According
to the study published in the journal Nature Astronomy on Monday, the water
vapour forms when ice from the moon’s surface turns from solid to gas.
Previous
studies have offered circumstantial evidence that Ganymede, the largest moon in
the solar system, contains more water than all of Earth’s oceans, NASA said.
However,
temperatures there are so cold that water on the surface is frozen solid,
according to the US space agency.
Ganymede’s
ocean would reside roughly 160 kilometres below the crust, therefore, the water
vapour would not represent the evaporation of this ocean.
Astronomers
re-examined Hubble observations from the last two decades to find this evidence
of water vapour.
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.
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