The land’s present topography is not just the frontier between the lithosphere and the atmosphere and hydrosphere, but where plants of many different kinds grow. Whether in the form of cyanobacteria, lichens or luxuriant tropical rain forest, vegetation affects weathering, erosion and the deposition of sediments. Animals – leaving out humans – also have some influence, whether they be subterranean rabbits, moles and worms, or heavy-footed beasts that force soils to move downslope. Inevitably life-land interactions affect landforms, although rock-type and active geological processes tend to dominate. Nonetheless, a planet with life ought to show different styles of surface shapes from one that is organically dead. The central issues for geomorphologists is whether or not it is possible to define absolutely the differences, and then to use them as a means of detecting the likely former influence of life on other worlds.
Central to such a venture (Dietrich, W.E. & Perron, J.t. 2006. The search for a topographic signature of life. Nature, v. 439, p. 411-418) is the ability to map in detail the variation of topographic elevation. Digital topographic elevation data is now available for most of the Earth’s land surface at a resolution of between 90 and 30 m, the second only publicly available for the USA, from the groundbreaking Shuttle Radar Topography Mission of 2000. Aerial photography and high-resolution stereoscopic images from satellite such as Quickbird and Ikonos, allow resolution as sharp as a few metres. Laser scanning from aircraft potentially can even improve that to the scale of a few tens of centimetres, but such high-resolution data are far from global. The planet Mars is now better endowed with elevation data than is our own planet, thanks to photogrammetric instruments carried by ESA’s Mars Express mission, and the shyness of various intelligence agencies to share publicly what they have gleaned from high-altitude aircraft and spy satellites. Nonetheless, it is now possible to analyse elevation data from the entire range of terrestrial biomes to see what signal vegetation has imposed on surface shape. An easy way to visualise that is simple – just use Google Earth (see The Digital Earth revolution above).
Dietrich and Perron review the mathematical approaches to modelling life’s topographic influences, beginning with an equation that relates elevation and time to rates of uplift, erosion and entry of sediment into storage, thereby expressing conservation of mass. All the variables are themselves governed by a variety of processes, theoretically amenable to quantification, summarised in Dietrich and Perron’s review. In each there will be some potential biotic influence. On Earth there are sufficient landscapes devoid of all but a minute veneer of organisms to assess both end-members clearly. Mars and Venus ought to be good tests. But, should such a rigorous quantification of lifeless and lively surfaces at a spectrum of scales be achieved, where would we deploy it?