We have solid evidence of water vapour in the
atmosphere of Jupiter's moon Ganymede – the solar system's biggest moon — for
the first time. The frozen water on Ganymede's surface may have sublimated,
moving from solid to gas without becoming liquid.
The surface of Ganymede is a mix of dark, cratered
regions and bright grooved terrain that produces fascinating patterns.
Researchers have long thought that Ganymede has a large amount of water —
possibly more than the Earth — but because Ganymede is so far from the Sun,
water could only remain liquid behind a thick covering of ice.
Ganymede is assumed to have three primary layers: a
metallic iron core, a rocky mantle, and a liquid and frozen layer of water. The
ice shell on the outer is extraordinarily thick (around 500 miles / 800
kilometres), and any liquid water might exist beneath it. Regardless, there is
water — and where there is water, there may be life.
For the first time, researchers have discovered
non-ice water on the surface.
As part of a larger observation program, Lorenz Roth
of the KTH Royal Institute of Technology in Stockholm, Sweden was using Hubble
to measure the amount of oxygen on Ganymede. Roth and his colleagues used 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.
In 1998, Hubble’s Space Telescope Imaging
Spectrograph (STIS) took the first ultraviolet (UV) images of Ganymede, which
revealed a particular pattern in the observed emissions from the moon’s
atmosphere. The moon displays auroral bands that are somewhat similar to aurora
ovals observed on Earth and other planets with magnetic fields. This was
illustrative evidence for the fact that Ganymede has a permanent magnetic
field. The similarities in the ultraviolet observations were explained by the
presence of molecular oxygen (O2). The differences were explained at the time
by the presence of atomic oxygen (O), which produces a signal that affects one
UV color more than the other. Credit: NASA, ESA, Lorenz Roth (KTH)
The UV data showed the presence of atomic oxygen —
at least that’s what the original interpretation from 1998 noted. But much to
their surprise, Roth’s team found hardly any evidence of atomic oxygen in
Ganymede’s atmosphere. If this is the case, there must be another explanation
for the apparent differences in these UV aurora images.
When the researchers took a closer look at the
relative distribution of the colorful ribbons of electrified gas called auroral
bands in the UV images, they found another piece of evidence: Ganymede’s
surface temperature varies strongly throughout the day. Around noon, the
equatorial parts of Ganymede may become sufficiently warm that the ice surface
releases (or sublimates) some small amounts of water molecules.
This fits excellently with the Hubble data. The
presumed oxygen (which Roth now believes to be water vapor) was found exactly
around the equator.
“So far only the molecular oxygen had been observed,”
explained Roth. “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 finding makes Ganymede a much more interesting
place, especially considering the European Space Agency’s upcoming mission.
JUICE (JUpiter ICy moons Explorer) is planned for launch in 2022, and arrival
at Jupiter in 2029. The mission will spend three years making detailed
observations of Jupiter and its largest moons — including Ganymede.
“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,” added Roth.
 |
Comparison between the Earth, the Moon (top) and
Ganymede (bottom). |
Astronomers are increasingly looking at frozen moons
around Jupiter and Saturn as places where life could emerge. They were once
discarded as barren, frozen wastelands, but the more we look at them, the more
the potential habitability of these moons seems increasingly likely. Of course,
just because there could be life on Ganymede doesn’t mean there is — that’s up
for future research to discover.
The study was published in Nature Astronomy.