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


23 6.49 pm moon at first quarter

25 Uranus at opposition

30 11.36 am full moon.

Suggested Topics
peopleGerman paper published pictures of 18-month-old daughter
Arts and Entertainment
'A voice untroubled by time': Kate Bush
musicKate Bush set to re-enter album charts after first conerts in 35 years
Life and Style
Arts and Entertainment
Robin Williams' life story will be told in a biography written by a New York Times reporter
arts + ents
Life and Style
food + drink
Life and Style
ebooksAn evocation of the conflict through the eyes of those who lived through it
Roger Federer is greeted by Michael Jordan following his victory over Marinko Matosevic
tennisRoger Federer gets Michael Jordan's applause following tweener shot in win over Marinko Matosevic
peopleJustin Bieber accuses paparazzi of acting 'recklessly' after car crash
Arts and Entertainment
Oppressive atmosphere: the cast of 'Tyrant'
tvIntroducing Tyrant, one of the most hotly anticipated dramas of the year
Travel Shop
the manor
Up to 70% off luxury travel
on city breaks Find out more
Up to 70% off luxury travel
on chic beach resorts Find out more
sardina foodie
Up to 70% off luxury travel
on country retreats Find out more
Have you tried new the Independent Digital Edition apps?
Independent Dating

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

iJobs Job Widget
iJobs General

Qualified Nursery Nurse

Negotiable: Randstad Education Crawley: This independent Nursery is looking fo...

Qualified Nursery Nurse

Negotiable: Randstad Education Crawley: This independent Nursery is looking fo...

Merger and Acquisition Project Manager

£500 - £550 Per Day: Clearwater People Solutions Ltd: Our client are currently...

SEN Teaching Assistant

£50 - £55 per day: Randstad Education Cheshire: SEN TAWe are looking to recrui...

Day In a Page

Kate Bush, Hammersmith Apollo music review: A preamble, then a coup de théâtre - and suddenly the long wait felt worth it

Kate Bush shows a voice untroubled by time

A preamble, then a coup de théâtre - and suddenly the long wait felt worth it
Robot sheepdog technology could be used to save people from burning buildings

The science of herding is cracked

Mathematical model would allow robots to be programmed to control crowds and save people from burning buildings
Tyrant: Is the world ready for a Middle Eastern 'Dallas'?

This tyrant doesn’t rule

It’s billed as a Middle Eastern ‘Dallas’, so why does Fox’s new drama have a white British star?
Rachael Lander interview: From strung out to playing strings

From strung out to playing strings

Award-winning cellist Rachael Lander’s career was almost destroyed by the alcohol she drank to fight stage fright. Now she’s playing with Elbow and Ellie Goulding
The science of saturated fat: A big fat surprise about nutrition?

A big fat surprise about nutrition?

The science linking saturated fats to heart disease and other health issues has never been sound. Nina Teicholz looks at how governments started advising incorrectly on diets
Emmys 2014 review: Can they genuinely compete with the Oscars

Can they genuinely compete with the Oscars?

The recent Emmy Awards are certainly glamorous, but they can't beat their movie cousins
On the road to nowhere: A Routemaster trip to remember

On the road to nowhere

A Routemaster trip to remember
Hotel India: Mumbai's Taj Mahal Palace leaves its darker days behind

Hotel India

Mumbai's Taj Mahal Palace leaves its darker days behind
10 best pencil cases

Back to school: 10 best pencil cases

Whether it’s their first day at school, uni or a new project, treat the student in your life to some smart stationery
Arsenal vs Besiktas Champions League qualifier: Gunners know battle with Turks is a season-defining fixture

Arsenal know battle with Besiktas is a season-defining fixture

Arsene Wenger admits his below-strength side will have to improve on last week’s show to pass tough test
Pete Jenson: Athletic Bilbao’s locals-only transfer policy shows success does not need to be bought

Pete Jenson: A Different League

Athletic Bilbao’s locals-only transfer policy shows success does not need to be bought
This guitar riff has been voted greatest of all time

The Greatest Guitar Riff of all time

Whole Lotta Votes from Radio 2 listeners
Britain’s superstar ballerina

Britain’s superstar ballerina

Alicia Markova danced... every night of the week and twice on Saturdays
Berlin's Furrie invasion

Berlin's Furrie invasion

2000 fans attended Eurofeurence
‘It was a tidal wave of terror’

‘It was a tidal wave of terror’

Driven to the edge by postpartum psychosis