Nature online, 27 Nov 2007

European mission reports from Venus

Venus Express, the first European mission to Venus, finds evidence for past oceans.


Two years after Venus Express left a launch pad in Kazakhstan, the first European mission to Earth's ‘evil twin’ is turning up results.

Scientists today publish a suite of eight papers in Nature 1-8 reporting the first observations from the craft, which arrived at Venus â€" the second closest planet to the Sun and Earth’s next-door neighbour â€" in 2006. The probe represents the first dedicated mission to the planet since NASA’s Magellan mission ended in 1994, after using radar to map the planet. “Venus has been a forgotten planet for more than a decade,” says HÃ¥kan Svedhem, project scientist for the European Space Agency mission.

Through the mission, scientists are gaining an understanding of venusian weather â€" how the Sun’s energy is driving the planet’s thick robe of sulphuric acid clouds. They coast along in an atmosphere of about 95% carbon dioxide, cloaking a blisteringly hot â€" about 460 ºC â€" surface. The probe has found that Venus is still losing water to space via this runway greenhouse effect â€" an extreme version of the Earth’s â€" which helps to support the theory that Venus once had oceans like Earth. It is one more piece in an enduring puzzle: how could two planets in the habitable zone, with roughly the same size, mass and composition, end up so different?

“For me, this is showing that Venus is a ‘twin Earth’ that has just taken a different evolutionary path,” says Fred Taylor, a mission scientist from the University of Oxford, UK.


Extreme weather

The probe has offered a three-dimensional view of the planet’s atmosphere, giving a clearer view of the strange ‘super-rotation’ of the planet’s upper atmosphere. On Earth, the planet's rotation drives terrestrial weather. But on Venus, winds in the upper atmosphere, blasting along at 350 kilometres per hour, are detached from the slowly rotating and windless surface.

The mission has also highlighted the way the solar winds erode the atmosphere, which is vulnerable because Venus lacks a strong magnetic field. The probe found hydrogen and oxygen ions escaping in a two to one ratio, meaning that water vapour in the atmosphere â€" the little that is left of the suspected oceans â€" is still disappearing. “It was suspected, but it was never seen before,” says Svedhem.

Further details on the gases escaping from the planet hint that the planet, which currently has the equivalent of about 3 centimetres of water on its surface, likely once had a deep ocean â€" deeper than the previous estimate of 4.5-metres.


Down to geology

The point at which the two planets’ paths diverged is unknown. On Earth, sea life, building calcium carbonate, sucked the carbon out of the atmosphere. Tectonic plates, lubricated by water, plunged into the mantle with the carbon. That kept the planet cool and wet.

At some point, Venus did the opposite, with atmospheric carbon triggering the runaway global warming that boiled away the oceans. Without oceans, its tectonic plates â€" if they ever existed â€" hardened into a stagnant lid of volcano-dotted crust. It's unclear how volcanically active Venus’s surface is today. Venus Express, built primarily to study the atmosphere, probably won’t be solving the geologic mysteries.

One of the craft’s instruments was supposed to peer through the clouds to spot narrow plumes of sulphur rising from volcanoes. But a mirror on that instrument is stuck, so the instrument only sees itself. A separate instrument, mapping in the infrared spectrum, may detect the hotspots of eruptions or lava flows, but scientists say they need more time to collect and analyse this data.

The â,¬220 million Venus Express, cobbled together quickly using hand-me-down Mars Express designs, has whetted scientists’ appetites for more missions; especially when the Venus community sees more expensive missions going to Mars almost every year, says David Grinspoon, a Venus Express scientist and astrobiology curator at the Denver Museum of Nature and Science in Colorado. The gap between what is known about Mars and Venus is now huge, Grinspoon says. “That deficit, that gap â€" that Venus gap â€" makes every bit of information from the surface of Venus that much more valuable.”


   
References
1. Markiewicz, W. J. et al. Nature 450, 633â€"636 (2007).
2. Piccioni, G. et al. Nature 450, 637â€"640 (2007).
3. Drossart, P. et al. Nature 450, 641â€"645 (2007).
4. Bertaux, J. - L. et al. Nature 450, 646â€"649 (2007).
5. Barabash, S. et al. Nature 450, 650â€"653 (2007).
6. Zhang, T. L. et al. Nature 450, 654â€"656 (2007).
7. Pätzold, M. et al. Nature 450, 657â€"660 (2007)
8. Russell, C. T. , Zhang, T. L. , Delva, M. , Magnes, W. , Strangeway, R. J. & Wei, H. Y. Nature 450, 661â€"662 (2007).