SOLAR SYSTEM FAMILY-FACTS ABOUT THE FOUR GALILEAN MOONS

 Galileo Galilei's discovery of the four Jovian moons in 1610 provided the greatest support in favour of the Copernican Heliocentric Model of our universe. Galileo's discovery was of particular importance because it showed that four celestial objects were in an orbit around a planet apart from Earth; which was completely unacceptable in those days. As of now, the mighty Jove has got some 79 known moons and counting, out of which the Galilean moons i.e., Io, Europa, Ganymede, and Callisto deserve special mention.

Formation: Based upon observational data and computer simulations, scientists estimate that the Jovian moons arose from the coalition of ice and dirt which were present within the disk of nebular material surrounding the young planet. When a typical planet is born from the accretion of material, some smaller bodies, which later becomes the former planet's satellites are also formed. In this way, when the young moons slowly gathered more material along with their parent planet, gravitational interaction locked them into their present positions. The initial debris disk also thinned out, thereby giving rise to the current orbital configurations. 

Ganymede(Jupiter III): Named after a boy who was made cupbearer for the Olympian Gods, Ganymede, with a mean diameter of 5268 km is the largest and the most massive among all the Solar System's known moons. It is the third Galilean moon and also the seventh natural satellite of Jupiter. Being situated at an average distance of 1.07million km, it completes an orbit in seven days and three hours. Ganymede is composed of equal parts rocky and water ice materials and observations from the Galileo spacecraft suggests an interior being fully differentiated into an iron-sulfide core, a silicate mantle, followed by outer layers of subsurface saltwater oceans, and finally a rocky or an icy crust. The Ganymedian surface is made up of two different types of terrain viz., very old, highly cratered, dark regions and somewhat younger and lighter regions filled with grooves and arrays. Observations made with the Hubble Space Telescope suggest that the moon has got a very tenuous oxygen atmosphere and generates a fairly strong magnetic field. It's tiny magnetosphere is further embedded within Jupiter's extensive magnetosphere. Magnetic interaction with the solar wind plasma generates a region of charged particles around Ganymede, followed by regions of intense radiation and spectacular aurorae.

Ganymede/Image Credits: NASA/JPL (edited by PlanetUser), Public domain, via Wikimedia Commons

Europa(Jupiter II): With a mean diameter of 3120 km, Europa is the smallest of the four Galilean moons. Named after a lover of the Greek god, Zeus, the former is situated at an average distance of 670,900 km from Jupiter's centre and has an orbital period of 3.5 days. Europa is quite similar to our moon or the terrestrial planets and is believed to be composed of silicate rocks, having an interior further differentiated into a molten iron core, a sub-surface liquid water ocean and a frozen crust. It has got the smoothest and a relatively young(based upon geologic time frames) surface which is adorned with numerous crisscrossing lines called ''lineae''. These ridges and groves, that can typically run for thousands of kilometers are a result of tidal gravitational forces. Their reddish colour is probably due to the presence of different minerals which rise above from the sub-surface ocean. Europa has an extremely thin atmosphere which is mostly made up of oxygen and traces of water vapor. 

Europa/Image Credits: NASA / Jet Propulsion Lab-Caltech / SETI Institute, Public domain, via Wikimedia Commons

Io(Jupiter I): It is the innermost and the third largest among the Galilean Moons, with a mean diameter of 3642 km. Named after the priestess of Hera, another of Jupiter's lover, Io is situated at an average distance of 421,700 km from the planet's centre and takes about 4.5 hours to complete a single orbit. It has a differentiated interior made up of a silicate rich crust and mantle, followed by an iron-sulfide rich core. Io is the most volcanically active world in the solar system. This active volcanism is due to the intense gravitational forces generated from orbital resonances with the other moons and of course the planet itself. Apart from volcanic and tectonic features, the Ionian surface is rather smooth without any craters, as they are wiped off due to regular volcanic eruptions and continuous flow of lava. This active volcanism and subsequently the deposit of sulfuric materials have given Io a rather colourful surface, analogous to a rotten orange or a pizza. Since Io lies within the intense radiation belt of Jupiter, strong electromagnetic interaction gives rise to a stream of charged and high energy particles which exerts a further influence on the Jovian magnetic field and the magnetosphere. Io has an extremely thin atmosphere which is primarily made up of sulfur dioxide, with traces of sulfur monoxide, sodium chloride, atomic sulfur and oxygen. But the atmosphere is being constantly stripped away due to intense magnetic activity.

Io/Image Credits: NASA/JPL/University of Arizona

Callisto(Jupiter IV): Named after one of Zeus' lover, Callisto, with a mean diameter of 4821 km is the second largest of the Galilean moons and is the third largest moon of the solar system. It is situated at an average distance of 1.88 million km from Jupiter and completes its orbit once in every 16.7 Earth days. It has a differentiated interior, with a crust composed of equal parts rock and water ice, a subsurface salty ocean, an icy lithosphere and the overall chemical composition is similar to carbonaceous asteroids. The surface is heavily cratered and the crater density is so high that formation of a new one usually results in wiping out another. The notable craters are the Valhalla, a ring like structure extending 1800 km radially and Asgard, another similar structure measuring about 1600 km in diameter. Apart from impact craters, the surface is also featured with fractures, scarps, deposits, and patches of pure water ice. Callisto has a very tenuous atmosphere which is probably made up of carbon dioxide, molecular oxygen and atomic hydrogen. 

Callisto/Image Credits: NASA/JPL/DLR(German Aerospace Center), Public domain, via Wikimedia Commons

Orbital Resonances: Io, Europa and Ganymede orbit Jupiter in such a way that they are in mean motion orbital resonance with each other. Orbital resonance occurs when orbiting bodies exert regular, periodic effects of mutual gravitational forces due to their orbital periods being related to each other by a simple ratio. Io is in a 2:1 resonance with Europa and 4:1 resonance with Ganymede, thereby completing two orbits of Jupiter for every one orbit completed by Europa and four orbits for every one orbit completed by Ganymede. 

Life and Biological Considerations: Except Io, all the Galilean moons are on the list of places where extraterrestrial life might exist. Based upon our current understanding, it can be speculated that perhaps Europa, Ganymede, and Callisto might have the conditions necessary for the proliferation of microbial life, assuming that liquid water, a few minerals and a little bit of energy derived from geothermal activity is all that is needed to have some microbes, alive and kicking. Thus, among all the Jovian moons, Europa has been nominated as the only celestial object with the highest possibility of harbouring life forms other than Earth.

Observations and Explorations: 

• Observations: On observing only three bodies near the plane of Jupiter, Galileo at first believed them to be fixed stars. But on the subsequent nights, he found that the so-called stars have changed their positions. Again, after the discovery of a fourth body near the plane of Jupiter, Galileo concluded them to be celestial bodies in an orbit around Jupiter. He decided to name them as the ''Medicean Planets'', in honour of the famous Medici brothers of the Medici family. Although everybody credited Galileo for the new discovery, there was another astronomer, Simon Marius who had independently observed the moons at the same time. Simon Marius, under the suggestion of Johannes Kepler, wanted to use the names Io, Europa, Ganymede and Callisto. But these names weren't accepted much later until further discovery of additional moons of Jupiter.

A montage of Jupiter's four Galilean Moons/Image Credits: NASA/JPL/DLR, Public domain, via Wikimedia Commons

• Explorations:

1. Pioneer Mission: The first set of spacecraft to explore the Jovian moons and their parent planet were the Pioneer 10 and 11 probes launched during 1973-1974. The Pioneer probes could not provide much data in addition to what was already known with the help of ground based telescopes. Nevertheless, some information was obtained about their masses and corresponding physical characteristics, in addition to a few low resolution images. 
2. Voyager Mission: The Pioneer space probes were succeeded by the Voyager 1 and 2, launched during 1975-1977. The Voyager probes were equipped with more advanced technology than the Pioneer probes and as a result, the former gathered much detailed and high resolution imagery. The images revealed a total of nine active volcanoes scattered across the surface of Io, numerous reddish streaks across the icy surface of Europa, and collected immense data on Ganymede and Callisto, like their size, structure, surface features, and etc.
3. Galileo Mission: The most detailed observations were however made by the Galileo spacecraft, launched in 1989. From numerous flybys, Galileo inferred that an underground ocean exists below the icy surface of Ganymede, Europa and Callisto, in addition to a load of data about their respective ionosphere, magnetosphere, internal composition, atmosphere, etc. Although much of the data on Io could not be gathered because of intense radiation, significant observations were made of the most volcanically active body in our solar system. 
4. Cassini Mission and New Horizons Mission: Cassini and the New Horizons probe were launched in 1997 and 2006 respectively. Both the probes interacted with the Jovian system and gathered some useful data en route to their respective destinations-Saturn and Pluto. 
5. Juno Mission: Juno was launched in 2011, and was entrusted with the task of gathering further data about Jupiter's interior, its magnetic field, aurorae and the polar regions. At present it has been placed on an extended mission so that it can perform some close flybys around the Galilean moons.  
6. Other future missions: Some future missions include the Jupiter Icy Moon Explorer by the ESA, Europa Clipper, Europa Lander, Io Volcano Observer

Other notable moons of Jupiter: Since the region around Jupiter looks like a mini solar system, it was necessary to categorize the moons into distinct groups. The group of regular satellites include those which are extremely close to their parent planet or the inner group of satellites viz., Metis, Adrastea, Amalthea and Thebe; and the main group or the Galilean moons which include Io, Europa, Ganymede and Callisto. The rest fall under the category of irregular satellites which have high orbital eccentricity, retrograde motion and are probably captured objects. 

References:

1. https://en.wikipedia.org/wiki/Galilean_moons
   
2. https://en.wikipedia.org/wiki/Io_(moon)
   
3. https://en.wikipedia.org/wiki/Europa_(moon)
   
4. https://en.wikipedia.org/wiki/Ganymede_(moon)
   
5. https://en.wikipedia.org/wiki/Callisto_(moon)
   
6. https://solarsystem.nasa.gov/moons/jupiter-moons/overview/