Ganymede - Just the facts (A article written in 2007)
Ganymede is Jupiter's largest moon, & the largest moon in the entire solar system; it is larger in diameter than Mercury but only about half its mass. Ganymede is one of four planetary satellites of Jupiter which can be seen with the naked eye — with good eyesight, a clear night & without the pollution haze of cities. Other than these, the only other planetary satellite visible with the naked eye is our own moon.
It may have been discovered
by Chinese astronomer Gan De in 364 BC. However, discovery of the moon is generally
credited to Galileo Galilei who documented its existence in 1610. The name Ganymede
was suggested soon after by Simon Marius, for the cup-bearer of the Greek gods,
beloved of Zeus (see Ganymede). This name & the names of the other Galilean
satellites fell into disfavor for a considerable time, & were not revived
in common use until the mid-20th century. In much of the earlier astronomical
literature, it is simply referred to by its Roman numeral designation as Jupiter
III or as the "third satellite of Jupiter". Ganymede is the only Galilean
moon of Jupiter named after a male figure.
Physical characteristics
of the Moon Ganymede
Interior of Ganymede
Internal structure
Ganymede
is composed of silicate rock & water ice, with an ice crust floating over
a warmer ice mantle that may contain a layer of liquid water. Indications from
the Galileo orbiter data suggest that Ganymede is differentiated into a three
layer structure: a small partially molten iron or iron/sulfur core surrounded
by a rocky silicate mantle with an icy shell on top. This metallic core suggests
a greater degree of heating at some time in Ganymede's past than had previously
been proposed. In fact, Ganymede may be similar in its internal structure to Io.
Surface features
The Ganymedean surface is a mix of two types of terrain:
very old, highly cratered dark regions & somewhat younger (but still ancient)
lighter regions marked with an extensive array of grooves & ridges. Their
origin is clearly of a tectonic nature, probably formed by the extension, stretching,
& faulting of the icy crust. Analogous tectonic features form the ridges &
valleys in the Basin & Range province of the southwestern United States. Features
reminiscent of old lava flows have also been observed. Similar ridge & groove
terrain is seen on Enceladus, Miranda & Ariel. The dark regions are similar
to the surface of Callisto.
Extensive cratering is seen on both types of terrain. The density of cratering indicates an age of 4 billion years for the dark terrain, similar to the highlands of the Moon, & a somewhat younger age for the bright grooved terrain (but how much younger is uncertain). Craters both overlay & are cross cut by the groove systems indicating that some of the grooves are quite ancient. Relatively young craters with rays of ejecta are also visible. Unlike on the Moon, however, Ganymedean craters are quite flat, lacking the ring mountains & central depressions common to craters on the Moon & Mercury. This is probably due to the relatively weak nature of Ganymede's icy crust which can flow & thereby soften the relief. Ancient craters whose relief has disappeared leaving only a "ghost" of a crater are known as palimpsests.
The largest feature on Ganymede is a dark plain named Galileo Regio, as well as a series of concentric grooves, or furrows, that are remnants of an ancient impact crater long since obscured by subsequent geological activity.
Atmosphere
In
the mid-1980s, a team of Indian & American astronomers working at Indonesia's
Lembang Observatory detected a thin atmosphere around Ganymede during an occultation
when Jupiter passed in front of a star. Evidence for a tenuous oxygen atmosphere
on Ganymede, very similar to the one found on Europa, has been found by the Hubble
Space Telescope. Note that this is not necessarily evidence of life: it is thought
that the oxygen is produced when water ice on Ganymede's surface is split into
hydrogen & oxygen by radiation & then the hydrogen is lost due to its
low atomic mass.
Magnetosphere
The Galileo orbiter's first flyby of Ganymede
discovered that Ganymede has its own magnetic field, embedded inside Jupiter's
huge field. Ganymede is the only moon known to have a magnetosphere. Ganymede's
intrinsic magnetic field is probably generated in a similar fashion to the Earth's:
as a result of conducting material moving in the interior, likely originating
in its metallic core. Ganymede also has an induced magnetic field component, indicating
that the satellite contains a subsurface layer that acts as a conductor. It is
thought that this conductive material is a layer of liquid water containing salt,
located at about 150 km depth & sandwiched between layers of different density
forms of ice.
Possibility of Life
It has been suggested that Ganymede
may be amenable to life. However, scientists say that the chances of life existing
on Europa are much higher than on Ganymede, because Europa's ocean is closer to
the surface than Ganymede's.
While temperatures are extremely low, &
there is very little atmosphere, it does have its own magnetic field that might
protect life from harmful radiation, & it lies in a relatively quiet radio
zone around Jupiter unlike Europa.
An Index with links to almost all our sites.
A Picture of Jupiter, & a Picture of Florida from Space A Top picture of Stars A Nebulas A Picture of Buzz Aldrin on the Moon
A site on space, & the records to do with this subject, even more than this site, to do with space
A list of Planets that could be colonised by humanity
A site saying the 10 most famous aliens ever
A Clock saying how many people have landed on each planet in our Solar System
Jokes to say to aliens that are offending you
The Highland Clearances & it's full terribleness
A site on the Belgian Congo, & how the king of that land killed 10s of millions of Congolese
Why the French Revolution was good
The most evil regimes of the 19th Century
What would happen if Marsians invaded
A Multiple Choice Quiz on Space
What were the nicest regimes ever
The Best regimes ever in terms of achievers
How Many people have landed on each of our Planets see, in this space clock
Worst 17th Century regimes ever
A site stating what have been the world's largest empires ever
What would happen in a war between these sides
What were the most evil regimes ever
Click here to Find a Cottage in Britain and Ireland
A list stating what were the worst 1990s regimes
What were the worst 16th Century regimes ever
What were the worst 15th Century regimes ever
What were the worst 2000s regimes
A site stating the 10 largest majority English speaking lands, as their main tongue in the world
A site on a time traveling revolutionary
The world's 10 most powerful countries in 2008
My Worst regimes of the 20th century essay
My worst regimes of the 20th century stats
Our History Lounge - Where you can peruse many fascinating historical articles.