Getting To Know the Moons of Jupiter

Jupiter is one of the most interesting discoveries made by humans. First, discovered by Galileo and Simon in 1610, Jupiter in total consists of 79 known moons. The biggest moons are the four Galilean moons. These moons were also the first objects that did not orbit either Earth or the Sun. 

Over the years, astronomers and scientists have been able to discover additional moons but the ones independently found by Galileo are the biggest. So let’s dive a bit deeper and discover what makes Jupiter and its moons so special. 

The Jupiter Moons


When it comes to studying the moons of Jupiter, the first thing astronomers notice that each moon differs from the other. The four Galileans tend to be 3,100 kilometers in diameter and the Ganymede is not only the largest but also the ninth largest object in the Solar System. It is smaller than the Sun but bigger than Mercury.

Other Jovian moons are not more than 250 kilometers in diameter. Additionally, the orbital shapes vary. Some moons have perfectly round orbital shapes while others have inclined and eccentric shapes. Furthermore, many of the moons revolve opposite to the rotation of Jupiter.

Origin and Evolution


It is believed that the regular satellites of Jupiter might have formed from a circumplanetary disk. The disk is basically a ring that comprises gas and solid debris, analogous to a protoplanetary disk. In addition to that, stimulations have led the astronomers to conclude that the disk might have a relatively high mass at any given moment. 

Over time, the mass captured by Jupiter from the Solar Nebula might have had a substantial fraction of it passed through Jupiter. Fortunately, scientists and astronomers require only 2% of Jupiter’s proto-disk mass to understand and explain the existing satellites. Since the disk features drag, it is estimated that several generations of Galilean-mass satellites might have spiraled into Jupiter. 

Meanwhile, the formation of new moons might have also occurred from the new debris captured from the Solar Nebula. By the time the fifth generation formed, the disk had become thinned to the point where it did not interfere with the orbits of other moons. 

However, bigger moons such as the Galilean moons were still affected but were partially protected due to the existence of orbital resonance between each other. This resistance still exists between Europa, Ganymede, and Lo. On the other hand, it is said that the captured asteroids might have given birth to the outer irregular moons. 



To understand the existence of Jupiter and its moons, it is important that history is re-visited. With that being said, Xi Zezong a Chinese Historian claims that the first record of a Jovian moon was actually a note regarding a “reddish star” by a Chinese Astronomer Gan De around 364 BC. However, it was Gallileo in 1609, who came out with the first certain observations of the satellites of Jupiter. 

A year later in 1610, Galileo used his 20x powerful telescope and sighted the four massive Galilean moons. As a result, he published his findings in March 1610. Additionally, Simon Marius had also sighted the moons one day after Galileo’s findings but did not make a final report until 1614. But the names of the moons we hear today were given by Marius including lo, Ganymede, Callisto, and Europa. 

After these discoveries, no major progress was made until Amalthea was observed by E.E. Barnard in 1892. Following the discovery, several discoveries were made during the 20th century using telescopic photography. For instance, Himalia was discovered in 1904 while Elara in 1905 along with Sinope in 1914 and Pasiphae in 1908, etc. 

Then, it was the Voyager spacecraft that discovered three additional inner moons in 1979, Adrastea, Thebe, and Metis. As a result, by the time the Voyager space probes had reached Jupiter in 1979, a total of 13 moons had already been discovered. 

For the next two decades, no significant discoveries were made until October 1999 and February 2003, when researchers by using sensitive ground-based detectors to find additional 34 moons. These are tiny moons mostly featuring an average diameter of 3 km while 9 km being the largest.

By 2015, 15 more moons were discovered. Then, in 2016 two more were discovered by Scott S. Sheppard and his team in 2016. This brought the total to 69 moons. On 17 July 2018, it was confirmed by the International Astronomical Union that Sheppard’s team had discovered ten more Jupiter moons increasing the tally to 79. 

The Galilean Moons



Amongst the Galilean Moons, the IO happens to be the most volcanic solar system active body. It features a surface that is a combination of different sulfur color forms. Since the IO travels in an elliptical orbit, the immense gravity of Jupiter gives birth to immense tides on the solid surface that rise as high as 300 feet, generating enough heat to remove water and any volcanic activity. 



The surface of Europa is mostly water ice. Over the years, scientists have collected enough evidence to suggest that it might be covering slushy ice or an ocean of water underneath. The moon interests many astronomers and scientists due to its potentially habitable zone. Since life forms have been discovered near subterranean volcanoes on Earth along with other extreme locations, it might be that Europa supports some types of life forms as well. 



Ganymede tends to be the biggest moon in the solar system and is the only moon until now known to have its own internally generated magnetic field. 



The Callisto Moon’s surface is heavily ancient and cratered. This might suggest the earlier historic events taking place in the solar system. Today, the surface of the Callisto features very few small craters indicating a current surface activity of small intensity. 

Final Word

Considering the ongoing discoveries made by astronomers and scientists, Jupiter remains one of the most exciting outer space phenomena. Scientists have been trying to discover and determine the history of Jupiter, which is crucial to understanding its current state. With that being said, NASA is planning to explore Jupiter’s moons using its Europa Clipper. It will help study Europa in detail, leading to additional discoveries in the future.