Europa's Optical Aurora

TL;DR

This study reports the first detection of Europa's visible-wavelength atomic oxygen aurora, revealing insights into its atmospheric composition and variability through observations with Keck and Hubble telescopes.

Contribution

It provides the first observation of Europa's visible oxygen aurora and constrains the atmospheric composition, particularly supporting O$_2$ as the dominant species.

Findings

Detected Europa's atomic oxygen aurora at 6300/6364 Å.

Variability in auroral brightness indicates atmospheric changes.

Results suggest O$_2$ as the main atmospheric component.

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 6300/6364 \AA{} 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 $<$500 R up to more than 2 kR between dates, a factor of 15 higher than the OI 1356 \AA{} brightness on average. The ratio of emission at 6300/5577 \AA{} is diagnostic of parent molecule; the 5577 \AA{} emission was not detected in our dataset, which favors O$_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$\times$10$^{14}$ cm$^{-2}$.