Summit observers Jen Miller and Joy Chavez first noticed something unusual about the start of the night’s observations at Gemini North on December 16th, 2014, with this excerpt from the night log, “…the target looks like a crescent, like Venus or the moon. …reslew. …Same thing, different direction to the crescent.”
Gemini staff observer Sunny Stewart also got a surprise reviewing the previous night’s data “That’s no moon! It looks like Venus,” said Stewart. Observations of Jupiter’s volcanically active moon Io, obtained that night as part of a program led by Katherine de Kleer of UC Berkeley to watch for volcanic outbursts, revealed an unusual event involving Io and another large jovian moon, Europa. According to de Kleer, the images captured an occultation event in which Europa briefly blocked some of the light from Io, “…giving Io a very un-Io-like appearance!” These sorts of events occur when we observe the moons’ orbits edge-on, and can occasionally view the moons passing in front of one another.
From Earth, Io’s volcanoes are generally only detectable at infrared wavelengths where we see emission from the hot lava. Using large telescopes with adaptive optics systems allows us both to see the shapes of the moons and to pick out individual features. The bright spot on the Io’s left limb is thermal emission from an active volcano, and as Europa moves off of Io’s disk it uncovers a second fainter volcano near the right limb.
Below is a diagram of the circumstances of the event and an image from the JPL simulator.