Io

Io is the innermost of the Galilean moons of Jupiter. It is the most volcanically active object in the solar system and the most dense of all planetary satellites. Io was named for an Argive princess and priestess of Hera who became a fairly prominent mistress of Zeus, after much suffering. That name, assigned quite by accident in the seventeenth century, might be quite apt today in view of Io's volcanic activity and the likely reason for it.

Io
True-color image of Io, from Galileo spacecraft
Date of discovery	January 7, 1610
Name of discoverer	Galileo Galilei, Simon Marius
Name origin	Argive princess and mistress of Zeus
Orbital characteristics
Primary	Jupiter
Order from primary	5
Perizene	420,071 km
Apozene	423,529 km
Semi-major axis	421,800 km
Orbital eccentricity	0.0041
Sidereal month	1.769 da
Avg. orbital speed	17.34 km/s
Inclination	0.040° to Jupiter's equator
Rotational characteristics
Sidereal day	1.769 da
Rotational speed	74.88 m/s
Physical characteristics
Mass	8.93194 * 1022 kg (1.49464% earth)
Density	3,528 kg/m³
Mean radius	1,821.6 km
Surface gravity	1.79611 m/s² (0.183152 g)
Escape speed	2.55805 km/s
Surface area	41,698,065 km² (8.17504% earth)
Mean temperature	130 K
Composition	silicate rock and iron
Color	Yellow
Albedo	0.63

Discovery and naming

Galileo Galilei observed Jupiter beginning on January 7, 1610, with his famous telescope. He at first thought he had discovered three stars near Jupiter, but on the next night those "stars" seemed to have moved. (Those "stars" were actually Io, Europa, and Callisto.) He continued to observe Jupiter and its companion "stars" for seven days, during which time a fourth "star" (Ganymede) appeared and all four of these objects appeared to move with Jupiter. Finally he concluded that these objects were not stars at all, but satellites of Jupiter. This was the first direct observation that provided evidence for Nicolaus Copernicus's heliocentric model of the solar system.

Simon Marius claimed to observe Jupiter and these satellites independently of Galileo and beginning five weeks earlier. However, he did not publish his findings, while Galileo published his. Furthermore, Galileo's notes were more reliable and extensive than those of Marius, which is why Ganymede and the three other satellites he observed (Io, Europa, and Callisto) are called the Galilean satellites and not the Galilean-Marian satellites.

Marius does, however, receive credit for providing the names that the satellites have today. He named Ganymede, the largest, for the mythical son of King Tros of Troy, carried aloft to Mount Olympus by Zeus to be the cupbearer to the gods. (The names Zeus and Jupiter refer to the same classical deity from whom Jupiter gets its name.) The other three satellites are named for three of Zeus' most famous mistresses. Marius propounded these names after Johannes Kepler suggested them to him.

Galileo, for his part, called them the "Medicean planets" after the Medici family and simply numbered the moons I, II, III, and IV. The satellites carried these names for two centuries until the discovery of other moons of Jupiter made that naming system untenably confusing.

Orbital and rotational characteristics

Orbital resonance of Io, Europa, and Ganymede

Io is fifth in order of all of Jupiter's moons, and orbits Jupiter at a semi-major axis of 421,800 km. Its orbit is very slightly eccentric and slightly inclined to Jupiter's equator. Io is in tidal lock (thus keeping the same face toward Jupiter as it revolves).

Io maintains a three-part orbital resonance or Laplace resonance with Europa and Ganymede. These three satellites complete their orbits around Jupiter in the ratio 1:2:4. They also conjoin with one another at their apsides, in a mutually self-correcting manner that allows the resonance to persist. (Most such resonances are unstable and end with one or all bodies changing their orbits.)

The origin of the resonance itself is controversial. Peale and Lee, in 2002, presented a model suggesting that this resonance is primordial and thus was part of the Jovian system since its formation. Their model, however, derives from the nebular hypothesis of the formation of the solar system and thus depends on uniformitarian assumptions. Showman and Malhotra suggest a model by which the resonance developed after the solar system had formed.

This orbital resonance, together with tidal stresses imposed by Jupiter itself, is widely held to be the most likely cause of Io's volcanism (see below).

Physical characteristics

Io cutaway

Io is slightly more massive than is Earth's Moon, though smaller in radius. These two factors, a consequence of Io's greater density, give Io a slightly higher surface gravity than the Moon has.

Volcanism

Io's most striking physical characteristic is its tremendous volcanic activity. The volcanoes of Io, which Voyager 1 helped discover in 1979, were the first extraterrestrial volcanoes ever discovered.

Observation and Exploration

Until the advent of space flight, observations of Io were perforce limited to those by Earth-based telescopes. Galileo Galilei had shown that Io was present, of course, but only in the late 19th and 20th centuries did telescope technology allow for any resolution of surface features on Io. Astronomers like Edward E. Bernard noted the differing brightness between Io's equatorial and polar regions and finally determined that they were strictly due to differences in color and albedo (a measure of surface reflectivity), and not because Io had an odd shape and not because Io was in fact two companion objects. More detailed observations followed, as recently (to the Voyager missions) as 1973.

The Pioneer missions 10 and 11 flew by Io in 1973 and 1974. They revealed little beyond a better estimate of Io's mass, and one good image from Pioneer 11. The heavy radiation in the torus of radioactive particles that surrounds Io compromised Pioneer's instruments, and all its observations were lost.

Voyager 1 established Io's volcanism. Moreover, Voyager 2, which passed Io five months later, demonstrated pronounced surface changes that showed that the volcanoes were highly active and constantly reforming Io's surface.

The Galileo Project spacecraft did not make any close passes at Io for years, primarily because mission controllers were concerned with the safety of the spacecraft and desired to examine several far less dangerous targets. The mission was extended twice, however, and in the last years of the mission, Galileo made several flybys of Io and took surface temperature measurements. These measurements clearly showed that Io's volcanoes tended to be hotter even than many volcanoes on Earth.

In September of 2003, the Galileo controllers deliberately crashed the spacecraft on Jupiter, in order to avoid crashing it on Europa and possibly doing irreparable damage to Europa's subsurface ocean. Since that time, observations have been limited largely to Earth-based telescopes (like the Keck Observatory in Hawaii and the Hubble Space Telescope--except that the New Horizons mission made a flyby of Io on February 28, 2007, and captured many remarkable images. No other missions to Io are planned for at least ten more years.

Related reference

John Gribbin, Companion to the Cosmos (Little, Brown & Company, 1996)