The mystery behind Triton's plumes.

The mystery behind Triton's plumes

The dark, geyser-like plumes photographed by Voyager 2 last year, rising about 8 kilometers above Neptune's moon Triton, are far less spectacular than the towering volanic eruptions witnessed by Voyager 1 on Jupiter's moon Io a decade earlier. Yet Triton's outbursts puzzle scientists more.

Researchers had predicted Io's eruptions -- which rose hundreds of kilomters and provided the first examples of active volcanism seen beyond Earth. Moreover, a now widely accepted explanation accompanied the prediction, suggesting that a tidal tug-of-war between Jupiter and its other big moons might drive Io's volcanic fury.

Triton's plumes, by contrast, were wholly unexpected (SN: 10/14/89, p.247). And the question of what propels them high into Triton's atmosphere still furrows many brows. Are Triton's plumes due to some version of the "greenhouse" effect, or to some extraterrestrial analog of the dust devils that whirl sand across desert landscapes on Earth?

Triton's mystery plumes consist primarily of nitrogen gas that has burst through a layer of frozen nitrogen covering Triton's extremely cold surface. Some Voyager researchers proposed last year that the sun's warming of dark particles trapped in the ice may ultimately heat the gas underneath. The resulting expansion of this gas create a pressure buildup that eventually relieves itself in eruptions through weak spots in the ice. It's also possible, though less likely, that radioactive elements in Triton's core might generate enough heat to expand and pressurize the gas, accoring to Laurence Soderblom of the U.S. Geological Survey in Flgstaff, Ariz.

Whatever the source, warming the plume material by as little as 4[degrees]C could "drive it out of the ground" with enough momentum to spew it 8 km into Triton's atmosphere, says Robert H. Brown of NASA's Jet Propulsion Laboratory in Pasadena, Calif. And two kinds of "greenhouse" effects occurring within Triton's frozen surface could store enough heat to power such plumes, Brown reports.

Under one possible scenario -- which he terms a "super-greenhouse" effect--dark, absorbing matter trapped with the nitrogen gas might capture and hold heat from the sun beneath a thin covering of icy nitrogen. Under a more "classical" greenhouse scenario, the sun's heat might merely build up throughout a pile of ice more than 60 meters deep. Either way, the heated gas would eventually escape.

Kimberly A. Tryka and Andrew P. Ingersoll of the California Institute of Technology in Pasadena counter that the curious plumes might instead represent a Tritonian version of dust devils -- swirling atmospheric vortices. Though the mean temperature at Triton's surface is a firgid-235[degrees]C, dust devils might arise if the sun created a relatively hot spot on Triton's surface. The temperature differential between that surface ice and the surrounding, colder terrain might create enough turbulence in the nitrogen atmosphere to drive formation of the plumes, the Caltech team concludes.

Dust devils on Earth can occur in environments well above 50[degrees]C, but there is evidence that they also arise in much colder climes. Besides Voyager's intriguing Triton images, photos taken in the late 1970s by the Viking craft recorded 6-km-high dust devils on Mars.