Earlier this month, astronomers surveying Jupiter's moon Io received an early Christmas surprise when another Jovian moon, Europa, unexpectedly passed in front of it.
This is what's called a transit — when a celestial object passes in front of another object along our visual plane of sight. In this case, Europa passed in front of Io, providing these unprecedented images.
As noted, it caught the astronomers completely off guard. Phil Plait from Slate's Bad Astronomy explains:
I had to laugh when I saw the animation. My first thought was, why didn't the astronomers know this would happen? The log records...show they really were surprised. So I looked up how often these transits happen, and it turns out to be a little complicated. First, they happen clustered in time when Earth passes through Jupiter's equatorial plane. Then you might only see a half-dozen or so from a given location per month (some happen during the day, or behind Jupiter, when you can't see them). That's not very many, and even then they only last for a few minutes at a time.
So really, from a given observatory, it's pretty unlikely at any given time to accidentally observe a transit. However, in this case, astronomers are engaging in a long-term campaign to observe Io, because the tides from Jupiter cause it to be the most volcanically active object in the solar system. Its volcanoes are constantly erupting, and when they do they're visible in the infrared. The Gemini telescope, which made these observations, is designed to look at these wavelengths, and in fact you can see an active volcano on the upper left part of Io's face. Since things on Io change all the time, lots of observations are made, and so it's inevitable a visible transit would happen eventually.
This is not the first time astronomers have captured eclipsing moons. Last year, the Mars Curiosity rover captured images of Photos passing in front of Diemos.
Be sure to read Plait's entire article.
Images: Gemini Observatory.