Faint echoes of what our neighbours are like: Peter Bond looks at advances in the technology

RADAR, the technique of bouncing microwaves off solid objects and listening for their echoes, is becoming an increasingly powerful tool for learning about our neighbours in the solar system.

In the next few weeks, radar astronomers hope to obtain the best images to date of 4179 Toutatis, the asteroid that makes its closest approach to Earth tomorrow. They hope these pictures will show details as small as 160m (525ft) across.

New techniques and modern equipment have allowed a revolution in the astronomical use of ground-based radar since the first tentative efforts in the late Fifties. Scientists at Goldstone in California and at Arecibo in Puerto Rico are now able to detect features as small as 70m across (250ft) on Mars, 120km (74 miles) on Mercury, and 1.5km (1 mile) on Venus.

Although Earth-based observations of Venus have now been surpassed by the incredibly detailed radar images from the Magellan orbiter, the same cannot be said of the other planets. Often hidden in the glare of the Sun, Mercury has always been a difficult world to explore and has long been recognised as one of the most inhospitable planets in the solar system. With a surface temperature of 430C, it would hardly seem to be the ideal place to find water ice. Yet this is just what American scientists announced at the end of last year.

One spacecraft, Mariner 10, has so far visited Mercury, but it was only able to send back pictures of the hemisphere it passed over. Until recently little was known about the planet's other side. In August last year a team led by Professor Duane Muhleman, of the California Institute of Technology, succeeded in obtaining the first detailed images of Mercury's 'hidden' hemisphere.

They used the 70-metre radio dish at Goldstone to transmit a 500kW signal over a period of eight hours. The faint echoes were then picked up by 27 linked radio telescopes in New Mexico. The resulting images were dominated by an elliptical feature 300km- 600km (186-370 miles) across, centred around Mercury's north pole.

Comparisons with similar studies of the Martian ice-caps and the frozen surfaces of Jupiter's satellites led to the startling conclusion that these enhanced echoes are caused by a highly fractured layer of ice.

Further studies this spring indicated a similar, though much smaller, feature close to the planet's south pole. According to Dr Martin Slade of the Jet Propulsion Laboratory (JPL), the location of the radar-bright patch corresponds to a 100km-wide (62-mile) crater named Chao Meng-Fuin, which can be seen in the Mariner 10 pictures.

Dr Slade is the first to agree that polar ice-caps are not very likely on a waterless, airless, Sun- blistered ball of rock such as Mercury. 'If the ice could be continually replicated, there would be no difficulty in explaining its presence, but it has to be an almost permanent feature. For this to be the case, there needs to be a temperature of around minus 160C.'

Such low temperatures possibly exist at Mercury's poles. The planet spins on its axis like an upright top, so the Sun never appears far above the polar horizon. If the poles are regions of rough topography, there may be a zone of permanent shadow.

But where did the ice come from? Volatile substances such as water could be driven from the rocks as they boil under the midday Sun. Such gases would then either escape into space or settle as 'snow' in the cold polar regions. Further supplies of water and carbon dioxide ice could be brought at irregular intervals by comets and asteroids which vaporise on impact with the planet.

Dr Slade explains how such ice might remain intact for a considerable period of time: 'Since radar can see through loose surface debris, it is possible that the ice is buried.' A blanket of material, perhaps 50cm (19in) thick, could protect the ice-cap from melting and from erosion by interstellar radiation.

The new radar maps also reveal huge impact basins on Mercury's hitherto unknown hemisphere. Two of these have diameters of about 800km (500 miles) and lie at the same latitude but on opposite sides of the planet's equator.

Not all discoveries are the result of bright radar echoes. In recent studies of Mars, some parts of the planet are invisible to radar. One huge blank on the maps, 2,000km (1,240 miles) across, displays no noticeable echo. Christened 'Stealth' by Professor Muhleman's team, the region seems to be covered in loosely consolidated ash, blown down slopes from nearby volcanoes.

The greatest challenge for radar astronomy is Titan, the giant moon of Saturn, whose surface is permanently covered by a dense orange smog. The vast distances involved result in a return echo of a mere 10-22 (a decimal point followed by 21 noughts and a one) watts being picked up by the New Mexico telescopes - very close to the limits of detection by existing equipment. Observations over the past three years have led Professor Muhleman to the conclusion that Titan does not copy our Moon by always keeping the same hemisphere towards its home planet, as previously believed.

Even more significant are variations in radar reflectivity, which suggest that Titan's surface consists of icy continents, perhaps coated in tars or hydrocarbons, with scattered lakes of ethane, a liquid rather like paraffin. Astronomers can also use radar to learn about relatively near, fast-moving objects. In New Zealand radar is used to track shooting stars, the tiny interstellar dust particles that burn up in the Earth's upper atmosphere.

Earth-approaching asteroids are the quarry of Dr Steven Ostro, of JPL. He has already had some spectacular success. Last year, his team announced that an asteroid, which had been discovered some five years before, called 1986 DA, was made of iron and nickel mixed with 10 parts per million of platinum and gold. In a body the size of 1986 DA, this would add up to 100,000 tons of precious metals.

Three years ago he obtained the most detailed radar images yet of an asteroid. This 3km-wide (1.8 miles) chunk of rock, now known as 4769 Castalia, turned out to comprise two irregular boulders, welded together like beads on a string and tumbling in a chaotic pirouette across the sky.

Dr Ostro is convinced that many more surprises are in store if sufficient funds can be provided. 'Radar promises a revolution in astronomy,' he says.

(Photographs omitted)

Start your day with The Independent, sign up for daily news emails
ebooks
ebooksAn introduction to the ground rules of British democracy
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
SPONSORED FEATURES
Independent Dating
and  

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

iJobs Job Widget
iJobs General

Recruitment Genius: Interactive / Mobile Developer

£40000 - £45000 per annum: Recruitment Genius: This digital production agency ...

Recruitment Genius: PHP Developer - Midweight

£40000 - £45000 per annum: Recruitment Genius: This digital production agency ...

Recruitment Genius: Junior Front End Developer

£20000 - £30000 per annum: Recruitment Genius: This digital production agency ...

Recruitment Genius: Front End Developer - Midweight / Senior

£35000 - £40000 per annum: Recruitment Genius: This digital production agency ...

Day In a Page

Giants Club: After wholesale butchery of Idi Amin's regime, Uganda’s giants flourish once again

Uganda's giants are flourishing once again

After the wholesale butchery of Idi Amin's regime, elephant populations are finally recovering
The London: After 350 years, the riddle of Britain's exploding fleet is finally solved

After 350 years, the riddle of Britain's exploding fleet is finally solved

Archaeologists will recover a crucial item from the wreck of the London which could help shed more light on what happened in the vessel's final seconds
Airbus has patented a jet that could fly from London to New York in one hour

Airbus has patented a jet that could fly from London to New York in one hour

The invention involves turbojets and ramjets - a type of jet engine - and a rocket motor
10 best sun creams for kids

10 best sun creams for kids

Protect delicate and sensitive skin with products specially formulated for little ones
Tate Sensorium: New exhibition at Tate Britain invites art lovers to taste, smell and hear art

Tate Sensorium

New exhibition at Tate Britain invites art lovers to taste, smell and hear art
Ashes 2015: Nice guy Steven Finn is making up for lost time – and quickly

Nice guy Finn is making up for lost time – and quickly

He was man-of-the-match in the third Test following his recall to the England side
Ashes 2015: Remember Ashton Agar? The No 11 that nearly toppled England

Remember Ashton Agar?

The No 11 that nearly toppled England
Turkey-Kurdish conflict: Obama's deal with Ankara is a betrayal of Syrian Kurds and may not even weaken Isis

US betrayal of old ally brings limited reward

Since the accord, the Turks have only waged war on Kurds while no US bomber has used Incirlik airbase, says Patrick Cockburn
VIPs gather for opening of second Suez Canal - but doubts linger over security

'A gift from Egypt to the rest of the world'

VIPs gather for opening of second Suez Canal - but is it really needed?
Jeremy Corbyn dresses abysmally. That's a great thing because it's genuine

Jeremy Corbyn dresses abysmally. That's a great thing because it's genuine

Fashion editor, Alexander Fury, applauds a man who clearly has more important things on his mind
The male menopause and intimations of mortality

Aches, pains and an inkling of mortality

So the male menopause is real, they say, but what would the Victorians, 'old' at 30, think of that, asks DJ Taylor
Man Booker Prize 2015: Anna Smaill - How can I possibly be on the list with these writers I have idolised?

'How can I possibly be on the list with these writers I have idolised?'

Man Booker Prize nominee Anna Smaill on the rise of Kiwi lit
Bettany Hughes interview: The historian on how Socrates would have solved Greece's problems

Bettany Hughes interview

The historian on how Socrates would have solved Greece's problems
Art of the state: Pyongyang propaganda posters to be exhibited in China

Art of the state

Pyongyang propaganda posters to be exhibited in China
Mildreds and Vanilla Black have given vegetarian food a makeover in new cookbooks

Vegetarian food gets a makeover

Long-time vegetarian Holly Williams tries to recreate some of the inventive recipes in Mildreds and Vanilla Black's new cookbooks