Moons

Jupiter has around 60 moons most of which have been discovered since 2000. Only 38 of them have been named and in all cases after the god’s descendents and those who attended him well, including his many lovers. The four largest moons were the first moons to be discovered after the Earth’s in 1610. That chilly January evening, Galileo pointed his small refractor at Jupiter and in that moment the 2,000 year old Aristotelian truth that all worlds revolved around the Earth essentially fall apart.

The Four Galilean Moons

Ganymede

Ganymede is the largest of Jupiter’s moons with a diameter of just over 5,000km, meaning that it is bigger that either Pluto or Mercury and just a little smaller than Mars. It is made up of mainly rock and ice, orbits at a distance of about 1 million km from Jupiter and is named after the cupbearer of the Olympian gods.

Callisto

Callisto is the second-largest moon and again is made up mainly of ice and rock. Its distance from Jupiter is just under 2 million km and was named after one of Zeus’ lovers.

Io

Io is just a little larger than our own moon and orbits Jupiter at just a little greater distance, about 422 thousand km. Close up images of Io show it looking like a pizza, a highly coloured world of volcanic eruptions, pits, lava flows and high-reaching plumes. At its volcanic hotspots, temperatures can reach to over 1,200ºC whilst elsewhere, in less active terrain, they can drop to as much as -150ºC. Io was named after another of Zeus’ lovers whom he changed into a cow to hide from his wife, Hera.

Europa

Europa is another ice-covered ball of rock with a diameter of about 3,000km and a distance from Jupiter of about 670 thousand km. Although the smallest of the moons, it is probably one of the most fascinating objects in the solar system. Below its covering crust of ice are seas and oceans estimated to contain more liquid than Earth’s combined and is believed to be potentially a haven for life, so much so, that when it was known that the Galileo space probe at the end of its mission in 2003 could collide with the moon and possibly contaminate or destroy life, it was put on a collision course with Jupiter. In ancient Greek myth, Europa was another lover of Zeus.

Jupiter

Jupiter hardly needs an introduction. It is the largest planet in the Solar System with over two and a half times the mass of all the other planets combined and in which you could fit as many as 1,300 Earths.

In times of yesterday, Jupiter was the supreme god for the Romans (in Greek he is known as Zeus). He was the King of Kings, personifying the divine right of rule and authority, law, justice and government. He was typically identified with a thunderbolt and the eagle, the Aquila, which the Roman Legion adopted as the symbol for their own war flag, or standard.

Although much has been written of Jupiter, we can point out the following highlights:

Orbit

Jupiter is the fastest spinning planet in the Solar System and lies anything between 925 (Aphelion) to 630 (Perihelion) million kilometers from Earth. Due to its slight axis tilt, neither of its hemispheres points markedly towards or away from the sun and in consequence, Jupiter does not appear to have any obvious season.

Structure

We can imagine Jupiter as essentially a huge ball of gas with four main structures.

The outer layer is a gaseous state made up of mainly hydrogen (90%) and helium (9%) where temperatures fall to around -110ºC. As we move downwards, pressure, density and temperature increase, so by about 7,000km deep the hydrogen and helium acts like a liquid gas reaching temperatures of about 2,000ºC and by about 14,000km deep, they have risen to over 5,000ºC and the hydrogen has compacted into molten metal. Deep inside, at a depth of about 60,000km, there is probably a solid core of metal and rock compounds.

Atmosphere

The mass of Jupiter’s atmosphere is made up of about 75% hydrogen, 24% helium, with the remaining one percent consisting of other compounds and elements, such as methane, ammonia and water. As these elements and compounds condense, different coloured clouds are formed giving Jupiter its distinctive appearance. As the circulating air falls, for example, it heats up creating those rusty brown regions known as belts, as it rises again it begins to cool giving Jupiter those creamy bands known as zones.

Weather

As already suggested, due to Jupiter’s tilt, the planet doesn’t appear to have any distinct season. Nevertheless, the rising and falling air produces winds which reach in the excess of 400kph. Solar heat, the winds and Jupiter’s spin combine to produce huge storms, the smallest of which would be apocalyptic on Earth. Some of these storms last only days, others endure for centuries, like the Great Red Spot, a high pressure storm.

Rings and Magnestic Fields

Jupiter’s ring system was discovered in 1979 by images taken by Voyager 1. It is thin and very faint and is composed of dust strewn from Jupiter’s four inner moons of which Io is one. Jupiter’s magnetic field is the strongest of any planet, about 20,000 times that of Earth’s.

Excellent Qualia

Magnificent as always - an interesting change in topic and a true astronomer's log showing the patience to observe a single target over hours.

Forgot to mention the orbital times for the 4 Galilean Moons, so here goes:

Orbital Period

Ganymede: 7.15 days (about 7 days, 3 hours and 36 minutes).

Callisto: 16.69 days (about 16 days, 16 hours and 33 minutes).

Io: 1.77 days (about 1 day, 18 hours and 28 minutes).

Europa: 3.55 days (about 3 days, 13 hours and 12 minutes).

Love reading your blog. Keep them up. Good to see you enjoying this hobby so much :D