Discovery and exploration of Jupiter’s 4 main moons

On January 7, 1610, Italian astronomer Galileo Galilei observed the planet Jupiter through his newly improved homemade 20x telescope. He observed three bright spots near the planet, and at first believed they were distant stars. Observing them for several nights, he noticed that they seemed to move in the wrong direction with respect to the background stars and that they remained in the vicinity of Jupiter, although they changed their position with respect to each other. Four days later, he observed a fourth bright spot near the planet with the same unusual behavior. On January 15, Galileo correctly concluded that he had discovered four moons orbiting Jupiter. This provided robust evidence in favor of Copernicus’s theory which correctly indicated that the vast majority of celestial objects do not revolve around the Earth.

The names that Galileo proposed for these four moons were derived from the Medicis surname of his sponsor, although in his own notes Galileo referred to these stars by the Roman numerals I, II, III and IV, in order of their distance from Jupiter.

The names that these satellites have today (Io, Ganymede, Europa and Callisto) were those proposed in 1614 by the German astronomer Johannes Kepler.

Collectively, today these four stars are called the “Galilean satellites” in honor of their discoverer.

No more satellites were discovered around Jupiter until 1892, when American astronomer EE Barnard found Jupiter’s fifth moon, Amalthea, much smaller than the Galilean moons and orbiting closer to the planet than Io. It was the last satellite in the solar system found by direct visual observation; all subsequent discoveries occurred through traditional photographs or through digital images. Today, there are 95 moons identified in orbit around Jupiter.

Mosaic of photographs of Jupiter’s four Galilean satellites, shown to scale. Clockwise and starting from the top left, they are: Io, Europa, Callisto and Ganymede. (Photos: NASA)

Although each of the Galilean satellites has unique features, such as the volcanoes of Io, the heavily cratered surface of Callisto, and the magnetic field of Ganymede, the astronomical community has focused more attention on Europa because of the fascinating possibility that it could host life. .

In the 1970s, NASA’s Pioneer 10 and 11 space probes, as well as Voyager 1 and 2, captured increasingly detailed images of Galilean satellites, including Europa, during their brief visits. to the Jupiter system. The photographs revealed that Europa’s surface is the smoothest of all the known worlds in the solar system, indicating that it has a relatively young crust, and also one of the brightest of all the satellites, therefore be highly reflective.

All these characteristics led to the hypothesis that Europa is covered by an icy crust floating on a salty ocean of liquid water. Additionally, tidal heating caused by Jupiter’s gravity was postulated to reshape the surface ice sheet in cycles of melting and freezing.

More detailed observations by NASA’s Galileo space probe, which orbited Jupiter from 1995 to 2003 and completed 11 flybys of Europa’s surface, revealed that certain elongated structures on its surface may indicate tectonic activity. The reddish-brown material along fissures and in spots elsewhere on the surface may contain salts and sulfur compounds transported from beneath the crust and modified by radiation. Observations from the Hubble Space Telescope and a new analysis of images captured by the Galileo spacecraft have revealed possible “geysers” expelling material from beneath the crust, lending credence to the above hypothesis. Although the exact composition of this material is unknown, it is likely to offer clues as to whether Europa supports life.

Future robotic explorers from Europe could answer some of the lingering questions about this unique moon of Jupiter. NASA’s Europa Clipper departed in October 2024 on a 5.5-year journey to Jupiter. Upon arrival in 2030, the spacecraft will enter orbit around the giant planet and perform 49 flybys of Europa during its four-year mission. Europa Clipper has nine instruments, including image capture systems and a radar, with which it is expected to better understand the structure of the icy crust. The data collected by the Europa Clipper will complement that collected by the JUICE (Jupiter Icy Moon Explorer) space probe of the European Space Agency (ESA). Launched in April 2023, JUICE will orbit Jupiter in 2031 and then orbit Ganymede in 2034. The ship will also carry out studies of Europe complementary to those carried out by the Europa Clipper. The two space probes should greatly increase our knowledge of Europa and perhaps provide new clues to the possibility of life in that moon’s subterranean ocean. (Fountain: NCYT by Amazings)