At this point, we’ve worked out the basics of the processes that produced the topography around us here on Earth. But other worlds in our solar system have very different landscapes that could partly be the result of foreign processes. The distant glimpses we get of these worlds make revealing those landscape histories a real challenge. Reconstructing a crime from a detailed inspection of a crime scene is one thing, doing it through a telescope is another.
Rivers are, in a way, topography bystanders that always flow downhill. The channels they carve certainly modify the landscape, but their paths reflect the elevations around them. They can also tell you about past topography if you know how to look. A team led by City University of New York researcher Benjamin Black sought to apply this concept not just to the Earth, but also to the two other worlds where we see river channels—Mars and Titan.
The researchers distinguished between long-wavelength topography (think continents and ocean basins on Earth) and short-wavelength topography (think mountain ranges within continents). The differing scales signify different processes, with smaller features resulting from local interactions between Earth’s tectonic plates rather than the fundamental difference between continental and ocean crust.
Playing with a simulation of an evolving landscape, the researchers demonstrate how rivers record these two processes. If you only deform an initially flat landscape to make long-wavelength topography, river channels dutifully trace simple, orderly lines from high elevation to low. But as you also push up localized topography, river channels deviate from that large-scale pattern more and more as “downhill” becomes a spatially variable concept.
Of course, if you stop pushing up localized topography, those rivers will eventually carve through the “bumps” and return to the orderly, large-scale pattern.
The researchers mapped out river channels on Earth, Mars, and Titan, and the team then compared the paths of those channels to a sequence of CSI-image-enhancement-style elevation maps. At the fuzziest level, only the very broadest elevation changes are apparent, and the correlation with river directions is assessed. The calculation is repeated for higher and higher resolution elevation maps to find out how much the correlation improves. If rivers coexisted with complex topography, the correlation will start out low and increase markedly as the hills and mountains diverting rivers come into focus. If rivers mainly flowed across long-wavelength topography, though, the correlation will start much higher and improve little as you increase the resolution.
For Mars and Titan, the correlation between river channels and the lowest-resolution elevation maps was pretty good. For Earth, though, the correlation started out very low. In other words, Earth’s plate tectonics have added a level of fine-scale complexity that Mars and Titan lack, and its rivers are more convoluted as a result.
On Mars, it looks like the present topography—dominated by a low-elevation northern hemisphere and a region of volcanic highlands near the equator—was already in place when water cut its river valleys. That implies that there weren’t any significant plate-tectonics-like processes active during Mars’ early days when water flowed across its surface.
The researchers describe Titan’s history as “the most enigmatic.” We have evidence that Titan is (or was relatively recently) geologically active, yet its hydrocarbon river channels fit the long-wavelength topography well. The northern pole is an exception, with river channels that deviate a little more from the broader pattern.
Overall, the researchers say this probably means that Titan’s recent geological activity is, itself, large scale. For example, it could be that elevation differences are being driven by global patterns of melting and freezing beneath its outer water ice layer, thickening and thinning different regions.
The northern pole is pretty puzzling, though. Titan’s atmospheric circulation carries hydrocarbons towards the pole. The river channels also drain towards the pole, so it’s not clear what process is sending material back southward.
While Mars and Titan are the only two other worlds where we’ve identified channels carved by liquid rivers, the researchers point out that a similar analysis could be done with other things that flow downhill—like the hot volcanoes of Venus or the cold “cryovolcanoes” of Pluto. Either could potentially preserve signs of shifting landscapes.
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