Saturn: Moons: Dione
Dione [dy-OH-nee] was discovered in 1684 by Giovanni Cassini. It is an icy body similar
to Tethys and Rhea. Its density is 1.43 gm/cm3
makes it the densest moon of Saturn other than Titan. Dione is probably composed of a rocky
core making up one-third of the moon's mass, with the rest water-ice. Its ice coverage is less
than that of Tethys and Rhea.
Dione's icy surface includes heavily cratered terrain, moderately cratered plains, lightly
cratered plains, and whispy material. The heavily cratered terrain has numerous craters
greater than 100 kilometers in diameter. The plains area tends to have craters less than 30
kilometers in diameter. Some of the plains are heavily cratered while others are not. Much of
the heavily cratered terrain is located on the trailing hemisphere, with the less cratered plains
area existing on the leading hemisphere. This is opposite from what some scientists
expected. Shoemaker and Wolfe proposed a cratering model for a tidally locked satellite with
the highest cratering rates on the leading hemisphere and the lowest on the trailing
hemisphere. This suggests that during the period of heavy bombardment, Dione was tidally
locked to Saturn in the opposite orientation. Because Dione is relatively small, an impact
causing a 35 kilometer (21 mile) crater could have spun the satellite. Since there are many
craters larger than 35 kilometers (21 miles), Dione could have been repeatedly spun.
Dione has probably been tidally locked in its current position for the past several billion
years. This is reflected in the average surface albedo of the leading and trailing hemispheres.
The surface albedo decreases from the leading to the trailing hemispheres due to a higher
micrometeor dusting on the leading hemisphere.
The origin of the bright whispy material is somewhat obscure. Apparently, it is material
with a high albedo and is thin enough that it doesn't obscure the surface feature underneath. It
might have formed from eruptions along cracks in Dione's surface that fell back to the surface
as snow or ash.
Copyright © 1997-1999 by Calvin J. Hamilton.