Using the spectral data collected by the Visual and Infrared Mapping Spectrometer (VIMS) onboard NASA’s Cassini spacecraft, a team of researchers has found a mixture of organic materials and water ice in nine impact craters on Titan, the largest moon of Saturn.
This colorized radar image, taken by NASA’s Cassini orbiter on February 15, 2005, shows a 60-km (37-mile) wide crater on Titan. The appearance of the crater and the extremely bright (hence rough) blanket of material surrounding it is indicative of an origin by impact, in which a hypervelocity comet or asteroid, in this case, roughly 5-10 km (3-6 miles) in size, slammed into the surface of the moon. The bright surrounding blanket is debris, or ejecta, thrown out of the crater. The asymmetric appearance of this ejecta blanket could be an effect of atmospheric winds associated with the impact itself. Although clearly formed by impact, the feature lacks a central peak, suggesting that it has been eroded or otherwise modified after formation. Rainfall, wind erosion, and softening of the solid material in which the crater formed are all possible processes that might have altered this impact feature. Image credit: NASA / JPL-Caltech / ASI.
“Like Earth, Titan has a thick atmosphere that acts as a protective shield from meteoroids,” said lead author Dr. Anezina Solomonidou of ESA and colleagues.
“Meanwhile, erosion and other geologic processes efficiently erase craters made by meteoroids that do reach the surface. The result is far fewer impacts and craters than on other moons.”
“Even so, because impacts stir up what lies beneath and expose it, Titan’s impact craters reveal a lot.”
In the study, Dr. Solomonidou and colleagues analyzed data from the VIMS instrument on Cassini, which operated between 2004 and 2017 and conducted more than 120 flybys of the hazy moon.
Their results show that Titan’s impact craters can be split into two categories: those in the fields of dunes around the moon’s equator and those in the vast plains at midlatitudes.
The equatorial dune craters the team studied — Selk, Ksa, Guabonito, and the crater on Santorini Facula — appear to be purely composed of organic material.
Titan’s midlatitude plain craters — Afekan, Soi, Forseti, Menrva, and Sinlap — are enriched in water ice within an organic-based mixture.
The scientists found that craters actually evolve differently, depending on where they lie on Titan.
Some of the new results reinforce what they knew about the craters — that the mixture of organic material and water ice is created by the heat of impact, and those surfaces are then washed by methane rain.
But while the authors found that cleaning process happening in the midlatitude plains, they discovered that it’s not happening in the equatorial region; instead, those impact areas are quickly covered by a thin layer of sand sediment.
That means Titan’s atmosphere and weather aren’t just shaping the surface of Titan. They’re also driving a physical process that affects which materials remain exposed at the surface.
“The most exciting part of our results is that we found evidence of Titan’s dynamic surface hidden in the craters, which has allowed us to infer one of the most complete stories of the moon’s surface evolution scenario to date,” Dr. Solomonidou said.
“Our analysis offers more evidence that Titan remains a dynamic world in the present day.”
The researchers also found that one impact crater, Selk, is untouched by the rain process that cleans the surface of other craters.
“Selk is in fact a target of NASA’s Dragonfly mission, set to launch in 2027,” they said.
“The rotorcraft-lander will investigate key astrobiology questions as it searches for biologically important chemistry similar to early Earth before life emerged.”
The results were published in the journal Astronomy & Astrophysics.
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A. Solomonidou et al. 2020. The chemical composition of impact craters on Titan. I. Implications for exogenic processing. A&A 641, A16; doi: 10.1051/0004-6361/202037866
