Defining the topography of a planetary body.

R ivers develop on planetary bodies from the interplay of climate and topography. In planetary contexts, rivers and their effects are used to constrain shortand long-term climate models. However, rivers are also controlled by the shortand long-wavelength topography of the planetary surface. Rivers thus provide a means to constrain the generation of topography. On page 727 of this issue, Black et al. (1) compare topography at a range of scales with mapped river drainages to provide new insights into the topographygenerating mechanisms on Earth, Mars, and Titan. Earth is particularly suited for comparing topography and river drainage. Its volatile cycle is hyperactive, relative to others in the solar system, and results in abundant river drainages, which are readymade arrows pointing the way downhill within the channels. The surrounding topography, however, need not trend downhill in the same direction, for example, when antecedent rivers persist in their original flow direction even during uplift of the regional topography (2). The eastern tributaries of the Tennessee River illustrate discrepant drainage and topography, although the causes for this are still under investigation (see the figure) (3). Quantifying such discrepancies between drainage and topography gives a new understanding of the mechanisms by which this topography is generated. Plate tectonics on Earth generates topography at plate margins, where two plates converge to create mountains, such as the Himalayas, or where subduction and melting of one plate generates volcanic orogens, such as those that formed the South American Andes Mountains. Although often extending thousands of kilometers along the end-to-end dimension, the plate-marginal topography is much shorter in width (or wavelength), typically less than a few hundred kilometers. Conversely, topography generated by other processes tends to be longer in wavelength, although exceptions do exist. Comparison of river drainages and landscape topography by Black et al. shows that on Earth, the downstream flow directions of rivers are not particularly well correlated with the landscape topography. This poor correlation means that rivers on Earth often drain across, or even upslope, through their surrounding landscapes. In other words, at locations of short-wavelength deformation,