Ganymede (moon)

The Jovian moon Ganymede was the first natural satellite, other than Earth’s Moon, to be discovered. It is also the largest satellite in the solar system—large enough to generate its own magnetic field, an unusual characteristic for a satellite.

Overview

Ganymede, the largest satellite of Jupiter, was discovered by Galileo Galilei with a telescope in 1610. He published the information in Sidereus Nuncius (starry messenger) and thereby initiated a dispute with the Roman Catholic Church that would eventually lead him to be placed under house arrest for the remainder of his life; it was heresy to say that anything revolved around something other than Earth. The satellite was named by Simon Marius for one of the lovers of the Roman god Jupiter.

Ganymede is approximately 5,260 kilometers in diameter and is just over one million kilometers from Jupiter; it is the third of Jupiter's four largest moons. It is common for a satellite to present the same face to its planet at all times—a relationship called synchronous—and Ganymede does this, just as Earth’s Moon does. The rotation of Ganymede is prograde; that is, it is in the same direction as that of Jupiter. Ganymede’s orbit is almost circular, meaning that its eccentricity (the measure of how close to a circular orbit the satellite travels) is small. A circular orbit has an eccentricity of zero. Ganymede’s angle of inclination is less than a degree, meaning that this moon rotates almost exactly in the plane of Jupiter’s equator.

Ganymede’s albedo, the amount of sunlight reflected, is large. This reflectivity is caused by ice mixed with carbon-rich soil on the surface of the satellite. When the ice underneath the surface is heated and melts, it erupts to the surface. The soil, which is denser than water, sinks below the water. The water freezes, causing a bright spot on the surface. The water is heated either by radioactive decay or by tidal flexing. Not only does the gravity of Jupiter and Callisto pull on Ganymede, the moon also has Laplace resonance, which occurs because of the forces from the satellites Io and Europa. Every time Ganymede rotates around Jupiter once, Europa, the satellite just inside Ganymede, goes around Jupiter twice, and Io, the moon inside Europa, goes around four times. Thus, during every orbit, the three satellites are aligned, magnifying the gravitational effect. This increased gravitational pull and then relaxation not only causes the orbits to become elliptical but also causes stresses within the satellites themselves. This tidal flexing generates heat that melts ice and causes the surface of Ganymede to be smoother than expected. Many of Ganymede’s impact craters have had their depths reduced by the changing surface of Ganymede.

The surface of Ganymede is a mixture of bright areas and dark regions. The dark regions are heavily cratered; the bright areas are less cratered but are often grooved. The bright areas consist of the soil left after the ice has sublimed away. The surface appears much like the surface would if there were lava flow. Water melted by the tidal flexing, then freezing as it reaches the surface, may still flow much as glaciers flow on Earth. The heavily cratered regions are older and often show fractures and grooves through the craters where water has broken through the surface at a crater. Ice on Ganymede causes the reflection of radar to be much greater than on most other satellites. Ice also allows radar to penetrate more deeply into Ganymede than if the surface were all silicates. The percentage of ice has been measured at 45–55 percent.

The bulk density of Ganymede is between that of ice and that of carbonaceous silicates, indicating a mixture of the two materials. Ganymede has differentiated; that is, the components have separated, producing a core of dense metals, probably iron or iron with sulfur. A molten iron core is usually the reason for a magnetic field, which Ganymede does have. One model, which agrees with the measured values, postulates a large core of iron with 10 percent sulfur. The moon has a core with a radius of 695 kilometers, a silicate mantle, and a 900-kilometer-thick ice-water shell. Bombardment by meteors causes a change in the albedo of Ganymede in two ways. First, the meteor causes new, darker material to be thrown up onto the surface (the underlying silicates are darker than the ice or residue left by subliming ice). Second, meteor impacts cause the loss of volatile material, leaving an opaque, dark material.

Ganymede has a thin atmosphere composed of electrically charged gases. One gas seen in the space around Ganymede is hydrogen. Water sublimed from the surface or escaping from a surface fracture condenses at the poles, producing a whitish polar cap down to latitudes of about 40°. Near-infrared spectra show the expected water and hydrated minerals. Unexpected are the indications of carbon dioxide, carbon bonded to hydrogen, carbon triple-bonded to nitrogen, sulfur bonded to hydrogen, and sulfur dioxide. The carbon dioxide appears to be trapped in the surface, perhaps in small bubbles. Jupiter’s magnetic field causes ions to be swept along the satellite's orbit, generating a current producing an auroral spotlight onto the poles of Jupiter.

Ganymede has an intrinsic magnetic field that is opposite to the field of Jupiter. It also displays an induced magnetic field caused by the strong rotating, angled field of Jupiter. The induced field is an indication of a conducting ocean deep under the icy surface. If the ocean has enough minerals dissolved in it to make it strongly conducting, it could generate the intrinsic magnetic field. Jupiter’s strong magnetic field causes Ganymede to be bombarded by charged particles. This bombardment is thought to cause the molecular oxygen, O², and ozone, O³, found on the surface of Ganymede.

Since Ganymede’s orbit is in the same plane as Jupiter, it is thought that they were formed by the same process. Ganymede is away from the very hot, dense region where the planet formed. Ganymede was formed in a cooler region, where water did not boil away but instead froze to form part of the satellite. Although Ganymede is locked into the same face toward Jupiter all the time, there are indications that this may not have always been true. One clue is that the number of meteor craters should be greater on the leading side of Ganymede, as is the case with Callisto, but this is not true of Ganymede. Another fact pointing to a change in the part of the ice shell facing Jupiter is the catenae that are found on the back side of Ganymede. Catenae are caused by a string of fragments from a comet that was broken up by the intense magnetic field of Jupiter but escaped capture to hit one of the satellites. They should occur only on the Jupiter-facing side of Ganymede.

Knowledge Gained

The Jovian system, as Jupiter and its moons are called, has been visited by several space missions. Pioneer 10 (1973), Pioneer 11 (1974), Voyager 1 and Voyager 2 (both in 1979), New Horizons (2007), and Juno (2016) flew through the system. They all used gravitational assists to gain speed travel on toward the outer part of the solar system.

The Pioneer spacecraft provided the first visual images of the surface of Ganymede, as well as a much better estimate of the size and mass of Ganymede. The Voyager spacecraft improved these images to a resolution of a kilometer and provided color by means of six filters. Scientists were able to develop ideas of how the satellite formed and its structure. The Voyager data on craters indicated either that Ganymede’s surface has changed and thus erased the early craters from meteor hits or that the surface was not firm enough to retain the early craters.

The Galileo mission arrived in orbit about Jupiter in 1996. The visual camera increased the resolution of the surface to twenty meters. The model of the moon, showing its core, mantle, and shell of ice, was developed after data from Galileo provided Ganymede’s mass, average density, and moment of inertia. The moment of inertia and average density required the data on gravitational fields produced by Galileo and how the flight path was perturbed as the craft flew by the satellite. Galileo also provided information on the magnetic fields using a magnetometer. The dual purpose of the magnetometer was to determine if Ganymede had a magnetic field of its own and how the satellites interacted with the strong magnetic field of Jupiter. Galileo used its Near Infrared Mapping Spectrometer (NIMS) to make a compositional map of the surface.

While studying Jupiter was not the main focus of New Horizons, astronomers did not let the opportunity escape. The data had been transmitted to Earth and analyzed from the New Horizons mission, added some interesting information. The spacecraft’s infrared Linear Etalon Imaging Spectral Array (LEISA) and its panchromatic Long-Range Reconnaissance Imager (LORRI) charge-coupled device camera mapped Ganymede’s composition. These instruments’ resolutions are better than any land-based instrument or Galileo’s NIMS. Low-temperature crystalline ice was found as expected, but asymmetric bands of non-ice were found, especially in the darker regions. More ice is found in bright regions and in craters and ejecta from meteor hits. This correlates with darker material on the surface, except where meteor strikes have brought ice to the surface. New Horizons could also map parts of Ganymede that Galileo could not see.

Not all information is gathered by spacecraft. The Hubble Space Telescope (HST) has taken pictures of the auroras of Jupiter. Other types of data, such as those gathered by eclipse radiometry, can be used from Hubble or from Earth. Eclipse radiometry is the measurement of thermal radiation just as the satellite is eclipsed by the planet. For Ganymede, these studies suggest that heat is lost rapidly; therefore, the surface material must be porous due to bombardment from meteors over millions of years.

Context

The National Aeronautics and Space Administration (NASA) is teaming with the European Space Agency (ESA) to launch an unmanned spacecraft that will conduct experiments and studies of Jupiter and three of its largest moons. The Jupiter Icy Moons Explorer (JUICE) mission launched in April 2023 on an eight-year journey to Jupiter, where NASA scheduled a three-year mission to study the planet and its moons, with the potential to extend the project if possible.

The more astronomers learn about large bodies like Ganymede, the more is revealed about how the solar system was formed and about Earth and its Moon. Ganymede may show the action of plate tectonics, which will, in turn, provide valuable information to scientists about Earth's tectonically rearranged continents.

Additionally, each space mission has returned valuable information on how to survive in space. Not only are meteorites a danger, but gravitational wells, and especially strong magnetic fields, can damage a spacecraft. When humans venture forth, all those dangers must be considered. The number of craters on Ganymede gives scientists some indication of the chance of a meteor hitting the Earth.

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