Triton 497
FIGURE 14 The geographic
distribution and orientation of
wind streaks, crescent streaks, and
plumes on Triton as seen by
Voyager. The latitude and
longitude of each feature source is
plotted as a dot; tails indicate
streak or plume length and
direction. [From C.J. Hansenet al.
(1990)Science 250 , 421–424.]back onto the ground, the methane frost would over time
darken from exposure to radiation, explaining the surface
wind streaks. Although this model ingeniously solves the
problem of how such a gently rising plume picks up or gener-
ates enough solids to become visible, there is the fundamen-
tal objection that such a plume would be blown sideways
(as dust devils on the Earth and Mars are). A final variation
on these types of plume model suggests that it is nitrogen
rather than methane that is ascending and condensing. Of
the same composition as the rest of the atmosphere, the
plume in this model would be buoyant only because it is
warm. Condensation during its ascent could release enough
heat to accelerate the plume substantially, but the nitrogen
must somehow start off fast enough to pick up dust and to
avoid being blown sideways by winds near the base of the
plume.
How could a plume of nitrogen gas get started? One
possibility is that they are geysers. Like geysers on earth,
which consist of water and water vapor, those on Triton
would be eruptions of volatile material that has been heated
underground. Whereas the water in terrestrial geysers starts
as a liquid and partially boils, however, Tritonian geysers
would start as hot gas that would partially condense as it
expanded to the ambient pressure. This expansion could
drive a gas flow powerful enough to pick up dust and form
the observed plumes. Solar-powered nitrogen geysers have
been studied in some detail. The pieces of the model are as
follows: