153 Space science ............. One of the stated goals of NASAs Astrobiology Institute is to investigate the possibility of whether life can spread beyond its home planet: What is the potential for survival and biological evolution beyond the planet of origin? This boils down to where we are going as a species, and the really big question is, could Mars have a biosphere once again? In October 2000 a two-day conference entitled The Physics and Biology of making Mars Habitable was organised by Chris McKay at the NASA Ames Laboratory to discuss the possibility of one day changing the climate of Mars to a more Earth-like environment, suitable for terrestrial species to flourish. Twenty six papers, by an international cast of authors, were listed on the programme and the attendance was so good that the venue had to be transferred to a larger auditorium. Serious science Over the past 30 years the concept of planetary engineering - more popularly known as terraforming - has moved from the arena of science fiction towards serious scientific attempts to determine its future practicality. The word terraforming was first coined by the science fiction writer Jack Williamson and can be defined as a process by which a barren extraterrestrial planetary environment can be altered to one that is suited for life. Any argument that such an idea remains fantasy is countered by our emerging appreciation of the fact that mankind already has the ability to alter the Earths global parameters and has been doing so for some time. For example, the annual amount of rock mobilised in mineral extraction and construction already exceeds that washed off the continents by natural erosion. The atmospheric composition is now partly man-made and emissions of greenhouse gases (largely as by-products of industry) have probably caused an increase in the mean global surface temperature of between 0.5 and 1 o C since pre-industrial times. It is true that seems certain that during their first billion years of existence the surface conditions on Earth and Mars resembled each other more closely than at present. The first scientific papers on the possibility of terraforming Mars appeared in the early 1970s and over the 30 years since then, as interest has grown, the number of papers in print has risen to over 50, dealing with a variety of aspects of the problem, such as technological and biological aspects of planetary engineering and the ethics involved in such an undertaking. During his opening address, McKay reminded the audience that terraforming studies have been summed up by one technical-level book per decade, each one over double the length of the last. In 1976, NASA published On the Habitability of Mars: An Approach to Planetary Ecosynthesis, edited by Mel Averner and Robert MacElroy; in 1981 New Earths by James Oberg appeared and in 1995 SAE International published Terraforming: Engineering Planetary Environments by Martyn Fogg. Judging by the quantity of new work presented at the conference, this doubling time looks set to fall. Engineering A naive view of planetary habitability would demand environmental conditions similar to those of the Earth: a mean global temperature of 15 o C, one atmospheric pressure composed of 20% oxygen and 80% nitrogen, and lots of water. However, the Earth is not the only way for a planet to be habitable. The requirements for Terraforming Mars Scientists discuss the feasibility of making the Red Planet habitable by Julian Hiscox and Martyn Fogg Gullies eroded into the wall of a meteor impact crater in Noachis Terra (above). This high resolution view from the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) shows channels and associated aprons of debris that are interpreted to have formed by groundwater seepage, surface runoff and debris flow. NASA/JPL This is the south polar cap of Mars as it appeared to the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) on 17 April 2000. In winter and early spring, this entire scene would be covered by frost. In summer, the cap shrinks to its minimum size, as shown here. these changes have been unplanned but if we consider adding intention to a proven technological ability, then planetary engineering on Mars does not seem such a wild proposition. Mars is the most obvious choice in the Solar System for considering future habitability, simply because it was probably habitable in the past. There is now abundant evidence that liquid water once existed on the surface of Mars, suggesting that the carbon dioxide atmosphere was thicker than today. Schools of thought are divided between whether ancient Mars was warm and wet or cold and icy; but whatever the truth, it