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Jupiter: Moons: Io
This global view of Jupiter's moon, Io, was obtained during the tenth orbit of Jupiter by NASA's Galileo spacecraft. Io, which is slightly larger than Earth's Moon,
is the most volcanically active body in the solar system.
Looking like a giant pizza covered with melted cheese and splotches of tomato and ripe olives, Io is the most volcanically active
body in the solar system. Volcanic plumes rise 300 km (190 miles) above the surface, with material spewing out at nearly half the
required escape velocity.
A bit larger than Earth's Moon, Io is the third largest of Jupiter's moons, and the fifth one in distance from the planet.
Although Io always points the same side toward Jupiter in its orbit around the
giant planet, the large moons Europa and Ganymede perturb Io's orbit into an
irregularly elliptical one. Thus, in its widely varying distances from Jupiter, Io is
subjected to tremendous tidal forces. These forces cause Io's surface to bulge
up and down (or in and out) by as much as 100 m (330 feet)! Compare these
tides on Io's solid surface to the tides on Earth's oceans. On Earth, in the place
where tides are highest, the difference between low and high tides is only 18 m
(60 feet), and this is for water, not solid ground!
This tidal pumping generates a tremendous amount of heat within Io, keeping
much of its subsurface crust in liquid form seeking any available escape route
to the surface to relieve the pressure. Thus, the surface of Io is constantly
renewing itself, filling in any impact craters with molten lava lakes and
spreading smooth new floodplains of liquid rock. The composition of this
material is not yet entirely clear, but theories suggest that it is largely molten
sulfur and its compounds (which would account for the varigated coloring) or
silicate rock (which would better account for the apparent temperatures, which
may be too hot to be sulfur). Sulfur dioxide is the primary constituent of a thin
atmosphere on Io. It has no water to speak of, unlike the other, colder Galilean
moons. Data from the Galileo spacecraft indicates that an iron core may form
Io's center, thus giving Io its own magnetic field.
An active volcanic eruption on Jupiter's moon Io was
captured in this image taken by NASA's Galileo
spacecraft.
Io's orbit, keeping it at more or less a cozy 422,000 km (262,000 miles) from Jupiter, cuts across the planet's powerful magnetic
lines of force, thus turning Io into a electric generator. Io can develop 400,000 volts across itself and create an electric current of 3
million amperes. This current takes the path of least resistance along Jupiter's magnetic field lines to the planet's surface, creating
lightning in Jupiter's upper atmosphere.
As Jupiter rotates, it takes its magnetic field around with it, sweeping past Io and stripping off about 1,000 kg (1 ton) of Io's material
every second! This material becomes ionized in the magnetic field and forms a doughnut-shaped cloud of intense radiation referred
to as a plasma torus. Some of the ions are pulled into Jupiter's atmosphere along the magnetic lines of force and create auroras in
the planet's upper atmosphere. It is the ions escaping from this torus that inflate Jupiter's magnetosphere to over twice the size we
would expect.
Discovery:
Io was discovered on 8 January 1610 by Galileo Galilei. The discovery, along with three other Jovian moons, was the first time a
moon was discovered orbiting a planet other than Earth. The discovery of the four Galilean satellites eventually led to the
understanding that planets in our solar system orbit the sun, instead of our solar system revolving around Earth. Galileo apparently
had observed Io on 7 January 1610, but had been unable to differentiate between Io and Europa until the next night.
How Io Got its Name:
Galileo originally called Jupiter's moons the Medicean planets, after the Medici family and referred to the individual moons
numerically as I, II, III, and IV. Galileo's naming system would be used for a couple of centuries.
It wouldn't be until the mid-1800s that the names of the Galilean moons, Io, Europa, Ganymede, and Callisto, would be officially
adopted, and only after it became apparent that naming moons by number would be very confusing as new additional moons were
being discovered.
Io was originally designated Jupiter I by Galileo because it is the first satellite of Jupiter. Io is named for the daughter of Inachus,
who was raped by Jupiter. Jupiter, in an effort to hide his crime from his wife, Juno, transformed Io into a heifer.