As many as 92 moons of Jupiter have been discovered so far, but four of them are particularly large. It is named after the discovery of Galileo Galilei in 1610.Galileo satelliteThese four moons are calledAyo」「Europa」「Ganymede」「Callisto” he is called.
All of the Galilean satellites have very thin atmospheres. Volcanic activity is active on Io due to tidal forces, and this is the source of the atmosphere. Ion’s atmosphere consists mainly of sulfur dioxide, which is denser than that of the other Galilean moons. On the other hand, the sources of the atmospheres of the three remaining moons, Europa, Ganymede, and Callisto, are not well understood. The atmosphere of these moons is mainly oxygen, with some water and carbon dioxide, but less than that of Io. Since the surfaces of the three moons are covered with water ice, the water molecules were initially split into hydrogen and oxygen by photolysis by sunlight, and the lighter hydrogen quickly escaped, creating an oxygen-based atmosphere, which is thought to have formed. However, it was later found that the concentration of oxygen cannot be explained by photodecomposition alone, so another explanation is that charged particles accelerated by Jupiter’s magnetic field break water molecules. This theory can generally explain the state of the satellite’s atmosphere However, the details remain vague. (※ 1) It cannot fully explain the subtle changes in the composition of the atmosphere resulting from
* 1 … Like Earth’s moon, the Galilean satellites also rotate synchronously with the same side facing Jupiter. Therefore, the period of rotation of the Galilean satellites coincides with the orbital period.
In order to accurately explain the origin of the atmosphere of the Galilean satellites, it is necessary to accurately measure the composition of the atmosphere. It is not easy either. but”twilight, you can know the atmosphere of the Galileo satellite indirectly even with a telescope on Earth. When a charged particle collides with a molecule that makes up the atmosphere, the molecule receives energy once, but then releases it immediately. This energy is emitted in the form of electromagnetic waves and is observed in the form of the aurora. The Galilean satellites produce aurorae when charged particles accelerated by Jupiter’s magnetic field collide with the satellite’s atmosphere. The color of the aurora depends on the wavelength of the emitted electromagnetic waves, which is determined by the type of molecule. In other words, by examining the wavelength and intensity of the aurora borealis, we can learn the exact atmospheric composition of the satellite.
But there are other difficulties in observing auroras. When you hear the word aurora, you might imagine something like a green curtain, but aurora light, which has wavelengths of visible light (electromagnetic waves that can be seen by the human eye), is very weak. In the case of the Galilean satellites, in addition to the distance of Jupiter’s moons, the reflected light and scattered light of Jupiter and the satellite itself become noise that masks the afterglow.
In fact, there have been limited observations of the aurora by Galileo satellites in the visible-light range, mostly from Io, where the intensity of the aurora is strong, and only a few from Europa. On the other hand, there are many observations of aurorae in the ultraviolet region, which are more intense than visible light and are easier to distinguish from noise. However, the visible aurora data was necessary to obtain more information. Also, Callisto was the only satellite that never saw the aurora borealis, and it lacks information about its atmosphere compared to other Galileo satellites.
A research team led by Catherine De Claire at Caltech used the aurora borealis to attempt to accurately analyze the composition of the Galileo satellite’s atmosphere.
There are two important points in this study. The first is an observatory for making accurate observations of the aurora, the W.M. Keck Observatory.RecruitmentsApache Point Lookoutthrush“The big-eyed telescope.”PepsiIt was used. All of these are spectroscopic devices installed in telescopes, and they can pick up the auroral light precisely by wavelength.
The second is the observation period of the Galileo satellites. In this research, the Galilean moons were “eclipsed” by Jupiter so that HIRES could capture the faint light of the afterglow. Just like a lunar eclipse, when the moon passes into Earth’s shadow, Galilean satellites can also pass into Jupiter’s shadow. When the satellite is in shadow, it is not illuminated by the sun, so noise such as reflected light is suppressed, making it easier to catch the faint light of twilight. In this study, 20 observations in the visible light range (including partially near infrared) of eclipses of Jupiter’s moons that occurred between 1998 and 2021 were analyzed. Because only one satellite can be observed in a single eclipse, the breakdown is 10 for Io, 4 for Europa, 4 for Ganymede, and 2 for Callisto. (※ 2) 。
*2 … However, each of the three satellites is included except for Io which did not provide good data due to bad weather. This was excluded from the detailed analysis.
the analysis’ results ,It has successfully detected auroral light at multiple wavelengths of visible lightSome wavelengths were first discovered by the Galileo satellites, andCallisto observes Twilight for the first timehad become.
As for Io, the aurora borealis have been observed in previous sightings, but there is some data revealed for the first time this time around. For example, it has been suggested that potassium chloride erupts from Io’s volcanoes, but it has so far only been observed at locations far from Io. This time, the light emitted by the potassium atoms was detected as an aurora,Potassium was found in Io’s atmosphere for the first time. We were also able to accurately monitor the concentration of sodium atoms before and after the Jupiter eclipse. Previous observations also noted a rapid change in sodium concentration before and after the eclipse, and this was thought to be due to a sudden change in temperature due to sunlight being blocked by Jupiter’s shadow. In this observation we found that the sodium concentration dropped sharply 10 minutes after entering the shade, while it took 2 hours to recover to the original level after leaving the shade. This asymmetry is due to the fact that, due to sunlight being blocked, the temperature drops and sodium drops to the surface rapidly, while sunlight heats the surface of Io and sublimates the sodium (phase changes from solid to gas). This is strong evidence that there is a time difference until atomic oxygen (O) was also detected in the atmosphere, but no significant changes were observed before and after the eclipse by Jupiter. This result indicates that the source of atomic oxygen is different from sodium.
For all three satellites other than Io, we obtained results that the main component of the atmosphere is molecular oxygen (O2), which is consistent with analyzes of the atmosphere by other methods. Atomic oxygen and water molecules (H2O) were also discovered in Europa and Ganymede.The concentration of water in the atmosphere is high, whose origin is subject to further discussion. The three moons are also expected to have inner oceans rich in liquid water under their icy crusts. Water flowing from the underground oceans can be a source of water supply to the atmosphere. However, the amount of water inferred from our analysis is limited, and although it provides weak, but not conclusive, evidence of a subsurface ocean, our analysis shows a very large amount of water. It may have been appreciated. To claim that water is provided by a subterranean ocean, we need to show that the amount cannot be explained by sublimation from surface ice alone, which requires additional observations.
On that note, all of the Galileo satellites detected aurora borealis in the visible-light region, and we also learned that each has unique properties. Additional observations will provide more data about the Galileo satellite’s atmosphere, revealing in more detail the satellite’s internal activities that influence atmospheric composition.
- Image credit: NASA, Schmidt, et al.
- Karl Schmidt et al. “Io optical aurora in Jupiter’s shadow”. (Journal of Planetary Sciences)
- Catherine de Claire and others. “Optical Twilight of Europa, Ganymede, and Callisto”. (Journal of Planetary Sciences)
- Mauna Kea, Hawaii. “New Northern Lights Discovered on Jupiter’s Four Largest Moons”. (W.M. Keck Observatory)
Text: Rare Aya
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