baalke@mars.jpl.nasa.gov (Ron Baalke) (11/14/90)
From the Science News - November 10, 1990
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 that the towering volcanic eruptions witnessed by Voyager 1
on Jupiter's moon Io a decade earlier. Yet Triton outbursts puzzle
scientist more.
Researchers had predicted Io's eruptions -- which rose hundreds of
of kilometers 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 it other
big moons might drive Io's volcanic fury.
Triton's plumes, by contrast, were wholly unexpected. 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 of 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 could 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, according
to Laurence Soderblom of the U.S. Geological Survey in Flagstaff, Arizona.
Whatever the source, warming the plume material by as little as 4 degrees
Celsius 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, California. 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 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 if a rigid -235 degrees
Celsius, 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
Celsius, 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.
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| | | | __ \ /| | | | Ron Baalke | baalke@mars.jpl.nasa.gov
___| | | | |__) |/ | | |___ Jet Propulsion Lab | baalke@jems.jpl.nasa.gov
/___| | | | ___/ | |/__ /| M/S 301-355 |
|_____|/ |_|/ |_____|/ Pasadena, CA 91109 |