Earth and Planetary Surface Processes

Self-organization of landscapes

Landscapes on Earth and other planets display many forms of self-organization. One of the most widespread and striking examples is the regular spacing of ridges and valleys, which frequently occurs even where valley locations are not influenced by bedrock structure. We developed a spectral technique to measure this characteristic "wavelength" of landscapes from high-resolution digital topography, as detailed in a recent paper: PDF. In a second paper PDF, we use a numerical model of landscape evolution to show that this basic characteristic of Earth's topography emerges from a competition between diffusive (soil creep) and advective (stream incision) erosion processes, and that the ridge-valley wavelength is a function of the relative magnitudes of these two processes. The movie below shows how uniform valley spacing emerges over time in the model. Finally, in a paper in the July 23, 2009 issue of Nature, we compare the numerical model with high-resolution laser altimetry (right) and show that our theory correctly predicts the valley spacing in sites across the United States. (See also the News & Views article by K.X. Whipple.)

Evolution of planetary landscapes

Mars

Ancient oceans: Several lines of evidence suggest that oceans might once have existed on Mars' surface, including geologic and topographic features near the margins of the northern lowlands that have been interpreted as shorelines. But topographic profiles along the shorelines do not follow surfaces of equal gravitational potential (i.e., sea level), as the margins of a standing body of water should. Instead, the shoreline elevations rise and fall by more than 2 km over distances of thousands of km. This observation has been used to argue that the features cannot be shorelines, and has therefore cast doubt on the idea that Mars once had oceans. In a recent paper in Nature (PDF; see also the News & Views article by M.T. Zuber PDF), we show that these long-wavelength topographic trends can be explained by deformation that occurred in response to true polar wander (TPW), a reorientation of the planet with respect to its rotation axis. Moreover, we show that the TPW path that explains the shoreline deformation satisfies a major constraint on Mars' rotational stability, because it keeps the large Tharsis volcanic rise at the equator. This result revives the possibility of an ancient martian ocean, and implies that the ocean would have been centered in the tropics rather than the north polar region.

Shifting poles: A related effort led by Isamu Matsuyama and Jerry Mitrovica seeks an updated theory for TPW that occurs in response to large perturbations to the inertia tensor of a planet with an elastic lithosphere. We have found that the magnitude of a TPW event resulting from a given load is a function of lithospheric thickness. In a recent paper PDF, we present this result and also provide a framework for predicting TPW that occurs in response to non-axisymmetric loads. We are using this theory to model the TPW that would have occurred in response to the combined influence of the Tharsis volcanic rise and a northern ocean. Edwin Kite also led a project in which we use this theory to show that TPW driven by young volcanic flows may be responsible for the offset of Mars' polar ice caps from its rotation poles: PDF.

Martian ice cores: Climate records in ice cores and ocean sediments on Earth record temperature changes, including repeated ice ages that bear a clear imprint of long-term variations in Earth's orbit. The martian polar caps contain hundreds of meters of layers composed of mixed ice and dust, and scientists have speculated that these layers formed in response to similar orbital variations. In a recent paper in Geology, [free access], Taylor Perron and Peter Huybers use spacecraft images and elevation measurements to construct virtual "cores" through the north polar cap, and ask whether the layers bear a recognizable orbital imprint. Our analysis reveals repeating layers 1.6 meters thick in many areas of the polar cap. While we do not rule out the possibility that these layers formed in response to orbital variations, we find that they could instead be evidence of a shorter-term process that affects the deposition of ice and dust at the poles, perhaps similar to the way the El Niño Southern Oscillation produces intermittent changes in Pacific Ocean temperatures and rainfall.

 

Titan

Methane rain: Branching valley networks near the landing site of the Huygens probe on Titan (Saturn's largest moon) imply that flowing fluid has eroded the surface. The fluid was most likely methane, and the eroded material was probably composed mostly of water ice. In a recent paper PDF, we show that the properties of these materials at Titan's surface and the morphology of the networks suggest that the valleys were eroded mechanically by surface runoff, and use the valley network morphology to estimate the methane precipitation rates required to form these features.

Limits to relief: In a paper published shortly before the Cassini spacecraft delivered the first close-up views of Titan PDF, Taylor Perron and Imke de Pater predicted the maximum topographic relief that Titan's icy crust could support over long periods of time.

Influence of life on topography

Landscapes are shaped by the uplift, deformation and breakdown of bedrock and the erosion, transport and deposition of sediment. Life is important in all of these processes, yet most current models of landscape evolution do not account explicitly for its effects. In a recent review article PDF, Bill Dietrich and Taylor Perron discuss our current understanding of the influence of biota on the processes controlling landscape form and evolution, at spatial scales ranging from that of a soil profile to that of a mountain range. We conclude that, although there does not appear to be a unique topographic signature of life, there may be ways of measuring the degree to which biota have influenced a landscape's development.