Mars has always held a special interest because of the possibility that life may have existed there, and its water history is crucial to understanding its geology, climatology, and biology. Moreover, recent studies in molecular phylogeny suggest that volcanic hot springs, which may have been common in early Mars, are also the most likely point of origin for life on Earth. In this book, Dr. Carr explores the history of water on Mars, including evidence that liquid water was once abundant at the planet's surface; ways in which the climate might have changed to accommodate liquid water; and what an abundance of water implies for the formation of Mars and other planets, including Earth. The book's argument rests on interpretation of data acquired on Viking missions, and on information from meteorites, found on Earth, that almost certainly originated on Mars. Because liquid water is universally regarded as essential for life, the water story has particular biological significance, with important implications for the future exploration of the planet, and should be a valuable study for geologists and planetary scientists.