Using new and archival datasets from Nasa’s Hubble Space Telescope, scientists and astronomers have for the first time uncovered evidence of water vapor in the atmosphere of Jupiter's largest moon Ganymede. This water vapor forms when ice from the moon's surface sublimates -- turning from solid to gas.
According to an official Nasa statement, previous research only offered circumstantial proof that Ganymede, the largest moon in the solar system, contains more water than all of Earth's oceans. “However, temperatures there are so cold that water on the surface is frozen solid. Ganymede’s ocean would reside roughly 100 miles below the crust; therefore, the water vapor would not represent the evaporation of this ocean,” the statement explains.
Astronomers re-examined Hubble observations from the last two decades to find this evidence of water vapor. The findings of this research were published in the journal Nature Astronomy this week.
More than two decades ago, in 1998, Hubble had captured the first ultraviolet (or UV) images of Ganymede, using its imaging spectrograph. These images showed colorful ribbons of electrified gas, known as “auroral bands”, and provided further evidence that Ganymede has a weak magnetic field, the Nasa statement explains: “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 the 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,” the statement adds.
Contrary to the original interpretations of the 1998 data, the researchers discovered there was barely any atomic oxygen in Ganymede's atmosphere, which meant there must be another explanation for the apparent differences in the 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,” the statement explains.
Roth says in the statement: “So far, only the molecular oxygen had been observed… This is produced when charged particles erode the ice surface. The water vapor that we measured now originates from ice sublimation caused by the thermal escape of water vapor from warm icy regions.”
Why are these findings important?
In 2022, the European Space Agency (ESA) will launch its JUICE mission, which stands for JUpiter ICy moons Explorer. It is the first large-class mission in ESA's “Cosmic Vision 2015-2025 program”. Once the spacecraft arrives at Jupiter in 2029, it will spend at least three years making detailed observations of Jupiter and three of its largest moons (Ganymede, Europa, Callisto), with a special focus on Ganymede as a potential habitat and planetary body.
The Nasa statement adds that Ganymede was identified for detailed investigation because it provides a natural laboratory for analysis of the nature, evolution and potential habitability of icy worlds in general. “Our results can provide the JUICE instrument teams with valuable information that may be used to refine their observation plans to optimize the use of the spacecraft,” Roth explains in the statement.