New Era of Planetary Missions : NASA Seeks Clues to Life in the Earth’s Back Yard
Each July and August, blue ice lakes form on the surface of glaciers near the U.S. research station at Sondre Stromfjord in southern Greenland. Belying the pristine Greenland air and the clean-swept beauty of the glaciers, the bottoms of the lakes are covered with black organic goo.
The tar is not the residue of an oil spill, nor is it any other detritus of civilization. It is comet dust, a melange of rock and organic materials formed between the planets, and between stars, and dating back perhaps to the formation of the solar system 4.5 billion years ago.
The dust falls to Earth continually--and on other planets and moons as well--becoming buried in the snow and ice. In the arctic regions, it is concentrated by the summer melt and then settles on lake bottoms.
Earth’s organic chemicals--the stuff of life--represent only a fraction of the total found throughout the solar system. In fact, “the universe is awash in organic chemicals” like those from which life evolved on Earth, said astronomer Dale Cruikshank of NASA’s Ames Research Laboratory in Mountain View.
Because of the apparent ease with which life was created on Earth, some researchers believe life could have arisen elsewhere in the solar system as well: on the surface of Mars, when water flowed freely there, or in the frozen oceans of Europa, a moon of Jupiter.
Now, with the launch of the Magellan probe to Venus in May and next month’s launch of Galileo to Jupiter, NASA has begun a new series of exploratory missions to Venus, Jupiter, Saturn, its satellite Titan and at least two comets. In addition to evidence about the origin of the solar system, they hope to discover primitive forms of life that are no longer present on Earth itself, as well as other evidence that would illuminate the origin of life.
At the very least, researchers hope that information gained from those missions will shed light on how primitive organic molecules were organized into the more complex forms that are now characteristic of life.
“The clues to life’s origin . . . are going to be found elsewhere in the solar system,” said Lynn Griffiths, head of NASA’s exobiology program, which studies life off the Earth. “Exobiology has matured quite a bit in the last 20 years and is now in its active young adulthood,” added cosmologist Harold P. Klein of the University of Santa Clara. “We have unlimited challenges. Unfortunately, at the present we have only limited opportunities.”
Researchers gathered recently in Sunnyvale for a NASA-sponsored conference to discuss the best places to look for life off the Earth. They concluded that a number of tempting targets exist within the solar system but that undoubtedly the chances of finding life are rather small. Many would be happy to find fossils of life that had once existed elsewhere, but others argue that more clear-cut evidence--including living organisms--is necessary to convince them.
Exobiologists once had high hopes for Mars, the fourth planet from the sun, and were crushed when the Mariner probes produced no evidence of life. But they have been reassessing what they know about the Red Planet and now think that perhaps the probe simply wasn’t looking in the right place. “Mars is still the best candidate for life in the solar system,” said astrophysicist Christopher McKay of the Ames lab.
The chief factor militating against life existing on Mars, which is about half the size of Earth, is the small amount of water that exists there, either frozen at the poles in winter or vaporized in summer. The fact that water is not present in liquid form is “a serious problem for contemplating life on Mars,” McKay said, “but it hasn’t always been that way.”
Photographs of Mars show large areas, such as Valles Marinaris and Playa Lake, that look as though they were shaped by water. Valles Marinaris, for example, has a delicate surface tracery of small channels that could have been formed by the slow flow of water on the surface, “perhaps rain finding a collection area,” McKay said.
This and other evidence suggest that during the period from 4.5 billion to 3.5 billion years ago Mars had a relatively thick atmosphere, much larger quantities of water and a surface temperature above freezing. “If life did exist then, there could be a record,” said Glenn Carle, chief of NASA’s Solar System Exploration Branch.
“What we really need to know is: If life didn’t evolve there, why not?” McKay said. If life didn’t evolve on Mars, “there is something wrong with our understanding of Mars or our paradigm for the evolution of life on Earth.”
Missions to Mars
The Soviet Union and the United States are discussing manned missions to the Red Planet. McKay and others are urging that any landings be made in the polar regions near the ice caps or in regions where there is evidence of past water flow because that is where evidence of life is most likely to occur.
A more remote possibility for life is Venus, the second planet from the sun. The planet, which is about the size of Earth, has an average surface temperature of about 900 degrees Fahrenheit, an atmosphere that is 95 times as dense as Earth’s and virtually no water.
“The idea of extant life can be ruled out by these data,” astrophysicist Larry Colin of San Jose State said.
But, like Mars, Venus could have been more hospitable to life early in its history when the sun was not putting out as much radiation as it does now. One key piece of evidence favoring a clement early Venus are mass spectrometer measurements made by the Pioneer Venus spacecraft of the ratio of deuterium to hydrogen in the planet’s atmosphere. Deuterium is a rare heavy isotope of hydrogen.
The deuterium/hydrogen ratio on Venus is about 100 times as great as Earth’s. Because water containing deuterium is heavier than normal water and thus evaporates more slowly, this ratio indicates that Venus probably had “significant amounts of water,” Colin said. As the planet warmed up, that water eventually rose into the upper atmosphere, where it was broken apart by sunlight, leaving behind the heavier deuterium as much of the lighter hydrogen escaped into space.
Venus could have had oceans during this period, and life could have gotten started, only to die out as the planet heated up, astrophysicist James Kasting of Ames said. Traces of life could still remain, he said. It may be very difficult to locate such evidence, however, because of Venus’ hostile environment.
Venus’ Past
Scientists hope to learn much more about Venus’ past from the Magellan probe, which is scheduled to reach the “Veiled Planet” Aug. 10, 1990. The probe will use sophisticated radar imaging to map at least 80% of the planet’s surface, a process that is expected to shed light on its geological history and the origin of the “runaway greenhouse effect” that caused its high surface temperature.
Perhaps the most tantalizing object in the solar system is Europa, a Jovian moon slightly larger than Earth’s moon. Voyager 2 pictures show that its surface is bright, smooth and highly fractured “like sea ice,” astronomer Steve Squyres of Cornell University said.
The whole surface of Europa has only about five craters, even though it has presumably been subject to the same intense bombardment that has left the Earth’s moon covered with craters. Researchers thus believe that Europa has continuing geological activity that obliterates the craters.
About 7% of the mass of Europa is water, he said, and the most likely explanation for the surface features is that the moon’s surface is covered by an ocean of liquid water topped with a thick layer of ice. The water is presumably kept liquid by tidal heating, a continual compression and relaxation of the moon caused by Jupiter’s massive gravity.
Furthermore, according to biochemist John Oro of the University of Houston, “there is more organic material on Europa than in the terrestrial biosphere,” as well as other elements, like phosphorus, that are crucial to life. “And if there is liquid water, then Europa has potential hot spots” similar to thermal vents in the Earth’s oceans.
His conclusion: “Could terrestrial life grow there? My answer is yes. We definitely need a close flyby and landing.”
No Plans for Mission
No such missions are planned at the moment, however, largely because of the difficulty of landing amid Jupiter’s massive gravity.
Another possibility, less likely but more accessible, is Titan, the largest moon of Saturn, said Thomas Scattergood of the State University of New York at Stony Brook. Titan, which is slightly larger than the Earth’s moon, looks like “a featureless billiard ball” because its extensive atmosphere hides its surface.
Although researchers have never seen the surface of Titan, Scattergood speculated that it is an ethane ocean containing both water-ice and frozen carbon dioxide. Large numbers of organic compounds could be formed from these raw materials through the action of sunlight, lightning and energetic particles trapped by Saturn’s magnetic field.
Whether or not Titan can accommodate life will depend on how much water is present, he said. But “it looks like there is interesting organic chemistry taking place there even today,” NASA’s Carle added.
Exobiologists are also interested in studying asteroids and comets, not because they think life might exist there but because these celestial bodies contain large amounts of organic chemicals and are presumably the source of most of the carbon-containing compounds now on Earth.
Asteroids, which early astronomers called the “vermin of the skies” because of their misleading motion, come in two categories, black and lighter, NASA’s Cruikshank said. The black material, he said, is very much like kerogen, the complex organic material found in oil shale. The lighter material is made up of less complex organic substances.
The asteroids and meteorites, added geochemist John Kerridge of UCLA, “are the only materials we have that date from the beginnings of the solar system. . . . Studies of carbon-containing meteorites that have fallen to Earth, called carbonaceous chondrites, have shown the presence of more than 400 different compounds, including most of the chemicals that are involved in life processes.
Studies of such meteorites allow researchers to determine what kinds of chemicals were present on Earth before life arose, and may lead to new insights into the events that sparked life’s formation. Researchers are thus eager to have more samples. Kerridge added: “We pray for more carbonaceous chondrite falls.”
Exobiologists may be able to get more information about life’s origin without waiting around for more meteorites, as NASA begins sending more probes out into the solar system after an 11-year hiatus. “The queen is ready to give us more ships,” Carle said.
The Magellan mission that will perform radar mapping of Venus was launched from the space shuttle Atlantis in May, and the Galileo spacecraft to Jupiter is scheduled to be launched Oct. 12.
NASA has also tentatively approved two other deep-space missions, Titan-Cassini and the Comet Rendezvous and Asteroid Flyby (CRAF). Both spacecraft, designed at JPL, would use versions of the successful Mariner Mark II spacecraft that would be outfitted with new instrumentation.
The Titan-Cassini probe, named after the 17th-Century French- Italian astronomer who discovered several of Saturn’s moons, would be launched from an expendable rocket in April, 1996. The following March, it would have a close flyby of asteroid Maja. It would swing by Jupiter in February, 2000, for a gravitational boost and reach Saturn in October, 2002.
The orbiter will perform radar mapping of both Saturn and Titan, identifying objects as small as 200 yards in diameter. It will occasionally fly through the upper reaches of Titan’s atmosphere and sample it, and will also send a probe to the moon’s surface.
Help on Financing
Last November, the European Space Agency agreed to build the Titan probe and help finance the mission. NASA has not yet received funding from Congress for the $550-million mission but is seeking $40 million for the next fiscal year to begin construction of the craft.
The CRAF mission, which is scheduled for launch in August, 1995, would intercept comet Kopff on Aug. 14, 2000, Marcia M. Neugebauer of JPL said. Unlike the flights to Halley’s comet, which flew past it at high speed, CRAF will rendezvous with Kopff 850 days before its nearest approach to the sun and accompany it through its orbit, measuring its output of gases and dust.
Tests on Comet
After it has collected data for a year, CRAF will launch a 4.5-foot-long, golf tee-shaped penetrator that will sink at least 1 1/2 feet into the comet, measuring its strength, density, temperature and composition.
On its way to the comet, CRAF will fly by an asteroid named Hamburga. “Perhaps we can get McDonald’s to sponsor the mission,” Neugebauer said. The craft will survey the asteroid’s size, shape and rotation and gather information about its chemical composition.
The CRAF mission has not yet been approved by NASA. Its cost would be about the same as Titan-Cassini.
