The moon hits your eye like a big round pizza pie...but most heavenly bodies are actually a bit squashed by rotation, with Saturn the most notable example. Titan's flattening is also notable because there's too much of it. The moon's rotation should cause Titan's diameter at the equator to be 1,345 feet (410 meters) longer than the pole-to-pole diameter. But the equatorial diameter is a whopping 2,231 feet (680 meters) larger. What's causing that additional 886 feet (270 meters) of flattening?
A study from 2009 suggested Titan's shape might have been formed when the moon was spinning faster. Depending on Titan's internal structure, Titan would have needed to spin from 15 to 30 percent faster in the past. Other researchers proposed in 2010 that Titan's icy shell varies in thickness, depending on the latitude.
Scientists can estimate about how long it would take for this mechanism to flatten Titan to its current shape. Titan's ethane is formed in the atmosphere at a known rate. If this rate was more or less constant over Titan's history, then it took 300 million to 1.2 billion years to create enough ethane to make Titan's poles sink. This surface "age" is consistent with many observations, such as the cratering seen on Titan's surface and the age of atmospheric methane derived from observing the prevalence of different isotopes. All in all, Choukroun and Sotin's story of how Titan has evolved is consistent with what is seen today with Cassini.
This subsidence process isn't unique to Titan. A similar process is seen on Earth in two different environments. In sedimentary basins, silt settles out from liquid water, forming new rocks that load the crust underneath the basins and make it sink. And, new crust formed underneath Earth's oceans becomes more and more dense as it cools down, while spreading away from the mid-oceanic ridges where it was created. The additional weight makes the ocean bottom sink under its own weight (and that of additional sediments).
Choukroun and Sotin's model would also provide an additional source of methane for Titan's atmosphere, though not enough to be the sole source. More methane might come from cryovolcanoes releasing methane, or impacts with the surface releasing methane from Titan's interior. Observing Titan's surface for these kinds of features during future Cassini flybys is needed to set better limits on these models. Cassini data on the extent of rainfall and shape models of higher accuracy would also be helpful.
There are two other bodies (besides Earth) where the observed flattening has been used to study interior structure and thermal evolution. The minor planet Ceres's flattening could mean that Ceres has an interior ocean, as described in a 2005 paper, and the shape of Saturn's moon Iapetus might have frozen into its current shape when it was spinning faster. It's fascinating to consider that the shape of a moon or planet could also tell us something about its weather. Choukroun and Sotin's work shows that there may be another body, besides Earth, Venus, and Mars, where atmosphere-surface interactions have detectable and large-scale implications. This is a completely new process for icy satellites, and is due to Titan's unique, dense, and chemistry-rich atmosphere. This kind of process can only happen on places with a significant atmosphere with active chemistry.
This Cassini Science League entry is an overview of science papers authored, or co-authored, by at least one Cassini scientist. The information above was derived from or informed by the following publications:
1)" Is Titan's shape caused by its meteorology and carbon cycle?," Mathieu Choukroun and Christophe Sotin, Geophysical Research Letters, 39 (2012) L04201-5.
2) "Size and Shape of Saturn's Moon Titan", Howard Zebker, Bryan Stiles, Scott Hensley, Ralph Lorenz, Randolph Kirk, and Jonathan Lunine, Science, 324 (2009), 921-3
3) "Ceres: Evolution and current state", Thomas McCord and Christophe Sotin, Journal of Geophysical Research, 110 (2005), E05009-23
4)" Iapetus' Geophysics: Rotation Rate, Shape, and Equatorial Ridge", Castillo-Rogez J., D. L. Matson, C. Sotin, T. V. Johnson, J. I. Lunine, P. C. Thomas Icarus ,190 ( 2007), 179-202.
-- Jo Eliza Pitesky, Cassini science communication coordinator