Europa's Optical Aurora

Auroral emissions provide opportunities to study the tenuous atmospheres of solar system satellites, revealing the presence and abundance of molecular and atomic species as well as their spatial and temporal variability. Far-UV aurorae have been used for decades to study the atmospheres of the Galilean satellites. Here we present the first detection of Europa's visible-wavelength atomic oxygen aurora at 6300/6364 angstrom arising from the metastable O(D-1) state, observed with the Keck I and Hubble Space Telescope while Europa was in eclipse by Jupiter on six occasions in 2018 February-April. The disk-integrated O(D-1) brightness varies from < 500 R up to more than 2 kR between dates, a factor of 15 higher than the O I 1356 angstrom brightness on average. The ratio of emission at 6300/5577 angstrom is diagnostic of the parent molecule; the 5577A emission was not detected in our data set, which favors O-2 as the dominant atmospheric constituent and rules out an O/O-2 mixing ratio above 0.35. For an O-2 atmosphere and typical plasma conditions at Europa's orbit, the measured surface brightness range corresponds to column densities of (1-9) x 10(14) cm(-2).