The Jovian System

Jupiter is the largest planet in our Solar System with a diameter of 43,441 miles, almost 11 times larger than Earth. The Jovian system also has more moons than any other world with an estimated 79 satellites. A large number of these moons appear to be captured asteroids as Jupiter not only orbits close to the asteroid belt it also shares its orbit with Trojan minor planets. These small bodies share Jupiter’s orbit at its Lagrangian points. Lagrangian points are points along Jupiter’s orbit measured at exactly 60º in front of and behind Jupiter. Of Jupiter’s moons, the four Galilean moons are some of the most well-known moons of our Solar System, since they are easily identifiable with a just a telescope. Galileo (whom they were named after of) discovered them in 1609.

Being a gaseous planet, Jupiter also has the most massive planetary atmosphere of our Solar System with 3,000 miles in altitude. Jupiter is believed not to have a surface. This massive atmosphere consists of hydrogen and helium with traces of methane, ammonia, hydrogen sulfide and water. One major question has if Jupiter has no surface, then how it is held together? This question goes for all gaseous planets. Let me explain.

A rocky or solid core is not required for gravity to hold a gaseous planet together. Our Sun doesn’t have a rocky core. The hydrogen of Jupiter is not necessarily the same “gas” we know it here on Earth. On Jupiter, is in a state of liquid metal.

Scientists believe Jupiter’s dense core consists of a mixture of elements, a layer of liquid metallic hydrogen, helium, and an outer layer composed of molecular hydrogen. You can think of Jupiter’s center as a highly pressurized gas compressor with pressure is 10,000 times greater than the bottom of the ocean and 10 billion times greater than water at the Earth’s surface.

History of Jupiter

Jupiter was being observed as far back as the 7th century BC by Babylonian astronomers. Along with these early astronomers, the Chinese were also among the first to observe the planet. These observations led to the Chinese Zodiac. In 1610, the four largest moons that orbit Jupiter were discovered by Galileo. These four moons are now more commonly referred to as the Galilean moons. A day later, another astronomer discovered these same moons but did not publish his findings for four more years. However, he did name these moons. They are called Io, Europa, Ganymede, and Callisto. A fifth moon Amalthea was discovered in 1892 by E.E. Barnard at Lick’s Observatory in California.

The Great Red Spot was first discovered in 1664. There were several different times that astronomers were unable to see the Red Spot until it became incredibly noticeable in 1878. Since that time, it has been seen to have faded and reappeared several times.

Radio signals were discovered coming from Jupiter in 1955. These radio wave were used to understand the rotation of Jupiter. Three different types of radio signals were observed from Jupiter. Thermal radiation, which is created in the atmosphere of the planet. The second was decametric radio waves, which were created by disturbances between the moon Io and the magnetic pull of the planet. The third are decimetric radio waves which come from the equator of Jupiter and are caused by radiation.

The Pioneer 10 space probe was the first to send back important data about Jupiter. The first automated mission to the planet began in 1973. Pioneer missions were the first to also capture information regarding the different planets. Next came the Voyager missions. They observed the Great Red Spot, the rings of the planet, and more about the moons. Pioneer 10 also discovered volcanoes on Io.

The Galileo probe was the first to orbit this planet back in 1995. It remained in orbit for more than seven years. In 2003, the probe was dropped into the atmosphere of Jupiter so that it could melt and possibly vaporize. This was done due to the concern that it may instead crash into Europa, which is thought to possibly be able to support life.

The Juno mission made it to Jupiter in 2016. It is expected to conduct nearly 40 orbits in just under two years. There are two future missions also being planned. One is called JUICE, which stands for Jupiter Icy Moon Explorer, which will be conducted by the European Space Agency. The next NASA mission will occur in 2025 and will be expected to observe Europa.


Jupiter is the fourth brightest object in the night sky coming in at a magnitude of -2.1. Only the Sun, Moon and Venus are brighter than Jupiter. The planet reaches a position for optimum viewing via telescope every thirteen months, and it makes its latest closest approach to Earth on June 12, 2019. Jupiter’s opposition happens on June 10, at 15 UTC. Opposition places Jupiter opposite the sun in our sky.

The Earth catches up to and passes, Jupiter every 398 days. As this occurs, it almost seems as though the planet moves backward across the sky in comparison to everything else. This “movement” in the sky is called “retrograde motion.” There is no need for powerful equipment to view the Earth. With a small telescope, you can see the moons and the various cloud belts that are on the surface.


It is also the largest planet in our solar system. The upper atmosphere is comprised mostly of hydrogen, along with helium. Frozen ammonia makes up the outer layer of the atmosphere. Both Jupiter and Saturn are thought to be very similar in their makeup.

This planet is so large that it is over two times larger than all of the other planets if they were combined. The radius of Jupiter is nearly 1/10 of the Sun, so it is a rather large planet. This planet gives off more heat than it takes in from our Sun.

Jupiter is thought to have a rocky core. Outside of this core should be a layer of metallic hydrogen, which is then wrapped in another layer of molecular hydrogen. Beyond this, it is very unclear as to what else may make up the planet in those deeper layers. Due to the nature of the elements in the atmosphere of Jupiter, it is often thought that as the elements precipitate, there could actually be rainstorms of diamonds falling. This planet also has over 3,000 miles in altitude associated with its atmosphere.

The most famous feature on this planet is the Great Red Spot. This is actually a storm that continuously rages across the face of the planet. It is actually larger than the Earth. It is actually thought that this storm is so stable that it is a permanent fixture on the planet. Since its discovery, the storm has become smaller and smaller. Jupiter’s magnetic field is the strongest of any planet in the solar system as well.

Explorations and Missions

The Pioneer program took place in 1973 and 1974. Pioneer 10 was the first craft to fly to the planet. The Pioneer 11 came exactly one year later. The first craft showed up-close pictures of the planet, as well as the moons. Pioneer 11 showed great images of the Great Red Spot. It also figured out the mass of the moon Callisto.

The Voyager program took place in 1979. Voyager 1 visited Jupiter in 1979 and immediately began taking pictures. Voyager two came and discovered the rings of the planet. It also found plate tectonics on Ganymede, volcanoes on Io, and many different craters on Callisto. These missions gave us a great understanding of those different moons. Two more, smaller moons were also discovered by these spacecraft. They are called Metis and Andrastea. Another moon called Thebe had also been discovered. It was the first time that an active volcano had been found somewhere other than Earth.

The Cassini probe was heading to Saturn when it visited the planet back in 2000. It took roughly 26,000 images of the planet as it was passing by. It found another major spot up near the north pole of Jupiter. It also showed pictures of the moon called Himalia.

The New Horizons probe was heading to Pluto when it came by Jupiter in 2007. It studied one of the outer moons called Elara. It studied a much smaller storm called the Little Red Spot that has been growing over time. This probe actually sent back more information about the Jovian System than Pluto, which it was sent out to examine.

NASA launched Juno in 2011. The probe arrived in July 5, 2016 after traveling roughly 2.8 billion kilometers (18.7 astronomical units). Juno’s primary mission is to study the gravity field, atmosphere, magnetic field, polar magnetosphere and possible composition of Jupiter. In the process, we have received some of the most beautiful photos of Jupiter as Juno orbits the planet. Juno is powered by solar arrays with the largest solar array wings ever deployed on a planetary probe. Usually solar panels are used for Earth orbiting satellites and inner Solar System probes as opposed to radioisotope thermoelectric generators used for deep Solar System missions.

Moons of Jupiter

The Galilean Moons are the main group of moons that are most often spoken about around Jupiter. They are also the largest planetary bodies in the solar system, with only the Sun and the eight planets being larger. The names of these four moons are Io, Ganymede, Europa, and Callisto. They were discovered by Galileo. All of these moons were names given that came from Greek as different names of lovers that belonged to Zeus. Io is the fourth-largest moon that is located within the solar system and has over 400 active volcanoes. It even has mountains that are larger than Mount Everest.

Europa is one of the most fascinating places in our Solar System. It’s slightly smaller than the Moon, and has the smoothest surface of any known solid object in the Solar System. Europa is thought to have liquid water located underneath the frozen exterior making it the best place to look for life outside of Earth in our Solar System.

It has a very thin atmosphere composed primarily of oxygen. Its surface is striated by cracks and streaks, whereas craters are relatively rare. In addition to Earth-bound telescope observations, Europa has been examined by a succession of space probe flybys, the first occurring in the early 1970s.

Ganymede is the largest moon in our Solar System and the only moon known to have a substantial magnetosphere. The magnetic field causes auroras in regions circling the moon’s north and south poles much like the auroras here on Earth. Like Europa and a few moons, Ganymede is thought to have an atmosphere and subterranean ocean, overlying a liquid iron and nickel core that is believed to generate Ganymede’s magnetic field.

Images from Voyager indicate the moon has had a complex geological history. Ganymede’s surface is a mixture of two types of terrain. 40% of Ganymede’s surface is highly cratered dark regions, and the other 60% is covered by a light grooved terrain which forms intricate patterns across Ganymede. These patterns or grooves in its surface are described as “sulcus,” which means meaning a groove or burrow.

Astronomical Symbol of Jupiter's moon Ganymede

Callisto is the second-largest moon of Jupiter and the third-largest moon in the Solar System. Callisto orbits about 1,170,000 miles from Jupiter taking about 17 Earth days to complete one orbit of Jupiter. Callisto’s surface is covered with impact craters such much so it is the most heavily cratered in our Solar System. Along with craters, Callisto’s surface is covered with small, sparkly, frost-like deposits at the tips of high spots, surrounded by low-lying, smooth blanket of dark material. Interestingly Callisto does not show any signs of subterranean processes such as plate tectonics or volcanism. This may be because Callisto is the only Galilean moon that is not in orbital resonance. Thereby, it’s not subjected to the same tidal heating as the other Galilean moons. Callisto also orbits slightly outside of Jupiter’s violent magnetosphere further saving it from structural stress due to external forces.

Astronomical Symbol of Jupiter's moon Callisto

Io is the innermost of the Galilean moons as well as the smallest. Io has some very unique features. It has the highest density of Galilean moons along with the least amount of water of any known object in our Solar System. But the most interesting feature of Io is that it has over four hundred active volcanoes making it the most geologically active object in the Solar System.

This extreme geologic activity is the result of tidal flexing from friction generated within Io’s interior as it is tugged on between Jupiter and the other Galilean satellites. Most of the volcanoes produce plumes of sulfur and sulfur dioxide that climb as high as 300 miles above the surface. Its surface is comprised of extensive plains coated with sulfur and sulfur-dioxide and dotted with hundreds of mountains that have been uplifted by extensive compression at the base of Io’s silicate crust.

Astronomical Symbol of Jupiter's moon Io
Io along with other Galilean moons are bright enough to be viewed from Earth with binoculars, but can easily be made out individually with a telescope. The moons have an apparent magnitude that ranges in between 4.6 and 5.6 when Jupiter is at opposition, and are about one unit of magnitude dimmer when Jupiter is in conjunction. You may have a slight difficulty in observing the moons since the moons are so close to Jupiter, and you are obscured by its brightness. The moons are separated by 2 to 10 arcminutes from Jupiter which is close to the limit of human visual acuity. Ganymede and Callisto, at their maximum separation, are easiest to point out.

Irregular Moons of Jupiter

The vast majority of Jupiter’s moons are irregular. “Irregular” means the moon follows a distant, inclined, and often eccentric and retrograde orbit. Of the irregular moons, Jupiter has five distinct family of satellites with two moons who do there own thing, Carpo and Themisto. The five groups are Himalia, Carme, Ananke, Pasiphae and Amalthea.

The Himalia group contains the fifth largest moon orbiting Jupiter, named Himalia. The other members the family include Leda, Elara, Dia, Lysithea, Jupiter LXXI and Jupiter LXV. The International Astronomical Union (IAU) reserves names for moons of Jupiter ending in -a for the moons in this group to indicate retrograde motions relative to Jupiter.

It is believed that the group is a remnant of the break-up of an asteroid from our Solar’s System’s asteroid belt. The radius of the parent asteroid is theorized to be around 89 km, slightly larger than that of Himalia, which retains approximately 87% of the mass of the original body.

The Carme group consists of eleven moons, Carme, Taygete, Eukelade, Jupiter LVII, Chaldene, Isonoe, Kalyke, Erinome, Aitne, Kale, Pasithee, and S/2003 J9. These moons orbit Jupiter relatively close to each other, separated by less than 700,000 km in semi major axis and less than 0.7° in inclination. This orbital dispersion suggests that the entire Carme group may once have been a single body that was broken apart by a major impact.
Astronomical Symbol of Jupiter's moon Metis
Astronomical Symbol of Jupiter's moon Adrastea
Astronomical Symbol of Jupiter's moon Amalthea
Astronomical Symbol of Jupiter's moon Thebe
The Ananke group consists of Ananke, Praxidike, Iocaste, Harpalyke, Thyone, and Euanthe. As with the other Jovian satellite families, this family is believed to be the remnants of an asteroid collision with Ananke accounting for more than 90% of the mass.

The Pasiphae group consists of Pasiphae, Sinope, Callirrhoe, Megaclite, Autonoe, Eurydome and Sponde. You notice that the moons of this group end with an “e”. That’s because the International Astronomical Union (IAU) reserves names ending in -e for all retrograde moons, including this group’s members. Pasiphae and Sinope are the largest moons in this group, nearby receiving the nod for symbols.

The Amalthea Group is another set of moons found around Jupiter. This group is also made up of four moons. These moons are Thebe, Andrastea, Metis, and Amalthea. These moons help make up the rings of Jupiter. Metis is the closest moon to Jupiter. Andrastea is the next closest moon and is the smallest. Amalthea is the third closest moon and has water ice on its surface. It has many other unknown elements and may contain many different craters and ridges. Thebe is the last inner moon and has many craters and mountains on its surface.
Astronomical Symbol of Jupiter's moon Leda
Astronomical Symbol of Jupiter's moon Lysithea
Astronomical Symbol of Saturn's moon Elara
Astronomical Symbol of Jupiter's moon Himalia
Carme was discovered by Seth Barnes Nicholson at Mount Wilson Observatory in California in July 1938. It is named after the mythological Carme, mother by Zeus of Britomartis, a Cretan goddess. Carme is relatively small with a radius of 23 km (14 m). However, Carme’s size makes it the largest member of the Carme group accounting for 99% of the mass for the sixteen moons within the group. Carme orbits Jupiter at a mean distance of about 14.5 million miles (23.4 million km), taking around 734 Earth days to complete an orbit around the massive gas giant.

Ananke is a retrograde irregular moon of Jupiter discovered by Seth Barnes Nicholson at Mount Wilson Observatory in 1951. Ananke received it’s name from the mythological Ananke, the personification of Necessity, and the mother of the Moirai (Fates) by Zeus. Upon discovery, Ananke’s original name was Jupiter XII, but was later named in 1975.It is believed that Ananke formed from the destruction of an asteroid which produced the Ananke group of satellites. Ananke is the largest of the Ananke satellites with a radius of around 14 km and orbits Jupiter at a distance of around 21.3 million miles. The satellite takes about 630 Earth days to complete one orbit.

Sinope has a highly-eccentric and inclined retrograde orbit. These orbits are continuously changing due to and planetary perturbations. Although Sinope belongs to the Pasiphae group, Sinope could be also an independent object, captured independently, unrelated to the collision and break-up at the origin of the group. Sinope is also known to be in secular resonance with Jupiter, similar to Pasiphae. However, Sinope can drop out of this resonance and has periods of both resonant and non-resonant behavior in time scales of 10 years.

Pasiphae is a retrograde, irregular satellite of Jupiter discovered in 1908 by Philibert Jacques Melotte. Pasiphae was named after the mythological Pasiphaë, wife of Minos and mother of the Minotaur from Greek legend. Pasiphae was discovered on a plate taken at the Royal Greenwich Observatory on a winter night in 1908. It received the provisional designation 1908 CJ, as it was not clear whether it was an asteroid or a moon of Jupiter. It was confirmed to be a moon of Jupiter later that year. Pasiphae is a part of a larger group of satellite that orbit Jupiter at distance around 23 million km from Jupiter.