Science: A close-up look at the biggest moon of all

Six months on from its triumphal arrival at Jupiter, Nasa's spacecraft Galileo is about to send back its first pictures of the giant planet and some of its moons, including the most detailed views ever seen of a moon that is almost as big as the planet Mars.

Galileo swings past Ganymede, the largest moon in the solar system, early on Thursday morning. It skims a mere 844 km over the surface - 70 times nearer than the previous closest encounter - with Voyager 2 in 1979. Galileo would be able to make out individual buildings on Ganymede, if any existed.

The main scientific return will be to understand this moon's peculiar geology. The pictures sent back by Voyager 2 reveal that 40 per cent of Ganymede's surface is covered by large dark patches, strangely reminiscent of our Moon. There are numerous craters, blasted out by cosmic impacts, although the craters of Ganymede are surprisingly flat.

Geologists believe the walls have slumped because the surface is made of a mixture of rock and ice, which can gradually flow (like a glacier on Earth) and flatten out under its own weight. The largest dark area is named Galileo Regio. Fittingly, the spacecraft bearing the same name will be homing in on this region.

Its cameras will also be investigating the strangest feature of Ganymede - the paler areas lying between dark regions. Voyager's cameras showed that they consist of long grooves separated by parallel ridges. Some ridges are 700 metres high, and stretch for thousands of kilometres. Geologists call these areas "sulcus", meaning a groove or burrow.

The sulcus areas probably formed as the dark regions moved apart. On Earth, a similar stretching of the crust has created the parallel mountain ranges of Nevada and Utah. But some geologists support a different theory - that the sulcus was caused when ice below Ganymede's surface melted; as the water escaped upwards, the surface collapsed into wrinkles.

Not only Ganymede will be in the frame this week. Galileo will be taking more distant views of Jupiter's other big moons - Io, Europa and Callisto, and its first close-up views of the planet. Until now, Galileo has been travelling blind.

Mission controllers have kept the cameras switched off so far because of two different problems. The umbrella-like main antenna, which sends radio signals back to Earth, failed to open properly as Galileo sped toward Jupiter, so pictures can only be sent back at a very slow rate by a smaller antenna. Galileo must store the pictures on a tape recorder and gradually send them back to Earth over a period of weeks. The images snapped this week, for example, won't be coming back to Earth until late in July.

As Galileo was set to take a preliminary set of pictures on its approach to Jupiter last December, however, the tape recorder stuck in 'rewind' position. For safety's sake, Nasa controllers cancelled the imaging session and switched off the recorder. They now believe they have figured out what went wrong and have found a way around it, so it should not be a problem in future.

Even though Galileo was blind as it swept past Io last December, it has provided interesting news about this moon. Nasa scientists have now analysed in detail how Io's gravity disturbed the spacecraft's path, and found it must have a very dense core. It probably consists of iron, like the Earth's core, and there are hints that Io's core may also be generating a magnetic field. If so, it is the first magnetic moon to be found in the solar system.

Over the past few months, Nasa's researchers have also been re-analysing results from the probe that Galileo dropped into Jupiter's atmosphere last December. They show that Jupiter has powerful winds blowing not just at cloud-top level, but reaching deep into its interior. Unlike Earth's weather, which is driven by the Sun's heat from above, it seems Jupiter's weather is controlled by heat coming up from deep in the planet's hot centre.

What's Up

With Jupiter in the news, take a look at the giant world for yourself. It's closest to the Earth this year on 4 July, shining more brilliantly than any star, low in the south during the late evening. (The outer planets Uranus and Neptune are also at their closest and brightest this month, but you won't see these with the naked eye.)

With a pair of binoculars held steadily, you should be able to make out Jupiter's four biggest moons. They appear as tiny specks of light, endlessly circling the parent world. At the beginning of this week, the outermost one, Callisto, is to the left of the planet, with huge Ganymede further in. By the weekend, Ganymede lies to the right of Jupiter, with Callisto further out on the same side.

A small telescope will give you a better view, though with 'left' and 'right' in the above descriptions reversed as a telescope inverts the image. It should also show the light and dark bands on Jupiter, and reveal that the planet is slightly flattened, to a tangerine shape, because it spins around so rapidly; a 'day' on Jupiter is less than 10 hours long.

Diary (all times BST)

July 1 4.59 am full moon

4 Jupiter at opposition

7 7.55 pm moon at last quarter

15 5.15 pm new moon

18 Neptune at

opposition

23 6.49 pm moon at first quarter

25 Uranus at opposition

30 11.36 am full moon.