Ganymede (moon)

Galileo Galilei

Ganymede ( Jupiter III) is the third and with a diameter of 5262 km the largest of the four large moons of Jupiter. He is still in front of Titan is the largest moon in the solar system, and larger than the planet Mercury. The moon was discovered by Galileo Galilei, which is why he is also known as one of the Galilean moons.

Discovery

Ganymede's 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.

Named the moon after the young man Ganymede, cupbearer of the gods and a lover of Zeus in Greek mythology. He is the only moon of Jupiter, which is named after a male figure. Although the name of Ganymede has been proposed shortly after its discovery by 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 Ganymede was Jupiter Moon III.

The Galilean moons are so bright that they can already be observed with binoculars or a small telescope. In particular, Ganymede is having a brightness of up to 4.6 like so bright opposition to times that he would freiäugig visible when he would not outshone by the nearby Jupiter.

Orbit and rotation

Ganymede orbits Jupiter at an average distance of 1.0704 million km in 7 days 3 hours and 42.6 minutes. Therefore he will be in resonance with its two inner neighbors Europe (1:2) and Io (1:4), an effect that contributes to relatively large eccentricities of the orbits of these moons. This fact has particular importance for the tidal friction in the interior of these moons and is thus an explanation for the volcanism on Io. The web has an eccentricity of 0.0015 and 0.21 ° inclined to the equatorial plane of Jupiter.

Ganymede rotates in 7 days, 3 hours and 42.6 minutes on its own axis, thus showing how the Earth's Moon and the other inner moons of Jupiter, a tidally locked to. Because of the low compared to the Earth's moon eccentricity of the orbit and the very small axial inclination Ganymede Librationseffekte are low. The libration of the maximum length which is proportional to the eccentricity of the orbit, is only about 10 ' (at the Earth's moon this is a maximum of about 7 °). The costs arising from this wobbling motion tidal effects, which are considered the main cause of the strong volcanic activity at Jupiter 's moon Io, are therefore ( and because of their much greater distance ) much lower.

Physical Properties

Ganymede has a mean diameter of 5262 km and is the largest moon in the solar system. It is slightly larger than Saturn 's moon Titan ( 5150 km ) and significantly larger than the planet Mercury ( 4878 km ), although he with its low density of 1.936 g/cm3, only half of Mercury's mass.

Surface

Ganymede's surface can be divided into two distinct regions: a geologically very old, dark region with a large number of impact craters and a slightly younger, brighter region with marked ditches and distortions. The two regions are due to tectonic activity. Ganymede's surface is composed of two continental plates that move independently of each other, with mountain ranges may be discussed at their edge zones. In addition, areas are visible, could be passed through the so-called aqueous lava, whose origin is due to last Kryovulkanismus. In terms of tectonics similar to Ganymede planet Earth, although the activities have come to a standstill on Ganymede. The dark regions are similar to the surface of Callisto, a similar system of trenches and faults exist on Saturn's moon Enceladus and the Uranian moons Miranda and Ariel.

Both regions have many impact craters. Their number and distribution result for Ganymede's surface an age of 3 to 3.5 billion years, comparable to the Earth's moon. The crater superimpose the trenches or be broken by this, which suggests that the trenches are also geologically old. There are also more recent strikes in which material was ejected from the underground in the form of rays systems. Unlike on the Earth's moon or Mercury most craters are relatively flat and have no ringwalls or central mountains on. Obviously, the ice crust has yielded over geological time and leveled these structures. Very old craters are visible only as dark reliefs.

The largest coherent structure on Ganymede is a dark level, called Galileo Regio. Furthermore, extended concentric elevations are visible, which are the remains of a massive impact event, which took place a long time ago.

Ganymede's albedo is 0.43, ie 43 % of the incident sunlight is reflected from the surface. Compared to the moons Io and Europa 's surface is relatively dark. The surface temperature is -160 ° C, on average,

Internal construction

The analysis of the data of the Galileo spacecraft indicates that it is Ganymede is a differentiated body, the shell structure of four layers: a relatively small core of iron or iron sulfide is surrounded by a shell of silicate rock. Above are a about 800 km thick layer of soft water ice and a hard outer crust of ice.

Furthermore, the orbital motion of the spacecraft showed small anomalies in the gravity field that indicate either an uneven rocky mantle or evidence of large amounts of trapped in the ice mantle rocks. Perhaps they are also caused by rock debris in near-surface layers of ice.

The metallic core is an indication that Ganymede in the early days of its formation in the interior higher temperatures exhibited than had been previously thought. In fact, Ganymede appears to be similar to Io, only that he is also surrounded by an outer ice mantle and a crust of ice.

Atmosphere

The first signs of the existence of an atmosphere around Jupiter's moon have been found in the year 1972 at the occultation of the star SAO 186800 by Ganymede. The pressure in the extremely thin atmosphere was then specified to be greater than 10-6 bar. Observations with the Hubble Space Telescope yielded 1997 references to the presence of an extremely thin atmosphere of oxygen. It is believed that the oxygen produced by the effect of solar radiation on the icy crust, where the water is split into hydrogen and oxygen. The volatile hydrogen escapes to space, the more massive oxygen is retained by Ganymede's gravity.

Magnetic field

During the first two flybys of the Galileo spacecraft on the moon Ganymede in June 1996 ( G1) at an altitude of 838 km and in September 1996 ( G2) at an altitude of only 264 km could be demonstrated that Ganymede has its own magnetic dipole field. In addition to the Earth and Mercury Ganymede is now the only solid planetary body in the solar system with an appreciable own dipole field, in particular the only moon. Speculation about just such a field of Jupiter's moon Io, however, have not been confirmed.

The magnetic field can be assumed in a first approximation as a simple superposition of a dipole field with the Jupiter field in vacuum. The Jupiter field can be considered constant while in an environment of the moon from about 10 Ganymedradien, the strength of this homogeneous field is about 120 nT. However, the orientation of the magnetic field can vary during the revolution around Jupiter. The model fits the data the flyby G1 when an equatorial field strength of the dipole field is assumed of 750 nT (shown in the adjacent figure ). The field strength is indeed much lower than that of the geomagnetic field (equatorial 30,000 nT ), but larger than the planet Mercury (equatorial 450 nT). The direction of the magnetic dipole deviates about 10 ° from the axis of rotation and shows the Ganymede -centered coordinate system in the direction of 220 degrees longitude ( the prime meridian shows because of the rotation always bound to Jupiter ).

The structure of the magnetic field is slightly different from the planetary magnetic fields of the earth or the gas planets. The ambient magnetic field of Jupiter is so strong that there is only a relatively small area are on the surface of Ganymede at the equator, where the magnetic field lines from the moon run back to the moon. In the relatively large polar regions, the field lines, however, to Jupiter or come from there. The green line in the picture - the " separatrix " - separates regions where the field lines of Ganymede to Ganymede, Ganymede to Jupiter and Jupiter run to Jupiter.

Better models of the magnetic pull into consideration the fact that Ganymede does not move in a vacuum by the magnetic field of Jupiter, but that there is a korotierendes with Jupiter ( two-sided rotary bonding ) plasma in which the Moon is located. The inclusion of the plasma in the model is done in the framework of magnetohydrodynamics and explains the formation of a magnetosphere. The existence of a magnetopause has been confirmed by Galileo, but there is no bow shock, in contrast to Earth's magnetosphere. For the formation of such a shock wave, the inflowing plasma would have a velocity relative to Ganymede, which is greater than the Alfvén speed. Unlike the Earth, in the solar wind at about eight times the Alfvén speed ( and ten times the speed of sound) is incident on the magnetopause, has the co-rotating plasma of Jupiter, although 2.4 times the speed of sound, but only about half the Alfvén speed. In the polar regions of Ganymede, where field lines of Jupiter Ganymede to run, plasma can penetrate to the atmosphere of the planet where it leads to northern lights that could be actually observed by the Hubble Space Telescope in ultraviolet light.

The fact that the direction of the dipole deviates from the direction of the rotation axis by only 10 °, suggests that the cause of the magnetic field is to be sought in a dynamo effect. As a possible carrier of the dynamo effect conductive liquids come into question. Are discussed as candidates both liquid metal in the core of the moon, as well as salt water in the jacket.

Exploration by special missions

The exploration of Ganymede 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 this moon. However, the majority of our knowledge about Ganymede comes from the Galileo orbiter, which reached the Jupiter system in 1995 and several close flybys fully introduced during the next eight years at Jupiter moon.

For 2020, the space agencies NASA and ESA had suggested the common Europe Jupiter System Mission Laplace, which provided at least two orbiters, which should each enter into orbit around Europa and Ganymede and explore the entire Jupiter system with a revolutionary draft.

NASA, who wanted to build the JEO, but got out of the project. However, the ESA realized the JGO with a slightly different mission planning as JUICE. JUICE will swing in 2032 in orbit around Ganymede after their arrival at Jupiter in 2030 after two flybys in Europe and twelve flybys of Callisto. Since the NASA probe is omitted, the Europa flybys were taken as a substitute in the mission plan for JUICE.

Science fiction

Ganymede plays an essential role in many novels and short stories by Philip K. Dick; represented most prominently the moon is in the novel The Ganymede Takeover ( " The invaders of Ganymede " ), which was written in 1967 along with Ray Nelson.

The novel Farmer in space (English Farmer in the Sky, 1950) by Robert A. Heinlein, the terraforming and colonization of Ganymede on the topic.

The German feature film Operation Ganymede is about the adventures of a fictional spaceship crew who returns from a trip to Ganymede to Earth.

In the anime series Cowboy Bebop the moon is the birthplace of the figure Jet Black.

In the series Babylon 5, a ship of the " shadow " beneath the surface of Ganymede was discovered and destroyed by the "White Star" ship.

In the Giants series by James P. Hogan says in Book 2: " A group of aliens from a race that was long gone, returns to cooperate with earthly scientists to the strange story of an earlier era of the solar system assemble before both species went their separate ways. "This happened on Ganymede.

The British- American scientist and science fiction author Charles Sheffield devoted an entire book series Ganymede and the other Galilean satellites. It is called The Dark Universe and includes three books, which are also present in German language edition:

  • Vol 1: Cold as Ice. 1992
  • Vol 2: The Ganymede Club. 1995
  • Vol 3: Dark as Day. 2002
360677
de