Science: The Sky At Night: Venus's rivers of molten chalk

The bright star you see in the sky isn't a star at all - it's a planet. Heather Couper and Nigel Henbest explain some unusual features of Venus.

The Christmas star is here! At least that's how it will seem in early December, as a brilliant light hangs low in the south west skies at dusk: 20 times brighter than any star, and second only to the Moon in lighting up the night sky.

Despite its seasonal appearance, this is not, in fact, the Star of Bethlehem. Astronomers still argue over the nature of the star that presaged Christ's birth: front-runners are a close approach of Jupiter and Saturn, or perhaps a stellar explosion (a nova). What we are seeing now is more mundane occurrence: an appearance of our nearest neighbour planet, Venus.

Through a small telescope - or even good binoculars, held steadily - you'll see Venus shaped as a crescent, as the Sun lights up its globe obliquely. But even the most powerful telescope reveals little more. Venus hides her secrets under an all-enveloping cloak of brilliant clouds.

Space probes have unveiled the mystery planet by using radar waves to penetrate the clouds. The most detailed radar views have come from Nasa's Magellan mission, which orbited Venus as an artificial moon. The latest result is the discovery of narrow, winding river-beds. The longest of the channels is Baltis Vallis, which bears the Syrian name for Venus. It winds for 6,800km, longer than any river on the Earth. And its sediments have covered a plain twice the size of the US.

But Baltis Vallis cannot be the bed of a river such as we know on Earth. For billions of years, the planet has been sweltering under a thick atmosphere of carbon dioxide, creating a greenhouse effect that has raised its temperature to 460 C. Water cannot flow on Venus: it would boil away instantly.

Venus is also a highly volcanic world, so one obvious culprit is a flow of lava. But ordinary lava is too viscous to carve such a thin and sinuous channel. And the planet's temperature - though hot for us - is comparatively cool for lava; it would solidify quite quickly, at least on the surface, and disappear into a subterranean lava tube.

Applying the Sherlock Holmes maxim that "When you have eliminated the impossible, whatever remains, however improbable, must be the truth", the Nasa team is now suggesting that the river consists of an incandescent, flowing stream of melted washing soda and chalk.

This is not quite as implausible as it sounds. The Earth has one volcano that erupts molten carbonates: Oldonyo Lengai in Tanzania. This type of lava - known as carbonatite - is much more runny than ordinary lavas, which are made of silicate rocks (such as basalt). And carbonatite can cool to almost the temperature of Venus's surface before solidifying, so there's no problem explaining how it can flow so far.

But why should Venus's volcanoes have such large supplies of carbonate rocks? Perhaps in the planet's early days it had a milder climate and vast oceans. The carbonates were laid down on the seabed, like chalk and limestone on Earth. Then the greenhouse effect took over, and the oceans boiled away. Venus's volcanoes have now melted its store of carbonates, and in the new hothouse climate they have flowed fast and far, to form the longest "rivers" in the Solar System.

More information about Venus can be found on the World Wide Web at http://pds.jpl.nasa. gov/planets/welcome/venus.htm

What's up

To the upper left of Venus lies the second-brightest planet - the gas giant Jupiter.

Over to the south, look out for the rather dimmer, ringed world of Saturn. In the east, the bright constellations of winter are rising: Orion (the hunter), Taurus (the bull) and Gemini (the twins), along with Canis Major and Minor (the two dogs).

The night of 13 December sees the annual Geminid shower of shooting stars, but this year the display will be marred by bright moonlight.

Diary (24-hour clock)

7th: 0610 Moon at first quarter

11th: Venus at greatest brilliance

13th: maximum of Geminid meteor shower

14th: 0238 full Moon

21st: 2007 winter solstice; 2144 Moon at last quarter

29th: 1657 new Moon

Ed Miliband received a warm welcome in Chester
election 2015
Life and Style
Apple CEO Tim Cook announces the Apple Watch during an Apple special even
fashionIs the iWatch for you? Well, it depends if you want for the fitness tech, or the style
Astronauts could be kept asleep for days or even weeks
scienceScientists are looking for a way to keep astronauts in a sleeplike state for days or weeks
Have you tried new the Independent Digital Edition apps?
Life and Style
ebookNow available in paperback
ebookPart of The Independent’s new eBook series The Great Composers
  • Get to the point
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating

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

ES Rentals

    iJobs Job Widget
    iJobs General

    Recruitment Genius: Project Implementation Executive

    £18000 - £23000 per annum: Recruitment Genius: They work with major vehicle ma...

    Recruitment Genius: Chiropractic Assistant

    £16500 per annum: Recruitment Genius: A Chiropractic Assistant is needed in a ...

    Recruitment Genius: Digital Account Executive - Midlands

    £18000 - £26000 per annum: Recruitment Genius: They work with major vehicle ma...

    Recruitment Genius: Web Developer

    £28000 - £30000 per annum: Recruitment Genius: This company provides coaching ...

    Day In a Page

    NHS struggling to monitor the safety and efficacy of its services outsourced to private providers

    Who's monitoring the outsourced NHS services?

    A report finds that private firms are not being properly assessed for their quality of care
    Zac Goldsmith: 'I'll trigger a by-election over Heathrow'

    Zac Goldsmith: 'I'll trigger a by-election over Heathrow'

    The Tory MP said he did not want to stand again unless his party's manifesto ruled out a third runway. But he's doing so. Watch this space
    How do Greek voters feel about Syriza's backtracking on its anti-austerity pledge?

    How do Greeks feel about Syriza?

    Five voters from different backgrounds tell us what they expect from Syriza's charismatic leader Alexis Tsipras
    From Iraq to Libya and Syria: The wars that come back to haunt us

    The wars that come back to haunt us

    David Cameron should not escape blame for his role in conflicts that are still raging, argues Patrick Cockburn
    Sam Baker and Lauren Laverne: Too busy to surf? Head to The Pool

    Too busy to surf? Head to The Pool

    A new website is trying to declutter the internet to help busy women. Holly Williams meets the founders
    Heston Blumenthal to cook up a spice odyssey for British astronaut manning the International Space Station

    UK's Major Tum to blast off on a spice odyssey

    Nothing but the best for British astronaut as chef Heston Blumenthal cooks up his rations
    John Harrison's 'longitude' clock sets new record - 300 years on

    ‘Longitude’ clock sets new record - 300 years on

    Greenwich horologists celebrate as it keeps to within a second of real time over a 100-day test
    Fears in the US of being outgunned in the vital propaganda wars by Russia, China - and even Isis - have prompted a rethink on overseas broadcasters

    Let the propaganda wars begin - again

    'Accurate, objective, comprehensive': that was Voice of America's creed, but now its masters want it to promote US policy, reports Rupert Cornwell
    Why Japan's incredible long-distance runners will never win the London Marathon

    Japan's incredible long-distance runners

    Every year, Japanese long-distance runners post some of the world's fastest times – yet, come next weekend, not a single elite competitor from the country will be at the London Marathon
    Why does Tom Drury remain the greatest writer you've never heard of?

    Tom Drury: The quiet American

    His debut was considered one of the finest novels of the past 50 years, and he is every bit the equal of his contemporaries, Jonathan Franzen, Dave Eggers and David Foster Wallace
    You should judge a person by how they peel a potato

    You should judge a person by how they peel a potato

    Dave Hax's domestic tips are reminiscent of George Orwell's tea routine. The world might need revolution, but we like to sweat the small stuff, says DJ Taylor
    Beige is back: The drab car colours of the 1970s are proving popular again

    Beige to the future

    Flares and flounce are back on catwalks but a revival in ’70s car paintjobs was a stack-heeled step too far – until now
    Bill Granger recipes: Our chef's dishes highlight the delicate essence of fresh cheeses

    Bill Granger cooks with fresh cheeses

    More delicate on the palate, milder, fresh cheeses can also be kinder to the waistline
    Aston Villa vs Liverpool: 'This FA Cup run has been wonderful,' says veteran Shay Given

    Shay Given: 'This FA Cup run has been wonderful'

    The Villa keeper has been overlooked for a long time and has unhappy memories of the national stadium – but he is savouring his chance to play at Wembley
    Timeless drama of Championship race in league of its own - Michael Calvin

    Michael Calvin's Last Word

    Timeless drama of Championship race in league of its own