Eerie molecular hydrogen cloud found on Europa

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Feb 6, 2017

Jupiter's moon of ice has always been something of an enigma to astronomers, but now NASA's Hubble Telescope has revealed another secret about Europa's tenuous atmosphere.

Hubble's Space Telescope Imaging Spectrograph observed the Jovian satellite in ultraviolet light between December 2014 and March 2015, capturing six images of it in transit of Jupiter — the moment the orbiting moon passes in front of and appears to obscure its planet from an observer's point of view. Europa would be impossible to make out through a telescope stationed on Earth because of its Lyman-alpha dayglow. On the far end of the UV spectrum, this type of radiation would be absorbed by the oxygen in Earth's atmosphere, which is why it must be imaged from space.

What these observations revealed was groundbreaking (or ice-breaking). While Hubble set out on a mission to search the ultraviolet hydrogen emissions in Europa's atmosphere for traces of water vapor that could signal the presence of vaporous plumes like those found in an earlier study, the images it transmitted back to Earth unexpectedly exposed an enormous hydrogen gas cloud shrouding the frozen moon. Previously predicted by astronomers, a corona of molecular hydrogen (molecules of two joined hydrogen atoms or H2) around Europa only existed in theory until this revelation.

Europa's molecular hydrogen corona is revealed in blue.

"The amount of hydrogen we observed was actually expected to exist and ultimately originates from the erosion of Europa water ice surface," explained Lorenz Roth of KTH Royal Institute of Technology in Stockholm, Sweden, whose team has been studying the phenomenon, in an article recently published in The Astronomical Journal.

Europa's thin atmosphere could be demystified more by the abundance of hydrogen gas. Molecular oxygen or O2 is its main and densest component and differs from Earth's oxygen in that it is of chemical rather than biological origin. This oxygen is generated by radiolysis, the process by which ionizing radiation — which gives atoms a positive negative charge by displacing electrons from their orbits — causes the molecular structure of a substance to decompose. Jupiter's magnetosphere also has influence over its moon. UV radiation from the sun, along with charged ions and electrons, crashes into Europa’s crust of solid ice. Frozen water is then broken down into oxygen and hydrogen absorbed by its atmosphere.

Though this study enabled astronomers to confirm that the previously theorized hydrogen in Europa's atmosphere is released as ice erodes, whether this cloud remains constant or undergoes any transformations depending on time and the moon's location in orbit is still a mystery trapped in ice.