Based on data from the Hubble telescope, astronomers have discovered the first evidence of water vapor in the atmosphere of Ganymede, the largest moon of Jupiter. This water vapor forms when the ice on the moon's surface changes from a solid solid state to a gas state.
Ganymede, the largest moon of Jupiter and the Solar System, contains more water than all of Earth's oceans. Much of this water forms an underground ocean found at a depth of about 150 km. On the surface, the temperatures are so cool that the water is only in a solid state. At least, that's what we thought so far. By analyzing several sets of new and archived data from the Hubble telescope, a team announces that it has isolated traces of water vapor on the surface. Details of the study are reported in the journal Nature Astronomy.
In 1998 , Hubble's Imaging Spectrograph had captured the first ultraviolet (UV) images of Ganymede, revealing auroral bands (colored bands of electrified gas) on the surface. These structures were very similar to the auroral ovals observed on Earth and on other planets with a magnetic field.
The similarities in these observations were explained by the presence of molecular oxygen (O2) in the moon's atmosphere. Nevertheless, some observed characteristics did not correspond to the emissions expected from a pure O2 atmosphere . At the time, the researchers concluded that these differences could be explained by higher concentrations of atomic oxygen (photolysis or light breakdown of molecular oxygen).
As part of a program to support the Juno mission, a team from the Royal Institute of Technology in Stockholm (Sweden) set out three years ago to measure this quantity of atomic oxygen with Hubble. To their surprise, the researchers then discovered that there was actually almost no atomic oxygen in the atmosphere of Ganymede.
A few months ago, the same team then took a closer look at the relative distribution of these famous auroras, suspecting that features seen in 1998 could be explained by water vapor concentrations (gas phase) released by sublimation . As on Earth, Ganymede's surface temperature indeed varies throughout the day, and around midday, near the equator, it can become warm enough for the surface of the ice to release small amounts of molecules of water.
Ultimately, the perceived differences in the UV images did indeed correlate to where water would be expected in the lunar atmosphere.
"So far, only molecular oxygen has been observed “, said Lorenz Roth, main author of this work. “This is produced when charged particles erode the surface of the ice. The water vapor we now measure comes from the sublimation of ice caused by thermal escape from warmer icy regions “.
The discovery of this water vapor thus adds to the understanding of the atmosphere of Ganymede. These results could also be of interest to the team of JUICE (Jupiter Icy Moon Explorer), ESA's next large-scale mission due to take off next year. Its objective will be to closely study the Jovian system from 2029, before focusing exclusively on Ganymede from 2032 . This data could therefore be used to refine observation plans.
