Neptune's Nereid: another mysterious moon.

Neptune's Nereid: Another mysterious moon

The moons of Neptune are strange bodies, indeed.

Triton has a retrograde orbit -- moving clockwise around the planet Neptune while most other bodies in the solar system revolve counterclockwise--and it may host what would be the only known liquid-Nitrogen ocean. Scientists have realized that Nereid, Triton's satellite sibling, is unusual too because it has a highly eccentric orbit, taking it 1.4 million to 9.7 million kilometers from Neptune. But since its discovery in 1949. Nereid has been largely ignored and Triton has basked in the astronomical limelight. Now two researchers have found that Nereid has other peculiar qualities that merit the spotlight as well.

In the June 2 NATURE, Martha W. Schaefer and her husband Bradley E. Schaefer, both at NASA's Goddard Space Flight Center in Greenbelt, Md., report the first detailed photometric measurements of the light reflected from Nereid. Observing over an eight-day period in June 1987, they found that Nereid's color is unlike that of any normal satellite or asteroid. Moreover, the brightness of light reflected from Nereid varied by a factor of four during their observations.

"There are not many things in the solar system that show as big a variation as that," says Martha Schaefer. One possibility is that Nereid has a highly irregular shape, so that when its largest part faces the earth, more light is reflected than when the smallest part is in front. But scientists know of no other satellites in nereid's size range that are that irregular. The Schaefers estimate Nereid's diameter to be at least 660 km, and theorists believe the gravity of any object larger than 400 km makes it spherical.

Alternatively, the light variation could arise from distinct spots on Nereid's surface that reflect light very differently. In this respect, Nereid might be similar to Iapetus, a moon of Saturn. Its brightness varies by a factor of 10, a change in intensity believed to be caused by the existence of ice on one side and black carbonaceous material on the other. How such distinct regions came to be, however, baffles scientists.

Researchers hope to gauge Nereid's shape, exact size and any prominent surface features in August 1989, when Voyager 2 passes within 4.7 million km of Nereid--close enough for Voyager's camera to resolve details of about 50 km, according to Joseph Veverka at Cornell University in Ithaca, N.Y. Whatever causes the light variation, Nereid is "not a run-of-the-mill object," he notes. "And whatever we find out is going to be interesting."

Unfortunately, Voyager is not equipped to shed much light on Nereid's mass, makeup or origin. Because of its eccentric orbit, scientists have suggested that Nereid is an asteroid captured by Neptune's gravitational field. If this were so, say the Schaefers, it would be one of the largest and most unusual captured asteroids in existence. Instead, they lean toward the idea that Nereid accreted into a moon around Neptune or another planet and that both it and Triton were knocked into their peculiar orbits by some large body or planet.

"We are far from knowing the explanation," Veverka says. But every bit of data whets the appetite because Nereid is among a number of eccentric satellites inhabiting the outer solar system, and Veverka says they may provide the only clues to what kinds of bodies glommed together to form Neptune and Uranus billions of years ago.