Io Volcano of the Week: Pillan - Part One

Io's Black Eye - Thanks to Pillan

Beginning this week, we will take a look at one of Io's hundreds of volcanoes each week.  For this premiere post, we will take a look at Pillan, a volcano that is notable for its large eruption during the Galileo Nominal Mission in 1997.  The eruption resulted in a dark, pyroclastic more than 400 kilometers (250 miles) across, a fresh lava flow with an area of 3100 square kilometers (1,200 square miles), and a deposit created by a dusty plume 100-120 kilometers (60-75 miles) in height that partially covered the iconic red ring plume deposit of Pele.  The massive eruption, one of the most significant observed by Galileo in terms of energy output and the areal surface coverage of the fresh lava, began in May 1997 and was in full swing by June 28 when the SSI camera onboard Galileo detected a bright hotspot at the volcano when the instrument observed Io while the satellite was in the shadow of Jupiter.  And to think, nothing strange had ever been noted at this volcano.

Voyager and Galileo images of Pillan before its eruption

Pillan Patera was first seen during the Voyager 1 encounter in March 1979, the first time Io's surface had been seen in any kind of detail.  Voyager scientists were astonished by the sheer number of volcanic features on Io's surface.  However, Pillan was not of much interest.  The volcanic depression was not even named until shortly before the eruption in 1997.  It was seen at a resolution of 500 meters (1,640 feet) per pixel in the large south polar mosaic, so the lack of interest wasn't for lack of good images of the region.  Image number 1 at right shows the region around Pillan. You can see Pele, a persistently active lava lake near lower left.  Pillan wasn't really distinguishable from the surrounding terrain, if it weren't for the two kilometer high margin that separates the floor of the patera from the surrounding plains.  The image does provide some details about the nearby terrain that was not seen at this kind of detail by Galileo.  To the north of the speech-bubble shaped patera is a mountain now named Pillan Mons.  This mountain is bisected by a pair of fractures and appears to be in a state of collapse with a landslide deposit on the northeast side of the mountain.  This deposit is frequently coated in bright sulfur dioxide frost, perhaps from sapping from the mountain.  Between Pillan Patera and Pillan Mons is the V-shaped source fissure of the 1997 eruption.  During the Voyager mission, this fissure was surrounded by bright material, likely older lava flows that have been coated in sulfur and sulfur dioxide after an earlier eruption.  Interestingly, some of these flows have the same shaped as the 1997 flows.  Once again on Io, eruptions have happened the exact same way before, and they will do so again.

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

Special attention should be paid to the clear filter images from November 1996, the highest resolution data set of the Pillan region from Galileo prior to the eruption. The image clearly shows the carrot-shaped ^ of the 1997 eruption source.  It appears as a thin, dark fissure in this image, which would be around six kilometers wide based on this data.  Surrounding this thin fissure is a diffuse region of darkish material.  A similar albedo pattern is seen at other Ionian fissures, such as within Lei Kung Fluctus, and most intriguingly at East Girru, shown paired with the November 1996 image of Pillan.  East Girru was the source of a bright eruption that was just getting going during the New Horizons flyby in late February 2007.

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.