It might seem intuitive that water always flows downhill. But that axiom provides important clues to the tectonic histories of Mars, Titan, and Earth. A team of researchers led by Benjamin Black (City College of New York) recently compared the topographies of these three bodies, all of which show evidence of fluvial, or river-based, influences on their surface features. The team’s study, published this week in Science, used global drainage patterns of each object’s surface to determine the likelihood of recent tectonic activity.
On Earth, a tectonically active body (hello earthquakes and subduction zones), the results seem to defy physics: water appears to flow along level surfaces or uphill about 40% of the time. Of course, this doesn’t actually occur. The misleading results arose when the researchers blurred topographic data from Earth and Mars to match the resolution of Cassini’s Titan data. At this lower resolution, the researchers suggest, only larger, continent-scale features that formed over a longer timescale will be detectable. “Short-wavelength” mountain ranges and other features formed by tectonic activity will be blurred out, sometimes creating the illusion that fluid is flowing against gravity.
Mars and Titan, on the other hand, have much better “topographic conformity”— that is, the fluvial features seem to flow downhill (65% of the time or better). On Mars, high conformity indicates that little topographic reshaping has occurred since Martian river networks formed. So apparently there’s been little tectonic activity or intense impact barrages to disrupt drainage patterns that had aligned with ancient topographic gradients.
On Titan, where rock-hard water ice shapes the landscape and liquid methane and ethane fill its rivers, the drainage networks follow the prevailing slopes in mid-latitude and equatorial regions. So the topography there has been stably in place since before the river networks formed. However, its north polar region doesn’t conform as well — hinting that some kind of deformation (cryovolcanoes?) happened in the geologically recent past.
Based on Cassini’s infrared and radar imagery, Titan does show evidence of recent or ongoing geologic activity — perhaps a consequence of tidal heating or melting deep down where its ice crust and rocky core meet — that results in global changes in the ice’s thickness. In general, surface material on Titan seems to migrate poleward — hydrocarbons in the atmosphere travel from mid-latitudes to the pole and five out of six rivers drain to the poles. Evidence also suggests that a substantial amount of sediment drifted from high locations to low ones, conceivably erasing some short-wavelength evidence of Titan’s geologic past.
In short, as far as Titan, Earth, and Mars are concerned, if you’ve got topographic conformity and your liquid flows downhill, then your topography is positively ancient.