Saturn's 18th Moon
Ron Baalke
"Five Year Hunt Locates Saturn's 18th Moon"
Add another moon to the list of natural satellites orbiting Saturn.
Temporarily designated 1981S13, the tiny moon apparently creates the 320
kilometer wide "Encke's gap" in the planet's A-ring, the outermost of its
clearly visible rings. Mark R. Showalter of NASA's Ames Research Center in
Mountain View, Calif., confirmed the moon's presence in an analysis of photos
taken by Voyager 2 during its 1981 Saturn flyby.
Indirect evidence of an undiscovered moon in the Voyager images first
emerged in 1985 when Jeffrey N. Cuzzi and Jeffrey D. Scargle of Ames noticed
wavy ripples, resembling the wake of a speedboat, along the inner and outer
edges of Encke's gap. The two suggested that the ripples could be due to a
small moon in the gap that pushed material away from its orbit. In 1986, Cuzzi
and Showalter joined two other researchers in determining the orbit and mass
of the proposed satellite. However, they found no trace of it in the best
of the photos.
"The '86 study pretty much made an ironclad case that the moon was there,
though it did not appear in any of the few dozen high-quality images we were
studying at the time," Showalter says. Four more years passed before
Showalter spotted the bright moonlet while using a computer program he wrote
to analyze 30,000 lesser-quality Voyager photos.
"The consensus at first was that we wouldn't find it till Cassini gets
there," he says. Cassini is a planned Saturn-orbiting spacecraft due to reach
the planet in 2002; it will also carry a probe to plumb the atmosphere of
Titan, Saturn's biggest moon.
The newly found moon measures about 20 kilometers in diameter and ranks
as Saturn's smallest known satellite, the 18th comfirmed to date. Showalter
bases his size estimate on an assumed albedo (reflectivity) of 0.5, which he
says resembles that of the other moon close to Saturn's rings. "An albedo
of one-half is pretty bright," he adds, "so there really isn't much it could
be covered with except water-ice."
Showalter would like to name the object Pan, after the Greek god of
shepherds. Space scientists use the term "shepherding" do describe
gravitational interactions among satellites and smaller particles in orbit
"whereby a moon can repel ring material and clear open a gap," he explains.
Official adoption of the name will require approval by the International
Astronomical Union.
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| | | | __ \ /| | | | Ron Baalke | baa...@mars.jpl.nasa.gov
___| | | | |__) |/ | | |___ Jet Propulsion Lab | baa...@jems.jpl.nasa.gov
/___| | | | ___/ | |/__ /| M/S 301-355 |
|_____|/ |_|/ |_____|/ Pasadena, CA 91109 |