First probes to land on Martian surface and photograph terrain

Ever since the 17th century, when Christiaan Huygens first sketched the face of Mars, people have wondered what the planet is like—and if it is home to life. To the eyes of early astronomers. Mars appeared to be a watery world, lush with vegetation. Advanced civilizations—builders of canals and cities—were thought to exist on the planet. Today, we know Mars is a cratered, desolate world. Space probes have mapped all of its barren surface. But Mars still has plenty of secrets. One of them may yet be life.

From the first telescopic observations to the latest probe pictures, our view of Mars has been under constant review. The Dutch astronomer Christiaan Huygens (1629-93) made the first sketch of the Red Planet in 1659. He believed that there was life on Mars— as well as on other planets—and the view through his crude telescope led him to think that Mars was similar to the Earth. Huygens could make out dark spots that he mistook for seas, and assumed the bright regions he observed were fields of ice.

Two centuries later, in 1877, Italian astronomer Giovanni Schiaparelli (1835-1910) drew the first good maps of Mars. He used a telescope that could resolve objects down to 50 miles in diameter to fill in the planet's surface features. The maps—characterized by dark tracks that Schiaparelli named canali—were controversial: Some astronomers could see his dark bands, others could not.

The wealthy U.S. businessman Percival Lowell (1855-1916) mistranslated Schiaparelli's Italian term, and "canali" became "canals." Excited by what he thought was evidence of extraterrestrial life, Lowell built himself a telescope to investigate further. High up in Arizona, Lowell's 24-inch refractor allowed him to make out Martian surface features just 35 miles across. Despite Lowell7 s conviction, most scientists remained skeptical and dismissed the idea of water canals—although they all continued to believe that dark regions on the planet were patches of vegetation. Not until 1965 were their ideas finally disproved.

In 1965, images from NASA's first Martian probe— Mariner 4—gave Mars its most dramatic costume change since Huygens first sketched the planet. Disappointingly for some, a dry, rocky world was clearly visible, with no sign of life. Over the next seven years, three more spacecraft paid visits. It was the last of these probes— Mariner 9—that discovered Mars' most spectacular features. As Mariner 9 orbited the planet, a huge dust storm masked the surface. When the storm subsided, the tops of volcanoes and the outline of huge chasms became clear through the orange haze. The atmospheric pressure of Mars was found to be too low for water to remain liquid, but these newly discovered channels suggested that water had once flowed across the surface.

In the mid-1970s, the red surface of Mars became visible from a completely different angle. Two Viking landers touched down and took images of the rock-strewn landscape that surrounded them.

Though the Mars Global Surveyor satellite has now photographed surface objects only five feet across, our view of Mars since the Viking missions has changed largely through exploration of our own planet. The discovery of life in hostile environments on Earth has reigned the idea that Martians could exist in nooks and crannies on the planet's surface—or perhaps underground. Orbital missions can tell us little more:

Only painstaking examination at ground level will coax Mars into giving up its tantalizing secrets.