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Five-year hunt locates Saturn's 18th moon.

Five-year hunt locates Saturn's 18th moon

Add another moon to the list of natural satellites orbiting Saturn. Temporarily designated 1981 S13, 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 pushes 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 confirmed to date. Showalter bases his size estimate on an assumed albedo (reflectivity) of 0.5, which he says resembles that of the other moons 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" to 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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Author:Eberhart, Jonathan
Publication:Science News
Date:Aug 4, 1990
Words:398
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