Polar Heat Flow On

Introduction: Recently, Galileo spacecraft data have revealed Io’s polar regions to be much warmer than previously expected. This unexpected development came from Photo-Polarimeter Radiometer (PPR) data which show that the minimum night temperatures are in the range of 90-95 K virtually everywhere on Io [1-4]. The minimum night temperatures show no dependence upon latitude and, when away from the sunset terminator, they show no dependence upon time of night. This is indeed bizarre behavior for surface units which generally had been assumed to be passive with respect to Io’s pervasive volcanism. Night temperatures of 90-95 K at high, polar latitudes are particularly hard to explain. Even assuming infinite thermal inertia, at these latitudes there is insufficient sunlight to support these warm night temperatures. Thus, through the process of elimination of other possibilities, we come to the conclusion that these surfaces are volcanically heated [5-8]. Taking previously passive units and turning them into new sources of heat flow is a radical departure from previous thermophysical model paradigms [911]. However, the geological interpretation is straight forward. We are simply seeing the effect of ‘old’, cool lava flows which cover most of the surface of Io but yet have some heat to radiate [12-14]. Under these new constraints, we have taken on the challenge of formulating a physical model which quantitatively reproduces all of the observations of Io’s thermal emission. In the following we introduce a new parametric model which suffices to identify a previously unrecognized polar component of Io’s heat flow. Model: We use a ‘Three Component’ background model for Io to address this polar conundrum as summarized in the Table. This model has the following general characteristics. The predicted infrared radiation is consistent with available data for Io at multiple wavelengths both day and night [15]. The background has both relatively high and low albedo components. The background has both very high and very low thermal inertia components. The very high thermal inertia component provides cooling of the day side as well as infrared radiation at night. We now include a warm volcanic unit in the polar regions which also contributes significant infrared radiation at night. Table: ‘Three Component’ Model for Io