Jupiter

79 Dwarf Moons

## **Overview**

**Jupiter** is the fifth and largest planet in the Sol System. Jupiter is a cold gas giant, being primarily composed of hydrogen and helium in it's atmosphere. It is the most explored gas giant in the entirety of it's respective system. Jupiter is the third-brightest natural object in the Earth's night sky after the Moon and Venus. It has been observed since pre-historic times and is named after the Roman god Jupiter, the king of the gods, because of its observed size.

Jupiter possesses a small-sized, solid and rocky core made up of heavier elements, but it lacks a well-defined solid surface like the other gas giants. Jupiter has the shape of an oblate spheroid because to it's quick rotational period of nine hours. At various latitudes, the outer atmosphere is separated into many differently colored bands, each containing turbulence and storms along their interacting edges.

## **Moon System**

Jupiter has 79 known natural satellites. Of these, 60 are less than 10 km in diameter. The four largest moons are Io, Europa, Ganymede, and Callisto, collectively known as the "Galilean moons", and are visible from Earth with binoculars on a clear night.

The interiors of Io, Europa and Ganymede have a layered structure (as does Earth). Io has a core, and a mantle of at least partially molten rock, topped by a crust of solid rock coated with sulfur compounds. Europa and Ganymede both have a core; a rock envelope around the core; a thick, soft ice layer; and a thin crust of impure water ice. In the case of Europa, a global subsurface water layer lies just below the icy crust. Layering at Callisto is less well defined and appears to be mainly a mixture of ice and rock.

Three of the moons influence each other in an interesting way. Io is in a tug-of-war with Ganymede and Europa, and Europa's orbital period (time to go around Jupiter once) is twice Io's period, and Ganymede's period is twice that of Europa. In other words, every time Ganymede goes around Jupiter once, Europa makes two orbits and Io makes four orbits. The moons all keep the same face towards Jupiter as they orbit, meaning that each moon turns once on its axis for every orbit around Jupiter.

### Io

Io is the most volcanically active body in the solar system. Io's surface is covered by sulfur in different colorful forms. As Io travels in its slightly elliptical orbit, Jupiter's immense gravity causes "tides" in the solid surface that rise 100 meters high on Io, generating enough heat for volcanic activity and to drive off any water. Io's volcanoes are driven by hot silicate magma.

### Europa

Europa is by far the most explored out of the three Galilean Moons. The moon used to be covered by a layer of ice, and a large life-filled subsurface ocean below it. Europa has been given a breathable atmosphere and most of the ice has been melted, creating a global ocean. The first sapient extraterrestrial organisms were discovered on Europa, named the Khavoids.

### Ganymede

Ganymede is the largest moon in the Sol System (larger than the planet Mercury), and is the only moon known to have its own internally generated magnetic field.

### Callisto

Callisto's surface is extremely heavily cratered and ancient—a visible record of events from the early history of the Sol System. However, the very few small craters on Callisto indicate a small degree of current surface activity.

## **Rings**

Jupiter has a faint planetary ring system composed of three main segments: an inner torus of particles known as the halo, a relatively bright main ring, and an outer gossamer ring. These rings appear to be made of dust, rather than ice as with Saturn's rings. The main ring is probably made of material ejected from the satellites Adrastea and Metis. Material that would normally fall back to the moon is pulled into Jupiter because of its strong gravitational influence. The orbit of the material veers towards Jupiter and new material is added by additional impacts. In a similar way, the moons Thebe and Amalthea probably produce the two distinct components of the dusty gossamer ring

## **History**

### **Information Age**

Jupiter received a high amount of attention and fame in the information age, largely due to it's immense size and it's unique moon system. A probe called Pioneer 10 flew past Jupiter in December of 1973, followed by Pioneer 11 exactly one year later. Pioneer 10 made the first ever close up images of Jupiter, studied its atmosphere, and detected its magnetic field.

Voyager 1 flew by in March 1979 followed by Voyager 2 in July of the same year. The Voyagers vastly improved the understanding of the Galilean moons and discovered Jupiter's rings. They also took the first close up images of the planet's atmosphere.

In February 1992, Ulysses solar probe performed a flyby of Jupiter at a distance of 450,000 km. The flyby was required to attain a polar orbit around the Sun. The probe conducted studies on Jupiter's magnetosphere. Since there are no cameras onboard the probe, no images were taken. In February 2004, the probe came again in the vicinity of Jupiter. This time the distance was much greater, about 240 million km.

The first spacecraft to orbit Jupiter was the Galileo orbiter, which went into orbit around Jupiter on December 7, 1995. It orbited the planet for over seven years, making 35 orbits before it was destroyed during a controlled impact with Jupiter on September 21, 2003. During this period, it gathered a large amount of information about the Jovian system; the amount of information was not as great as intended because the deployment of its high-gain radio transmitting antenna failed.

In 2000, the Cassini probe, en route to Saturn, flew by Jupiter and provided some of the highest-resolution images ever made of the planet of that time.

In 2007, Jupiter was also visited by the New Horizons probe, en route to Pluto. Due to the immense size of the Jupiter system and the relative closeness of the Jovian system to Earth in comparison to the closeness of Pluto to Earth, New Horizons sent back more data to Earth from the Jupiter encounter than the Pluto encounter.

NASA launched Juno on August 5, 2011 to study Jupiter in detail. It entered a polar orbit of Jupiter on July 5, 2016. The spacecraft studied the planet's composition, gravity field, magnetic field, and polar magnetosphere. Juno was also searching for clues about how Jupiter formed, including whether the planet had a rocky core, the amount of water present within the deep atmosphere, and how the mass is distributed within the planet. Juno also studied Jupiter's deep winds.

### **Interplanetary Age**

Jupiter started continually getting more fame in the interplanetary age, mostly toward it's icy moon Europa, which had advanced and complex life living in it's subglacial oceans, which was still undiscovered for the early interplanetary age.

European Space Agency's Jupiter Icy Moon Explorer was launched in 2024. However, it arrived near Jupiter 8 years later, after a series of flybys in the inner Sol System. The Jupiter Icy Moon Explorer orbiter performed detailed investigations on Ganymede and evaluate it's potential to support life. Investigations of Europa and Callisto also took place, and completed a comparative picture of these Galilean moons. For Europa, the focus was on the chemistry essential to life, including organic molecules, and on understanding the formation of surface features and the composition of the non-water-ice material. Furthermore, the orbiter provided the first subsurface sounding of the moon, including the first determination of the minimal thickness of the icy crust over the most recently active regions.

Europa Clipper was launched in 2024 and reached Jupiter in 2026. The goals of Europa Clipper were to explore Europa, investigate its habitability and aid in the selection of a landing site for the future Europa Lander. This exploration was focused on understanding the three main requirements for life: liquid water, chemistry, and energy. Specifically, the objectives were to study:

* Ice shell and ocean: Confirm the existence, and characterize the nature, of water within or beneath the ice, and processes of surface-ice-ocean exchange.
* Composition: Distribution and chemistry of key compounds and the links to ocean composition.
* Geology: Characteristics and formation of surface features, including sites of activity.