In addition to being the largest moon in the solar system, it is also larger than even the planet Mercury. It is the only satellite in the solar system known to possess a magnetosphere, has a thin oxygen atmosphere, and much like its fellow-moons, Europa and Callisto is believed to have an interior ocean.
Though Chinese astronomical records claim that astronomer Gan De may have spotted a moon of Jupiter probably Ganymede with the naked eye as early as BCE, Galileo Galilei is credited with making the first recorded observation of Ganymede on January 7th, using his telescope.
Simon Marius, a German astronomer and contemporary of Galileo's who claimed to have independently discovered Ganymede, suggested alternative names at the behest of Johannes Kepler. However, the names of Io, Europa, Ganymede and Callisto — which were all taken from classical mythology — would not come to formally be adopted until the 20th century. Following the discovery of the moons of Saturn, a naming system based on that of Kepler and Marius was used for Jupiter's moons.
In Greek mythology, Ganymede was the son of King Tros aka. Ilion , the namesake of the city of Troy Ilium. With a mean radius of However, with a mass of 1.
This is due to Ganymede's composition, which consists of water ice and silicate rock see below. Ganymede's orbit has a minor eccentricity of 0. Ganymede takes seven days and three hours to completes a single revolution. Like most known moons, Ganymede is tidally locked, with one side always facing toward the planet. Its orbit is inclined to the Jovian equator, with the eccentricity and inclination changing quasi-periodically due to solar and planetary gravitational perturbations on a timescale of centuries.
These orbital variations cause the axial tilt to vary between 0 and 0. Ganymede has a orbital resonance with Io and a resonance with Europa. Essentially, this means that Io orbits Jupiter four times and Europa twice for every orbit made by Ganymede.
The superior conjunction between Io and Europa occurs when Io is at periapsis and Europa is at apoapsis, and the superior conjunction between Europa and Ganymede occurs when Europa is at periapsis. Such a complicated resonance a resonance is called the Laplace Resonance. With an average density of 1. Near-infrared and ultra-violet surveys have also revealed the presence of carbon dioxide, sulfur dioxide, and possibly cyanogen, hydrogen sulfate and various organic compounds.
More recent data has shown evidence of salts such as magnesium sulfate and possibly sodium sulfate, which may have originated from the subterranean ocean see below. Ganymede's interior appears to be fully differentiated, consisting of a solid inner core made of iron, a liquid iron and iron-sulfide outer core, a silicate mantle, and a a spherical shell of mostly ice surrounding the rock shell and the core.
The core is believed to measure km in radius, and has a temperature of about — K and pressure of up to 10 GPa. The most compelling evidence for the existence of a liquid, iron-nickel-rich core is Ganymede's intrinsic magnetic field. 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 believed to be 5. This mantle is composed of silicate materials, most likely chondrites and iron. The outer ice shell is the largest layer of all, measuring an estimated km miles thick.
The precise thicknesses of this and other layers in the interior of Ganymede depends on the assumed composition of silicates and amount of sulfur in the core. Scientists also believe that Ganymede has a thick ocean nestled between two layers of ice — a tetragonal layer between it and the core and a hexagonal layer above it.
The presence of this ocean has been confirmed by readings taken by orbiters and through studies of how Ganymede's aurora behaves. In short, the moon's auroras are affected by Ganymede's magnetic field, which in turn is affected by the presence of a large, subsurface salt-water ocean.
Ganymede's surface is a mix of two types of terrain. There's the very old, highly cratered, and dark regions, and the somewhat younger, lighter regions marked with an extensive array of grooves and ridges. In a way that is similar to Europa, Ganymede's surface is asymmetric, with the leading hemisphere being brighter than the trailing one. The dark terrain, which comprises about one-third of the surface, is so-colored because the surface ice in these regions contains clays and organic materials.
It has been theorized that these have been left behind by impactors, which accords with the fact that impact craters are far more extensive in the areas of dark terrain. Meanwhile, the grooved terrain is believed to be tectonic in nature; which could be due in part to cryovolcanism, but is thought to be mostly the result of tidal heating events.
Though craters are more common in the darker areas, they are seen all over the surface. Ganymede may have experienced a period of heavy cratering 3. If true, the vast majority of impacts happened in that epoch, whereas the cratering rate has been much smaller since. Io is the most volcanically active moon in our solar system, with plumes of material reaching up to kilometres from the surface, spewing out masses of what is possibly either silicate rock or sulfur-rich material into space.
However, scientists believe that water may have been present on Io early in its formation, and there is a possibility that some form of life may exist underground.
Underneath a spiky, icy crust , Europa is thought to have a huge, planet-wide liquid water ocean. All life on Earth requires liquid water, and so what better place to look than an ocean?
This sort of activity is a plus for life. Ganymede has three main layers : A metallic iron core at its center, a spherical shell of rock mantle surrounding the core, and a spherical shell of mostly ice surrounding the rock shell. The surface is the very top of the ice shell. These irregular masses may be rock formations, supported by Ganymede's icy shell for billions of years.
Computer models show Ganymede might have ice and oceans stacked up in several layers like a club sandwich. Scientists first suspected Ganymede had an underground ocean in the s. The Galileo spacecraft discovered Ganymede's magnetic field in , providing evidence to support the theory. Spacecraft images of Ganymede show its surface is a mix of two types of terrain. Forty percent of the surface is covered by highly cratered dark regions , and the remaining sixty percent is covered by a light grooved terrain , which forms intricate patterns across the moon.
Some of the groove ridges are as high as 2, feet meters , and the grooves run for thousands of miles across Ganymede's surface. The large craters on Ganymede are mostly flat, probably due to settling in the soft icy surface.
Both bright and dark rays of ejecta can be seen around Ganymede's craters. In , scientists discovered irregular lumps beneath the icy surface of Ganymede. The irregular masses may be rock formations, supported by Ganymede's icy shell for billions of years. This tells scientists that the ice is probably strong enough, at least near the surface, to support such rock masses from sinking to the bottom of the ice.
However, this anomaly could also be caused by piles of rock at the bottom of the ice. Astronomers using the Hubble Space Telescope have found evidence of a thin oxygen atmosphere on Ganymede. Ganymede is much colder than Earth, with daytime surface temperatures ranging from to degrees Fahrenheit 90 to Kelvin.
This moon also has sulfur dioxide snowfields, leading to its characterization as a moon of fire and ice. Io has an iron or iron sulfide core and a brown silicate outer layer, which gives it a splotchy orange, yellow, black, red, and white appearance. Moving outward from Jupiter is Europa. Cracks and streaks crisscross the entire icy surface, which is marked with very few craters. Europa has a high degree of reflectivity, making it among the brightest moons in the solar system.
At 20 to million years old, the surface is fairly young. It is possible that an extensive ocean beneath the surface harbors life.
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