The Galilean satellites of Jupiter

One of the most interesting aspects of Jupiter is its system of satellites (moons). It has 16 in all, but the four Galilean satellites are the most interesting. These are Io, Europa, Ganymede, and Callisto. All are about the size of the Earth’s moon .

They are one of the favorite objects of amateur astronomers, since they are very intriguing to see in a small telescope. At about 8PM our time when we look at Jupiter, it will look like the picture below.

The two dots off to the right side are two of the moons. The other two will be right in front of Jupiter.

When I began studying astronomy in the 1960s, almost nothing was known about their surface features. The representation of them in the film 2001: A Space Odyssey was plausible, but didn’t anticipate the variety that actually exists.

A family picture of them is given below, which shows them to scale. You can see immediately that each one is distinctive in appearance. Almost everything we know about them comes from exploration in the “space age”. For reference, the Earth’s Moon is about the size of Io (upper left).

A simulation of the Galileo spacecraft swinging by Europa is given in http://www.jpl.nasa.gov/galileo/europa/e19anim.html

Let’s look at their distances and orbital periods (around Jupiter) first.

Moon	Distance (km)	Period (days)	Density (g/cc)
Io	422,000	1.77	3.6
Europa	671,000	3.55	2.87
Ganymede	1,070,000	7.16	1.94
Callisto	1,883,000	16.69	1.86

There are a couple of things worth emphasizing at the start about the Galilean satellites. (1) There is a great deal of excitement about them nowadays. In fact, Europa is considered one of the main targets (along with Mars) of the search for life elsewhere in the solar system. (2) To state the conclusion at the start, the geology of the Galilean satellites is determined by tides from Jupiter. The strong gravitational force of Jupiter causes tides which flex the satellites, causing internal heating. The inner satellites are much more active objects than they would be if they were just out in space.

The Galileo orbiter has provided huge amounts of information on these objects. What we know now is vastly more than was available ten years ago. Let’s take a tour.

Callisto is the furthest out .

It has a density of 1.86 grams/cubic cm. This means it can’t be rock, and must be largely composed of ice. At the temperature of Callisto (130K) ice is a mineral. The surface of Callisto is heavily cratered, and you can see shatter-marks around some of the big craters (see Figure 14.21). From the nearly continuous crater cover, we conclude that not much has happened on Callisto since the “good old days”.

A surprise in the last year is that even though Callisto appears geologically dead, it may have a buried ocean. A magnetometer (instrument that measures magnetic fields) showed an anomaly when it passed near Callisto. One explanation for this is if there were a shell of salt water (a good conductor of electricity) beneath the surface.

Ganymede is the biggest Moon in the solar system.

The density of 1.94 grams per cubic centimeter means it has somewhat more rock, but must also be mainly ice. Unlike Callisto, Ganymede shows geological diversity. Although there are heavily cratered parts, there are also parts where craters are few and far between. There are stress marks on the surface that some people interpret as evidence of tectonic forces on Ganymede. There also seem to be features which might be the result of water gushing from the interior of the world. This increased internal activity is due to the fact that Ganymede is closer to Jupiter, and the tides are stronger.

Europa has become one of the most interesting objects in the solar system.

It has been known since the Voyager flyby missions in the 1970’s that the moon is cased in a covering of water ice. This is responsible for its cue-ball like appearance in figure 14.3. Its density is 2.87, which is nearly the density of rock. Apparently Europa is like a terrestrial planet with an icy case.

The Galileo orbiter has made many close flybys of Europa, and in some of these passes it almost could have seen aliens on the surface (those pictures have been suppressed in a government coverup, however).

Let’s look at a couple of pictures of Europa, showing different levels of detail. Remember that you are looking at the number 2 object in the search for life in the solar system.

The first picture shows a detail of the surface. This picture is about 780 miles on a side.

Since the Galileo spacecraft flew within a few hundred miles of the surface of Europa, it took pictures with stunning detail. It showed that the icy planes are criss-crossed with strange cracks and fractures, which often cross over earlier cracks. In many cases, it looks as though fluid as seeped through these cracks and on to the surrounding surface. The picture below shows an region that is about 12 miles on a side, and shows the details of the surface.

The exact nature of these cracks in the ice are unknown, and will probably be unknown until we get a lander on the surface of Europa.

The evidence has tended to corroborate speculations that there might be a liquid ocean under the ice.

There are long cracks in the ice and evidence that water has flowed up through them and on to the surface. (See figure 14.16)
There are features which look like icebergs in the frozen arctic ocean on Earth. This suggests that these blocks are moving around on top of a liquid ocean..
Some of the ( rare) craters look like the projectile punched through to a liquid layer.
Crater counts indicate that the surface that we now see is less than 10 million years old. Melting and refreezing of the ice is one of the easiest ways to accomplish this.

Educated guesses are that the thickness of the ice is from a few kilometers to a few tens of kilometers in thickness. A current estimate of the interior structure of Europa is seen in Figure 14.14 of the book, or a slightly different representation below.

What makes Europa particularly exciting is recent discoveries on Earth as well as in space. It has been found that life abounds around “black smoker” vents at the bottom of the ocean. It is unclear if some of these species might have originally evolved down there. If this could have happened on Earth, geothermal vents on a tidally-heated Europa might have served as home to horrible, monstrous alien forms of life. NASA has plans for a Europa orbiter to try and learn more about this planet. In the future, we can practically count on robots landing and drilling through the ice.

Io is the closest and the most vigorously tidally heated.

Because of this anomalous interior heating, the moon has vigorous volcanic activity. The rate of these processes is so furious that the landscape changes on timescales of only a few years (see Figure 14.11). Not surprisingly, there are no craters on visible on Io, although geological robots could doubtlessly find evidence of paleocraters.

The Galileo spacecraft returned close up pictures of the surfaces of all of the Galilean satellites. In these pictures, we see the great differences in landscapes on these moons. Differences that point to quite different histories for these objects. In the picture below, the top row shows a global view of the moon. The middle row shows a region on the moon about 600 miles on a side. Finally, the bottom row shows a scene about 60 miles on a side.

These pictures show a range from dormant, cratered Callisto to volcanically reworked Io. The Galilean satellites are a solar system in miniature.

There is an intriguing speculation about Europa. Very early in the history of the solar system, it appears likely that Jupiter was an emitter of radiation, that is, it shown like a little star. In that case, the Galilean satellites would have truly been like a miniature solar system. Some scientists believe that there may have been a period in which the ice we now see covering Europa was in the form of an ocean. This is illustrated in an artist’s conception in a painting by Don Davis.

One can then go on to speculate as to whether life may have gotten a start in that ocean, and whether it has survived to the present time in the ocean under miles of ice on the surface of Europa. The answer is out there.