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
Voices
The Sumatran tiger, endemic to the Indonesian island of Sumatra, is an endangered species
voicesJonathon Porritt: The wild tiger population is thought to have dropped by 97 per cent since 1900
News
The Swiss Re tower or 'Gherkin' was at one time the UK’s most expensive office when German bank IVG and private equity firm Evans Randall bought it
news
Arts and Entertainment
tv
News
Gardai wait for the naked man, who had gone for a skinny dip in Belfast Lough
newsTwo skinny dippers threatened with inclusion on sex offenders’ register as naturists criminalised
PROMOTED VIDEO
Sport
Van Gaal said that his challenge in taking over Bobby Robson's Barcelona team in 1993 has been easier than the task of resurrecting the current United side
football
Sport
Moeen Ali wearing the 'Save Gaza' and 'Free Palestine' wristbands on his left arm
cricket
Arts and Entertainment
The Great British Bake Off 2014 contestants
tv
Life and Style
tech
Arts and Entertainment
filmThe Battle of the Five Armies trailer released
News
news
Arts and Entertainment
Beast would strip to his underpants and take to the stage with a slogan scrawled on his bare chest whilst fans shouted “you fat bastard” at him
musicIndie music promoter was was a feature at Carter gigs
Arts and Entertainment
Story line: Susanoo slays the Yamata no Orochi serpent in the Japanese version of a myth dating back 40,000 years
arts + entsApplying the theory of evolution to the world's many mythologies
News
ebookA unique anthology of reporting and analysis of a crucial period of history
Extras
indybest
News
Performers dressed as Tunnocks chocolate teacakes, a renowned Scottish confectionary, perform during the opening ceremony of the 2014 Commonwealth Games at Celtic Park in Glasgow on July 23, 2014.
news
Life and Style
Popular plonk: Lambrusco is selling strong
Food + drinkNaff Seventies corner-shop staple is this year's Aperol Spritz
Life and Style
Shake down: Michelle and Barack Obama bump knuckles before an election night rally in Minnesota in 2008, the 'Washington Post' called it 'the fist bump heard round the world'
newsThe pound, a.k.a. the dap, greatly improves hygiene
Arts and Entertainment
La Roux
music
Independent
Travel Shop
the manor
Up to 70% off luxury travel
on city breaks Find out more
santorini
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
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

Chemical Engineer/Project Coordinator

£40000 - £45000 per annum + competitive: Progressive Recruitment: Chemical Eng...

Project Manager,Conduct Risk,London,£5-600pd

£500 - £600 per day + competitive: Orgtel: My client, a leading bank, is curre...

PMO Manager (Portfolio Management, ExCel, Cost Benefit Analysis)

£450 - £500 Per Day: Clearwater People Solutions Ltd: PMO Manager - 6 month co...

Senior Fund Administrator - Edinburgh - £22 p/hr

£20 - £22 per hour + competitive: Orgtel: Senior Fund Administrator, Top Four ...

Day In a Page

The children were playing in the street with toy guns. The air strikes were tragically real

The air strikes were tragically real

The children were playing in the street with toy guns
Boozy, ignorant, intolerant, but very polite – The British, as others see us

Britain as others see us

Boozy, ignorant, intolerant, but very polite
Countries that don’t survey their tigers risk losing them altogether

Countries that don’t survey their tigers risk losing them

Jonathon Porritt sounds the alarm
How did our legends really begin?

How did our legends really begin?

Applying the theory of evolution to the world's many mythologies
Watch out: Lambrusco is back on the menu

Lambrusco is back on the menu

Naff Seventies corner-shop staple is this year's Aperol Spritz
A new Russian revolution: Cracks start to appear in Putin’s Kremlin power bloc

A new Russian revolution

Cracks start to appear in Putin’s Kremlin power bloc
Eugene de Kock: Apartheid’s sadistic killer that his country cannot forgive

Apartheid’s sadistic killer that his country cannot forgive

The debate rages in South Africa over whether Eugene de Kock should ever be released from jail
Standing my ground: If sitting is bad for your health, what happens when you stay on your feet for a whole month?

Standing my ground

If sitting is bad for your health, what happens when you stay on your feet for a whole month?
Commonwealth Games 2014: Dai Greene prays for chance to rebuild after injury agony

Greene prays for chance to rebuild after injury agony

Welsh hurdler was World, European and Commonwealth champion, but then the injuries crept in
Israel-Gaza conflict: Secret report helps Israelis to hide facts

Patrick Cockburn: Secret report helps Israel to hide facts

The slickness of Israel's spokesmen is rooted in directions set down by pollster Frank Luntz
The man who dared to go on holiday

The man who dared to go on holiday

New York's mayor has taken a vacation - in a nation that has still to enforce paid leave, it caused quite a stir, reports Rupert Cornwell
Best comedians: How the professionals go about their funny business, from Sarah Millican to Marcus Brigstocke

Best comedians: How the professionals go about their funny business

For all those wanting to know how stand-ups keep standing, here are some of the best moments
The Guest List 2014: Forget the Man Booker longlist, Literary Editor Katy Guest offers her alternative picks

The Guest List 2014

Forget the Man Booker longlist, Literary Editor Katy Guest offers her alternative picks
Jokes on Hollywood: 'With comedy film audiences shrinking, it’s time to move on'

Jokes on Hollywood

With comedy film audiences shrinking, it’s time to move on