Ganymede: Moon Of Jupiter, Internal Structure, Atmosphere, Exploration

Ganymede(Moon Of Jupiter)

(Ganymede: Moon Of Jupiter, Internal Structure, Atmosphere, Exploration)

Ganymede /ˈɡænᵻmiːd/ (Jupiter III) is the largest and most massive moon of Jupiter and in the Solar System. The ninth largest object in the Solar System, it is the largest without a substantial atmosphere. It has a diameter of 5,268 km (3,273 mi) and is 8% larger than the planet Mercury, although only 45% as massive. Possessing a metallic core, it has the lowest moment of inertia factor of any solid body in the Solar System and is the only moon known to have a magnetic field. It is the third of the Galilean moons, the first group of objects discovered orbiting another planet, and the seventh satellite outward from Jupiter, Ganymede orbits Jupiter in roughly seven days and is in a 1:2:4 orbital resonance with the moons Europa and Io, respectively.

Ganymede is composed of approximately equal amounts of silicate rock and water ice. It is a fully differentiated body with an iron-rich, liquid core, and an internal ocean that may contain more water than all of Earth's oceans combined. Its surface is composed of two main types of terrain. Dark regions, saturated with impact craters and dated to four billion years ago, cover about a third of the satellite. Lighter regions, crosscut by extensive grooves and ridges and only slightly less ancient, cover the remainder. The cause of the light terrain's disrupted geology is not fully known, but was likely the result of tectonic activity due to tidal heating.

Ganymede's magnetic field is probably created by convection within its liquid iron core. The meager magnetic field is buried within Jupiter's much larger magnetic field and would show only as a local perturbation of the field lines. The satellite has a thin oxygenatmosphere that includes O, O2, and possibly O3 (ozone). Atomic hydrogen is a minor atmospheric constituent. Whether the satellite has an ionosphere associated with its atmosphere is unresolved.

Ganymede's discovery is credited to Galileo Galilei, who was the first to observe it on January 7, 1610. The satellite's name was soon suggested by astronomer Simon Marius, for the mythological Ganymede, cupbearer of the Greek gods and Zeus's lover. Beginning with Pioneer 10, several spacecraft have explored Ganymede. The Voyager probes refined measurements of its size, while Galileodiscovered its underground ocean and magnetic field. The next planned mission to the Jovian system is the European Space Agency's Jupiter Icy Moon Explorer (JUICE), due to launch in 2022. After flybys of all three icy Galilean moons, the probe is planned to enter orbit around Ganymede.

History

Chinese astronomical records report that in 365 BC, Gan De detected what appears to have been a moon of Jupiter, probably Ganymede, with the naked eye. However, Gan De reported the color of the companion as reddish, which is puzzling since the moons are too faint for their color to be perceived with the naked eye. Shi Shen and Gan De together made fairly accurate observations of the five major planets.

On January 7, 1610, Galileo Galilei observed what he thought were three stars near Jupiter, including what turned out to be Ganymede, Callisto, and one body that turned out to be the combined light from Io and Europa; the next night he noticed that they had moved. On January 13, he saw all four at once for the first time, but had seen each of the moons before this date at least once. By January 15, Galileo came to the conclusion that the stars were actually bodies orbiting Jupiter. He claimed the right to name the moons; he considered "Cosmian Stars" and settled on "Medicean Stars".

The French astronomer Nicolas-Claude Fabri de Peiresc suggested individual names from the Medici family for the moons, but his proposal was not taken up. Simon Marius, who had originally claimed to have found the Galilean satellites, tried to name the moons the "Saturn of Jupiter", the "Jupiter of Jupiter" (this was Ganymede), the "Venus of Jupiter", and the "Mercury of Jupiter", another nomenclature that never caught on. From a suggestion by Johannes Kepler, Marius once again tried to name the moons:

“ Then there was Ganymede, the handsome son of King Tros, whom Jupiter, having taken the form of an eagle, transported to heaven on his back, as poets fabulously tell  the Third, on account of its majesty of light, Ganymede ”

This name and those of the other Galilean satellites fell into disfavor for a considerable time, and were not in common use until the mid-20th century. In much of the earlier astronomical literature, Ganymede is referred to instead by its Roman numeral designation, Jupiter III (a system introduced by Galileo), in other words "the third satellite of Jupiter". Following the discovery of moons of Saturn, a naming system based on that of Kepler and Marius was used for Jupiter's moons. Ganymede is the only Galilean moon of Jupiter named after a male figure—like Io, Europa, and Callisto, he was a lover of Zeus.

Internal structure

Ganymede appears to be fully differentiated, with an internal structure consisting of an iron-sulfide–iron core, a silicate mantle and outer layers of water ice and liquid water. The precise thicknesses of the different layers in the interior of Ganymede depend on the assumed composition of silicates (fraction of olivine and pyroxene) and amount of sulfur in the core. Ganymede has the lowest moment of inertia factor, 0.31, among the solid Solar System bodies. This is a consequence of its substantial water content and fully differentiated interior.

Subsurface oceans 

In the 1970s, NASA scientists first suspected that Ganymede has a thick ocean between two layers of ice, one on the surface and one beneath a liquid ocean and atop the rocky mantle. In the 1990s, NASA's Galileo mission flew by Ganymede, confirming the moon's sub-surface ocean. An analysis published in 2014, taking into account the realistic thermodynamics for water and effects of salt, suggests that Ganymede might have a stack of several ocean layers separated by different phases of ice, with the lowest liquid layer adjacent to the rocky mantle. Water–rock contact may be an important factor in the origin of life. The analysis also notes that the extreme depths involved (~800 km to the rocky "seafloor") mean that temperatures at the bottom of a convective (adiabatic) ocean can be up to 40 K higher than those at the ice–water interface. In March 2015, scientists reported that measurements with the Hubble Space Telescope of how the aurorae moved over Ganymede's surface suggest it has a subsurface ocean. A large salt-water ocean affects Ganymede's magnetic field, and consequently, its aurora.

Core

The existence of a liquid, iron–nickel-rich core provides a natural explanation for the intrinsic magnetic field of Ganymede detected by Galileo spacecraft. The convection in the liquid iron, which has high electrical conductivity, is the most reasonable model of magnetic field generation. The density of the core is 5.5–6 g/cm3 and the silicate mantle is 3.4–3.6 g/cm3. The radius of this core may be up to 500 km. The temperature in the core of Ganymede is probably 1500–1700 K and pressure up to 10 GPa (99,000 atm) (GPa).

Exploration

Completed missions

Several probes flying by or orbiting Jupiter have explored Ganymede more closely, including four flybys in the 1970s, and multiple passes in the 1990s to 2000s.

Pioneer 10 approached in 1973 and Pioneer 11 in 1974, and they returned information about the satellite. This included more specific determination on physical characteristics and resolving features to 400 km (250 mi) on its surface. Pioneer 10's closest approach was 446,250 km.

Voyager 1 and Voyager 2 were next, passing by Ganymede in 1979. They refined its size, revealing it was larger than Saturn's moon Titan, which was previously thought to have been bigger. The grooved terrain was also seen.

In 1995, the Galileo spacecraft entered orbit around Jupiter and between 1996 and 2000 made six close flybys to explore Ganymede.These flybys are G1, G2, G7, G8, G28 and G29. During the closest flyby—G2—Galileo passed just 264 km from the surface of Ganymede. During a G1 flyby in 1996, the Ganymedian magnetic field was discovered, while the discovery of the ocean was announced in 2001. Galileo transmitted a large number of spectral images and discovered several non-ice compounds on the surface of Ganymede. The most recent close observations of Ganymede were made by New Horizons, which recorded topographic and compositional mapping data of Europa and Ganymede during its flyby of Jupiter in 2007 en route to Pluto.

Upcoming mission concepts

The Europa Jupiter System Mission (EJSM) had a proposed launch date in 2020, and was a joint NASA and ESA proposal for exploration of many of Jupiter's moons including Ganymede. In February 2009 it was announced that ESA and NASA had given this mission priority ahead of the Titan Saturn System Mission. EJSM consisted of the NASA-led Jupiter Europa Orbiter, the ESA-led Jupiter Ganymede Orbiter, and possibly a JAXA-led Jupiter Magnetospheric Orbiter. ESA's contribution faced funding competition from other ESA projects, but on 2 May 2012 the European part of the mission, renamed Jupiter Icy Moon Explorer (JUICE), obtained a L1 launch slot in 2022 with a Ariane 5 in the ESA's Cosmic Vision science programme. The spacecraft will orbit Ganymede and conduct multiple flyby investigations of Callisto and Europa.

The Russian Space Research Institute is currently evaluating the Ganymede Lander (GL) mission, with emphasis on astrobiology. The Ganymede Lander would be a partner mission for JUpiter ICy moon Explorer (JUICE). If selected, it would be launched in 2024, though this schedule might be revised and aligned with JUICE.

A Ganymede orbiter based on the Juno probe was proposed in 2010 for the Planetary Science Decadal Survey. Possible instruments include Medium Resolution Camera, Flux Gate Magnetometer, Visible/NIR Imaging Spectrometer, Laser Altimeter, Low and High Energy Plasma Packages, Ion and Neutral Mass Spectrometer, UV Imaging Spectrometer, Radio and Plasma Wave sensor, Narrow Angle Camera, and a Sub-Surface Radar.

Another canceled proposal to orbit Ganymede was the Jupiter Icy Moons Orbiter. It was designed to use nuclear fission for power, ion enginepropulsion, and would have studied Ganymede in greater detail than previously. However, the mission was canceled in 2005 because of budget cuts. Another old proposal was called The Grandeur of Ganymede.