Observations of Europa’s Tenuous Atmosphere

Abstract

Europa is known to possess a predominantly molecular oxygen atmosphere produced by sputtering of its icy surface. This atmosphere, which is diagnostic of surface composition and processes, has been characterized by Hubble Space Telescope, Galileo, Cassini and groundbased observations. The primary means of detecting Europa's atmosphere is via emission from the atomic constituents. The relative strengths of the atomic oxygen emission lines allow inference of a dominant O 2 component. Oxygen, sodium and potassium are known minor constituents. A relatively dense ionosphere has also been detected on several occasions by Galileo radio occultation measurements, the presence of which appears to require a sunlit trailing hemisphere. Neither the spatial distribution of the oxygen emission associated with the atmosphere, nor the obvious variability of the atmospheric emissions and the ionospheric densities, has been adequately explained to date.

Orbital apocenter is not a sufficient condition for HST/STIS detection of Europa’s water vapor aurora

“…Previous STIS observations revealed an anticorrelation of the Ly-α morphology to the brightness in visible images (14,25). On the more contaminated trailing hemisphere, which has a more pronounced brightness dichotomy in the visible imges, Ly-α bright areas were found to coincide with darker visible terrain and vice versa (14).…”

“…Like in the previous three HST/STIS visits to Europa in 1999 and 2012 (14,25), the moon was observed during five consecutive HST orbits. The STIS was used in first-order spectroscopy mode with the G140L grating and the 52 arcseconds × 2 arcseconds slit, to map the ∼1-arcsecond-wide disk of Europa at wavelengths between 1190 Å and 1720 Å.…”

Polymerization of Building Blocks of Life on Europa and Other Icy Moons

“…However, it is quite tenuous, that is, 0.1-1 lPa (McGrath et al, 2009). Most icy moons have very thin atmospheres, too, with the exception of Saturn's moon Titan.…”

Probing for Evidence of Plumes on Europa With HST/Stis

“…Europa possesses a tenuous atmosphere and, because of its location deep within the Jovian magnetosphere, auroral activity in the form of far ultraviolet hydrogen and oxygen line emission occurs from the interaction of plasma electrons with the thin gases around the satellite (Hall et al 1995;McGrath et al 2009). Though the exosphere of Europa arises from material escaping from the surface, its basic properties such as areal coverage, density, thermal structure, and spatial and temporal variability are poorly understood (McGrath et al, 2009;Roth et al, 2015).…”