Saturn's largest moon Titan is the only place other than Earth known to have an atmosphere and liquids in the form of rivers, lakes and seas on its surface. Because of the extremely cold temperature, the liquids on Titan are made of hydrocarbons such as methane and ethane, and the surface is made of solid water ice. A new study , led by planetary scientists at the University of Hawaiʻi at Mānoa, revealed that methane gas may also be trapped within the ice, forming a distinct crust up to 6 miles thick, which warms the underlying ice shell and may also explain Titan's methane-rich atmosphere.
Shallow impact craters lead to crust hypothesis
The research team, led by research associate Lauren Schurmeier, that includes Gwendolyn Brouwer, doctoral candidate, and Sarah Fagents, associate director and researcher, in the Hawaiʻi Institute of Geophysics and Planetology in the UH Mānoa School of Ocean and Earth Science and Technology (SOEST), observed in NASA data that Titan's impact craters are hundreds of meters shallower than expected and only 90 craters have been identified on this moon.
"This was very surprising because, based on other moons, we expect to see many more impact craters on the surface and craters that are much deeper than what we observe on Titan," said Schurmeier. "We realized something unique to Titan must be making them become shallower and disappear relatively quickly."
Researchers tested in a computer model how the topography of Titan might relax or rebound after an impact if the ice shell was covered with a layer of insulating methane clathrate ice, a kind of solid water ice with methane gas trapped within the crystal structure. Since the initial shape of Titan's craters is unknown, the researchers modeled and compared two plausible initial depths, based on craters of similar size on an icy moon.
"Using this modeling approach, we were able to constrain the methane clathrate crust thickness to five to ten kilometers [about 3 to 6 miles] because simulations using that thickness produced crater depths that best matched the observed craters," said Schurmeier. "The methane clathrate crust warms Titan's interior and causes surprisingly rapid topographic relaxation, which results in crater shallowing at a rate that is close to that of fast-moving warm glaciers on Earth."
Methane-rich atmosphere
Estimating the thickness of the methane clathrate crust is important because it may explain the origin of Titan's methane-rich atmosphere and helps researchers understand Titan's carbon cycle, liquid methane-based "hydrological cycle," and changing climate.
"Titan is a natural laboratory to study how the greenhouse gas methane warms and cycles through the atmosphere," said Schurmeier. "Earth's methane clathrate hydrates, found in the permafrost of Siberia and below the arctic seafloor, are currently destabilizing and releasing methane. So, lessons from Titan can provide important insights into processes happening on Earth."
