Imagine you were a scientist on the Voyager Imaging Team 30 years ago today. Only days before, Voyager 1 had passed through the Jovian system, returning thousands of images of Jupiter and its moons. The Galilean moons Voyager revealed had strange and fascinating vistas: cratered Callisto, grooved Ganymede, streaked Europa. Perhaps the strangest of these four worlds was Io. It was clear from the images that were coming down in the days following Voyager's close pass that Io was a world that was constant renewing its surface. Instead of impact craters that were prevalent on most worlds, Io had none and was covered in apparent volcanic features. Given the lack of impact features on the surface, Io was likely even active today.
On Friday, March 8, 1979 at 13:28 UTC, Voyager 1 pointed its narrow-angle camera at Io, 4.5 million kilometers away, one last time. Thirty-eight minutes later, the image was received on Earth. The image was taken to help establish the position of Voyager in space. By comparing the position of Io in the image with known background stars also found in the image, the navigation team could determine if additional maneuvers were needed to keep the spacecraft on track for an encounter with Saturn in November 1980. This task was assigned to Linda Morabito, the cognizant engineer of the Optical Navigation Imaging Processing System at JPL. Among the first things she saw when looking at this image, just as you might have when you saw the same image above, was the crescent-shaped feature just off the limb of Io. Over the next day, she and other Voyager navigation engineers and scientists worked to eliminate the possibilities of what this feature could be. When the crescent, now thought of as a cloud was shown to be assocated with a possible volcanic feature (now known as Pele), this seemed to nail this down as a volcanic plume. There was even another plume (now known as Loki) just beyond the terminator, catching the first bits of morning sunlight.
A more detailed account than I can ever give of the discovery of volcanism on Io can be found on the Planetary Society website and was told by none other than Linda Morabito Kelly.
The discovery of active volcanism on Io was announced at a press conference on Monday, March 12, 1979. By then more plumes had been found as earlier images of Io were re-examined and areas of high thermal emission were being found by Voyager's Infrared Interferometer Spectrometer (IRIS) . For example, in the mosaic I posted earlier today (and if you haven't checked that out, I couldn't recommend to do so more!), two plumes are visible along the limb: Masubi and Pele. I've created a special version showing these plumes in the mosaic. Masubi is at lower left and Pele is at right. These two plumes show the two main types of plumes on Io. Masubi is a smaller, dust plume, also known as a Prometheus-type plume. It has an umbrella-like shape, with a dense central column (in this case TWO central columns) and a bright shock canopy. Pele is the archtype of the Pele-type or gas plumes. In this case, there is no central, dense, eruption column and the shock canopy has a filamentary structure.
I guess for me, the discovery of active volcanism on Io was perhaps more significant for me personally than the Apollo landings, or any of the other major events in the history of spaceflight. While I was not around for either event (I hate to make some of you feel any older, but yes, I was still 4.5 years away when Voyager 1 flew by Jupiter), that discovery, along with the Galileo and Cassini missions were most significant for me to decide to work in this field. Seriously, I could have been in law school right now. Thanks Voyager 1!
Later today, I will be working on a few more mosaics from Voyager 1, though these won't be anywhere nearly as big as the one I posted earlier today. Seriously, have you checked it out yet‽ One will be a reprocessing of the PPS support imaging strip and the other will be a higher resolution mosaic over Io's south polar region. I am trying to stick using the Voyager imaging teams different mosaic designs when deciding which images to use for these mosaics. That's why Loki is missing in the Southern Hemisphere mosaic; it wasn't covered in that mosaic design. It WAS covered in the lower resolution, 4-color northern hemisphere mosaic, but that one has a lot more missing images and smeared images, so it will take me a bit longer to figure out how to parse that one down.
And have you checked out that mosaic I did earlier ;)
Link: Discovery of Io's Volcanoes [members.fortunecity.com]