Europa, the second of Jupiter's four large inner moons, appears to be able to support organic life.
In reporting this, Ray Reynolds and Steven Squyres of the NASA Ames Research Center emphasize that they make no claim that such life actually exists.
However, by a combination of theoretical calculation and data from ground-based studies and the Voyager spacecraft, they claim to show that Europa probably is covered by an ocean of liquid water beneath a relatively thin shell of ice. They note that this ice shell should be thin enough in some places for sunlight to penetrate and support simple organisms such as the microscopic ocean plants that live beneath Antarctic ice on Earth.
''This in itself is interesting,'' Dr. -Reynolds says. He adds, ''We think it's intriguing that conditions that can support life should exist so far out in the solar system.''
Reynolds further explains that the main purpose of this research is not to speculate about extraterrestrial life, but to ''lay a framework for understanding'' satellites of the outer planets.
Europa has been especially challenging. Before the Voyager 1 and 2 flyby missions in March and July 1979, many scientists had expected it to resemble Earth's moon. It turned out to be quite different.
Europa has a diameter of 3,130 kilometers (1,945 miles) - about 15 percent smaller than our moon. Its density, three times that of water, suggests a basically rocky body. But it could also have a lot of water. Seen telescopically from Earth, it appears white, like a giant snowball. So scientists were prepared to find an ice-covered body. But they were surprised when Voyager photos showed it to have a smooth, billiard-ball surface instead of a heavily cratered face like that of our moon.
In fact, only three craters have been identified in photos of Europa. This indicates an active surface with something obliterating craters rapidly after they are formed by incoming meteorites. Scientists have been further puzzled by long dark streaks on the satellite's smooth surface.
The model for Europa that Reynolds and Squyres have worked out would, among other things, explain the absence of craters.
This model envisions an ocean 50 km. (31 mi.) deep with an ice crust perhaps 5 km. (3 mi.) thick on average. The ocean would receive heat energy from radioactive decay within the satellite, perhaps from some volcanic action (although there is as yet no evidence for this), and by the action of tidal forces of Jupiter. Heat flowing outward would keep the ice at a subsurface temperature around 133 degrees below zero C (-271 degrees F). This would be warm enough for the ice to be supple and able to deform to smooth out craters, Reynolds says.
The two National Aeronautics and Space Administration (NASA) scientists say another process also affects the surface. According to their theory, Jupiter's tide-raising forces crack open the ice shell in various places. Water boils up through the cracks into the vacuum of space only to freeze and fall back onto the surface as frost. This would account for the coating of what appears to a fluffy substance on top of the ice.
This fluffy coating evidently builds up rapidly, Reynolds says. Jupiter's innermost large moon, Io, which has very active sulfur volcanoes, leaves a sulfur trail in space. Some of this sulfur falls onto Europa. But there is very little of it to be seen. To judge from the rate at which it should accumulate, this implies that the frost is covering up the surface very quickly, Reynolds says. The surface cracking is also the key to the possibility of life-supporting conditions. Reynolds and Squyres suggest that, when the cracks freeze over, the new ice should be thin enough to let in sufficient sunlight to support some forms of aquatic life.
The puzzling dark streaks may be regions where some of the cracks open up, and they are large cracks themselves. Some scientists have wondered if the darkening may be due to some kinds of organic chemicals that form in these regions. Reynolds notes that, were there life in regions where cracks form, there might be some organic products coming onto the surface with the frost. But he emphasizes that this is pure speculation.
Within five to seven years' time, new ice formed over cracks would become too thick to act as a window for photosynthesis. Nevertheless, the two scientists conclude that enough new cracks would be constantly opening to maintain something like 25 to 50 square km. (10 to 20 square mi.) of livable habitat around the satellite.