No other planet has so fired the human imagination as Mars. The possibility of life on the planet was an obsession in the 19th century. It is not surprising to find that it has become so once again, because of all the planets in the solar system (apart from Earth), Mars has always been regarded as the most likely to serve as the abode of life. Now that there are hints that the planet may have held "liquid water" (a prerequisite for life, in contrast...