Astronomers tracking volcanic activity on Jupiter’s moon Io using the Gemini North telescope in Hawaii on December 16th were very surprised to find it momentarily shaped like a crescent. Turns out Europa, another one of the giant planet’s moons, was briefly blocking some of Io’s light, “giving Io a very un-Io-like appearance,” UC Berkeley’s Katherine de Kleer explains in the Gemini blog.
From Earth, we can only see these sorts of transit events (also called an occultation) when the moons’ orbits are observed edge-on. And they’re not that common, at least not when viewed from here. In fact, the sighting was so unexpected, Gemini staff observer Sunny Stewart exclaimed, “That’s no moon! It looks like Venus,” when reviewing the night’s data. These transits happen clustered in time when Earth passes through Jupiter’s equatorial plane, Bad Astronomy explains, and only half a dozen or so can be seen from a given location per month. Some happen during the day or behind Jupiter (when you can’t see them), not to mention they only last for a few minutes.
Here are the individual images from this Io and Europa event obtained using the Gemini Altair adaptive optics system and the Gemini Near-InfraRed Imager (NIRI). Since the telescope was tracking Io, Europa appears to move from left to right.
You can see a little animation of the progression of the occultation here. During the actual transit, the moons were imaged in the L band, where Europa is nearly invisible. Water ice on its surface absorbs almost all of the incident sunlight. The frozen sulfur dioxide that covers Io’s surface reflects more sunlight at that wavelength, so Io appears brighter. After the occultation, they’re imaged in the K band. Both Io and Europa reflect sunlight at this shorter infrared wavelength, so they’re both visible.
Images via Gemini