No. 92: Mar-Apr 1994
In July, 1994, the comet Shoemaker-Levy 9 is scheduled to meet a fiery end as it plunges into Jupiter's atmosphere. Since this cataclysm is predicted to occur on Jupiter's far side, the pyrotechnics will be largely hidden from our telescopes. Yet, if any of Jupiter's four large Galilean satellites are swinging behind Jupiter during the comet's impact, but still visible to us by virtue of their distances from Jupiter, we might see one or more of these moons suddenly brighten due to light reflected from the incineration below.
This very well might happen, and something similar has happened before. On July 26, 1983, just 6 minutes after it emerged from behind Jupiter, the Galilean satellite, Io, suddenly brightened by 50% -- a "flash" that lasted 118 seconds. Now, Io is notoriously fickle brightness-wise. Its post-eclipse brightening has long puzzled astronomers, but this short, intense flash was even more anomalous than usual. H.B. Hammel and R.M. Nelson suggest that this 1983 flash might have been the reflection of some catastrophic event occurring on the hidden half of Jupiter -- possibly the impact of some large object -- or, even more intriguing, Jovian lightning.
(Hammel, H.B., and Nelson, R.M.; "Bright Flash on Jupiter in 1983," Nature, 366:117, 1993.)
Comment. Could this Jovian "lightning" actually have been an electrical spark? This thought dovetails nicely with the pair of "ghostly" infrared spots that race across Jupiter's surface in synchronism with Io's orbital motion. (See SF#91.) The two spots are believed to be the moving terminals of a gigantic electrical circuit that stretches from Jupiter's surface to Io and back again. The 1983 flash might have been a current surge in this cosmic circuit.
Reference. More on Io's post-eclipse brightning can be found in AJX6 in our catalog: The Moon and the Planets. For description of the book, see here.