Optimal Wavelengths for Studying Thermal Emission from Active Volcanoes

Introduction: Our goal is to identify the optimal wavelengths for studying volcanic activity on Io. While better temporal, spatial, and spectral resolution is always desired, practical limitations lead one to limit both the wavelength range and spectral resolution of an observation. Our primary motivation is to assist the development of future spacecraft observations, but these results are also relevant to Earth-based telescopic observations of Io. We focus on the Io Volcano Observer (IVO) concept [1]. IVO is proposed to study Io from the vantage point of a highly inclined elliptical orbit around Jupiter with close Io flybys every 30-200 days. While data collection at closest approach will be limited by the blistering ~18 km/s ground speed, more distant monitoring is planned over a ~2 day encounter period. The two instruments that would observe thermal emissions are the radiation-hard camera (RCam) and Thermal Mapper (ThM) [1]. RCam is planned to have a 10 μrad IFOV, giving better than 10 m/pixel at closest approach and ~1 km/pixel for the more distant, global observations. Lava fountains above fissure vents would be spatially resolved near closest approach, but the global synoptic views would spatially resolve only large lava flow fields and paterae. ThM should have a spatial resolution of about 10 km/pixel global and better than 1 km/pixel over selected targets. This should allow different parts of a lava flow field to be spatially resolved near closest approach, but most active volcanic centers will be contained in a few pixels in the global views. While the approximate spatial resolution is known, the choice of spectral bandpasses is ongoing [1]. Science Questions: The science questions related to thermal emissions are (1) what is the eruption temperature of the lava, (2) what are the different styles of eruption on Io, and (3) what is the global heat flow. Eruption temperature is important to measure because it provides the best constraints on lava composition and the state of Io's mantle [2]. If, as intially suggested by Galileo SSI data, the eruption temperature can exceed 1600 °C, then the upper mantle is largely molten and Io should contain a magma ocean [3]. Such a magma ocean would imply that Io is far from thermal equilibrium because tidal dissipation within the ocean would be much less than the observed heat flow [4]. Reanalysis of the uncertainties in the Galileo data suggests that eruption temperatures could be closer to 1400 °C [5]. More and better observational data are essential in order to answer fundamental questions about the basic nature of Io's interior. The least adulterated view of erupting lava is provided by spatially resolved images of skylights or lava fountains obtained in darkness. Constraining Peak Temperature (RCam): The peak temperature is best determined by measuring the steep short wavelength side of the Planck Function (Fig. 1). Cooler emitting surfaces have a relatively weak effect on this part of the spectrum. Furthermore, this part of the emission spectrum is nearly linear, so its slope (and thus the temperature) can be well constrained with limited spectral resolution. Figure 1. Plank function for emissivity of 1.