Three years ago this summer, one of the largest volcanic eruptions to occur on Jupiter's moon Io in the 1990's took place at a little known volcano called Pillan Patera.  In this article, I will discuss the current research being done to understand this most studied of Ionian eruptions.

Pillan Patera is an 80-km wide caldera northeast of the persistently active volcano named Pele.  During the Voyager flybys and the first Galileo flyby, Pillan was just another caldera, one of many on violently active Io.  In fact, until early 1997, Pillan didn't even have a name.  What made it remotely different from other calderas was its bright surface, indicating it was covered in sulfur dioxide frost or ice.  The Voyager 1 spacecraft, also imaged a mountain northwest of Pillan.  Voyager also imaged a dark fissure running NW-SE from the southeast side of the mountain. 

Images from the first orbit of Galileo in late June 1996, also called G1, also showed that there was an extensive white deposit northwest of the mountain.  However, when Galileo imaged the area during its next pass in early September 1996, G2, Pillan's caldera had darkened.  At the time scientists speculated whether this maybe caused by a flooding of lavas in the caldera or an effect of the change in phase angle.  On the next pass, C3, Galileo again imaged the Pillan region, this time at 3.0 km/pixel.  However, instead of seeing the changes that had occured during the summer, scientists saw that Pillan had returned to the way it was during G1 and during the Voyager mission.  This gave credience to the idea that some effect of the light had caused Pillan to look different during G2.  Interestingly, images taken during C3 in November 1996 showed that the fissure(s) had darkened between G2 and C3.  Also there was a new extension to the fissure, running NE-SW along the southeast of the mountain from the northwest tip of the fissure that was seen during the Voyager mission.

Pillan was again imaged in orbits E6 (February 1997) and G7 (April 1997) showing that Pillan shifted between being dark and being bright.  However, it did not change due to phase angle, as shown in E6, so the color changes had to be due to changes at Pillan.  What could be happening at Pillan?  Was an eruption imminent?

The God of Thunder Speaks

During the eighth orbit by Galileo, SSI only imaged Pillan from a distance but it did show that Pillan was in its bright phase.  But what the SSI camera didn't show was the beginning of a major eruption at the volcano.  The Near-Infrared Mapping Spectrometer, or NIMS, measures Io's thermal energy as well as its composition.  The NIMS observed the Pillan region in G8 and showed a hotspot, or an active volcano, at Pillan.  An active hotspot had been seen at Pillan since G2, when the caldera darkened, but it had never been as bright.A major eruption had begun.

Things began to heat up around the Galileo's C9 encounter in late June 1997.  On June 28, 1997, Bob Howell of the Wyoming Infrared Observatory observed Io during a mutual event.  A mutual event is when another Jovian moon gets between Io and the sun or earth.  In this case, Ganymede came between Io and Earth.  Analysis of this event shows that Loki was active and so was another bright hotspot, but they were unable to determine which volcano it was.  On July 5, 1997, John Spencer and his collegues observed Io with the Hubble Space Telescope hoping to image the Pele plume.  They were unable to image the Pele plume but they did image another plume that they thought might be Reiden Patera, a volcano east of Pillan.  Determination of which volcano it was would have to wait until data from Galileo were transmitted to earth.  The first images from Galileo returned from the C9 encounter showed the same plume that HST as well as the shadow from the Prometheus plume.  This was when they made the connection that the hotspot seen by Bob Howell and the plume seen by HST and Galileo were one in the same.  It was then decided to return an eclipse image that Galileo had taken of the region during C9.  Instead of showing a hotspot at Reiden as they expected, the images showed a hotspot over the volcano Pillan.  The volcano that had been brightening and darkening for a year now had just "blown its top."  NIMS data from C9 as well as the SSI eclipse image were combined to show that the lava erupting at that time were hotter than any lava that had erupted on earth in a billion years, possibly up to 2000 K.  This temperature indicates that the lavas erupting from Pillan were ultramafic or magnesium-rich.  The Pillan plume was imaged again by HST on July 22, indicating that the volcano was still active.  The images were also able to show that a significant amount of SO2 snow was in Ionian plumes.

The Black Eye of Io

On it's C10 pass in September 1997, Galileo imaged the Pillan region from a distance of 563,000 km.  Instead of the varying colored caldera surrounded by red and yellow plains, Galileo saw a large dark spot surrounding a fissure north of the caldera.  Surrounding this dark spot was a bright ring.  This dark spot had covered parts of the Pele plume ring as well as changed the shape of Reiden's deposits.  The Pillan caldera itself was bright.  Large, new lava flows were emanting from were the two fissures near the mountain northwest of Pillan met.  SSI again observed Pillan during an eclipse and showed that it had cooled to a meager 930 K.

Even though the data from C10 appeared to show that Pillan was done, data from E11 showed it wasn't.  The lava had actually increased in temperature and had also formed two hotspots, one at the fissures (the vent) and one near the caldera.  SSI images from E14 showed more lava flows than had been seen in C10 and they also showed that the lava had flowed into the caldera in multiple places, creating magnificent lava falls that make Niagra look like the faucet for the kitchen sink.  This two hot spot system has been seen at other volcanoes like Amirani-Maui and Prometheus.  In this case, the lava erupted from the vent and then went underground through insulated tubes before reaching the caldera wall, cascaded 2 km, and flooded the caldera floor.  The lava cascading onto the caldera floor were not much cooler than the lava erupting from the vent, 1300 K at the lava falls compared to 1450 K.

As The Volcano Cools

Over the next two years, Pillan continued to cool after its eruption during the summer of 1997.  By July 1998, there was still a small part of the lava flow at 990 K but otherwise the volcano was shutting down.  By July 1999, the plume was no longer there.  Also, despite comparisons to the long-lasting Babbar's dark deposit, the pyroclastic deposit left by Pillan was fading.  The bright deposit northwest of the mountain had returned by July 1999.  The cause of the fading is unknown but it might have something to do with Pele.   Pele is a monster of a volcano who forces SO2 and S2 400 km into space before it returns to the surface as a 1400 km wide red ring.  The red ring covers most of the old Pillan deposit and the caldera itself.   The ring maybe interacting with the new surface materials and covering them, makeing them seem to disappear.  The sulfur in Pele's plume is also interacting now with the warm lavas in the Pillan caldera, creating a green film over much of the caldera.

Lessons from the Eruption

Much about the Pillan eruption is a mystery.  What caused the second outburst in October and November 1997?  How large WAS the fire fountain at Pillan?  Estimates put it in the ten's of kilometers, perhaps as much as 100 km.  What makes Pillan's deposit different from other dark deposits on Io which seem to never fad or change shape?  One idea, as stated above is that Pele's plume might be changing it, but how?  Many of these questions could be answered with modeling of the eruption or future analysis of the available data, but for now these mysteries linger over the largest eruption seen on Io close up.