Scientists have discovered in data collected by the Galileo interplanetary station 20 years ago, new evidence of the existence of water geysers on Jupiter's icy moon Europa. The article was published in the journal Nature, briefly described in a press release on the NASA website.
Europa is the smallest of Jupiter's four largest moons, called the Galilean moons. Its radius is 200 kilometers less than the radius of the Moon, and its orbit is about 670 thousand kilometers from the gas giant. Particular interest in this celestial body arose in the late 1990s, when the existence of an under-ice ocean was predicted based on the analysis of the magnetic field of the satellite carried out by Galileo. It is assumed that the ocean is under 25 kilometers of ice, and its depth reaches hundreds of kilometers in depth. The ocean on Europa does not freeze due to the heating of the interior of the satellite, which arises under the influence of the tidal forces of Jupiter. In addition, scientists suggest the existence of geothermal activity at the bottom of the ocean, which may be a sufficient condition for the emergence of life.
One of the proofs of the presence of an ocean on Europa is geysers emitting water vapor, which was recorded by the Hubble telescope in 2012, 2014 and 2016. The ejections were detected over the equatorial region and the south pole of the satellite, extended up to 200 kilometers in height and had an unstable character - the greatest activity was observed when the satellite was as far away from Jupiter as possible. However, these discoveries were made at the limit of the telescope's capabilities and astronomers would like to get evidence of the existence of geysers in situ.
Astronomers led by Xianzhe Jia claim to have found such evidence by analyzing data collected by the magnetometer and the PWS (Plasma Wave Spectrometer) instrument on the Galileo spacecraft during two close flyby over equatorial and southern Europe in 1997 and 2000 years. During these flights, the minimum distance from the spacecraft to the icy surface of the satellite was almost 200 kilometers, which is currently a record low.
Galileo magnetometer data obtained during a close flyby in December 1997 over Europe and their comparison with models.
Data from the PWS Galileo instrument obtained during a close flyby in December 1997 over Europe, and the calculated dynamics of changes in plasma density.
During a flight over the equatorial region of Europe in December 1997, about one minute before passing the closest point to the satellite's surface, the magnetometer recorded a change in the magnetic field strength by hundreds of nanotastes in 16 seconds. At this time, the PWS instrument recorded local changes in the electric field and electron density of the plasma surrounding the apparatus. To test whether such phenomena can be associated with the passage of the device through the water plume (or plume) of the geyser, astronomers built a three-dimensional magnetohydrodynamic model that describes the effect of the ejection on the properties of plasma and fields in the vicinity of the satellite. Simulation tracks the behavior of ions O + (representative of magnetospheric plasma), O2 + (representative of ions originating from Europa's surface) and electron plasma,and takes into account the processes of ionization, recharge and recombination in the atmosphere of Europe, as well as the parameters of water emissions obtained during observations by different telescopes. It was assumed that the shape and structure of emissions on Europa is similar to the emissions of geysers on Saturn's moon Enceladus.
Estimated location of a water release from Europa's surface recorded during the Galileo close flyby in December 1997.
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As a result, it turned out that the models describe well the observational data and allow us to limit the area of the plume source, through which the Galileo flew in December 1997. In the case of a close flyby in 2000, the observational data on changes in the magnetic field cannot be interpreted as the effect of a geyser ejection. These findings highlight the value of collecting data at short distances from or on the satellite surface. This challenge faces a new generation of missions to the Jupiter system - the Europa Clipper, Jupiter Icy Moon Explorer and the Joint Europa Mission, which are expected to launch in the early 2020s.
Alexander Voytyuk