摘要：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 6300/6364Å arising from the metastable O(1D) 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(1D) brightness varies from <500 R up to more than 2 kR between dates, a factor of 15 higher than the O I 1356Å brightness on average. The ratio of emission at 6300/5577 Å is diagnostic of the parent molecule; the 5577Å emission was not detected in our data set, which favors O2 as the dominant atmospheric constituent and rules out an O/O2 mixing ratio above 0.35. For an O2 atmosphere and typical plasma conditions at Europa’s orbit, the measured surface brightness range corresponds to column densities of (1–9)×1014 cm−2.
关键词：planets and satellites: atmospheres;planets and satellites: aurorae
