From January 7, 1610 Galileo Galilei spotted three, then four, points of light near Jupiter in his telescope. Apparently they orbited this planet. This contradicted the traditional view that literally everything have to rotate around the earth. In the light of this discovery, Galileo publicly professed the Copernican world view for the first time.
In a bow to the Medici, he christened the moon quartet “Medicean Stars”. The princely family thanked him by appointing him court philosopher. The Frank Simon Marius saw the moons very shortly after Galileo and proposed the term “Brandenburg stars”. In 1614, however, Marius picked up an idea from Johannes Kepler: he chose four figures to name the moon, which Zeus (Roman: Jupiter) had once directed his desire to.
According to the myths, the god lured them Io into the forest and dishonored her. She was also involuntarily pregnant Callisto, who treacherously approached Zeus in the form of the moon and hunting goddess. The Phoenician king’s daughter Europa, the mythical namesake of our continent, appeared to Zeus in the form of a bull. When Europa unsuspectingly climbed onto his back, he swam with her to Crete. About the young shepherd Ganymede to abduct, the god slipped into the plumage of an eagle. From then on, Ganymede served as cupbearer in Olympus.
Sparkling slices
The orbital times of the four moons increase from the inside (1.8 days at Io) to the outside (16.7 days at Callisto). Staying in Linz, Johannes Kepler saw his third planetary law confirmed in her movement game. To determine the longitude at sea, you needed an instrument to measure the height of the stars – and a clock that told the corresponding time. Galileo suggested using Io as a “celestial clock,” since this nimble moon regularly disappears into Jupiter’s shadow. Olaus Römer remarked: The observed eclipses were delayed compared to the forecast when Jupiter and Io were further from the earth. The light then obviously needed more time to bring us the news of the event. In 1676, the Dane measured the speed of light for the first time.
In 1610, Galileo reported for the first time on the moons of Jupiter in “The Star Messenger” (here a reprint from 1718).
– © Pinter
The four Galilean moons would be bright enough to see with the naked eye. But the blazing Jupiter drowns them in its light. A pair of binoculars, mounted on a tripod without blurring, detaches them from their brilliance. As Galileo apparently noticed, the fine points of light sparkle less than fixed stars. In more powerful telescopes, they appear as tiny discs – and as such, they are less susceptible to the disruptive effects of Earth’s air turmoil.
However, it is as if we wanted to examine a 2 euro coin from a distance of 3 to 5 kilometers. At a distance of 600 million kilometers from Earth, the moons shrink terribly, despite their diameters of 3,122 kilometers (Europe) to 5,268 kilometers (Ganymede). Using large telescopes of the 19th and 20th centuries, sharp-sighted astronomers nevertheless made out diffuse shading on them, especially on Ganymede. Infrared spectra showed water ice signatures. Only Io stepped out of line: her warmer hue had been noticed before. Recently, the spectrometer at Keck Observatory, Hawaii, detected auroras on all four moons.
From 1973 NASA shot scouts past Jupiter for closer exploration. On Pioneer 10 and 11 followed Voyager 1 and 2: These frenzied reporters presented the four major moons in 1979 as highly individual worlds. In 1995, a spacecraft entered orbit around Jupiter for the first time: Galileo came close to the moons several times. The NASA probe has been exploring since 2016 Juno the gas planet. She radioed new images of Ganymede and Europa to Earth. If the Galilean moons orbited directly around the sun, they would pass as planets. This does not apply to the 91 small moons of Jupiter that were discovered later: Most measure only 1 to 5 kilometers.
Similar to our Earth moon, Jupiter’s major moons are locked in locked rotation. They always present the same hemisphere to their planet. The orbital periods of Io, Europa, and Ganymede are like the numbers 1, 2, and 4. Because of this resonance, the orbits deviate somewhat from circularity. While orbiting the 318 Earth mass “heavy” giant planet Jupiter, the lunar balls are “kneaded” by the tidal forces.
Two types of volcanism
The resulting heat transforms Io into the most volcanically active body in the solar system! Over a hundred volcanoes regurgitate lava onto their surface. Io’s landscapes are constantly being redesigned. Their age is only measured in millions of years. Sulfur deposits provide the yellowish complexion, interspersed with white patches of sulfur dioxide frost and adorned with reddish flecks. A global magma ocean may extend beneath the crust.
The faces of the moons further out, on the other hand, have solidified to ice. At temperatures of minus 170 degrees Celsius, Europe’s surface ice becomes as hard as rock. However, since it melts under pressure, the moon has no real mountains. Europe’s “mountain giants” are only a few hundred meters high. Their particularly shiny, smooth crust is less than 90 million years old. Volcanism once existed here too – but not with lava, but with melted ice!
Close-ups show fractures where fresher ice once gushed to the surface. The red-brown tint comes from sodium chloride. One sees jagged broken clods that have shifted and rotated. Apparently they were swimming on water or sliding on muddy ice.
Europe’s weak magnetic field is “borrowed”. It comes from electric currents induced by Jupiter’s powerful magnetic field – into a saline, conductive liquid. So Europa is very likely hiding a global ocean of water under the icy crust. Lakes could also be stuck in the ice at a depth of 4 to 8 kilometers, which could still drive water vapor fountains.
Ganymede with the dark Galileo Regio – amateur photo from a distance of 600 million kilometers.
– © Pinter
Jupiter is by far the most powerful planet, and its Ganymede is the largest moon in the entire solar system. Nevertheless, ice tectonics and ice volcanism have long been history here – Ganymede’s brown-grey ice crust is three and a half billion years old on average. Dark clods like Galileo Regio or Marius Regio differ sharply from somewhat younger and lighter areas. Unlike Europa, Ganymede generates its own magnetic field. Its iron core is still partially liquid. Charged particles from Jupiter’s magnetosphere destroy water ice molecules on the moon’s surface. The hydrogen escapes into space, the oxygen gives Ganymede the paper-thin caricature of an atmosphere. From the movement of the polar lights dancing in it, one also concludes that there is an ocean underground. Several layers of ice and water may even alternate, similar to onion skins.
The older a surface is, the more impact craters accumulate on it. Craters are thus the “age spots” of the solar system. In the case of the Galilean moons, their number increases from the inside outwards: the abundance of craters on the Callisto already reveals a landscape age of four billion years. The radiation-induced destruction of the surface ice also leaves behind dark debris over time. As a result, Kallisto’s brownish-grey ice reflects little sunlight.
Standing far from Jupiter, it does not participate in the warming 1:2:4 resonance game of the other moons. At a young age, these were melted during the decay of radioactive isotopes. Iron cores arose in their centers, encased in stone mantles. Not so with Kallisto: it consists entirely of a mixture of siliceous rock and water ice. However, 100 to 300 kilometers below the surface, the pressure and temperature should be sufficient to turn Kallisto’s ice into liquid water.
Do the lightless seas of such icy moons have to be hostile to life and sterile? On Earth, simple creatures can withstand the most unusual levels of temperature, pressure, or pH. Some defy desert-like drought, high concentrations of toxins or brutal doses of radiation. With the increasing complexity of living beings, however, their tolerance for extremes dwindles.
underground life?
Hydrothermal vents were found at the dark bottom of the earth’s deep sea. There, heated water enriched with sulphides and other salts seeps out of the ground. Minerals that are entrained form chimneys dozens of meters high. Certain bacteria, archaea, mussels, worms or crabs thrive in the environment of these “black smokers”. Extremophilic creatures may also dwell beneath icy moons. While the icy crusts block sunlight, they protect hidden oceans and their potential inhabitants from the vacuum, unrelenting cold, and harsh radiation of space.
Sample of a “black smoker” from the floor of the earth’s deep sea.
– © Pinter
Further insights are expected from Jupiter Icy Moons Explorer (JUICE). Die ESA-Sonde, whose thermal protection comes from Berndorf in Lower Austria, is scheduled to start on April 13, 2023 – from Kourou in French Guiana. To gain momentum, it will whiz past Earth three times and Venus once. Reached Jupiter JUICE in 2031. Then there are 35 flybys of the three icy moons Europa, Ganymede and Callisto.
For example, the ten on-board instruments research the exact composition of the surface ice. They measure the deformation of the Ganymede caused by the tidal forces or look for water vapor fountains on Europa. The radar can look about 9 kilometers deep into the crust to look for transitions from ice to liquid water. In 2034, the European scout even swung into an orbit around Ganymede. Twelve months later, it smashes on its crust.