This book is aimed at several distinct audiences: first, the upper division science major who wants an up-to-date appreciation of the present state of the planetary sciences for 'cultural' purposes; second, the first-year graduate student from any of several undergraduate disciplines who intends to take graduate courses in specialized areas of planetary sciences; and third, the practicing Ph. D. scientist with training in physics, chemistry, geology, astronomy, meteorology, biology, etc. , who has a highly specialized knowledge of some portion of this material, but has not had the opportunity to study the broad context within which that specialty might be applied to current problems in this field.