Hubble finds evidence of persistent water vapor in one hemisphere of Europa

NASA's Galileo spacecraft took this photograph of Europa, the Jovian moon. It was taken at 776,700 miles. Europa is slightly smaller than Earth's moon. It has a smooth surface, and its solid ice crust gives it the appearance of an eggshell. It has a global ocean that contains more water than the Earth's. It may be home to life as we know. Hubble Space Telescope observations have shown that Europa has persistent water vapor. This is despite its extremely fragile atmosphere. Hubble observations from 1999 to 2015 show that water vapor is continually replenished in one hemisphere on the moon. This is in contrast to Hubble's 2013 observations, which found water vapor localized from subsurface ocean geysers. This water vapor is a result of a completely different process. Sunlight causes surface ice to melt, transforming it into gas. This Galileo composite Galileo view includes infrared, violet, and green images. The moon view is available in both natural and enhanced colors. This enhances subtle color differences on the surface. Europa's bright white and bluish surface is mainly composed of water ice. There are very few other ice materials. Some fractures in the crust are long and dark, some measuring more than 1,850 miles. Credit: NASA, NASA JPL, University of Arizona
NASA's Hubble Space Telescope observations on Jupiter's icy moon Europa revealed persistent water vapour, but only in one hemisphere.

Europa may have a vast ocean beneath its icy surface that could be suitable for life. This finding advances the understanding of astronomers of the atmosphere of icy moons and allows for science missions to the Jovian to explore whether an environment half a billion miles from the Sun might support life.

Hubble photographed in 2013 the plumes that erupt through the ice as a possible explanation for previous observations of water vapour on Europa. They are similar to geysers in Earth but they reach more than 60 miles. They emit a temporary blob of water vapor in the lunar atmosphere. This is one-billionth of the atmosphere's surface pressure.

However, the new Hubble observations from 1999 to 2015 show that similar amounts of water vapour are found over a greater area of Europa. This indicates that Europa's trailing, or the portion of the moon facing in opposite direction to its orbital motion, has a long-term water vapor atmosphere. It is unclear what causes this asymmetry in the trailing and leading hemispheres.

This discovery was made by Lorenz Roth of KTH Royal Institute of Technology Space and Plasma Physics in Sweden.

Roth stated that "The observation of water vapour on Ganymede and on the trailing edge of Europa advances our understanding about the atmospheres of icy lunars." "But, it is more surprising to see a stable water abundance in Europa than on Ganymede due to Europa's lower surface temperatures than Ganymede.

Europa has a higher level of sunlight reflection than Ganymede. This keeps the surface 60 degrees Fahrenheit lower than Ganymede. Europa's daytime temperature is below 260°F. However, even at this lower temperature, new observations suggest that water ice may be sublimating. This is when it transforms directly from solid to liquid without any liquid phaseoff Europa's surfaces, much like Ganymede.

Roth searched through archival Hubble data and selected ultraviolet observations of Europa during different orbital positions in 1999, 2012, 2014, 2015 to make this discovery. All of these observations were made using Hubble's Space Telescope Imaging Spectrograph. Roth was able to use the ultraviolet STIS observations to determine the amount of oxygenone in Europa's atmosphere. He also used the strength of emission at different wavelengths to infer the existence of water vapor.

This discovery opens the door to future probes, including NASA's Europa Clipper (NASA) and the Jupiter Icy Moons Explorer mission (ESA), which will allow for more detailed studies of Europa. Astronomers can also gain insight into Jupiter-like planets orbiting other stars by understanding the formation and evolution Jupiter and its moons.

These results were published in Geophysical Research Letters.

Continue exploring Hubble's first evidence of water vapour on Jupiter's Moon Ganymede

More information: Lorenz Roth. A stable H2 O atmosphere on Europa’s trailing hemisphere using HST images. Geophysical Research Letters (2021). Information for Journal: Geophysical Research Letters Lorenz Rot, A stable H2 O atmosphere on Europa’s trailing hemisphere using HST images (2021). DOI: 10.1029/2021GL094289

Space Telescope Science Institute