Neptune: a new page in the book of worlds.

Neptune: A new page in the book of worlds

Scientists working with Voyager 2's closeup measurements of Neptune have barely begun their in-depth study of the data. This week, however, several presented their first reports of findings since the late August flyby, at a meeting of the American Astronomical Society's planetary Division in Providence, R.I.

One striking result from the mission was the discovery on Neptune's big moon Triton of two towering plumes of gas, probably nitrogen, one leaping up about 8 kilometers from the surface (SN: 10/14/89, p.247). Scientists found photographic evidence of the plumes well after the Voyager flyby. Laurence A. Soderblom of the U.S. Geological Survey in Flagstaff, Ariz., suggested this week that the nitrogen may become heated by sunlight absorbed in dark material in Triton's surface ice and escape through vents or fissures in an ice layer 2 to 3 meters thick. Though the nitrogen has a pressure of only 1 millibar, he says, it would push up against a thin atmosphere with a surface pressure 100 times less than that.

A hallmark of any planetary encounter's early data analysis is the struggle to extract subtle details from photos of the surface, and Triton proved no exception. According to Voyager assistant project scientist Ellis Miner of Jet Propulsion Laboratory in Pasadena, Calif., many of the shapes visible on the surface are probably water ice, since ices of methane and nitrogen "would not retain the large vertical structures that we see," such as cliffs 100 to 300 meters high.

Triton and Nereid were Neptune's only known moons before the Voyager encounter, but the spacecraft's photos revealed six more. The number has not increased since, giving Neptune the fewest known moons of the four major outer planets. But Carolyn Porco of the University of Arizona in Tucson notes that photo interpreters are reanalyzing Saturn 2's pictures for signs of a small moon suspected outside Triton's orbit.

The researchers still seek to refine the length of Neptune's day. In August, scientists put it at 16 hours and 3 minutes. Now a Neptunian day appears a little longer, perhaps 16 hours and 6 to 7 minutes.

Since a deep atmosphere hides Neptune's surface, the key to fixing the length of a Neptunian day lies in analyzing the radio emissions produced by its magnetic field. Studying the emissions in detail, however, turns out to be tricky. Voyager 2 has detected b oth brief, or "bursty," emissions and longer-term smooth ones, says Michael L. Kaiser of NASA's Goddard Space Flight Center in Greenbelt, Md.

The bursty ones, he says, often appear in unsually narrow frequency bands, typically showing up in only one channel at a time of Voyager's planetary radio astronomy instrument, with each burst no more than 20 kilohertz wide. Uranus has some similarly narrow bursts but at much lower frequencies -- below 0.1 megahertz at Uranus, compared with greater than 1 megahertz at Neptune.

One curious aspect of the radio signals, Kaiser says, is the intricate polarization pattern of the smooth ones, showing emissions with both left - and right-hand polarization even though they seem associated with just one of Neptune's magnetic poles -- "the weak one, the north pole," according to Kaiser. Scientists would expect right and left polarization to go hand in hand with observations from two poles.

The planet displays a remarkably complex magnetic field. The simplest form of a planetary magnetic field is called a dipole, like that of a bar magnet. At Neptune, says Miner, the field is "not easily represented by a multipole model, or even by a dipole plus quadrupole plus octopole."