Europa’s Optical Aurora

Abstract

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( 1 D) state, observed with the Keck I and Hubble Space Telescopes while Europa was in eclipse by Jupiter on six occasions in February-April 2018. The disk-integrated O( 1 D) brightness varies from <500 R up to more than 2 kR between dates, a factor of 15 higher than the OI 1356Å brightness on average. The ratio of emission at 6300/5577Å is diagnostic of parent molecule; the 5577Å emission was not detected in our dataset, 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 1-9×10 14 cm −2 .

A Stable H2O Atmosphere on Europa’s Trailing Hemisphere From HST Images

“…Considering various aspects of the variable plasma environment and the atmospheric distribution, the minimum O 2 column density in Europa's atmosphere is estimated to be (de Kleer & Brown, 2018; Hall et al., 1998; Roth et al., 2016). The upper limit for the O abundance derived above and this minimum O 2 abundance imply a maximum O/O 2 ratio in the atmosphere of 0.03 (red dash‐dotted line in Figure 2f).…”

“…For the estimates of absolute neutral abundances, we assume an electron density of 160 following de Kleer and Brown (2018). Although this is 4× higher than the density of 40 assumed in some previous studies of Europa's far‐UV emissions (Hall et al., 1995, 1998; Roth et al., 2016), it is in better agreement with the latest studies of the Europa plasma environment (Bagenal & Dols, 2020; Bagenal et al., 2015).…”

The Space Environment of Io and Europa

“…The resulting picture is an O 2 ‐dominanted atmosphere up to 900 km (1.6 R Eu ) where the O mixing ratio varies from 6% to 30%. de Kleer and Brown (2018) measured optical emissions in Europa's atmosphere (dominated by electron impact dissociation of O 2 leaving atomic products in an excited state) and concluded that the global O/O 2 ratio is less than 35%. Roth et al (2017) detected an atomic hydrogen corona with the HST.…”

The Origin and Fate of O 2 $\mbox{O}_{2}$ in Europa’s Ice: An Atmospheric Perspective

“…That these observations do exhibit considerable scatter about the average is not surprising considering the spatial and temporal variability discussed above as well as the difficulty of observing near eclipse. Because of their significant scatter, the ratios near eclipse (0 • /360 • longitude) could be ignored, although de Kleer and Brown (2018) observed an enhanced brightness in the visible aurora on the trailing dusk quadrant after Europa had entered eclipse consistent with an enhanced dusk/dawn emission ratio. Below we consider the implications of the result in Fig.…”

“…This led to the prediction that Europa could have a tenuous O 2 atmosphere due to the radiolytic decomposition of its icy surface by the incident Jovian plasma particles Johnson 1990). Their rough estimate of the magnitude of the atmosphere has been borne out by extensive Hubble Space Telescope (HST) observations in the UV (e.g., Hall et al 1995Hall et al , 1998McGrath et al 2004McGrath et al , 2009Roth et al 2016) and most recently in the visible during eclipse (de Kleer and Brown 2018). Although the physical processes have also been borne out by additional laboratory data (e.g., see summaries in Johnson 2011;Teolis et al 2017), a large number of uncertainties remain as to how the laboratory data should be applied at Europa (e.g., see Plainaki et al 2018).…”