's surface.
Conditions on Mars don't rule out the possibility of microscopic life. Frozen or even liquid water may exist beneath the Martian soil, perhaps providing a home for living organisms. The Viking landers looked for evidence of such organisms, but their experiments were inconclusive.
The fleet of spacecraft now at Mars — and more heading out in the coming years — will try to sniff out signs of life. And scientists are studying meteorites from Mars — chunks of Martian rock here on Earth — for signs of ancient Martian life. Scientists continue to debate the results as they continue the long quest to find life on Mars.
Is there life on Mars?
The possibility of life on Mars has intrigued skywatchers throughout the ages, but not until 1996 was the belief in Martians anything more than hopeful speculation. On August 6, 1996, a team of NASA scientists announced that they had evidence, which, though inconclusive, compelled them to believe that life had once existed on Mars. The evidence consisted of samples from a Martian meteorite -- named ALH84001 -- containing various organic chemicals and what appear to be clumps of "microfossils," similar to microscopic fossil bacteria found on Earth.
Most other scientists disagree that the meteorite shows evidence of life. Their tests indicate that most of the "evidence" was created by basic physical or chemical processes. But the original team remains convinced, and even says it's found evidence of life in other meteorites from Mars.
The study of the Martian surface and the search for possible life existing there — past or present — remains a high priority for the world's space programs. Five spacecraft remain in operation at Mars: the Spirit and Opportunity rovers on the surface, and Mars Odyssey, Mars Reconnaissance Orbiter, and Mars Express in orbit. With luck and hard work, this question may soon have a definitive answer.