Neptune marvels emerge from data deluge.

Neptune marvels emerge from data deluge

Four months after Voyager 2's flight past Neptune, researchers have published in the Dec. 15 SCIENCE their first summary of results from that encounter. But the gap between submission and publication gave them time to glean some newer findings, which they reported last week at the American Geophysical Union meeting in San Francisco.

For example, project scientists now think the solar system's fastest winds may whip through Neptune's atmosphere. Voyager 2 had discovered several large features, such as the Great Dark Spot, a counterclockwise storm about the size of Earth. But at last week's meeting, Andrew P. Ingersoll of the California Institute of Technology in Pasadena said the craft's photos also reveal a series of much smaller cloud features that appeared only during a single rotation of the planet. By comparing the cloud movement as captured on film to the length of a 16-hour, 11-minute Neptunian day, Ingersoll and his co-workers conclude that the little clouds may have scudded along at about 600 meters per second (m/s), or about 1,340 miles per hour.

Neptune's mostly hydrogen atmosphere includes a little helium, about 1 percent methane and a trace of ammonia, Voyager found. In a hydrogen atmosphere at a temperature like Neptune's 60 kelvins, Ingersoll says 600 m/s is "mach 1" -- the speed of sound. In other words, the clouds appear supersonic. In comparison, he says, the fastest winds Voyager detected at Saturn moved about 500 m/s, while those of Jupiter and Uranus were clocked via film at 150 to 200 m/s.

Neptune's supersonic winds are not a certainty, Ingersoll cautions. The altitudes of the different cloud features, for example, are difficult to confirm, making it hard to compute the clouds' speeds. Moreover, he says, it is difficult to tell from the photos whether the movements represent actual fluid motion of atmospheric mass or merely a wave moving through the atmosphere.

One of Voyager 2's most striking discoveries was a pair of huge plumes towering about 8 kilometers above the surface of Neptune's big moon Triton, apparently representing geyser-like eruptions of nitrogen gas that were blown sideways by the winds (SN: 10/14/89, p.247). Since then, Torrence V. Johnson of the Jet Propulsion Laboratory in Pasadena told SCIENCE NEWS, researchers studying the photos by various methods have identified "several other puffs of stuff at the same altitude" in the area of one plume, though the vertical portions of those puffs do not show in Voyager's photos.

Other Voyager scientists led by Robert M. Nelson of Jet Propulsion Lab report that the albedo, or reflectivity, of Triton appears to indicate it has been resurfaced by material laid down atop the original terrain. "The very high albedo of Triton is consistent with a surface that has been recently renewed, such as [Jupiter's moon] Europa, or a surface that may still be undergoing regeneration, such as [Jupiter's] Io," the group writes in SCIENCE. "These data are consistent with a tectonically active Triton."

The new surface could have been smoothed by the heat of radioactive elements at the satellite's interior. But even without them, the finding would be consistent with the plumes, which provide their own evidence that Triton is geologically active.

In addition, the Voyager team has more accurately sized Neptune's eight known satellites, six of them discovered by Voyager 2. In SCIENCE, a team led by Bradford A. Smith of the University of Arizona in Tucson reports Triton's diameter is 2,700 km, while another study suggests Triton harbors a rocky, silicate core about 2,000 km across. The other moons and their diameters are Nereid, 340 km; 1989N1, 400 km; 1989N2, 190 km; 1989N3, 150 km; 1989N4, 180 km; 1989N5, 80 km; and 1989N6, 54 km.