Jupiter is prominent in our night skies at the moment.
Look for a bright, untwinkling object in the south-east after dark and moving westward overnight. It is worth getting out the telescope. Even binoculars will show Jupiter as a disc with up to four starlike objects, all in a line, like beads on a wire. They are Jupiter's four largest moons.
Because they were discovered by Galileo, back in 1610, they are known as the Galilean moons. Moving outward from the planet, they are Io, Europa, Ganymede and Callisto. Over several nights, you will see the moons moving around the planet.
Jupiter is the fifth planet out from the Sun. We live on the third. It lies 5.2 times further from the Sun than our world, and consequently receives less than 4% of the light and heat we receive from the Sun. Out there, water should be a permanently frozen rock mineral. In addition, over the 4.5 billion years since the Solar System formed, small bodies, like the moons of Jupiter, should be solid all through, and geologically dead, like our Moon.
In 1979, the Voyager spacecraft turned our ideas about Jupiter's moons upside down. Our rather prosaic ideas about what they were like were replaced by facts that were far more exciting. This space mission, along with others that visited the system later, revolutionized our ideas as to where we might look for life beyond the Earth.
Io is the most volcanic and geologically active planet in the Solar System. As it approached, the Voyager spacecraft cameras showed great plumes of material being shot hundreds of kilometres up against a surface pocked with volcanoes, lava flows and great patches of yellow and orange sulphur.
The reason Io turned out to be so very different from what was expected is that as it orbits Jupiter, its inside is being kneaded and stretched by Jupiter's gravity, producing an enormous amount of heat, enough to melt rock. Io's interior is a lot hotter than the interior of Earth. Under the crust, the Earth's mantle is a sort of hot, plastic stuff, circulating slowly. Io's mantle is completely molten, hence the enthusiastic level of volcanism.
Europa, the next moon out, also looked spectacularly different. Instead of a cratered, rocky surface Europa is covered in an envelope of ice. There are cracks and what look like huge ice floes frozen in younger ice, just like what we see in the Arctic. There are few craters. This means the surface is continually being renewed. This, we believe, is due to there being a deep, salty ocean beneath the ice, kept from freezing by Jupiter's kneading the moon's interior. Since Europa is further from Jupiter than Io, the effect is less, maintaining an ocean rather than volcanoes. We believe life is a real possibility on Europa, maybe clustering around volcanic vents on the sea floor.
Ganymede, the next moon out, looks more like what we expected. Its surface is heavily cratered in some places, but there are areas of less cratered ice. This suggests that even though the level of Jupiter's tidal heating will be less than it is on Europa, there is some renewing of the surface, which raises the possibility of life there too.
Callisto is the outermost of the four and does not seem to be enjoying any of Jupiter's tidal heating. It is so heavily cratered it is likely that there has been little or no renewal of the surface since the moon formed 4.5 billion years ago. This moon turns out to be as we expected before Voyager.
Jupiter's moons have changed our ideas about where to look for life, and suggests it could be more widespread in the universe than we thought.
Although we continue to look for evidence of life on Mars, that world is perceived as dying, whereas Europa may be very much alive.
• Saturn lies in the south after sunset, with Jupiter in the south-east.
• Venus rises shortly before dawn.
• The Moon will be new on Dec. 12.
This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.