Introduction to Probing the Planets

When American astronaut Neil Armstrong piloted the Apollo 11 lunar module to a soft landing on the surface of the moon in July 1969, he gazed upon a landscape that was, in some respects, already familiar to him. As the spacecraft descended toward the surface, Armstrong scanned the ground below, looking for landmarks he had learned to recognize from photographs. By the time Armstrong walked on the moon, it had been 10 years since Luna 2, an unmanned probe from the Soviet Union, became the first spacecraft to reach the moon. In the decade that followed, more than a dozen other Soviet and United States space vehicles had studied the moon in fine detail. Photographs and data radioed to Earth by these probes were instrumental in the success of the six U.S. manned missions to the lunar surface.

Since then, probes have visited every planet in the solar system except the dwarf planet Pluto, and some have been visited several times. In November and December 1996, NASA sent two probes named Mars Global Surveyor and Mars Pathfinder on the first survey missions to the red planet since 1975, when the Viking 1 and Viking 2 spacecraft were launched. The new spacecraft were scheduled to reach Mars in the summer of 1997, the Surveyor to observe the planet from orbit and the Pathfinder to explore its surface. Coincidentally, these missions were launched on the heels of a 1996 announcement that a meteorite of Martian origin contains the apparent remains of bacterialike microorganisms, raising the possibility that primitive life forms lived on Mars billions of years ago. Although their development was already too far along to alter their missions, the Surveyor and Pathfinder may uncover evidence that points toward the existence of life on Mars.

With Mars back on the public's radar screen, some space-exploration enthusiasts advocated sending a manned mission to the red planet. The cost of such a venture, however, would be enormous—up to $400 billion by some estimates. In contrast, probe missions to Mars or other planets can be more than 1,000 times cheaper. Increasingly limited budgets have forced space programs of all nations to abandon the large, complex probe designs of the past in favor of smaller, simpler spacecraft that employ more advanced technologies. Engineers are now building sophisticated probes far more economically than ever before. The Mars Pathfinder Mission, for example, was projected to cost only about one-tenth as much as a Viking mission. Although human beings may set foot on faraway planets and moons at some time in the future, for now probes are by far our best option for exploring the solar system.

Probe Designs and Functions

On a cosmic scale, the solar system is not vast: The closest star to our sun, Alpha Centauri, is more than 1,000 times more distant from us than is the farthest planet of our solar system, Pluto. But distances within the solar system, though insignificant on the scale of the universe, are enormous on a human scale. Thus, to study the solar system in any detail from Earth is a difficult task. Recent advances in telescope technology have allowed some information about the composition of the atmospheres and surfaces of planets to be discerned with Earth-based telescopes. As advanced as these data and images are, however, the information they reveal is still limited. Sending probes to the various planets not only enables scientists to obtain much better images, it also permits them to directly study physical samples at the same time.

Probes carry many types of instruments. Cameras and other devices that are sensitive to different wavelengths of light, including infrared and ultraviolet, reveal surface features and atmospheric conditions of planets and their moons. Spectrometers divide sunlight that passes through a planet's atmosphere or is reflected from its surface into its component colors, producing patterns that reveal the makeup of soils, clouds, and atmospheric gases. Radiometers measure the intensity of infrared waves, from which scientists can calculate among other things the temperature of an object. Magnetometers measure the streams of electrically charged particles and magnetic fields emanating from the planets. All these data are radioed in digital form (as a series of 1's and 0's) back to Earth, where they are picked up by sensitive antennas.

Even a probe's radio system can be used to make observations. By bouncing radio signals off of a planet or moon's surface and measuring the time it takes for the signal to be reflected back up to the spacecraft, detailed computer-generated images can be made of a planet's surface features. This technique is known as radar mapping. A planet's atmosphere can be studied using a technique known as radio occultation, whereby a radio signal, aimed at the Earth, is beamed through the atmosphere of a planet. Scientists on Earth compare the signal with one sent by the probe through space. By analyzing the ways in which the signal has been changed by the atmosphere, they can infer what conditions (such as temperature, clouds, or charged particles) might have caused the changes.

Basic Types of Space Probes

Five basic types of space probes have been sent to examine planets and other bodies in the solar system: fly-by probes, orbiters, atmospheric probes, landers and rovers.

  1. A fly-by probe makes its observations as it passes a celestial body from a distance. Fly-by missions enable a spacecraft to visit more than one object.
  2. An orbiter is designed to park itself in a stable orbit around a particular planet or moon for an extended period of time. An orbiter closely circling a body with a substantial atmosphere is gradually slowed by atmospheric friction, which causes it to lose altitude and eventually crash.
  3. An atmospheric probe is a package of instruments that descends into the atmosphere of a planet, taking readings on its way down. The probe continues to transmit data until it reaches the surface or is destroyed by heat or atmospheric pressure.
  4. A lander is designed to land safely on a planet or moon and analyze soil samples and surface conditions.
  5. A rover is a robot vehicle with wheels or treads that roams across the surface. Carried to the surface by a lander, a rover has the advantage of not being confined to one spot.

Investigating the Mysteries of Venus

The first spacecraft to travel beyond Earth's moon, the U.S. probe Mariner 2, passed within 34,760 kilometers (21,600 miles) of Venus in December 1962. Mariner 2's microwave radiometer detected a surprising amount of heat, in the form of infrared radiation, seeping up from beneath the layer of clouds that surrounds the planet. This discovery shattered the theory that the climate of Venus was similar to the Earth's. Measurements by subsequent spacecraft showed that the atmosphere of Venus consists mainly of carbon dioxide and that the atmospheric pressure at the surface is 90 times higher than sea-level air pressure on Earth. This enormous blanket of carbon dioxide produces an intense greenhouse effect: The atmosphere lets sunlight reach the surface but prevents that energy—converted to infrared radiation by the heated planet—from escaping back into space. Due to the greenhouse effect, the average surface temperature of Venus is 462 °C (864 °F), hot enough to melt lead.

But had it always been like that? The answer to this question had to await the December 1978 arrival of two NASA spacecraft—Pioneer Venus 1, an orbiter, and Pioneer Venus 2, which carried four atmospheric probes. During its descent through Venus's atmosphere, one of the atmospheric probes measured a surprisingly large amount of deuterium—an isotope (variant form) of hydrogen—relative to normal hydrogen. This discovery led scientists to theorize that there had once been large amounts of water on Venus.

Several billion years ago, many scientists now believe, an ocean existed on Venus, and the climate was perhaps only somewhat hotter than Earth's is today. The heat, however, was sufficient to cause water from Venus's ocean to evaporate in large quantities. The vapor drifted into the upper atmosphere, where the water molecules were broken apart into hydrogen and oxygen by ultraviolet light from the sun. Only a tiny fraction of ocean water contains deuterium rather than normal hydrogen. The normal hydrogen, however, because it is lighter than deuterium, escaped Venus's atmosphere more easily. As a result, the atmosphere of Venus today is left with a large proportion of deuterium—the ghostly echo of an ocean long gone.

While the ocean evaporated, carbon dioxide from within the planet was building up in the atmosphere. On Earth, carbon dioxide is constantly being removed from the atmosphere by rainfall and plants and by the weathering of rocks to form compounds called carbonates. With none of these processes operating on Venus, the result was an overabundance of carbon dioxide and a runaway greenhouse effect.

Although the Pioneer Venus probes told scientists much about Venus's atmosphere, the surface of the cloud-shrouded planet remained a mystery. It was not until 1984 that the veil began to lift. In that year, twin orbiters launched by the Soviet Union—Venera 15 and Venera 16—successfully mapped about a fourth of Venus's surface using cloud-penetrating radar. The maps revealed a geology unlike anything seen on Earth. Volcanic activity was revealed in several huge regions of fractures on the surface, large volcanoes, and lava flows. But there was virtually no sign of the kinds of long, organized ridges and trenches that on Earth are characteristic of plate tectonics (the slow movement of rock plates making up our planet's crust).

More extensive mapping of Venus was carried out by NASA's Magellan spacecraft, which began orbiting Venus in 1990. Armed with synthetic aperture radar—an advanced kind of radar that can scan a planet's surface in much greater detail—Magellan mapped virtually the entire surface of Venus. The mountains of Venus shown in Magellan's images have an eerie incompleteness to them; they lack the canyons, gullies, and valleys that water carves into the mountainous regions of the Earth. The absence of such features confirmed that Venus has been without liquid water for most of its geologic history.

Magellan's radar maps also showed that the surface of Venus, compared to other rocky, waterless bodies in the solar system, is pocked by relatively few meteorite craters. Furthermore, the cratering is uniform across the entire surface—no one area has significantly more or fewer craters than any other area. To some geologists, these findings indicate that the surface of Venus is probably relatively young in geologic terms and that the entire present surface of Venus probably formed at about the same time. But what kind of cataclysmic event could have caused the resurfacing of an entire planet remains a mystery. One theory holds that heat builds up within the planet and that every few hundred million years, the entire crust breaks apart, melts, and then re-forms.

Mercury and Our Moon

Inward from Venus lies Mercury, a tiny planet about 1.5 times the diameter of our moon. Because of its close proximity to the sun, Mercury is difficult to approach, and only one spacecraft, the U.S. probe Mariner 10, has flown past it. Images returned from Mariner 10 in 1974 revealed a heavily cratered surface, similar to that of our moon. Most surprising was the discovery of a magnetic field, which scientists had believed could be generated only by larger, rapidly spinning planets like Earth and Jupiter. Mercury spins so slowly that it takes about 59 Earth days for a single rotation. The existence of a magnetic field, along with information on the planet's mass, indicated that Mercury is composed largely of iron.

The body nearest to Earth—our own moon—was most closely explored by the Apollo astronauts, who returned hundreds of pounds of lunar samples. But their expeditions were limited in area, and the orbiting command modules never reached the lunar poles. A U.S. probe, Clementine, was launched on a mission to examine the poles of the moon in 1993. In 1996, radio signals from Clementine that were bounced off the poles and received on Earth indicated the presence of frozen lakes. Some scientists believe that comets colliding with the moon could have deposited water ice at the lunar poles, but it remains for future probes to confirm the presence of water on the moon.

Missions to Mars

Of all the wanderers in the night sky, the planet Mars has haunted humanity the most, for centuries stimulating conjecture about extraterrestrial civilizations. Even many scientists speculated that Mars might harbor at least primitive life. But early fly-bys of Mars by NASA's Mariner probes in the 1960s showed a cratered landscape little different from the Earth's moon, and most scientists gave up hope that Mars was anything but a dead world. Those spacecraft, however, photographed only a small portion of the Martian surface. When Mariner 9 reached Mars in 1971 and began to send back photographs of the entire surface, opinions about the planet changed quickly. Evident in the new images were valleys and channels that appeared to have been carved by flowing water, as well as giant canyons and huge volcanoes. These discoveries showed that Mars had once been surging with great geologic upheavals and running water. But was there life on the red planet?

An interplanetary armada of two Viking orbiters and two landers from the United States arrived at Mars in 1976. The landers were equipped with chemical laboratories to detect signs of metabolic (living) processes in the soil, and to search for organic (carbon-containing) molecules that might be the product of life. No organic compounds were found, but the soil seemed unusually reactive in a way that was initially interpreted as a possible sign of life. Further study of the results, however, showed that the chemistry of the soil itself, which is rich in certain iron oxides, had caused the reactions—the landers had not detected life.

The Viking orbiters, meanwhile, studied Mars from high above. Their images of the Martian surface showed even more dramatically the valley networks, channels, volcanoes, and canyons seen earlier by Mariner 9. The much finer detail of the Viking images enabled scientists to suggest that some geological features on Mars were the remnants of glacial erosion and to identify sediments that may have been left behind by ancient lakes that had long since dried up.

Given the abundant evidence for liquid water on Mars in the past, scientists began to develop theories to explain why Mars today is a cold, dry planet. Because Mars is considerably farther from the sun than the Earth is, its early wet climate must have been sustained by a much thicker atmosphere. That atmosphere may have been dominated by carbon dioxide, which is still the main component of the now-thin envelope of gases surrounding Mars. Some of the early atmosphere was probably lost as the result of large impacts by asteroids. A larger portion of it may be locked in the crust as carbonates that formed when the planet had abundant water. Like Venus, Mars shows no sign of active plate tectonics, which on Earth returns carbon dioxide to the atmosphere by melting and squeezing carbonates in the crust. On Mars, carbon dioxide, once locked in carbonates, could not be recycled in this way.

To test this theory, NASA in 1992 launched a probe called Mars Observer check for the presence of carbonate minerals on the planet. But the spacecraft was lost when its propulsion system failed in August 1993, just days before reaching Martian orbit, and the search for carbonates on Mars was put on hold. Instead, a series of Mars orbiters, beginning with the one launched in 1996, will conduct the search.

Jupiter and Its Moons

The first probe to venture to the outer planets was Pioneer 10, which flew past Jupiter in 1973. Jupiter, 318 times the mass of the Earth, is essentially a huge ball of gas, though it has a hot, dense—perhaps solid—core made up of heavy elements. Jupiter's powerful magnetic field traps high concentrations of charged subatomic particles (such as protons and electrons), generating lethal levels of radiation. In 1974, Pioneer 11 encountered Jupiter on its way to Saturn. These missions proved that spacecraft could safely traverse the asteroid belt (a band of rocky debris orbiting the sun between Mars and Jupiter) and survive the onslaught of radiation at Jupiter to return data to Earth. Following in their path, the more advanced Voyager 1 and Voyager 2 probes flew past Jupiter in 1979 on their way to Saturn.

The Voyagers showed that Jupiter has a roiling, turbulent atmosphere, with gigantic storms appearing and then being swallowed up again. They identified hydrogen, helium, and methane in Jupiter's atmosphere and measured their abundance. The probes also found that the planet is circled by a thin ring of dust. The probes also discovered three small moons that were previously unknown—Adrastea, Metis, and Thebe.

But the most startling findings by the Voyager mission at Jupiter centered on the planet's four largest satellites: Io, Europa, Ganymede, and Callisto. The probes showed that the inner two moons, Io and Europa, are composed mostly of rock and are about the size of Earth's moon. The outer two, Ganymede and Callisto, are larger—about 5,000 kilometers (3,000 miles) in diameter—but they are less dense than the inner moons and appear to be composed of roughly equal parts of rock and ice. Although they are similar in size and mass, Ganymede and Callisto seem to have had vastly different geologic histories. Callisto is covered by craters, with nothing to break the monotony of its battered surface. In contrast, Ganymede—the solar system's largest moon—has vast regions where craters have been “erased” by geologic processes and replaced by complex networks of grooves.

Europa's rocky surface, Voyager data reported, is covered by a bright, smooth layer of water ice, which is cracked in places like an eggshell. Even more startling was Io, revealed by Voyager's cameras to be covered by active volcanoes that spew tremendous plumes of smoke and lava flows consisting of both molten sulfur and molten rock.

Scientists wondered what the source of energy was that powered all this activity. The answer, they concluded, was gravity. Jupiter's tremendous gravity exerts alternately pulling and squeezing forces on the larger moons as they travel around the planet in their slightly elliptical (oval) orbits. Close-in Io is most affected by this gravitational “kneading”—the constant stresses have heated Io to the melting point of rock, sealing its fate as a world of explosive volcanism. Farther out, Europa is also stressed, but only enough internal warming has occurred to melt underground ice to a slush, which then rises to the surface and refreezes. Most intriguing is the possibility that an ocean of liquid water, warmed by gravitational stresses, still exists beneath the icy surface of Europa.

To study Jupiter further, NASA in 1989 launched the Galileo spacecraft on a voyage to the giant planet. Galileo, which consisted of an orbiter and an atmospheric probe, arrived at Jupiter in December 1995. The probe, released from the orbiter several months earlier, plunged into Jupiter's atmosphere, surviving acceleration, pressure, and heating for 57 minutes until it melted and then vaporized. During its descent the probe found no evidence of the water clouds that scientists had expected to find. Concentrations of water vapor in the atmosphere were also lower than expected. One possible explanation for these findings is that the probe fell into an unusually dry, cloudless patch of Jupiter's atmosphere. The probe also reported that wind speeds increased at lower elevations. This indicated that, unlike Earth, where the winds are faster in the upper atmosphere because they are driven by heat from the sun, Jupiter's winds are driven by heat from within the planet.

After the probe mission was completed, the main part of the spacecraft rocketed into orbit around Jupiter to begin a survey of the planet and its moons. Information from Galileo was still being collected in mid-1997 and would be the subject of study for many years. But the spacecraft had already made several significant discoveries about Jupiter's satellites. A close approach to Ganymede revealed evidence that, unlike other moons encountered by probes, it possesses a magnetic field. And a survey of Io, with careful measurement of how the moon's gravity deflected the spacecraft's path, enabled scientists to deduce that this volcanic moon has a distinct, metallic inner core.

Galileo also revealed that the fractured sheath of ice enveloping Europa is divided into an incredibly complex network of cracks. The cracks crisscross one another at many angles, with many cracks obliterated by fresh flows of ice. The newest cracks might be places where liquid water lies only a few kilometers beneath the surface. In April 1997, a scientific panel declared that evidence gathered by the Galileo probe had confirmed this theory. In fact, observations indicate that there is probably more water beneath the surface of Europa than in all the oceans on Earth combined. This subsurface ocean might even support forms of life that rely on heat from within the moon or small amounts of sunlight penetrating the cracks in the surface.

Saturn, Uranus and Neptune

Voyagers 1 and 2 reached Saturn in 1980 and 1981, respectively. The second-largest planet in the solar system, Saturn has only a third of the mass of Jupiter, though it is still more than 90 times as massive as the Earth. Saturn sports a ring system that has been visible to telescopes on Earth for centuries. Voyager 1's close-up images revealed, however, that the planet's seven major rings are divided into hundreds of thousands of subrings, each divided by a narrow space, like the grooves on a phonograph record. The rocky and icy particles making up the rings are organized in this way by the effects of Saturn's gravity.

Voyager data on Saturn's atmosphere revealed little helium in comparison with Jupiter's atmosphere. Saturn has a colder interior than Jupiter—cold enough so that helium in the atmosphere liquefies. Since helium is denser than hydrogen, it falls through the atmosphere in the form of little droplets until it reaches—and becomes part of—the planet's dense core. The result is that there is now little helium remaining in the higher layers of the atmosphere.

Perhaps the most extraordinary part of the Voyager mission was Voyager 1's exploration of Titan, the largest of Saturn's 18 known moons. Scientists knew that Titan had an atmosphere, but they had not been able to determine its extent or density. Voyager 1's cameras showed that Titan's surface is completely hidden by a thick, orange haze suspended in the atmosphere. The probe revealed that nitrogen is the most abundant gas in the atmosphere. The temperature of Titan is so low—a frigid 178 °C (289 °F) at its surface—that water is completely frozen out of the atmosphere. At that temperature, however, methane—also plentiful around Titan—can exist as either a liquid or a gas, and it may form clouds in the atmosphere, much as water does on Earth.

The Voyager probes also detected many organic molecules in Titan's atmosphere. These probably form when ultraviolet light from the sun breaks apart methane and nitrogen molecules, some of which then recombine to form more complex substances, including hydrocarbons (molecular chains of carbon and hydrogen) and nitriles (molecular chains of carbon, hydrogen, and nitrogen). This photochemical process creates a “smog” that condenses and settles onto the moon's surface. Scientists speculate that pools of liquid organic molecules, or clusters of solid organic molecules, may exist on Titan's surface. Life on Earth may have arisen from reactions in similar collections of organic molecules, and some of the same reactions could be occurring on Titan.

After leaving Saturn, Voyager 1's course took it out of the solar system and into deep space. Voyager 2 continued on to Uranus and Neptune, reaching those planets in 1986 and 1989. Both of these large, gaseous planets are less than a fourth the mass of Saturn, but they still contain thick atmospheres of hydrogen and helium. They are so far from the sun—19 and 30 times the Earth's distance, respectively—that they receive little heat from solar radiation. At Uranus, Voyager 2 discovered a hazy, sluggish atmosphere. At Neptune, it found an atmosphere containing clouds and storms whose patterns change continuously. They concluded that Neptune's atmosphere must be powered by a much stronger source of internal heat, a result confirmed by Voyager 2 measurements.

Voyager 2 also found the magnetic fields of both Uranus and Neptune to be unusual. Unlike the magnetic fields of Earth, Saturn, and Jupiter, which are roughly aligned with the planets' axis of rotation, the fields of Uranus and Neptune are tilted. In Neptune's case, the field is shifted well away from the axis of the planet. The origin of these odd shifts remains poorly understood.

Voyager 2 discovered that Neptune has at least eight moons; only two had been known of previously. Neptune's largest moon, Triton, contained its own surprises. Voyager found that a bright cap of nitrogen and methane ice at the moon's south pole prevents sunlight from heating the surface, keeping temperatures below 233 °C (388 °F), the coldest surface yet measured by a probe. But despite such low temperatures, there is activity on Triton. Voyager spotted two geysers that were shooting plumes of dark dust to an altitude of 8 kilometers (5 miles). Scientists speculate that the eruptions are powered by underground pockets of nitrogen that are warmed just enough by sunlight to expand and burst through the surface. The plumes were an unexpected final discovery before Voyager 2 followed its sister ship on a perpetual journey beyond the solar system.

Plans for the Future

In 1997, with the Mars probes already underway, preparations were being made in the United States, Europe, and elsewhere to launch probe missions to revisit Saturn and to rendezvous with a comet. Several more Mars missions were also on the drawing boards at NASA, as well as in Russia and Japan, for the coming decade. One of their major tasks will be to look for evidence of life on Mars—either presently living microorganisms or remnants of ones from the distant past. Among the spacecraft being planned by NASA was a lander that would bring a sample of Martian soil back to Earth sometime around 2010.

Saturn's moon Titan was also due for a return visit. The possibility that organic molecules exist on the surface of Titan led to speculation that this intriguing moon could serve as a laboratory for examining how life begins. A joint U.S.-European mission named Cassini/Huygens, scheduled for an October 1997 launch, was to explore this possibility further. Plans called for the American-built Cassini orbiter to reach Saturn in 2004. There, if all goes according to plan, it will release a European-built atmospheric probe, Huygens, that will plunge through Titan's atmosphere making chemical measurements all the way down to the moon's mysterious surface. Cassini will then begin a four-year orbital tour of Saturn, making repeated passes by Titan to map its surface with advanced radar. If the mission is successful, Cassini and Huygens could greatly contribute to scientists' understanding of how life emerged on Earth and how it may do so elsewhere.

Future probe missions will also continue to target extraterrestrial objects besides planets and moons. The last major encounter with a comet in deep space took place in 1986, when Halley's Comet was visited by the Giotto probe, sent by the European Space Agency, and two Soviet Vega spacecraft. But far more exploration is needed to tell scientists what comets are made of and whether, as some suspect, the ice they contain has remained largely unaltered since the formation of our solar system. To help answer these questions, the Stardust mission, scheduled by NASA for launch in 1999, is intended to intercept a comet named Wild-2 at a point inside the orbit of Mars in early 2004. Scientists especially want to know whether comets contain organic molecules. Some researchers theorize that Earth and other planets and moons where organic compounds abound obtained those compounds from collisions with comets. The Stardust probe is designed to collect cometary dust and return it to Earth in 2006. More ambitious comet missions were to follow.

There is also interest in sending a mission to Pluto, the last planet in the solar system that remains unexplored by a probe from Earth, and to the Kuiper Belt. The latter is a collection of icy debris, just beyond the orbit of Pluto, that is left over from the formation of the outer planets. A NASA project named Pluto Express was being developed in 1997 to achieve both these goals. Pluto Express would fly past Pluto and then enter the Kuiper Belt to explore what lies at the very edge of the solar system. Scientists now know that at least some of the comets that make their way into the inner solar system originate in the Kuiper Belt. They are pulled from their distant orbits beyond Pluto by the gravity of the giant planets. Pluto Express could be launched sometime in the early 2000's, reaching Pluto around 2010.

The relatively low cost and increasing sophistication of unmanned space probes ensure that they will serve as our eyes and ears in the solar system for some time to come. The next generation of probes will add to the already enormous body of knowledge about the solar system amassed by their predecessors. When human beings finally set foot on other planets and their moons, the terrain, geology, and atmospheric conditions of those worlds will already be known to them. Unmanned probes will have blazed the trail.