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How crater shapes are revealing more about Titan's icy crust

Titan, Saturn's largest moon, is a fascinating world that is unique among moons of the outer solar system. It's shrouded in a thick, hazy atmosphere rich in nitrogen and methane and it's the only moon with a substantial atmosphere and the only place besides Earth known to have stable bodies of surface liquid.

These aren't water lakes and seas, however, but collections of liquid hydrocarbons (primarily methane and ethane) that form a complex cycle similar to Earth's water cycle. Beneath this alien landscape lies a mysterious interior: likely a water-ice crust floating atop a subsurface ocean of liquid water mixed with ammonia.

A reveals how a team of researchers from Imperial College London, U.K. have compared real craters on Titan with computer-simulated ones to determine the thickness of its icy shell. This information is important for understanding Titan's interior structure, how it evolved thermally, and its potential to produce organic molecules—making it significant for astrobiology research.

Impact simulations for Titan used special hydrodynamic code that simulates crater impact processes on planetary surfaces. They ran simulations with vertical impact velocities at 10.5 km/s, testing three impactor sizes (2, 5, and 10 km). The models incorporated strength and damage parameters for methane clathrate (where methane gas is trapped inside water) and water ice based on previous studies, using a model that simulates how rock and debris behaves like a fluid during high-energy impact events.

They also employed an ANEOS equation of state to describe how water ice behaves under extreme conditions. This was also used for methane clathrate too since there is limited data on this state. The simulations used adaptive resolution (starting at 40 cells per projectile radius) and continued until crater dimensions stabilized, with error margins of about 15% for dimensions and two grid cells for depth measurements.

All of the simulated impact craters appeared deeper than those actually observed on Titan. Among the tested models, the 10 km methane clathrate-capped scenario produced craters closest to reality, though still hundreds of meters too deep. Pure ice models performed worst, creating craters over a kilometer deeper than observed, but results improved as the ice lid thickness decreased.

When comparing Titan's actual craters to computer simulations, researchers found the 10 km methane clathrate model best matched reality. This model produced craters with central peaks and sharp rims like the observed Selk crater, though slightly deeper—likely due to sand filling in the craters over time. Pure ice models created much simpler yet significantly deeper craters that couldn't be explained by erosion or infill. The most accurate model appears to be a 10 km methane clathrate layer above 5 km of conductive ice, with warm convective ice beneath at 256.5 K.