Attempts to investigate the origin of giant wedge-shaped sedimentary structures on Saturn’s moon Titan using analogues in Death Valley, California has thrown up new questions about the surface processes and intense rainfall that might have formed them.
A vast alluvial fan in the XinJiang province of China. The right side is the active part of the fan, and appears blue from water currently flowing in the many small streams. The image was acquired on May 2, 2002. Image credit: NASA / GSFC / METI / ERSDAC / JAROS / US-Japan ASTER Science Team.
Alluvial fans form in areas of rare but intense rainfall. The resultant flash floods drive mass movements of sediment that run downslope through canyons before spreading out to form wedge-shaped deposits on valley floors.
On Earth, fans come in two main types; debris flow fans, in which sediment and fluid move downslope as landslides, and sheetflood fans, where fluid carries the sediment (as in a flooded river). The type of fan that forms depends largely on the sediment available — debris flows form where clay is abundant, whilst sheetfloods are found where larger grained sand is more prevalent.
As evidence of both recent sediment transport, and the type of surface material being transported, the discovery of fans around the Solar System has created a lot of excitement. Their identification on Mars helped confirm the presence of flowing water.
“With fans, we are talking about occasional but catastrophic precipitation driving large-scale sediment transport,” says Richard Cartwright, a planetary scientist at the University of Tennessee. “These aren’t formed by your average afternoon rain shower.”
In more recent years, the search for fans has turned to Saturn’s moon Titan.
On this active world, liquid methane rains down from an aerosol-rich atmosphere during intense storms that replenish surface lakes of the same hydrocarbon mix. Just last week NASA announced the return of wispy clouds above Titan’s northern latitudes.
Despite its thick atmosphere, Cassini’s radar instruments observed fan-like, wedge-shaped features on the surface.
Sam Birch and colleagues at Cornell University carried out the first comprehensive survey of possible fan features, demonstrating an interesting spread across the surface that might increase the likelihood of these being alluvial fans.
“The distribution seemed to matched predictions from the Titan climate modelers of where it was predicted to rain most based on concentrations of clouds,” says Birch.
However, with just low resolution grainy pics and no imminent prospect of ground surveys, any confirmation of Titanian fans, let alone a better understanding of the processes that formed them, remained out of reach.
With a new approach required, Cartwright and his colleague, Devon Burr, looked closer to home – 2,000 miles due west of their Knoxville office to be exact, in California’s Death Valley.
Death Valley contains its own fan systems, which continue to experience the intense flooding events that formed them. The Furnace Creek fan in the region flooded in 2004 killing two people, whilst several cars were crushed and filled with rocks.
One particular chain of Death Valley fans emerging from the Panamint Mountain are of particular interest. Containing both debris flow and sheetfloods fans in close proximity, the area provides an excellent natural laboratory to investigate radar’s ability to distinguish between different types of fans.
“The other advantage of the Death Valley fans is they have been extensively imaged using radar at a range of frequencies. There’s lots of data to compare to Cassini’s,” says Cartwright.
It’s difficult to directly compare radar images of fans on Earth with images from Cassini due to differing surface materials and the different radar frequencies used.
However, in their new paper, published in the journal Icarus, Cartwright and Burr looked to assess radar’s ability to differentiate fan types using the data from Death Valley, and see how that might be applied to the Saturnian moon.
Possible fans on Titan. Image credit: R.J. Cartwright & D.M. Burr, doi: 10.1016/j.icarus.2016.11.013.
Whilst the focus was on assessing the technique, the team did have a hypothesis to test as well. The presence of large ‘sand seas’ straddling Titan’s equator led the team to suggest that nearby fans would more likely be sheetflood fans, whilst debris fans might be more prevalent at higher latitudes, where sand is scarce.
Cartwright hoped he might be able to see evidence of such a distribution in the radar data using clues from analysis of the Death Valley fan images. Here, debris flow fans’ rougher surfaces (when compared to sheetfloods) produced noticeably brighter radar readings.
However, if similar patterns between surface roughness and alluvial fan formation mechanisms are to be expected on Titan, the data didn’t fit with the team’s predicted distribution at all. Instead it was the fan-shaped objects at lower latitudes that were the brighter by far.
“The results weren’t even close. Our hypothesis was turned on its head,” Cartwright says.
The exact reasons for the unexpected results are uncertain, with various potential contributing factors explored by the team.
Firstly, the Earth-based radar data used came from only one fan system, including only two types of fans, whilst ignoring others such as mud-flow fans.
It could also be the case that these Titan features aren’t actually fans at all. On Earth, fan-shaped features can form without alluvial fan processes.
“These results are suggestive of alluvial fans on Titan, but with only poor resolution radar images, no surface samples to analyze, and only two types of fans on Earth analyzed using radar, subsequent work is needed to be more certain.”
Sam Birch is also cautious about making any statements around the processes that formed these features on Titan.
“At least until we have better modeled the planet’s climate and sediment transport,” he says.
Looking further forward, beyond Cassini, Birch believes the only way to truly confirm Titan has fans is to go back.
“Ideally with higher resolution cameras or radar instrumentation with 10 times the resolution.”
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R.J. Cartwright & D.M. Burr. 2017. Using Synthetic Aperture Radar data of terrestrial analogs to test alluvial fan formation mechanisms on Titan. Icarus 284: 183-205; doi: 10.1016/j.icarus.2016.11.013
