The third largest and fifth closest moon of Jupiter,
and the innermost and densest Galilean satellite;
also known Jupiter I. Although similar in diameter and density to our own
Moon, Io is radically difference in appearance. It is the most volcanically
active body in the Solar System, its yellow and orange sulfur-rich surface
peppered with hundreds of active volcanic calderas. Huge eruptions, accompanied
by plumes 300 km high, have been observed by the Voyager
and Galileo spacecraft. The energy for all
this activity comes from tidal interactions between Io, Europa,
Ganymede, and Jupiter. The three moons
are locked in resonant orbits such that Io orbits twice for each orbit of
Europa which in turn orbits twice for each orbit of Ganymede. Though Io,
like our own Moon always faces the same side toward its planet, the effects
of Europa and Ganymede cause it to wobble a bit. This wobbling stretches
and bends Io by as much as 100 m (by comparison, the highest ocean tides
on Earth only reach about 18 m) and generates heat in the same way that
repeatedly flexing a rubber band causes it to warm up (see tidal heating).
Some of these hot spots on Io may reach temperatures as high as 2,000 K,
compared with an average surface temperature of about 130 K, and are the
principal means by which Io loses its heat.
Hundreds of volcanic calderas have been observed on the surface. Because
the surface is continually being relaid, there are no visible impact craters.
The surface colors of mottled red, orange, yellow, brown, and white are
due to sulfur and frozen sulfur
dioxide. The terrain consists mostly of flat plains rising to less than
1 km with some mountain ranges up to 9 km high. Io's biggest volcano, Loki,
is the most powerful volcano in the Solar System and consistently gives
out more heat than all of Earth's volcanoes put together. Its enormous caldera,
larger than the state of Maryland, is continually flooded with lava.
|An active volcanic eruption seen on Feb. 22, 2000,
by the Galileo spacecraft. It occurred in Tvashtar Catena, a chain
of giant volcanic calderas.
However, not all of Io's mountains are volcanic. Some appear to have been
formed by uplift and thrust faulting, and, in some cases, may rise to great
heights; Euboea Montes, for example, tops out at an altitude of 13 km. The
sheer size and steepness of these peaks argue that the material underlying
them is rock and not some form of sulfur. Images of Euboea Montes suggest
that it formed from the uplift of a large crustal block, which caused a
landslide that has left an enormous debris apron at the mountain's base.
The debris flow is 200 km wide and contains an estimated 25,000 cubic km
of rock – 10,000 times larger than the landslide that accompanied
the Mt. St. Helens eruption in 1980. Only on the flanks of Olympus
Mons on Mars have avalanches of this size been observed elsewhere.
As Io travels around its orbit, it cuts across Jupiter's magnetic field
lines, generating an electric current. Though small compared to the tidal
heating, this current may carry more than 1 trillion watts. It also strips
some material away from Io which forms a doughnut-shaped cloud of plasma
in Io's orbit. This cloud, consisting mostly of oxygen, sodium, and sulfur
ions, emits strongly in the extreme ultraviolet but also radiates at wavelengths
long enough to be detected by ground-based telescopes. Data from Galileo
also indicate that Io may have its own magnetic field as does Ganymede.
It has an iron core and a high-altitude ionosphere.
||Jan. 7, 1610, by Galileo Galilei
||421,700 km (262,100 miles)
||3,643 km (2,264 miles)
|equatorial diameter (Earth=1)
|surface gravity (Earth=1)
||2.56 km/s (9,216 km/h, 5,728 mph)
||1.769 days (1 day 18 hr 27 min.)
||-135°C (-216°F) with hot spots of 27°C
||Sulfur / frozen sulfur dioxide
in mythology (Encyclopedia of History)