Jupiter, taken by the Cassini spacecraft in 2000

Jupiter, taken by the Cassini spacecraft in 2000     [Source]

Jupiter is the largest of the planets in the Solar System, more than double the mass of all the other planets combined. Its gravity is able to trap hydrogen and helium (unlike the terrestrial planets), forming layers of these gases on its surface. Were Jupiter about 75 times more massive, it would be a star in its own right, able to fuse hydrogen into helium in its core.

Jupiter is typically the third brightest object in the night sky (after the Moon and Venus), with a peak apparent magnitude of -2.94. Though large in size, the planet has a rotation rate of just 10 hours. The resulting centrifugal forces (directed outward) are responsible for the planet’s equatorial bulge. Jupiter’s equatorial radius is actually 7% greater than its polar radius, a characteristic noticeable to keen-eyed observers.

Jupiter displays a broad variety in its cloud patterns, including several prominent bands parallel to the Jovian equator. The easiest to catch sight of are Jupiter’s two dark cloud belts on either side of the equator: the north equatorial belt (NEB) and the south equatorial belt (SEB). A particularly notable feature is the Great Red Spot (GRS), a raging storm more than double the size of Earth that has persisted in Jupiter’s south equatorial belt for centuries. But the GRS is not always obvious, as its color can range from a vibrant red or orange to a pale salmon tint.

Just as exquisite are its four largest moons (often called the Galilean moons): Ganymede, Callisto, Io, and Europa. All can be spotted in just binoculars and they are easily seen in a telescope.

The Galilean moons, ordered by increasing orbital distance: Io, Europa, Ganymede, and Callisto

The Galilean moons, ordered by increasing orbital distance: Io, Europa, Ganymede, and Callisto     [Source]

All four of these moons are tidally locked to Jupiter (like the Moon is to Earth). Ganymede is the largest moon in the entire Solar System, larger than Mercury in size (though less massive). Owing to its size, Ganymede even possesses its own magnetic field. Callisto—the outermost of the four moons—is the most heavily cratered object in the Solar System. Its surface is so densely cratered that new impacts will often erase old ones.

The inner two Galilean moons are a “fire and ice” duo. Spurred by the gravity of Jupiter, Io has over 400 active volcanoes. Europa, on the other hand, features a highly reflective icy surface, marked by cracks. Europa displays few craters, the signs of a young surface. Most notable, perhaps, are the liquid water oceans thought to exist underneath. Of all of the Solar System bodies aside from Earth, it is thought to have the greatest potential for harboring extraterrestrial life.

A transit of Io and its shadow, as seen by the Hubble Space Telescope

A transit of Io and its shadow, as seen by the Hubble Space Telescope   [Source]

The Galileans satellites regularly pass in front of or behind Jupiter. Four different events can occur: (1) a satellite transit, when moon is in front of Jupiter’s disk, (2) a shadow transit, when the moon casts a shadow on Jupiter, (3) an eclipse, when the moon temporarily hides in Jupiter’s shadow, and (4) an occultation, when Jupiter’s disk obscures our view of the given moon. Typically these events will come in pairs (either the first two or the last two together), though not necessarily. On rare occasion, it is possible for multiple moons to transit simultaneously.


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