Observations by the NASA/ESA Hubble Space Telescope in far-ultraviolet light of Jupiter’s icy moons were used in the past to detect molecular oxygen in their tenuous atmospheres. The results of an analysis of images and spectra of Jupiter’s moon Ganymede have recently shown that the same observations also contain information that water vapor is abundant in the atmosphere in addition to oxygen. A planetary researcher from the KTH Royal Institute of Technology has now used the same analysis for Jupiter’s moon Europa and found a water vapor atmosphere as well, but, mysteriously, only above the icy moon’s trailing hemisphere (the portion of the moon that is always opposite to its direction of motion).
Water vapor is present in the atmosphere of Jupiter’s icy moon Europa. Image credit: NASA / ESA / Hubble / J. da Silva.
Europa possesses a tenuous atmosphere that is thought to be constantly replenished by erosion of its water ice surface.
The main species in the bound atmosphere are expected to be molecular oxygen (O2), hydrogen (H2), and water (H2O).
The first evidence for this atmosphere was provided by a far-ultraviolet spectrum of Europa taken by Hubble in the 1990s.
“The new observations of water vapor on Ganymede and on the trailing side of Europa advance our understanding of the atmospheres of icy moons,” said Dr. Lorenz Roth, a researcher at the KTH Royal Institute of Technology.
“The detection of a stable H2O abundance on Europa is surprising because the surface temperatures are so low.”
To make this discovery, Dr. Roth delved into archival Hubble datasets, selecting ultraviolet observations of Europa from 1999, 2012, 2014 and 2015, while the moon was at various orbital positions.
These observations were all taken with Hubble’s Space Telescope Imaging Spectrograph (STIS).
The ultraviolet STIS observations allowed the scientist to determine the abundance of oxygen — one of the constituents of water — in Europa’s atmosphere, and by interpreting the strength of emission at different wavelengths he was able to infer the presence of water vapor.
Previous observations of water vapor on Europa have been associated with transient plumes erupting through the ice.
The phenomena seen in these plume studies were apparently transient inhomogeneities or blobs in the atmosphere.
The new results, however, show similar amounts of water vapor to be present spread over a larger area in observations spanning from 1999 to 2015.
This suggests the long-term presence of a water vapor atmosphere on Europa’s trailing hemisphere.
On the leading hemisphere, the Hubble data are consistent with a pure oxygen atmosphere everywhere across the moon disk.
“This result lays the groundwork for future science based on upcoming missions to the Jovian moons,” Dr. Roth said.
“The more we can understand about these icy moons before future spacecraft like JUICE and Europa Clipper arrive, the better use we can make of our limited observing time within the Jovian system.”
The findings were published in the journal Geophysical Research Letters.
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Lorenz Roth. A Stable H2O Atmosphere on Europa’s Trailing Hemisphere from HST Images. Geophysical Research Letters, published online September 13, 2021; doi: 10.1029/2021GL094289
