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An Amazing Occultation of Io by Europa

David Dickinson, Correspondent
May 12, 2015, 17:37 UTC

Sen—Sometimes, an image or animation makes your jaw drop in amazement. Such an amazing image made its way to us recently, when the Large Binocular Telescope atop Mount Graham in southern Arizona released a short animationof Jupiter’s moon Europa occulting (passing in front of) Io.

This amazing feat of imaging shows just what a massive ground-based telescope is capable of, and definitely places the Large Binocular Telescope at the forefront of the latest generation of up-and-coming large telescopes.


Europa passes across the disk of Io on March 7, 2015. Image credit: LBTO

The images were shot by LMIRcam, (The Large Binocular Telescope Mid-InfraRed Camera) which operates in the near-infrared band at 3 to 5 micrometres. A similar instrument known as NIRCam or the Near InfraRed CAMera will be featured on the James Webb Space Telescope set to launch in 2018. Though often touted as the “successor to the Hubble Space Telescope,” James Webb will also primarily perform work in the infrared range of the spectrum.

You can see several volcanic features on the surface of Io on the LBT image, including the massive Loki Patera region, which is represented as a bright patch. This lava lake is about 200 kmacross. The four Galilean moons of Jupiter (Io, Europa, Callisto and Ganymede) range in diameter from Europa at 3,121 km to Ganymede at 5,262 km, and yet only appear as star-like points of light less than 1.7” to 0.8” in size as seen from Earth.


A comparision of the expected simultated view of Io (left) and the actual view (right) obtained by the LBTI. Image credit: LBTO

The LBT consists of two mirrors 8.4 metres in diameter working in concert via a method known as interferometry to give the instrument the effective resolution of a single 22.8 metre mirror. “We combine the light from two very large mirrors coherently so that they become a single, extremely large mirror,” said LBT researcher and lead scientist on the study Al Conrad in a recent press statement.

Views of Jupiter’s moons were previously only possible during Jupiter flyby missions such as the Voyager 1 and 2 and New Horizons spacecraft. To date, the Galileo spacecraft is the only mission that has parked in orbit around Jupiter, though that will change when NASA’s Juno spacecraft enters orbit next year.

“We built LBTI to form extremely sharp images,” said Phil Hinz, the LBTI project lead for the University of Arizona Steward Observatory in the recent press release. “It is gratifying to see the system work so well.” Researchers are working to turn the eyes of the Large Binocular Telescope on stars to hunt for exoplanets and dusty protoplanetary disks. There has been some early success in this regard directly imaging the HR 8799 and planetary system and dust around the Sun-like star Eta Corvi.

We’re also currently in the tail-end of “mutual season eclipse season” for the Jovian moons, a period during which the planes of the orbits of the Jovian moons appear edge-on as seen from the Earth and the four large moons pass in front of one another.


A simulation of the transit of Europa across the disk of Io as seen from Earth on March 7, 2015. Image credit: Starry Night Education software

This year long plane crossing season only occurs about once every five to six years, and will not begin again until 2020. You can check out this table for notable eclipses and occultations of Jupiter’s moons. Contrary to what we first thought, the foreground object in the LBT animation is not Europa’s shadow, but Europa itself. Turns out, icy Europa is much darker than volcanically active Io in the near-infrared.

And did you know: it’s possible to see solar eclipses similar to those witnessed on the Earth from the surface of the Jovian moons during mutual eclipse season:

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Maybe the recently rumored Europa landerwill witness such as spectacle. Congrats to the team at Large Binocular Telescope for giving us such an amazing view!