Planetary scientists have long wondered why some regions of Saturn’s largest moon, Titan, exhibit few impact craters. Now, a new study suggests that the areas where craters are sparse or missing were once sediment-saturated wetlands or shallow seas that swallowed up evidence that impacts occurred.
The relatively smooth face of Titan is nothing like the pockmarked surface of our moon. The scars of impact craters are noticeably absent from Titan’s polar regions, for example. And the craters that are present on Titan appear to be much shallower than expected, based on their diameter. Titan has a thick atmosphere, which protects the orb against the impacts of small objects (they’re pulverized as they blaze through the atmosphere) and supports weather and erosion that can help hide or erase craters that do form. To date, researchers have tallied 61 definite or potential craters on the entire surface of Titan, most of them 20 kilometers across or wider, says Catherine Neish, a planetary scientist at Florida Institute of Technology in Melbourne. Of that number, 11 have been dubbed “certain” craters, and the remainder are considered either “near certain” or “probable.”
To explain the clear patches, previous studies have suggested various crater-erasing scenarios. For instance, some scientists have proposed that large amounts of sediment carried by streams of liquid methane and ethane from highland areas could have masked lowland craters. But that scenario doesn’t explain the presence of several craters in Xanadu, a broad area near Titan’s equator that is far from any hills.
Similarly, some researchers have proposed that windblown sand may have smothered craters. But that explanation doesn’t wash, Neish says, because most of Titan’s sand dunes are found in highland areas, and there’s no evidence of sand in the crater-free polar regions. Widespread cryovolcanism—the eruption of water, liquid methane, or other volatile substances rather than molten rock—doesn’t explain why craters appear in some lowland areas but not others. Neither does the languid rain of hydrocarbon particles produced by photochemical reactions in Titan’s hazy skies. Those particles pile up at an estimated rate of 6 meters every 1 billion years, not nearly fast enough to obscure a 1-kilometer-or-more-deep crater in the 4.56 billion years since our solar system formed.
But previous studies haven’t considered a scenario in which objects slamming into Titan land in a surface layer of liquid, such as a shallow sea, or in porous, soggy sediments—such as those in the region where the Huygens probe landed in 2005. In such areas, layers of mushy material could be hundreds of meters thick or more, Neish says. Both shallow seas and soggy wetlands—areas where flowing fluids would naturally collect—would be found more often in lowland areas, she notes. Impacts in an ocean, as on Earth, wouldn’t leave a noticeable scar. And an impact that occurred in a wetland would quickly be erased. Almost immediately, the soggy material around the crater walls would slump in to fill the hole. Impacts at wet or soggy sites also typically don’t create a crater with rim that stands high above the surrounding terrain—another reason the cosmic pockmark could be more quickly erased.
Titan’s topographical data bolster this notion, Neish and her colleague Ralph Lorenz of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, contend in the 15 January 2014 issue of Icarus. On average, Titan’s craters are located at higher-than-average elevations. Specifically, half of Titan’s craters lie on terrain 100 meters or more higher than Titan’s average elevation.
The presence of lowland craters in Titan’s Xanadu lowlands—some of the oldest terrain on the satellite—can likely be explained by the age of the impacts, Neish and Lorenz contend. If the craters were blasted before Titan’s atmosphere formed, they appeared when the surface was dry and craters would have remained relatively intact. The weather and other processes that tend to erase large craters wouldn’t have had enough time to hide these pockmarks.
The team’s scenario “is an interesting model,” says Jonathan Lunine, a planetary scientist at Cornell University. “It’s certainly possible that the preference of craters for higher elevations suggests that liquid methane/ethane was present in significant amounts in lowlands, but it doesn't prove that such liquids were present,” he notes.
Buried craters would be difficult to discern from an orbiting craft, the researchers say. So discovering any scars from past impacts that are now filled in might not be possible until rovers with sediment-penetrating radar meander the terrain beneath Titan’s hazy skies.
*Correction, 2 December, 12:35 p.m.: This article has been updated to reflect the as-of date for Titan's crater census.