THE ATMOSPHERIC CONTROL OF COOLING RATE OF CRYOLAVAS AND IMPACT MELTS ON TITAN'S SURFACE

As on Earth, Titan’s atmosphere plays a major role in the cooling of heated surfaces. We use a new finite-element cooling and solidification model to model the behaviour of H₂O-NH₃ “lavas” on Titan, and compare cooling and solidification rates with other lavas on other planetary bodies [2]. We have also assessed the mechanisms by which Titan’s atmosphere, dominantly N₂ at a surface pressure of 1.5 x 10⁵ Pa, cools a heated surface (e.g., a lava flow or impact melt) [1, 2]. These heated areas can be caused by impacts generating melt sheets and (possibly) by endogenic processes emplacing cryolavas (a low-temperature liquid that freezes on the surface). We find that for a cooling cryolava flow or impact melt, heat loss is mainly driven by atmospheric convection. Radiative heat loss, a dominant heat loss mechanism with terrestrial silicate lava flows, plays only a minor role on Titan. Long-term cooling and solidification are dependent on melt sheet or flow thickness, and also local climate, because persistent winds will speed cooling. Cooling caused by winds reduces the detectability of these dynamic events by instruments measuring surface thermal emission. As surface temperature drops by ≈50% within ~1 day of emplacement, the detectability of fresh flows or impact melt via thermal emission may be difficult unless an active eruption is directly observed. Cooling of flow or impact melt surfaces are orders of magnitude faster on Titan than on airless moons (e.g., Enceladus or Europa). Although upper surfaces cool fast, as the internal cooling and solidification process is relatively slow, cryolava flow lengths are more likely to be volume (effusion) limited, rather than cooling-limited. More detailed modeling awaits constraints on the thermophysical properties of the likely cryomagmas and surface materials. This work was performed at the Jet Propulsion Laboratory-California Institute of Technology, under contract to NASA, with support from the NASA Outer Planets Research Program. © 2009. All rights reserved. References: [1] Davies et al., 2009, LPSC-40 abstract 1906. [2] Davies et al., 2009, Atmospheric Control of the Cooling Rate of Impact Melts and Cryolavas on Titan’s Surface, Icarus, submitted.