Animation of Io's photometric behavior Options

[Bjorn Jonsson]

This is an animation showing Io's weird photometric behavior:

 io_photom.avi ( 3MB ) Number of downloads: 257


Note added later: A significantly improved version of this animation is available later in this thread. See this message for details.

To model the brightness changes as a function of phase angle I used a global map from a JGR paper by Simonelli et al. published back in 2001. The map shows the Henyey-Greenstein g parameter and provides a measure of how strongly backscattering or forward scattering individual regions are. The bright terrain near the equator is strongly backscattering while the dark terrain is far less backscattering. The result is that as the phase angle increases the equatorial terrain appears no brighter than the terrain that appears fairly dark at low phase.

The animation should be reasonably accurate, except that the brightness of the equatorial terrain relative to darker terrain may be a bit overestimated at very low phase angles. Another problem is that the bright terrain is "overexposed" at low phase - dynamic range is frequently a problem when you want high photometric accuracy (I really need a monitor that can get as bright as the sun ;-) ). Also beware that the images are "normalized", meaning that the brightest point in each image has a fairly constant intensity throughout the animation. What this really means is that the high-phase images appear too bright. Despite these "errors" I think this animation gives a fairly good idea of how Io changes with phase angle and this certainly turned out better than I expected when I decided to use the Henyey-Greenstein parameter map I mentioned.

I might do a more realistic version of this in the future - this really is work-in-progress.