The Planet Mars

Resurgent interest in Mars, coinciding with ten missions to the planet anticipated between 1996 and 2003, makes William Sheehan’s most recent history of astronomy quite timely. Sheehan is an amateur astronomer who has won awards for his publications concerning astronomy. The bulk of the text concerns early observations of Mars and the resulting theories and calculations concerning the planet. Sheehan provides short biographies of major astronomers, explaining how they came to their work and how their work fit into the development of knowledge about Mars. These biographies provide life to what otherwise might be a subject as dry as the surface of Mars itself.

The dryness of Mars was long a subject of debate. That debate centered on the Martian “canals,” first discovered (and named, misleadingly, as “canali” in Italian) by Giovanni Schiaparelli in 1877. The possibility of water on Mars raised speculations about life on the planet. Sheehan traces debates about the existence of canals through the century after Schiaparelli’s discovery; the debate continues whether life ever existed on Mars. Water still exists, but in the form of an underlying layer of the northern polar cap, the upper layer of which is frozen carbon dioxide.

Satellite and lander observations of the planet receive less attention. Sheehan describes the United States’ Mariner and Viking expeditions, mentioning unsuccessful missions in passing. He explains the experiments performed by the Viking landers to determine whether Mars ever hosted life; he takes neither side in the debate concerning the results of those experiments. The Mars expeditions did determine that water once flowed on the planet.

The concluding chapter describes how a reader might observe Mars, from use of a small telescope to sophisticated camera technology. Sheehan describes what an observer should look for and how to render maps or obtain useful photographs. Appendices provide statistical information about the planet and its moons, along with the best dates for viewing. Renditions of early hand-drawn maps and photographs show the progress made in observing the planet. The book is extensively footnoted and indexed.