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

Space probe 'Galileo' is going low to examine one of Jupiter's satellites. By Heather Couper and Nigel Henbest

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.

Suggested Topics
Start your day with The Independent, sign up for daily news emails
PROMOTED VIDEO
ebooks
ebooksA special investigation by Andy McSmith
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

Tradewind Recruitment: PMLD Teacher

Negotiable: Tradewind Recruitment: PMLD Teacher A specialist primary school i...

Recruitment Genius: Online Media Sales Trainee

£15000 - £30000 per annum: Recruitment Genius: Now our rapidly expanding and A...

Recruitment Genius: Public House Manager / Management Couples

£15000 - £20000 per annum: Recruitment Genius: Are you passionate about great ...

Recruitment Genius: Production Planner

£20000 - £30000 per annum: Recruitment Genius: This fast growing reinforcing s...

Day In a Page

As in 1942, Germany must show restraint over Greece

As in 1942, Germany must show restraint over Greece

Mussolini tried to warn his ally of the danger of bringing the country to its knees. So should we, says Patrick Cockburn
Britain's widening poverty gap should be causing outrage at the start of the election campaign

The short stroll that should be our walk of shame

Courting the global elite has failed to benefit Britain, as the vast disparity in wealth on display in the capital shows
Homeless Veterans appeal: The rise of the working poor: when having a job cannot prevent poverty

Homeless Veterans appeal

The rise of the working poor: when having a job cannot prevent poverty
Prince Charles the saviour of the nation? A new book highlights concerns about how political he will be when he eventually becomes king

Prince Charles the saviour of the nation?

A new book highlights concerns about how political he will be when he eventually becomes king
How books can defeat Isis: Patrick Cockburn was able to update his agenda-setting 'The Rise of Islamic State' while under attack in Baghdad

How books can defeat Isis

Patrick Cockburn was able to update his agenda-setting 'The Rise of Islamic State' while under attack in Baghdad
Judith Hackitt: The myths of elf 'n' safety

Judith Hackitt: The myths of elf 'n' safety

She may be in charge of minimising our risks of injury, but the chair of the Health and Safety Executive still wants children to be able to hurt themselves
The open loathing between Barack Obama and Benjamin Netanyahu just got worse

The open loathing between Obama and Netanyahu just got worse

The Israeli PM's relationship with the Obama has always been chilly, but going over the President's head on Iran will do him no favours, says Rupert Cornwell
French chefs get 'le huff' as nation slips down global cuisine rankings

French chefs get 'le huff' as nation slips down global cuisine rankings

Fury at British best restaurants survey sees French magazine produce a rival list
Star choreographer Matthew Bourne gives young carers a chance to perform at Sadler's Wells

Young carers to make dance debut

What happened when superstar choreographer Matthew Bourne encouraged 27 teenage carers to think about themselves for once?
Design Council's 70th anniversary: Four of the most intriguing prototypes from Ones to Watch

Design Council's 70th anniversary

Four of the most intriguing prototypes from Ones to Watch
Dame Harriet Walter: The actress on learning what it is to age, plastic surgery, and her unease at being honoured by the establishment

Dame Harriet Walter interview

The actress on learning what it is to age, plastic surgery, and her unease at being honoured by the establishment
Art should not be a slave to the ideas driving it

Art should not be a slave to the ideas driving it

Critics of Tom Stoppard's new play seem to agree that cerebral can never trump character, says DJ Taylor
Bill Granger recipes: Our chef's winter salads will make you feel energised through February

Bill Granger's winter salads

Salads aren't just a bit on the side, says our chef - their crunch, colour and natural goodness are perfect for a midwinter pick-me-up
England vs Wales: Cool head George Ford ready to put out dragon fire

George Ford: Cool head ready to put out dragon fire

No 10’s calmness under pressure will be key for England in Cardiff
Michael Calvin: Time for Old Firm to put aside bigotry and forge new links

Michael Calvin's Last Word

Time for Old Firm to put aside bigotry and forge new links