# Rocks/Rocky objects/Ganymede

(Redirected from Ganymede)
A true color image of Ganymede is acquired by the Galileo spacecraft on June 26, 1996. Credit: NASA/JPL.

"If Ganymede rotated around the Sun rather than around Jupiter, it would be classified as a planet."[1]

The Galilean Moons is a "name given to Jupiter's four largest moons, Io, Europa, Callisto & Ganymede. They were discovered independently by Galileo Galilei and Simon Marius."[2]

## Planetary sciences

This is global pictoral map of Ganymede. Credit: National Oceanic and Atmospheric Administration/USGS.

"Ganymede has a very distinct surface with bright and dark regions. The surface includes mountains, valleys, craters and lava flows. The darker regions are more heavily littered with craters implying that those regions are older. The largest dark region is named Galileo Regio and is almost 2000 miles [3200 km] in diameter. The lighter regions display extensive series of troughs and ridges, thought to be a result of tectonic movement."[1]

"A notable attribute of the craters on Ganymede is that they are not very deep and don’t have mountains around the edges of them as can normally be seen around craters on other moons and planets. The reason for this is that the crust of Ganymede is relatively soft and over a geological time frame has flattened out the extreme elevation changes."[1]

## Planetary astronomy

Jupiter and the Galilean moons are seen with SCUBA-2. Credit: University of British Columbia.
Jupiter with Europa (on the uttermost left), Callisto (nearest to Jupiter), and Ganymede is on the right. Credit: Stephen Rahn.{{free media}}
Galilean moons around Jupiter   Jupiter ·   Io ·   Europa ·   Ganymede ·   Callisto. Credit: Phoenix7777.{{free media}}

"Jupiter and the Galilean moons [are] seen with SCUBA-2 [in the image above]. Rather than seeing the sunlight reflected off the surface of these moons, such as Galileo did 400 years ago, this SCUBA-2 image shows the energy being radiated from the moons themselves."[3]

The order of submillimeter intensity appears to be Ganymede, Callisto, Io, and Europa.

On the right is a visual image of a portion of the Jupiter-Ganymede system.

## Color astronomy

This is a color image of Jupiter's moon Ganymede from 6 million km away by Voyager 2. Credit: NASA.

A "Voyager 2 image [is at the right] of Jupiter's largest moon Ganymede. This image was taken on 2 July 1979, from 6 million km, 6 days before Voyager 2's closest approach to the satellite. The light bluish regions near the north and south poles are visible in this image, possibly a result of water ice or frost."[4] The dark brown features at the top of Ganymede are shown on other images of this side of Ganymede.

## Theoretical Ganymede

Ganymede, larger than the planet Mercury, is the largest Jovian satellite. Credit: NASA/JPL/DLR.{{free media}}

Def. one of the Galilean moons of Jupiter, the seventh closest satellite to the planet and the largest satellite in the solar system with a diameter of 3,268 miles (5,260 km) is called Ganymede.

"Ganymede, larger than the planet Mercury, is the largest Jovian satellite. Its distinctive surface is characterized by patches of dark and light terrain. Bright frost is visible at the north and south poles. The very bright icy impact crater, Tros, is near the center of the image in a region known as Phrygia Sulcus. The dark area to the northwest of Tros is Perrine Regio; the dark terrain to the south and southeast is Nicholson Regio. Ganymede's surface is characterized by a high degree of crustal deformation. Much of the surface is covered by water ice, with a higher amount of rocky material in the darker areas. This global view was taken in September 1997 when Galileo was 1.68 million kilometers from Ganymede; the finest details that can be discerned are about 67 kilometers across."[5]

## X-rays

"Apart from the Sun, the known X-ray emitters now include planets (Venus, Earth, Mars, Jupiter, and Saturn), planetary satellites (Moon, Io, Europa, and Ganymede), all active comets, the Io plasma torus, the rings of Saturn, the coronae (exospheres) of Earth and Mars, and the heliosphere."[6]

## Violets

In this image of Ganymede's trailing side, the colors are enhanced to emphasize color differences. Credit: NASA/JPL/DLR.

"In this global view of Ganymede's trailing side, the colors are enhanced to emphasize color differences. The enhancement reveals frosty polar caps in addition to the two predominant terrains on Ganymede, bright, grooved terrain and older, dark furrowed areas. Many craters with diameters up to several dozen kilometers are visible. The violet hues at the poles may be the result of small particles of frost which would scatter more light at shorter wavelengths (the violet end of the spectrum). Ganymede's magnetic field, which was detected by the magnetometer on NASA's Galileo spacecraft in 1996, may be partly responsible for the appearance of the polar terrain. Compared to Earth's polar caps, Ganymede's polar terrain is relatively vast. The frost on Ganymede reaches latitudes as low as 40 degrees on average and 25 degrees at some locations. For comparison with Earth, Miami, Florida lies at 26 degrees north latitude, and Berlin, Germany is located at 52 degrees north."[7]

"North is to the top of the picture. The composite, which combines images taken with green, violet, and 1 micrometer filters, is centered at 306 degrees west longitude. The resolution is 9 kilometers (6 miles) per picture element. The images were taken on 29 March 1998 at a range of 918000 kilometers (570,000 miles) by the Solid State Imaging (SSI) system on NASA's Galileo spacecraft."[7]

## Oranges

This image of Ganymede is from Voyager 1. Credit: NASA.

If taken through the clear filter, the image at right suggests much of the surface of Ganymede was reflecting in the orange portion of the visual spectrum.

## Infrareds

Jupiter appears in pastel colors in this photo because the observation was taken in near-infrared light. Credit: NASA, ESA, and E. Karkoschka (University of Arizona).
This montage compares New Horizons' best views of Ganymede gathered with the spacecraft's Long Range Reconnaissance Imager and its infrared spectrometer. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.{{free media}}
This image shows an infrared reflection from Ganymede. Credit: Nolanus.

The image at right shows Jupiter in the near infrared. "Five spots -- one colored white, one blue, and three black are scattered across the upper half of the planet. Closer inspection by NASA's Hubble Space Telescope reveals that these spots are actually a rare alignment of three of Jupiter's largest moons -- Io, Ganymede, and Callisto -- across the planet's face. In this image, the telltale signatures of this alignment are the shadows [the three black circles] cast by the moons. Io's shadow is located just above center and to the left; Ganymede's on the planet's left edge; and Callisto's near the right edge. Only two of the moons, however, are visible in this image. Io is the white circle in the center of the image, and Ganymede is the blue circle at upper right. Callisto is out of the image and to the right. ... Jupiter appears in pastel colors in this photo because the observation was taken in near-infrared light. Astronomers combined images taken in three near-infrared wavelengths to make this color image. The photo shows sunlight reflected from Jupiter's clouds. In the near infrared, methane gas in Jupiter's atmosphere limits the penetration of sunlight, which causes clouds to appear in different colors depending on their altitude. Studying clouds in near-infrared light is very useful for scientists studying the layers of clouds that make up Jupiter's atmosphere. Yellow colors indicate high clouds; red colors lower clouds; and blue colors even lower clouds in Jupiter's atmosphere. The green color near the poles comes from a thin haze very high in the atmosphere. Ganymede's blue color comes from the absorption of water ice on its surface at longer wavelengths. Io's white color is from light reflected off bright sulfur compounds on the satellite's surface. ... In viewing this rare alignment, astronomers also tested a new imaging technique. To increase the sharpness of the near-infrared camera images, astronomers speeded up Hubble's tracking system so that Jupiter traveled through the telescope's field of view much faster than normal. This technique allowed scientists to take rapid-fire snapshots of the planet and its moons. They then combined the images into one single picture to show more details of the planet and its moons."[8]

The second image at the right shows an infrared image of Ganymede.

"This montage [on the left] compares New Horizons' best views of Ganymede, Jupiter's largest moon, gathered with the spacecraft's Long Range Reconnaissance Imager (LORRI) and its infrared spectrometer, the Linear Etalon Imaging Spectral Array (LEISA)."[9]

"LEISA observes its targets in more than 200 separate wavelengths of infrared light, allowing detailed analysis of their surface composition. The LEISA image shown here combines just three of these wavelengths -- 1.3, 1.8 and 2.0 micrometers -- to highlight differences in composition across Ganymede's surface. Blue colors represent relatively clean water ice, while brown colors show regions contaminated by dark material."[9]

"The right panel combines the high-resolution grayscale LORRI image with the color-coded compositional information from the LEISA image, producing a picture that combines the best of both data sets."[9]

"The LEISA and LORRI images were taken at 9:48 and 10:01 Universal Time, respectively, on February 27, 2007, from a range of 3.5 million kilometers (2.2 million miles). The longitude of the disk center is 38 degrees west."[9]

## Auroras

NASA Hubble Space Telescope images of Ganymede's auroral belts (colored blue in this illustration) are overlaid on a Galileo orbiter image of the moon. Credit: NASA/ESA.{{free media}}

"This discovery marks a significant milestone, highlighting what only Hubble can accomplish. In its 25 years in orbit, Hubble has made many scientific discoveries in our own solar system. A deep ocean under the icy crust of Ganymede opens up further exciting possibilities for life beyond Earth."[10]

"Ganymede is the largest moon in our solar system and the only moon with its own magnetic field. The magnetic field causes aurorae, which are ribbons of glowing, hot electrified gas, in regions circling the north and south poles of the moon. Because Ganymede is close to Jupiter, it is also embedded in Jupiter’s magnetic field. When Jupiter’s magnetic field changes, the aurorae on Ganymede also change, “rocking” back and forth."[11]

## Atmospheres

NASA's Hubble Space Telescope found ozone's spectral "fingerprint" during observations of Ganymede made by Keith Noll and colleagues at the Space Telescope Science Institute in Baltimore, Maryland. Credit: NASA.

"Though ozone may be diminishing on Earth, it is being manufactured one-half billion miles away, on Jupiter's largest satellite, Ganymede."[12]

"NASA's Hubble Space Telescope found ozone's spectral "fingerprint" during observations of Ganymede made by Keith Noll and colleagues at the Space Telescope Science Institute in Baltimore, Maryland. These Hubble Faint Object Spectrograph results were presented at the American Astronomical Society's 27th Annual Meeting of the Division of Planetary Sciences in Kona, Hawaii."[12]

## Craters

The image shows a chain of craters on Ganymede. Credit: Galileo Project, Brown University, JPL, NASA.
The craters Gula and Achelous (bottom) are shown in the grooved terrain of Ganymede, with ejecta "pedestals" and ramparts. Credit: NASA/JPL/Brown University.
A sharp boundary divides the ancient dark terrain of Nicholson Regio from the younger, finely striated bright terrain of Harpagia Sulcus. Credit: NASA/JPL/DLR.

The image at right shows a chain of 13 craters (Enki Catena) on Ganymede measuring 161.3 km in length. The Enki craters formed across the sharp boundary between areas of bright terrain and dark terrain, delimited by a thin trough running diagonally across the center of this image. The ejecta deposit surrounding the craters appears very bright on the bright terrain. Even though all the craters formed nearly simultaneously, it is difficult to discern any ejecta deposit on the dark terrain.

Cratering is seen on both types of terrain third image at right, but is especially extensive on the dark terrain: it appears to be saturated with impact craters and has evolved largely through impact events.[13] The brighter, grooved terrain contains many fewer impact features, which have been only of a minor importance to its tectonic evolution.[13] The density of cratering indicates an age of 4 billion years for the dark terrain, similar to the highlands of the Moon, and a somewhat younger age for the grooved terrain (but how much younger is uncertain).[14] Ganymede may have experienced a period of heavy cratering 3.5 to 4 billion years ago similar to that of the Moon.[14] If true, the vast majority of impacts happened in that epoch, while the cratering rate has been much smaller since.[15] Craters both overlay and are crosscut by the groove systems, indicating that some of the grooves are quite ancient. Relatively young craters with rays of ejecta are also visible.[15][16] Ganymedian craters are flatter than those on the Moon and Mercury. This is probably due to the relatively weak nature of Ganymede's icy crust, which can (or could) flow and thereby soften the relief. Ancient craters whose relief has disappeared leave only a "ghost" of a crater known as a palimpsest.[15]

The second image at right is an "[o]blique view of two fresh impact craters in bright grooved terrain near the north pole of Jupiter's moon, Ganymede. The craters postdate the grooved terrain since each is surrounded by swarms of smaller craters formed by material which was ejected out of the crater as it formed, and which subsequently reimpacted onto the surrounding surface. The crater to the north, Gula, which is 38 kilometers (km) in diameter, has a distinctive central peak, while the crater to the south, Achelous, (32 km in diameter) has an outer lobate ejecta deposit extending about a crater radius from the rim. Such images show the range of structural details of impact craters, and help in understanding the processes that form them."[17]

"North is to the top of the picture and the sun illuminates the surface from the right. The image, centered at 62 degrees latitude and 12 degrees longitude, covers an area approximately 142 by 132 kilometers. The resolution is 175 meters per picture element. The images were taken on April 5, 1997 at 6 hours, 33 minutes, 37 seconds Universal Time at a range of 17,531 kilometers by the Solid State Imaging (SSI) system on NASA's Galileo spacecraft."[17]

In the third image at right "[t]he ancient, dark terrain of Nicholson Regio (left) shows many large impact craters, and zones of fractures oriented generally parallel to the boundary between the dark and bright regions of Jupiter's moon Ganymede. In contrast, the bright terrain of Harpagia Sulcus (right) is less cratered and relatively smooth."[18]

"The nature of the boundary between ancient, dark terrain and younger, bright terrain, the two principal terrain types on Ganymede, was explored by NASA's Galileo spacecraft on May 20, 2000. Subtle parallel ridges and grooves show that Harpagia Sulcus's land has been smoothed out over the years by tectonic processes."[18]

"North is to the top of the picture. The Sun illuminates the surface from the left. The image, centered at ?14 degrees latitude and 319 degrees longitude, covers an area approximately 213 by 97 kilometers (132 by 60 miles.) The resolution is 121 meters (about 250 feet) per picture element. The images were taken on May 20, 2000, at a range of 11,800 kilometers (about 7,300 miles)."[18]

## Gravity astronomy

This animation of Ganymede was created from a mosaic of images taken by the Voyager spacecraft. Credit: Calvin J. Hamilton, Voyager, NASA.{{free media}}
The gravity anomalies or lumps inferred from the Galileo radio Doppler data are shown in red. Credit: NASA/JPL.{{free media}}

In the general case only planets (with their satellites) and stars could be considered "as free, as possible" to be used as some "antenna" to the gravitational waves detection. Spherical megascopic bodies have the folloving characteristic impedance:

${\displaystyle \rho _{GS}={\frac {v_{c}r_{s}}{3m_{s}}},\ }$

where

${\displaystyle v_{c}\ }$ is the equatorial velocity,
${\displaystyle r_{s}\ }$ spherical body radius and
${\displaystyle m_{s}\ }$ body mass.
Body name Body radius, m Equator velocity, m/s Body mass, kg Body impedance ${\displaystyle \rho _{G}/\rho _{G0}\ }$
Moon ${\displaystyle 1.738\cdot 10^{7}}$ 4.36 ${\displaystyle 7.35\cdot 10^{22}}$ ${\displaystyle 3.437\cdot 10^{-17}}$ ${\displaystyle 1.23\cdot 10^{1}}$
Titan ${\displaystyle 2.44\cdot 10^{7}}$ 276.8 ${\displaystyle 1.37\cdot 10^{23}}$ ${\displaystyle 1.643\cdot 10^{-15}}$ ${\displaystyle 5.875\cdot 10^{2}}$
Ganymede ${\displaystyle 2.6\cdot 10^{7}}$ 1116 ${\displaystyle 1.54\cdot 10^{23}}$ ${\displaystyle 6.28\cdot 10^{-15}}$ ${\displaystyle 12.245\cdot 10^{3}}$
Callisto ${\displaystyle 2.36\cdot 10^{6}}$ 252.7 ${\displaystyle 9.5\cdot 10^{22}}$ ${\displaystyle 2.092\cdot 10^{-17}}$ ${\displaystyle 7.48\cdot 10^{2}}$

Satellite data are taken from the textbook[19] As may be seen from the table, only the Moon has the closest value of characteristic impedance about 12-times greater than for free space.

"[I]rregular lumps [have been] discovered beneath the icy surface of Jupiter’s largest moon, Ganymede. These irregular masses may be rock formations, supported by Ganymede’s icy shell for billions of years."[20]

"This mosaic of Jupiter’s moon Ganymede [in the second image down on the right] consists of more than 100 images acquired with NASA’s Voyager and Galileo spacecrafts. The gravity anomalies or lumps inferred from the Galileo radio Doppler data are shown in red. The mosaic shows the surface of Ganymede with its geographic coordinate system and the Galileo gravity results superimposed."[20]

"The trajectory path of Galileo’s second Ganymede flyby on September 6, 1996, is shown in green. There are no obvious geologic features associated with the anomalies."[20]

## Astrogeology

Global Geologic Map of Ganymede is shown. Credit: USGS (United States Geological Survey).{{free media}}
NASA's Galileo spacecraft took this image of dark terrain in Nicholson Regio, near the border with Harpagia Sulcus on Jupiter's moon Ganymede. Credit: NASA/JPL/Brown University.
This mosaic (right) served as the base map for the geologic map of Ganymede (left). Credit: USGS Astrogeology Science Center/Wheaton/NASA/JPL-Caltech.{{free media}}

"Ganymede is the largest satellite of Jupiter, and its icy surface has been formed through a variety of impact cratering, tectonic, and possibly cryovolcanic processes. The history of Ganymede can be divided into three distinct phases: an early phase dominated by impact cratering and mixing of non-ice materials in the icy crust, a phase in the middle of its history marked by great tectonic upheaval, and a late quiescent phase characterized by a gradual drop in heat flow and further impact cratering. Images of Ganymede suitable for geologic mapping were collected during the flybys of Voyager 1 and Voyager 2 (1979), as well as during the Galileo Mission in orbit around Jupiter (1995–2003). This map represents a synthesis of our understanding of Ganymede geology after the conclusion of the Galileo Mission."[21]

"NASA's Galileo spacecraft took this image of dark terrain within Nicholson Regio, near the border with Harpagia Sulcus on Jupiter's moon Ganymede. The ancient, heavily cratered dark terrain is faulted by a series of scarps."[22]

"The faulted blocks form a series of "stair-steps" like a tilted stack of books. On Earth, similar types of features form when tectonic faulting breaks the crust and the intervening blocks are pulled apart and rotate. This image supports the notion that the boundary between bright and dark terrain is created by that type of extensional faulting."[22]

"North is to the right of the picture and the Sun illuminates the surface from the west (top). The image is centered at -14 degrees latitude and 320 degrees longitude, and covers an area approximately 16 by 15 kilometers (10 by 9 miles). The resolution is 20 meters (66 feet) per picture element. The image was taken on May 20, 2000, at a range of 2,090 kilometers (1,299 miles)."[22]

"To present the best information in a single view of Jupiter's moon Ganymede, a global image mosaic [on the left] was assembled, incorporating the best available imagery from NASA's Voyager 1 and 2 spacecraft and NASA's Galileo spacecraft. This image shows Ganymede centered at 200 west longitude. This mosaic (right) served as the base map for the geologic map of Ganymede (left)."[23]

## Astroglaciology

Voyager 2 image mosaic of Ganymede's anti-Jovian hemisphere. The ancient dark area of Galileo Regio lies at the upper right. It is separated from the smaller dark region of Marius Regio to its left by the brighter and younger band of Uruk Sulcus. Fresh ice ejected from the relatively recent Osiris Crater created the bright rays at the bottom. Credit: .

Ganymede is composed of approximately equal amounts of silicate rock and water ice.

The average density of Ganymede, 1.936 g/cm3, suggests a composition of approximately equal parts rocky material and water, which is mainly in the form of ice.[13] The mass fraction of ices is between 46–50%, slightly lower than that in Callisto.[24] Some additional volatile ices such as ammonia may also be present.[24][25]

Water ice seems to be ubiquitous on the surface, with a mass fraction of 50–90%,[13] significantly more than in Ganymede as a whole. Near-infrared spectroscopy has revealed the presence of strong water ice absorption bands at wavelengths of 1.04, 1.25, 1.5, 2.0 and 3.0 μm.[26] The grooved terrain is brighter and has more icy composition than the dark terrain.[27]

The forces that caused the strong stresses in the ganymedian ice lithosphere necessary to initiate the tectonic activity may be connected to the tidal heating events in the past, possibly caused when the satellite passed through unstable orbital resonances.[13][28] The tidal flexing of the ice may have heated the interior and strained the lithosphere, leading to the development of cracks and horst and graben faulting, which erased the old, dark terrain on 70% of the surface.[13][29] The formation of the grooved terrain may also be connected with the early core formation and subsequent tidal heating of the moon's interior, which may have caused a slight expansion of Ganymede by 1–6% due to phase transitions in ice and thermal expansion.[13]

Ganymede also has polar caps, likely composed of water frost. The frost extends to 40° latitude.[30] These polar caps were first seen by the Voyager spacecraft. Theories on the caps' formation include the migration of water to higher latitudes and bombardment of the ice by plasma. Data from Galileo suggests the latter is correct.[31]

Additional evidence of the oxygen atmosphere comes from spectral detection of gases trapped in the ice at the surface of Ganymede. The detection of ozone (O3) bands was announced in 1996.[32] In 1997 spectroscopic analysis revealed the dimer (or diatomic) absorption features of molecular oxygen. Such an absorption can arise only if the oxygen is in a dense phase. The best candidate is molecular oxygen trapped in ice. The depth of the dimer absorption bands depends on latitude and longitude, rather than on surface albedo—they tend to decrease with increasing latitude on Ganymede, while O3 shows an opposite trend.[33] Laboratory work has found that O2 would not cluster or bubble but dissolve in ice at Ganymede's relatively warm surface temperature of 100 K.[34]

Heavy ions continuously precipitate on the polar surface of the moon, sputtering and darkening the ice.[35]

## Astrognosy

This is a diagram of the internal structure of Ganymede. Credit: Kelvinsong.
The cut-out reveals an artist's impression of the interior structure of Ganymede. Credit: NASA/JPL.
This artist's concept of Jupiter's moon Ganymede, the largest moon in the solar system, illustrates the "club sandwich" model of its interior oceans. Credit: NASA/JPL-Caltech.

A variety of models have been suggested for the interior constitution of Ganymede. The first diagram at the right suggests ice, water, and an iron-based core.

The first image at the left suggests another interior: "The cut-out reveals the interior structure of this icy moon. This structure consists of four layers based on measurements of Ganymede's gravity field and theoretical analyses using Ganymede's known mass, size and density. Ganymede's surface is rich in water ice and Voyager and Galileo images show features which are evidence of geological and tectonic disruption of the surface in the past. As with the Earth, these geological features reflect forces and processes deep within Ganymede's interior. Based on geochemical and geophysical models, scientists expected Ganymede's interior to either consist of: a) an undifferentiated mixture of rock and ice or b) a differentiated structure with a large lunar sized "core" of rock and possibly iron overlain by a deep layer of warm soft ice capped by a thin cold rigid ice crust. Galileo's measurement of Ganymede's gravity field during its first and second encounters with the huge moon have basically confirmed the differentiated model and allowed scientists to estimate the size of these layers more accurately. In addition the data strongly suggest that a dense metallic core exists at the center of the rock core. This metallic core suggests a greater degree of heating at sometime in Ganymede's past than had been proposed before and may be the source of Ganymede's magnetic field discovered by Galileo's space physics experiments."[36]

The second image at the right suggests a variety of pressure-related ices similar to the model directly above it.

This "new model, based on experiments in the laboratory that simulate salty seas, shows that the ocean and ice may be stacked up in multiple layers, more like a club sandwich."[37]

"Ice comes in different forms depending on pressures. "Ice I," the least dense form of ice, is what floats in your chilled beverages. As pressures increase, ice molecules become more tightly packed and thus more dense. Because Ganymede's oceans are up to 500 miles (800 kilometers) deep, they would experience more pressure than Earth's oceans. The deepest and most dense form of ice thought to exist on Ganymede is called "Ice VI." [...] With enough salt, liquid in Ganymede can become dense enough to sink to the very bottom of the seafloor, below Ice VI. [...] What's more, the model shows that a strange phenomenon might occur in the uppermost liquid layer, where ice floats upward. In this scenario, cold plumes cause Ice III to form. As the ice forms, salt precipitates out. The salt then sinks down while the ice "snows" upward. Eventually, this ice would melt, resulting in a slushy layer in Ganymede's club sandwich structure."[37]

## Astrography

Derivative work of File:Map of Ganymede by Björn Jónsson.jpg, is centered on latitude 0 and longitude 0. Credit: NASA (original photographies)/Björn Jónsson/Feldo.{{free media}}

The image above is a centered astrographic project of the surface of Ganymede.

## Astromorphology

The image shows ridges and grooves penetrated by two apparent impact craters. Credit: NASA/JPL/Brown University.

"Complex sets of ridges and grooves are visible in this image of the Nippur Sulcus region on Jupiter's largest moon Ganymede. NASA's Galileo spacecraft imaged this region as it passed Ganymede during its second orbit through the Jovian system. The Nippur Sulcus region is an example of Bright Terrain on Ganymede which is typified by multiple sets of ridges and grooves. The intersections of these sets reveal complex age relationships. North is to the top of the picture and the sun illuminates the surface from the southeast (lower right). In this image a younger sinuous northwest-southeast trending groove set cuts through and apparently destroys the older east-west trending features on the right of the image, allowing scientists to determine the sequence of events that led to the region's formation. The area contains many impact craters. The large crater in the bottom of the image is about 12 kilometers (8 miles) in diameter."[38]

"The image, centered at 51 degrees latitude and 204 degrees longitude, covers an area approximately 79 kilometers (50 miles) by 57 kilometers (36 miles) across. The resolution is 93 meters (330 feet) per picture element. The images were taken on September 6, 1996 at a range of 9,971 kilometers (6,232 miles) by the solid state imaging (CCD) system on NASA's Galileo spacecraft."[38]

## Astromathematics

The Laplace resonances of Ganymede, Europa, and Io is illustrated. Credit: User:Matma Rex.

An orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. The physics principle behind orbital resonance is similar in concept to pushing a child on a swing, where the orbit and the swing both have a natural frequency, and the other body doing the "pushing" will act in periodic repetition to have a cumulative effect on the motion. Orbital resonances greatly enhance the mutual gravitational influence of the bodies, i.e., their ability to alter or constrain each other's orbits. In most cases, this results in an unstable interaction, in which the bodies exchange momentum and shift orbits until the resonance no longer exists. Under some circumstances, a resonant system can be stable and self-correcting, so that the bodies remain in resonance. Examples are the 1:2:4 resonance of Jupiter's moons Ganymede, Europa and Io, and the 2:3 resonance between Pluto and Neptune. Unstable resonances with Saturn's inner moons give rise to gaps in the rings of Saturn. The special case of 1:1 resonance (between bodies with similar orbital radii) causes large Solar System bodies to eject most other bodies sharing their orbits; this is part of the much more extensive process of clearing the neighbourhood, an effect that is used in the current definition of a planet.

## Exploratory astronomy

Pioneer 10 on its kick motor prior to encapsulation before launch. Credit: NASA Ames Resarch Center (NASA-ARC).
Ganymede appears more like Mars than the Moon or Mercury in this close-in spin-scan image from Pioneer 10. Credit: NASA.{{free media}}
The charged particle instrument (CPI) is used to detect cosmic rays in the solar system. Credit: NASA.
Ganymede is seen by Pioneer 10 on 1 July 1973. Credit: NASA Ames Resarch Center (NASA-ARC).{{free media}}
The cosmic-ray telescope collects data on the composition of the cosmic ray particles and their energy ranges. Credit: NASA.
Best images are of the four largest moons of Jupiter taken by the Pioneer 10 and Pioneer 11 spacecraft in 1973 and 1974. Credit: NASA.{{free media}}
The launch of Pioneer 10 aboard an Atlas/Centaur vehicle. Credit: NASA Ames Resarch Center (NASA-ARC).
This diagram shows the interplanetary trajectory for Pioneer 10. Credit: NASA.

"The two Pioneer spacecraft for the mission to Jupiter each weighed only about 570 pounds [~259 kg], yet carried eleven highly sophisticated instruments capable of operating unattended for many years in space. The spacecraft consumes less electrical power than a standard 100 watt lamp yet is able to accept instructions from Earth to control numerous operating modes of its scientific payload, process observations from these scientific instruments and format the observations into information usable on Earth. Even more remarkable, the spacecraft transmits a radio signal of only 8 watts power - equal to a nightlight - yet the information carried by the radio signal is received back on Earth from a distance of several billion miles."[39] and became the first spacecraft to escape the Solar System.

Pioneer 10 was launched on March 2, 1972 by an Atlas-Centaur from the John F. Kennedy Space Center, Florida.[39]

"On July 15, 1972, Pioneer 10 became the first spacecraft to enter the asteroid belt. Since the belt is too thick to fly over without prohibitively expensive launch vehicles, all missions to the outer planets must fly through it."[39]

"Based on a variety of analyses, project officials expected a safe passage, but the risk was always present that analyses from Earth could be wrong. Pioneer's closest approach to any of the known asteroids, visible by telescope, was 8.8 million km (5.5 million miles). One was a 1 km (1/2 mile) diameter asteroid on August 2, and the other was Nike 24 km (15 miles) in diameter on December 2, 1 972."[39]

"The Pioneers provided new information about the physical characteristics of the large Jovian satellites [...]. In terms of the mass of Earth's Moon (1/81 of Earth's mass), the masses of the satellites in order of distance from Jupiter are determined as: IO, 1.22; Europa, 0.67; Ganymede, 2.02; and Callisto, 1.44 lunar masses. [...] The density of the satellites decreases with increasing distance from Jupiter and was refined as the result of Pioneer 11 observations Io's density is 3.52 times that of water; Europa's, 3.28; Ganymede's, 1.95; and Callisto's, 1.63."[39]

"The Ganymede picture [first image on the left] resolves features to 400 km (240 mi.) and shows a south polar mare and a central mare, each about 800 km (480 mi.) in diameter, and a bright north polar region [...]. These isolated dark areas may, however, be areas where frost is not being formed as fast (by upwelling from the liquid watery interior) as evaporation takes place from a surface that is essentially without an atmosphere."[39]

"Probed by radar from Earth, Ganymede appears to have a surface rougher than Mercury, Mars or the Moon. This Jovian satellite may have a surface consisting of rocky or metallic material embedded in ice. While weathered smooth on the surface, the blocks of material within the ice would present a rough surface to the radar probing since the ice is relatively transparent to radar of the wavelengths used."[39]

"The dynamics of the Jovian magnetosphere with its charged particles appear to differ in important ways from those of the Earth. First the presence of substantial intensities of electrons having energies greater than 20 MeV in the outer magnetosphere cannot be explained by trapping of particles from the solar wind. Such solar wind particles could only reach about 1 keV. The corotation of energetic particles with Jupiter persists out to the magnetopause, whereas for the Earth corotation terminates at the outer boundary of the plasmasphere, far inside the magnetopause. The inner satellites, Amalthea, Io, Europa, and Ganymede, produce a fluctuating and complex structure of energetic particles."[39]

"Pioneer 11 measured large reductions in electron flux for energies below 560 keV and in proton flux for energies around 2.1 MeV as the spacecraft crossed the orbit of lo. Smaller effects were observed at the orbit of Amalthea, and only a rather feeble effect was seen at the orbit of Europa. However, near the orbit of Ganymede, Pioneer 11 detected strong transient anisotropic bursts of 1 MeV protons. One sequence of one-minute bursts continued for several hours. These particles appear to be locally accelerated."[39]

"The Pioneer 11 spacecraft discovered a high electron current flow at the orbit of Ganymede. Such an increase in electron flow had not been observed at the other passages of the Pioneers through satellite orbits. Near the Io flux tube the magnetic field line of Jupiter extending to Io along which scientists had speculated that large currents should flow- Pioneer 11 detected an increase of about ten times the flux of electrons with energies above 0.46 MeV."[39]

## Hypotheses

1. Ganymede is primarily a rocky object.

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