A Search for Water Vapor Plumes on Europa using SOFIA

TL;DR

This study used SOFIA/EXES spectroscopy to search for water vapor plumes on Europa, setting upper limits on vapor emission and comparing with previous HST observations, but found no direct evidence of plumes during the observations.

Contribution

First mid-infrared spectroscopic search for Europa's water plumes using SOFIA, providing quantitative upper limits and comparing with prior HST data.

Findings

No water vapor plumes detected during observations.

Upper limits constrain plume activity below previous HST estimates.

Potential plume activity could have been detected under ideal conditions.

Abstract

We present mid-infrared SOFIA/EXES spectroscopy of Europa, seeking direct evidence of the presence of water vapor arising from plumes venting from the surface of Europa. We place quantitatively useful upper limits on the strength of water vibrational-rotational emission lines. Conversion to water mass limits is dependent on the rotational temperature of the vapor. For low rotational temperature, the limits lie below the inferred water mass from previous HST plume observations. For higher temperatures, the limits are comparable. We also present coordinated HST transit observations obtained close in time to the SOFIA observations. There is evidence for a feature close to the location of the previously seen feature north of the crater Pwyll in one of the HST images, although it was not observable by EXES given its location. We conclude that if a water plume had been active at the time of the SOFIA observation, with the strength implied by previous HST observations, then under the right Earth atmospheric and geometric conditions, the plume could have been detected by EXES, however no IR water vibrational-rotational emission was detected.