the north and south poles provided water content could be constrained to avoid a runaway greenhouse effect. Moon Although the gravity on Earth's moon is too low to hold an atmosphere for geological spans of time, if given one, it would retain it for spans of time that are long compared to human lifespans. Landis and others have thus proposed that it could be feasible to terraform the moon, although not all agree with that proposal. Landis estimates that a 1 PSI atmosphere of pure oxygen on the moon would require on the order of two hundred trillion tons of oxygen, and suggests it could be produced by reducing the oxygen from an amount of lunar rock equivalent to a cube about fifty kilometers on an edge. Alternatively, he suggests that the water content of "fifty to a hundred comets" the size of Halley's comet would do the job, "assuming that the water doesn't splash away when the comets hit the moon." Likewise, Benford calculates that terraforming the moon would require "about 100 comets the size of Halley's." Earth It has been recently proposed that due to the effects of climate change, an interventionist program might be designed to return Earth to pre-industrial climate parameters. In order to achieve this, multiple solutions have been proposed, such as the management of solar radiation, the sequestration of carbon dioxide using geoengineering methods, and the design and release of climate altering genetically engineered organisms. Other bodies in the Solar System Other possible candidates for terraforming (possibly only partial or paraterraforming) include large moons of Jupiter or Saturn (Titan, Callisto, Ganymede, Europa, Enceladus), and the dwarf planet Ceres. Other possibilities Biological terraforming Many proposals for planetary engineering involve the use of genetically engineered bacteria. As synthetic biology matures over the coming decades it may become possible to build designer organisms from scratch that directly manufacture desired products efficiently. Lisa Nip, Ph.D. candidate at the MIT Media Lab's Molecular Machines group, said that by synthetic biology, scientists could genetically engineer humans, plants and bacteria to create Earth-like conditions on another planet. Gary King, microbiologist at Louisiana State University studying the most extreme organisms on Earth, notes that "synthetic biology has given us a remarkable toolkit that can be used to manufacture new kinds of organisms specially suited for the systems we want to plan for" and outlines the prospects for terraforming, saying "we'll want to