Comet dirt, magma oceans, meteorites from Mars--all the extraterrestrial grit and goop that geologists love--brought wittily down to earth. McSween (Geological Sciences/Univ. of Tennessee) picks subjects that might ordinarily numb most readers--the regolith, or loose mantle of rock and dust on the moon; volcanism on the Jovian moon of Io, etc.--and then proves that a dash of humor, a pound of anecdote, and a ton of crystalline explication can turn lead into gold. He notes, for instance, how books about the solar system are usually organized, either by planetary size or distance from the sun, and then ferrets out more unusual possibilities: Why not by temperature (Uranus is mysteriously cool) or shape (Saturn has a strangely rocky core) or density (``Saturn would float in water, if you could find a big enough bathtub'')? With similar panache, McSween uses a corker from Yogi Berra (``you can observe a lot by just watching'') to sum up the history of our understanding of geologic time scales on the moon. The Giotto spacecraft's flyby of Halley's comet (a ``sixteen-by-eight-by-eight kilometer potato'') raises the question ``Could you eat a comet''? The answer is no- -comets are indeed ``dirty snowballs,'' but too dirty for consumption. McSween's froth never obscures his lessons, each of which delivers key facts and concludes with an annotated bibliography. Also covered are the mysteries of volcanism on Neptune's moon Triton (the latter, as the coldest object in the solar system, is an odd place for volcanoes); the creation of diamonds in supernovas; the core composition of Earth-like planets; the fate of Mars's once-abundant water (locked into permafrost and the polar caps); the mineralogy of asteroids and planetary rings; and the origins of life on Earth (McSween plumps for the organic soup theory). As energetic as Sagan, without the pontificating; once McSween finds a richer theme, the moon's the limit.