I just realized:
You're seeing the effects of the bumpmap bug that's in v1.3.0. Mercury and Mars will cause problems for you, too.
You should consider upgrading to one of the "prerelease" versions of Celestia, or wait for the official release of v1.3.1, although we don't know when that will be.
I'd suggest Celestia prerelease v1.3.1pre5, which is available at
http://www.shatters.net/celestia/files/celestia-win32-1.3.1pre5.exe (an 11MB download)
You also will see problems with shadows cast by Saturn's rings on the planet, but those are due to bugs in ATI's graphics device drivers for their older cards. It only works on Radeon 9500 or better.
Of course, make sure you have installed the most recent graphics drivers that ATI provides on their Web site. Celestia v1.3 uses OpenGL v1.4, which is relatively new.