Fluids have shaped the surfaces of both Mars and Titan, creating many landforms that bear a strong resemblance to those on Earth. I describe two case studies that illustrate what we, as remote observers with some knowledge of terrestrial surface processes, can learn about the present states and evolutionary histories of other bodies by studying their surfaces. First, I present evidence that the formation and subsequent disappearance of oceans early in Mars' history significantly affected Mars' global-scale topography, and perhaps even influenced the planet's rotational stability. Second, I analyze dendritic valley networks near the landing site of the Huygens probe on Titan, and obtain an estimate of the minimum rate of methane rainfall required to erode the valleys.
Geologic and topographic features near the margins of the northern plains of Mars have been interpreted as shorelines formed by ancient oceans. The ocean hypothesis was called into question, however, when topographic data revealed that elevation profiles along the putative shorelines do not follow surfaces of equal gravitational potential, as the margins of a standing body of water should. Long-wavelength (thousands of km) trends, with amplitudes of hundreds of meters, are especially apparent in the shoreline topography. I show that these topographic trends can be explained by deformation associated with true polar wander (a reorientation of Mars with respect to its rotation axis), and that the inferred true polar wander could have resulted from the redistribution of the ocean water.
Branching valley networks near the landing site of the Huygens probe on Titan imply that flowing fluid has eroded the surface. The morphology of the valley networks is most consistent with mechanical valley erosion by methane precipitation and surface runoff. If mechanical erosion did occur, the methane flows must first have been able to mobilize any sediment accumulated in the valleys. I use sediment transport relations scaled from Earth to Titan to estimate the minimum methane precipitation rate required to mobilize sediment and initiate erosion. For the sediment grain sizes observed at the Huygens landing site, the estimated precipitation rates are Earthlike, and are quite plausible given the large mass of methane in Titan’s atmosphere and the considerable potential for the formation of convective storms.