Water vapor revealed on Jupiter’s moon, Ganymede
In one of the year’s big space discoveries, KTH researchers Lorenz Roth and Nickolay Ivchenko uncovered evidence of water vapor in the atmosphere of Jupiter’s moon, Ganymede.
Publishing in Nature Astronomy in July, Roth and his team drew on new—and decades old—datasets collected from Hubble space telescope images and spectra.
The finding contributes to understanding the Jovian system and unravelling its history, from its origin to the possible emergence of habitable environments.
While Ganymede is believed to have water deep below its surface—possibly more than in all of the Earth’s oceans—the water vapor discovered on the surface is not a result of evaporation from the underground ocean. It is generated instead from a rock-hard layer of ice on the moon’s surface.
The discovery of the vapor results from the KTH researchers’ efforts to measure the amount of atomic oxygen in Ganymede’s atmosphere—an investigation that was included in a larger observing effort to support the NASA Juno mission in 2018.
High concentrations of atomic oxygen (O) were previously believed to explain ultraviolet (UV) spectral images of the moon’s aurora, taken by the Hubble in 1998. Roth and Ivchenko looked at oxygen spectral lines excited by sunlight, and found there was hardly any atomic oxygen in Ganymede's atmosphere, as the lines did not disappear when the moon moved into Jupiter’s shadow. This led to further investigation and the realization that the lines were coming from water molecules sublimating from the icy surface.
"Up to that point only the molecular oxygen had been observed," Roth says. "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."
The discovery adds excitement to the upcoming ESA mission, JUICE, which will send a spacecraft to make detailed observations of Jupiter and three of its largest moons, with particular emphasis on Ganymede as a planetary body and potential habitat. The JUICE mission has strong Swedish participation, providing two instruments: The Radio and Plasma Wave Instrument (short RPWI) and the Particle Environment Package (PEP), both led by Swedish Institute of Space Physics. The team at KTH is involved in the RPWI effort, and the discovery will be important for planning the operations of the instrument, Ivchenko says.
"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," Ivchenko says.
David Callahan