Planets, life and the production of entropy

Two thermodynamic principles offer considerable insight into the climatic and geological settings for life on other planets, namely (1) that natural systems tend to actually achieve the ideal (‘Carnot’) limit of conversion of heat into work and (2) if a fluid system such as an atmosphere has sufficient degrees of freedom, it will choose the degree of heat transport that maximizes the generation of work (equivalently, that which offers maximum entropy production). The first principle agrees well with results on terrestrial cumulus convection, and the mechanical energy released by tectonic activity. The second principle agrees with the observed zonal climates of Earth, Mars and Titan, and shows promise for planetary interiors too; I discuss applications in the investigation of paleoclimates and habitability. I compare the work performed by planetary atmospheres and interiors on the terrestrial planets and thereby predict a weakly eroded landscape on Titan. The association of life with the production of entropy is also noted, and the possibility of evaluating planetary entropy production by telescopic observation is discussed. Received 25 September 2001