Welcome to Cheshire Starwatch,
a weekly column by Keene, NH amateur astronomer James Maynard.
Stardate 07:28:97
Since Jupiter will be ruling the summer skies this year, let us,
this week, continue in our study of the king of the planets. Last week,
we looked at Jupiter itself, and only breezed over it's companion satellites.
This week, we will look at the moons of Jupiter, and next week we will discuss the exciting new findings from the Galileo probe now orbiting Jupiter. The moons we will discuss in the most detail will be Jupiter's four largest moons, discovered by the astronomer Galileo in 1610. It is for this reason these four moons of Jupiter are known as the "Galilean moons" of Jupiter.
The innermost of these is called Io (correctly pronounced Ee-oo, not Eye- oo, as most people (including professional astronomers) are want to say.
This small worldlet is 1816 km (1135 miles)
across, and has a density similar to that of our Moon. This is a dynamic
world of changing colors, and is the only body in the solar system, other
than the Earth, known to have active volcanoes. The really exciting bit
about this is what the volcanoes are 
spewing out - sulfur dioxide.
This means the volcanoes on Io glow in the dark! In fact, the volcanoes
of Io are throwing so much material above the surface, that Jupiter is surrounded
by a ring of sulfur, oxygen, and sodium circling Jupiter at the same distance
as Io. Be careful not to confuse this gaseous ring with the thin, dark ring
discovered by Voyager in 1979. That ring is composed of small, dark pebbles.
In addition, a smaller, non-Galilean moon inside the orbit of Io, Amalthea, seems to be covered with sulfur deposited by the volcanism of Io. There are very few craters to be found on Io, so it seems that the volcanoes are acting as a method of erosion. Surprisingly, the volcanoes of Io release about three times as much energy as they should for the temperature there. Io is far too small for there to be any heat left over from it's formation, or for there to be enough radioactive materials left in the core to account for the amount of energy being released.
So, where is the excess energy coming from? The answer seems to be tidal forces as it orbits Jupiter, The surface of Io I lifts just about 100 meters (325 feet) at "high tide"- Compare this to the Earth, where the seas rise only about 90 cm (3 feet) and the land rises about 30 cm (1 foot) at high tide.
The next Galilean satellite we encounter on our journey away from
Jupiter is Europa. This is a rocky world, with a density just slightly less
than that of Io. It seems likely that the same tidal forces which are giving
Io extra energy may also be adding heat to Europa- New findings from the
Galileo spacecraft suggest that Europa may be covered by a liquid water
ocean, under a layer of ice. Since the building blocks of life are hydrocarbons
(literally, molecules made from hydrogen and carbon), and there have been
hydrocarbons found in the vicinity of Europa, we now know the building blocks
of life are to be found fixating around in an ocean of liquid (possibly
warm!) water. More on that next week when we discuss the Galileo findings
Like Io, Europe shows no signs of cratering, and must have some method of
erosion.
The third large satellite on our journey away from Jupiter is the Galilean moon Ganymede. This is the largest of the four moons, 2638 km (about 1650 miles) in diameter. This makes Ganymede about 50 percent larger than our own Ione natural satellite. Ganymede has a density only about twice that of water, This suggests that Ganymede may be composed largely of ice, of one sort or another.
The last moon we encounter before leaving Jupiter for another week is Callisto. Nearly the same size as Ganymede, Callisto is even less dense than the former moon. Callisto has a dirty, heavily cratered surface, suggesting an old surface, without any major form of erosion to rework the surface.
Clear skies, and good viewing.
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