Researchers Find Evidence of Water Vapor on Ganymede in Historic Hubble Data

A view of Ganymede assembled from images captured by NASA’s Galileo spacecraft in June 1996. Credit: NASA/JPL-Caltech/Jason Major

News from NASA on July 26, 2021

For the first time, astronomers have uncovered evidence of water vapor in the atmosphere of Jupiter’s moon Ganymede, the largest moon in the Solar System. This water vapor forms when ice from the moon’s surface sublimates — that is, turns from solid to gas.

Astronomers re-examined Hubble observations from the last two decades to find this evidence of water vapor, using new and archival datasets from NASA’s Hubble Space Telescope to make the discovery, published in the journal Nature Astronomy.

“The higher H2O/O2 ratio above the warmer trailing hemisphere compared with the colder leading hemisphere, the spatial concentration in the subsolar region and the estimated abundance of ~1015 molecules of H2O per cm2 are consistent with sublimation of the icy surface as source.”

Roth, L., Ivchenko, N., Gladstone, G.R. et al. A sublimated water atmosphere on Ganymede detected from Hubble Space Telescope observations. Nat Astron (2021). https://doi.org/10.1038/s41550-021-01426-9
In 1998, Hubble’s Space Telescope Imaging Spectrograph took these first ultraviolet images of Ganymede, which revealed a particular pattern in the observed emissions from the moon’s atmosphere. The similarities in the ultraviolet observations were explained by the presence of molecular oxygen. The differences were explained at the time by the presence of atomic oxygen, which produces a signal that affects one UV color more than the other.
Credits: NASA, ESA, Lorenz Roth (KTH)

In 1998, Hubble’s Space Telescope Imaging Spectrograph took the first ultraviolet (UV) images of Ganymede, which revealed colorful ribbons of electrified gas called auroral bands, and provided further evidence that Ganymede has a weak magnetic field.

The similarities in these UV observations were explained by the presence of molecular oxygen (O2). But some observed features did not match the expected emissions from a pure O2 atmosphere. At the same time, scientists concluded this discrepancy was likely related to higher concentrations of atomic oxygen (O).

As part of a large observing program to support NASA’s Juno mission in 2018, Lorenz Roth of the KTH Royal Institute of Technology in Stockholm, Sweden led the team that set out to measure the amount of atomic oxygen with Hubble. The team’s analysis combined the data from two instruments: Hubble’s Cosmic Origins Spectrograph in 2018 and archival images from the Space Telescope Imaging Spectrograph (STIS) from 1998 to 2010.

To their surprise, and contrary to the original interpretations of the data from 1998, they discovered there was hardly any atomic oxygen in Ganymede’s atmosphere. This means there must be another explanation for the apparent differences in these UV aurora images.

Roth and his team then took a closer look at the relative distribution of the aurora in the UV images. Ganymede’s surface temperature varies strongly throughout the day, and around noon near the equator it may become sufficiently warm that the ice surface releases (or sublimates) some small amounts of water molecules. In fact, the perceived differences in the UV images are directly correlated with where water would be expected in the moon’s atmosphere.

Read the full news story from NASA here.

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