Thermal Emission from Lava Flows on Io.
Robert R. Howell
Icarus 127: 394-407, June 1997.
Abstract: The thermal emission from Loki, Ulgen, and Amaterasu Paterae can be modeled remarkably accurately with a simple two parameter model based on analytical expressions for the cooling of a silicate lava flow. In this model the flow is assumed to produce new surface at a constant rate, then the surface cools purely by radiation. The highly nonlinear (T^4) dependence of radiative flux on temperature paradoxically produces simple approximations for the temperature of the surface vs. time, and also for the distribution function which gives the fraction of the surface at any given temperature. This work is based upon a modification of the Stefan model which describes the solidification of lava flows. The first of the two parameters which describe the system, R_A', is the product of a flow rate R_A (given in m^2/sec) and a term set by material properties and related to thermal inertia. The second parameter t_0' is the product of a time scale t_0 and another term set by material properties. The time t_0 can be interpreted either as the time the flow has been active, or the time it takes for old surface to be covered by new flow. The analytical model predicts a spectrum with F_lambda proportional to lambda^3 in the near infrared, with an exponential cutoff at short enough wavelengths and a slower turnover at long wavelengths. Slightly more elaborate versions of the model predict the way in which the spectrum should change when the flow rate varies with time. Measurements of such variations will provide a critical test of the model.