Published: October 2008
Meet the Neighbors
Exploring the solar system
By Michael Lemonick

I was nearly 16 when Neil Armstrong first stepped onto the moon in 1969, and like most Americans who were alive at the time, I will never forget that moment. But I also remember another earlier space-related milestone equally well. On July 31, 1964, an unmanned space probe called Ranger 7 slammed into the moon at more than 5,800 miles an hour. The crash destroyed the spacecraft, of course, but the mission was a complete success: As it plunged to its death, Ranger snapped a series of photographs of the lunar surface. And while the last one, shot from an altitude of about 1,700 feet, could only be partially transmitted before final impact a fifth of a second later, it showed craters just a couple of feet in diameter.

When I saw that image in the newspaper, I was blown away. Until that moment, the only way to photograph the moon was from Earth, through a telescope—and the most powerful telescope in existence at the time couldn't see surface details less than a mile across. Now, for the first time in history, we'd gotten a close-up look at another world.

And that was just the beginning. Over the more than four decades since, NASA and other space agencies have taken us to vicarious close encounters with all eight planets (and if you refuse to accept Pluto's recent demotion to "dwarf planet," the New Horizons mission will reach a ninth in 2015). Robotic explorers have boldly gone where fragile human spacefarers can't, at least not anytime soon, sending back extraordinarily detailed pictures and measurements of dozens of moons and even a few asteroids. They've touched down on Venus, on Saturn's giant moon Titan, and most significantly, on Mars—the first time for the U.S. when Viking 1 landed in 1976, and most recently this past May, when the Phoenix probe arrived to dig into the Martian surface.

The result of this sustained campaign of exploration is a wealth of information about the nature and origin of our solar system. During the 1960s and 1970s, for example, Ranger 7's crash landing was followed by a series of soft lunar landings and orbital mapping missions, beginning with the Soviet Union's Luna 9 in 1966. Most of the early flights were designed to gather information for the Cold War goal of being first to put humans on the moon. But after the U.S. won that race, the Soviets continued to send unmanned missions that collected surface samples and returned them to Earth. In the 1990s the U.S.'s Clementine and Lunar Prospector probes found hints that the moon might hold deposits of ice in shadowed crater floors at the lunar poles—a potentially crucial resource for any future moon base.

Impressive as the moon probes have been, they merely filled in the details about a world we knew relatively well. As soon as Galileo pointed his telescope skyward in 1609, the general topography of our nearest heavenly neighbor, with its craters, mountains, and plains, was evident. But earthbound telescopes couldn't tell us very much about the more distant planets. It was only with the first flyby of Venus by Mariner 2, in 1962, that scientists began to understand that the planet's atmosphere is principally made of carbon dioxide, that Venus rotates in the opposite direction from Earth, and that its surface temperature, thanks to the heat-trapping ability of CO₂, is a withering 900°F. Later probes, including the Magellan orbiter, which reached Venus in 1990, have shown that beneath its permanent shroud of clouds, the planet that is Earth's near twin in size is covered mostly by hardened lava and is probably still geologically active. Mariner 10 was the first space probe to visit Mercury, in 1974 and 1975, revealing that the tiny planet's surface looks very much like the moon's, while its dense, iron-rich core is more like Earth's.

During the 1960s and '70s, NASA also sent missions in the opposite direction, toward the outer solar system. Mars, the first target, had long held special intrigue for humanity, though by the 1960s astronomers had decided that life, at least on the planet's surface, wasn't possible. Mars has such a thin atmosphere that any liquid water would vaporize or freeze—and as far as anyone knew then, or knows now, life can't exist in the absence of water. The first few probes to reach Mars, starting with NASA's Mariner 4, in 1965, showed a cratered, moonlike terrain. But six years later Mariner 9 revealed previously unsuspected features, including giant extinct volcanoes, canyons, and, most intriguingly, what looked suspiciously like dried-up river channels. At some time in the distant past, Mars clearly did have more favorable conditions for life to arise. And if it had, it was at least conceivable that microbial remnants could be hanging on, not out in the open air but under the soil.

That's why the twin Viking missions in 1976 carried experiments to look for that life. The Vikings each had an orbiter and a lander; the latter took the first, stunning photographs from the surface of Mars (one panoramic shot presided for a time over the main waiting room in Grand Central Terminal in New York from a billboard). While the orbiters whirled overhead mapping the planet, the landers scraped up some Martian dirt and performed four tests that might betray the existence of microbes. Three of the four were ultimately negative; the fourth was positive, but could be explained away as an unusual but purely chemical reaction (although the principal investigator on that experiment, Gilbert Levin, insisted it shouldn't be).

By the late 1990s, biologists had come to understand that life can flourish in more extreme environments than anyone had thought—deep underground, for example, or in unfrozen salty pockets within ice sheets (it turns out that some earthly bacteria can live in extraordinarily salty water). At the same time, the evidence for abundant surface water in Mars's past had become even stronger thanks first to the wheeled Pathfinder rover, which rolled over the surface in 1997 and photographed rocks that had been transported by huge floods of water. The Spirit and Opportunity rovers that began crawling over Mars in 2004 found evidence of rocks formed in and shaped by once free-flowing water, and in 2006 the Mars Reconnaissance Orbiter began taking the highest resolution pictures ever of ancient channels and possible lakeshores from space. In 2002 the Mars Odyssey orbiter had detected what may be the remains of that water: Its neutron spectrometer revealed massive amounts of water ice (as distinct from the carbon- dioxide, or dry, ice that forms on the surface of the planet's visible ice caps) lying just beneath the surface near the planet's north and south poles. It is here, if anywhere, that Martian organisms might have flourished most recently. This year the Phoenix spacecraft landed near the north pole and began digging with its robotic arm. Shortly thereafter—in a historic breakthrough—Phoenix confirmed the presence of water ice on Mars.

The growing understanding that water exists in great abundance beyond the Earth was further strengthened by the push to explore the giant gaseous planets— Jupiter, Saturn, Uranus, and Neptune. With the Pioneer probes in the early 1970s and the Voyagers later that decade and throughout the 1980s, the Galileo probe from 1995 through 2003, and the Cassini-Huygens arrival at Saturn in 2004, planetary scientists have gotten close-up looks not only at the planets but also at their moons. Features like Jupiter's Great Red Spot, the rings of Saturn, and the bands of clouds on both planets were laid bare in enough detail to begin teasing apart their complex structures and dynam�ics. These missions sent back all sorts of surprises as well—the discovery that all four gas planets have rings; that three of them have enormous lightning storms; that Jupiter's moon Io sports sulfur-spewing volcanoes; that the moons of the giant planets come in a bewildering variety of shapes, sizes, and surface features.

But the most intriguing discovery was that the rules about where scientists might look for life beyond Earth had been far too restrictive. Even from terrestrial telescopes, Jupiter's moon Europa has always looked impres�sively bright, suggesting a surface covered in ice. When the Voyagers snapped the first detailed close-up pictures, it became clear that this was the case, but also that Europa was covered with a network of linear features. Galileo's much closer and more prolonged look provided enough high-resolution evidence to prove an audacious hypothesis: The linear features are caused by cracks in a sheet of ice, perhaps many kilometers thick, floating on a subsurface, moon-spanning ocean of liquid water. In a part of the solar system so frigid that every bit of water should be frozen solid, Jupiter's tidal forces squeeze Europa like a rubber ball, creating heat through friction. Basic organic chemicals may swirl through that ocean, given that comets are rich in organics (the European Space Agency's Giotto mission proved this during its 1986 encounter with comet Halley) and that comets crash into moons and planets. And if life is reasonably easy to get started given the right raw materials—something biologists don't know yet—then Europa may be a hotbed of life. A possible future mission, called the Europa Astrobiology Lander, still in the early planning stages, could eventually find out.

Astronomers had also long been intrigued by Titan, the largest of Saturn's moons (and the second largest moon in the solar system—bigger, in fact, than the planet Mercury). Where Europa is white, Titan is orange, suggesting to experts, including the late Carl Sagan, that the moon's atmosphere is rich in methane and other organic compounds. Although Titan is far colder, the chemical mix may resemble Earth's atmosphere when life first arose here. The prospect of seeing Titan up close was one of the things that led to the Cassini-Huygens mission to orbit Saturn (Cassini) and send a lander down to Titan's surface (Huygens).

When Huygens finally parachuted down in January 2005, its cameras showed astonished scientists hills and river channels in a weird analogue of Earth. Later obser�vations by Cassini revealed hundreds of lakes perhaps filled with liquid methane and ethane. More surprising still was the discovery in March 2008 that Titan, like Europa, may have an ocean at least partly made of liquid water that could contain organic compounds.

In 2005 Cassini found yet another place in the solar system aside from Earth that evidently plays host to water in life-supporting liquid form. As the spacecraft swung by the Saturnian moon Enceladus, its cameras spotted icy plumes jetting from near the moon's south pole. Cassini's close look detected organic chemicals mixed in with the plumes, which may originate from a liquid water reservoir not far below the surface.

None of these discoveries proves that life exists, or has ever existed, on Mars, Europa, Titan, or Enceladus. If liquid water is found on Pluto by New Horizons, or by future missions to the moons of Neptune and Uranus, or to the swarm of icy bodies in the Kuiper belt and the Oort cloud that surround our planetary system, it won't necessarily mean life exists there either. But if robotic probes to Mars and beyond had proved, con�versely, that water in liquid form is exclusive to Earth, then the relatively new science of astrobiology—the search for life beyond Earth—would long since have run out of steam. Instead, buoyed by the discovery that planets are common around other stars, it has become the hottest subject in the cosmos. You might say it's out of this world.