After a year of analyzing data and photographs from the Voyager 2 spacecraft, scientists are not sure they can explain the strange streaks that appear like dark brush strokes on the bright canvas of Triton, the large icy moon of Neptune. They proposed at least two possible explanations in an issue of the journal Science devoted mainly to Voyager's first explorations of Triton.
Either the streaks or plumes are geysers of nitrogen gas, scientists suggested, or powerful "dust devils," wind-driven gusts of hydrocarbons that leave dark smudges on the moon's otherwise crystalline surface of nitrogen ice.
In the first reconnaissance of Triton, in August 1989, Voyager's cameras detected several of the mysterious plumes toward the south pole of the moon.
Geologists immediately suspected they were evidence of gaseous eruptions, even though they were surprised to see such dynamic forces on a world that would seem to be frozen solid at temperatures more than 400 degrees below zero Fahrenheit.
A team of Voyager geologists reported identifying at least four active geyserlike eruptions occurring at the time of the spacecraft's encounter.
Two of the best documented eruptions appeared as columns of dark material rising to an altitude of five miles, where winds stretched the clouds out more than 60 miles.
After weighing many hypotheses, the geologists decided that the most probable cause was what they call a solid-state greenhouse effect operating on Triton.
The nitrogen gas coating the surface is mostly crystal clear, reflecting nearly all of the dim sunlight reaching the moon. But there could be patches of dark particles embedded in the ice, the residue of methane that has been exposed to cosmic rays or ultraviolet radiation and thereby converted to black hydrocarbons, much like soot.
As geologist Laurence Soderblom explained it, the hydrocarbons in the ice could absorb enough sunlight to warm subsurface layers, vaporize some of the nitrogen ice and produce reservoirs of trapped gases. A key to the hypothesis is that the observed plumes occur in a region cocked toward the sun.
Scientists cannot agree whether the pressurized gases erupt explosively and form a long dark cloud, or if they vent quietly and are swept higher and into elongated clouds by a strong west wind.