Titan's Lithospheric Strength Envelope and Brittle–Ductile Transition: The Importance of Crustal Pore Fluids, Organics, and Clathrates

Abstract

The strength and thickness of Titan's lithosphere is likely influenced by its unique organic-rich crust. Titan's crust may be composed of water ice or a more insulating layer of methane clathrate (MC), and topped by potentially insulating surface deposits such as sand, labyrinth terrains, undifferentiated plains, and methane/ethane seas and pore fluids. We investigate the thermal and mechanical influences of potential crustal materials on Titan's lithosphere. We present a suite of lithospheric strength envelopes using two strain rates and three ice grain sizes to calculate the brittle–ductile transition depth, the thickness of the brittle lithosphere. We found that the brittle lithosphere could be as thin as 3–5 km for the thinnest MC crust cases explored (5 and 10 km thick crusts, respectively). A clathrate-free ice shell could have a lithosphere as thin as 10–12 km when convecting, or 20–26 km without convection, depending on the assumed heat flow. Highly insulating surface layers could thin the local lithosphere by a few kilometers. Generally, Titan's lithosphere is strong, of order a few MPa near the surface to >100 MPa at depth for the thermal profiles explored. When pore fluid pressure is considered, the stress required for brittle failure can be reduced to <1 MPa near the surface. To form the observed mountains via tidal stresses (of order 10s kPa), we suggest that a combination of crustal properties co-occur, which locally weakens the lithosphere and focuses stress.