Scientists have long suspected the presence of an extremely rarefied atmosphere in this huge natural satellite of Jupiter. Until recently, it was assumed that it consists of oxygen, which is knocked out by charged particles from the icy surface of Ganymede. New data show that there is another mechanism for replenishing its gas shell with matter - in the process of sublimation of water as a result of heating by sunlight.
Ganymede is a curious subject for research. It is the largest of all the moons of the planets in the solar system. It is larger than Mercury in size, although it is more than twice lighter than a rocky planet. And in comparison with the far from small Moon, this object seems colossal at all - it is more than twice as massive as a natural satellite. Thanks to such impressive characteristics, Ganymede looks more like a planet: it has its own magnetic field and differentiated bowels. So the question of the presence of an atmosphere, albeit a very rarefied one, arose long ago.
She was found with ground-based instruments in 1972, when Ganymede covered the star. Then the surface pressure on this satellite was estimated at 0.1 pascal. Nevertheless, just seven years later, data from the Voyager 1 spacecraft flying past the Jupiter system diminished the enthusiasm of astronomers. The probe instruments have determined the upper limit of the concentration of molecules at the surface of Ganymede, typical for a gas pressure of no more than 2.5 micropascals. And this is almost five orders of magnitude less matter. The refinement is all the more valuable because it was made thanks to measurements in the far ultraviolet range, and not in visible light, which gives a much clearer picture.
Until 1995, new information about the atmosphere of Ganymede could not be obtained. But the Hubble Space Telescope has provided new data. He found a weak concentration of molecular oxygen (O2) within the limits previously set by Voyager. It was possible to clarify these measurements three years later, when the near-earth apparatus received new instruments. At the same time, a slight oddity was revealed: some spectral lines went beyond the limits characteristic of molecular oxygen.
Interpreting these results, the scientists decided that some of the gas above Ganymede's surface was atomic oxygen. This rarefied atmosphere is replenished by high-energy particles from the solar wind, penetrating through the satellite's magnetosphere and knocking molecules or atoms out of the water ice. Hydrogen is very light and evaporates almost immediately, but oxygen manages to stay for a short time. Its concentration is sufficient for some of the electrons captured by the lines of the magnetic fields of Ganymede and Jupiter to excite individual atoms, which, in turn, emit photons. As a matter of fact, from these auroras in the ultraviolet range, "Hubble" and "saw" the desired oxygen.
However, an international team led by Swedish astrophysicist Lorenz Roth decided to study the issue a little deeper.In their research, the scientists combined archival data from two different Hubble instruments, as well as relatively recent observations from 2018, and analyzed them together. The results of the study are published in the journal Nature Astronomy. The authors of the scientific work were specialists from the Royal Technological Institute (Sweden) and the Southwest Research Institute (USA), as well as Cologne (Germany) and Liege (France) universities.
A more detailed look at the impressive amount of spectral measurements and images in the ultraviolet range allowed specialists to clarify the composition of Ganymede's atmosphere. In addition to molecular oxygen, it also contains water vapor, which was noticed by Hubble in 1998. But then scientists did not have enough data to accurately establish this, so they chose the more likely option (atomic oxygen). This conclusion was preferable also because the proposed mechanism of gas formation - knocking out molecules and atoms from water ice - cannot explain the presence of vapor in the atmosphere.
But thanks to the large amount of data that is also collected during long-term observations, Roth's team is confident that there is relatively much steam at the surface of Ganymede, and it appears there as a result of the sublimation of water from ice. The fact is that during the local day, the surface temperature of Jupiter's largest moon fluctuates within significant limits. On the side illuminated by the Sun, conditions arise when there is enough heat to transfer some water molecules from a solid to a gas. No, there is never a positive temperature there, but in conditions close to absolute vacuum, this process begins even at only one hundred kelvin.
This discovery greatly adds to the treasury of human knowledge about the icy moons of Jupiter, especially in light of the imminent launch of the JUICE mission. The mission of this apparatus, created by the European Space Agency, will be to study both the gas giant itself and three of the Galilean satellites: Ganymede, Callisto and Europa. The start is scheduled for 2022, and the start of the research campaign is scheduled for 2029. Scientists' interest in the largest satellites of Jupiter is explained by the incredibly favorable conditions for the emergence of life on them. So, in Ganymede and Europe, the presence of a subsurface ocean of liquid water is confirmed with great reliability, and in Callisto it is suspected.