|Io's Black Eye - Thanks to Pillan|
|Voyager and Galileo images of Pillan before its eruption|
Galileo entered orbit around Jupiter in 1995 and imaged Io on a semi-monthly basis starting in June 1996. Many of these early images, intended to monitor changes in the plume deposit around Pele, also revealed apparently changes at Pillan Patera. Were these real changes? During Galileo's first orbit in June 1996, Pillan Patera appeared brighter than its surroundings (see image 2 above). However, it had darkened considerably by September 1996 during the next orbit. Was this due to an eruption? Galileo would observed Pillan at even higher resolution during C3, the next orbit in November. This time, Pillan had the same albedo as its surroundings. Was this due to rapid surface changes? Turns out, it was due to an odd phase function of the surface materials on the floor of the patera. When Pillan was viewed at low phase angles, like G2 and later in E6 in February 1997, the surface appears dark. When viewed at higher phase angles, the surface appears brighter. This is the result of a thin layer of bright sulfur dioxide frost coating dark silicate materials on the patera floor. This makes the floor of Pillan quite forward scattering. You can see a similar effect taken to extremes near Loki in New Horizons images. In terms of volcanic activity, some slight activity was seen by the Near-Infrared Mapping Spectrometer (NIMS) in November 1996 and February 1997, but neither detections approached the level of activity seen later in the nominal mission.
|Two Fissures: Pillan from Nov 1997; East Girru from Oct 1999|
Unfortunately, I am going to have to cut this installment short (I want to get to Best Buy for the midnight sale of Starcraft II). Have no fear, tomorrow, when I am not working (or playing Starcraft II), I will continue this tale of fire fountains and black eyes tomorrow evening. We will explore the significance of the shape of the Pillan eruption source vent. We will also look at Pillan's impact on our knowledge of Io and its interior. Finally, we will look at how the model it inspired was undone.