River channels on three worlds reveal a history of shifting landscapes
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Earth, Mars, and Titan all have channels, but different geologic pasts.
Scott K. Johnson
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 toward the pole. The river channels also drain toward 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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