An Accidental Eclipse From 700m Km surprised Astronomers

What they saw was another of Jupiter’s moons, Europa, passing straight in front of Io, what astrophysicists call a transit (or more generally a “mutual event”).

Jupiter’s moons orbit the massive planet right above its equator, and a few times a year all lines up just right so that, from Earth, we perceive the orbits edge-on. When that occurs the moons can pass ahead of each other, producing a transit (or an eclipse if the more distant moon is totally, or closely entirely, covered).

I had to laugh when I saw the simulation. My first believe was, why didn’t the astrophysicists know this would happen? The log records show they actually were stunned. So I saw up how frequently these transits happen, and it turns out to be a little complex. First, they happen crowded in time when Earth passes through Jupiter’s equatorial plane. Then you might only grasp a half-dozen or so from a given situation per month (some occur during the day, or behind Jupiter, when you can’t see them). That’s not very many, and even then they only last for an amount of time.

So indeed, from a given telescope, it’s pretty doubtful at any given time to unintentionally witness a transit. However, in this case, astrophysicists are engaging in a long-term drive to observe Io, because the tides from Jupiter affect it to be the most volcanically active object in the solar system. Its volcanoes are continually erupting, and when they do they’re observable in the infrared. The Gemini telescope, which made these studies, is designed to look at these wavelengths, and in fact you can see an live volcano on the upper left part of Io’s face. Since things on Io alter all the time, lots of observations are made, and so it’s predictable a visible transit would happen ultimately.

Europa’s superficial is water ice, which is pretty good at absorbing the specific color of infrared observed, so it appears dark. After the transit, the astrophysicists swapped filters so that Europa can be faintly seen moving off to the upper right.

Occasionally there can be unexpected science from such things; for example, the precise timing can be used to test equations forecasting locations of volcanoes on Io and the locations of the moons themselves.

But one thing that I’m pretty certain will come of all this: In the future, I bet the astrophysicists making these observations will check for mutual events earlier the observations start!



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