Io (moon)

Galileo Galilei

Io ( Jupiter I) is the innermost of the four large moons of Jupiter and with a diameter of 3643 km the third- largest moon of Jupiter and the fourth largest moon in the solar system.

Its peculiarity is an extreme volcanism, which is unbeatable by any other celestial body in the solar system. This moon was known by the first measurement of the speed of light by Ole Rømer in 1676 based on observed Verfinsterungszeiten of Io as a function of the position of Jupiter to Earth.


Ios discovery is the Italian scholar Galileo Galilei attributed to the simple set up his telescope at Jupiter in 1610. The four large moons Io, Europa, Ganymede and Callisto are therefore also referred to as the Galilean moons.

It was named after the moon Io, in Greek mythology, a mistress of Zeus ( Zeus corresponds to the Roman Jupiter). Although the name of Io has been proposed shortly after the discovery of Simon Marius, he could not prevail over time. Only in the middle of the 20th century he came again into use. Previously, the Galilean moons were commonly referred to by Roman numerals and Io was the moon of Jupiter I.

The Galilean moons are so bright that they can already be observed with binoculars or a small telescope.

Orbit and rotation

Io orbits Jupiter at an average distance of 421 600 kilometers in 1 days 18 hours and 27.6 minutes. The track has a numerical eccentricity of 0.004 and is inclined at 0.04 degrees to the equatorial plane of the Jupiter.

Io rotates within 1 day 18 hours and 27.6 minutes around its own axis, thus showing how the Earth's moon and the other Galilean moons of Jupiter, a tidally locked to.

Physical Properties

Io has an average diameter of 3643.2 kilometers and has a relatively high density of 3.56 g/cm3. Thus, it has a slightly higher density and a slightly larger diameter than the Earth's moon.

Unlike the other Galilean satellites, Io can be found on virtually no water. This could be due to the fact that Jupiter in the early days of its formation had high temperatures that made escape of water and other volatiles on the innermost moon.

Ios albedo is 0.61, that is, 61 % of the incident solar light is reflected from the surface. The surface temperature is -143 ° C, on average,

Before the missions of unmanned space probes science was convinced that the Galilean moons dotted with craters body similar to the Earth's moon are. Based on the number and distribution of craters inferences should be drawn on the age of the moon. As the probes Voyager 1 and Voyager 2 first sent detailed images to Earth, they were surprised that the moons showed a completely different look. The reason for this is at Io of hitherto unknown volcanic.


Io's surface has an age of only a few million years and is subject to constant change. It is essentially very flat. With height differences of less than one kilometer, but there are also mountains of up to nine kilometers above that are not volcanic in origin and are probably caused by tectonic processes Comparisons of images of the Voyager probes and the 20- years-younger images of the Galileo probe also indicate rapid decay processes that are already visible in this short period of time.

However, the most prominent structures of the surface are hundreds of volcanic calderas, which are up to 400 kilometers in size and up to several miles deep in diameter. There are also numerous lakes of molten sulfur. The deposits of sulfur and its compounds have a broad spectrum of hues, giving the moon an unusually colorful appearance.

Furthermore, a low viscosity liquid lava flows extend across several hundred kilometers. Evaluations of Voyagerdaten suggested that the lava flows are mainly composed of sulfur and sulfur compounds. In contrast, ground-based infrared studies show the so-called hot spots with temperatures up to 2000 K. This is much too hot for molten sulfur. There may be lava flows of molten silicates. Recent observations from the Hubble Space Telescope indicate that the material is rich in sodium.


Io's surface has virtually no impact crater on how, but it is marked by active volcanism and ever changing. Io is the most volcanically active body in the solar system by far.

In eruptions of liquid sulfur and sulfur dioxide at speeds up to 1 km / s and temperatures of 1000 to 1300 ° C are ejected, which can reach up to 300 km altitude due to the low gravity. The materials fall back to the surface and form massive deposits.

The volcanic activity was first detected in 1979 on photographic images of the Voyager 1 spacecraft, which at the time created a great stir, as this was the first discovery of active volcanism on another celestial body as the earth. The eruptions vary greatly. Already over a period of only four months that had passed between the arrival of Voyager 1 and Voyager 2, was determined that eruptions had come to a standstill in some areas, while in other places had started new. The deposits around the volcanic crater had also changed significantly.

By comparison with the recorded 20 years later pictures of the Galileo spacecraft is seen that the permanent volcanic eruptions change the surface of Io by deposits of ejected material constantly. Io has the youngest planetologisch surface in the solar system. Your age is estimated at about 10 million years. Therefore, hardly any impact craters are visible, as they are leveled by the planetologischen processes.

The volcanic activity is caused by tidal forces exerted by the moon literally knead and heat up thereby. But the tidal forces of Jupiter on Io are more than 6000 times stronger than the earth's moon to the earth. The additional tidal forces of Europa and Ganymede are still the order of the moon on the earth. Due to the synchronous rotation of Io, however, is not the absolute strength of the tidal forces of Jupiter decisive, but only their change. Io is forced through a resonance effect with the moons Europa and Ganymede, whose orbital periods are in the ratio 1:2:4 to each other on a slightly elliptical orbit around Jupiter, so that the variation of the tidal forces of Jupiter solely by the variation of the distance still 1000 times as large as the influence of the tidal effect of the moon on the earth. The elliptical orbit of Jupiter varies from the perspective of an observer on Io during a revolution in the sky in addition gently back and forth. Due to the short distance to Jupiter this libration in longitude of the satellite leads to periodically migrating tides mountains of up to about 300 meters. The corresponding deformations of the earth's crust amount to only 20 to 30 centimeters. If the orbit of Io were circular, then its tidal mountains were unmoved and there on her no volcanism.

Significant volcanic mountains are the Culann Patera, the tupan Patera, the Ra Patera and the Loki Patera. Other named volcanoes are about Marduk Patera, Pele Patera or Prometheus.

Internal construction

Unlike the icy moons of the outer solar system, Io therefore seems to be more like the terrestrial ( Earth-like ) planets composed predominantly of silicate rock. Data from the Galileo spacecraft suggest that Io may have a core of iron, with interests in iron sulfides, of at least 900 kilometers in diameter. When re- evaluation of data from the Galileo spacecraft, researchers discovered that Io has under the entire surface of magma, which is molten at 20 % in a 50 km thick layer.


Io has an extremely thin atmosphere of 120 km altitude, which is composed of sulfur dioxide and possibly traces of other gases. The 700 km deep layer ionosphere consists of sulfur, oxygen and sodium ions. It is constantly renewed by the volcanic activity, so that the particle loss is compensated by the interaction with the magnetosphere of Jupiter.

Magnetic field and radiation

Io moves in its orbit by the strong magnetic field of Jupiter, whereby electric currents are induced. Here, around 1000 gigawatt be generated with a voltage potential of 400,000 volts. Under these conditions, atoms are ionized in the upper atmosphere and back into space. Io suffers through this stream of particles a mass loss of several tons per second.

The ions form along Ios train a torus around Jupiter, the intense lights in infrared light. Particles, which are carried away by the solar wind from the torus could be partially responsible for Jupiter's unusually extensive magnetosphere.

The ions are accelerated to such an extent in the Jovian magnetosphere that the resulting radiation would cripple the current (2011) strahlungssicherste electronics of a spacecraft.

In addition, according to the same mechanism, caused by the well auroras below Io in Jupiter's atmosphere forms a luminous phenomenon that draws a tracer by itself. Why the red dot hasten further, weaker luminous points, is so far not physically explained.

The position of Io greatly influences the transmission of radiated by Jupiter system radio waves (Jupiter bursts ). If Io from Earth is visible, the intensity of the radio emission increases significantly. Speculation about its own dipole field of Io as it has the Jupiter 's moon Ganymede, were refuted by the Galileo spacecraft.

Exploration by special missions

The exploration of Io by space probes began in the years 1973 and 1974 with the Jupiter flybys of Pioneer 10 and Pioneer 11, Voyager 1 and Voyager in 1979 were 2 first make precise observations of the moon. However, the majority of our knowledge of Io derived from the Galileo orbiter, which in 1995 reached the Jupiter system and during the next eight years, several close flybys fully led by Jupiter Moon.

On August 5, 2011, NASA's Juno probe is launched to explore Jupiter and its magnetosphere from a polar orbit. You could take pictures of Io.

In 2020, NASA and the ESA had the common Europe proposed Jupiter System Mission / Laplace, which provided at least two orbiters that will enter orbit around Europa and Ganymede, respectively, and explore the entire Jupiter system, including Io, with a revolutionary draft should.

However, the NASA stressed its share of the project. ESA will fly past with its now called JUICE Orbiter for Research on Ganymede and Callisto also in Europe. Flybys of Io are not included in the timetable of JUICE.

Furthermore, a study for an Io Observer said probe that could explore Io in several dense flybys exists. Whether it is realized, however, is not clear.