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More evidence for Venus' watery past
Posted: September 16, 2009

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The Venus Express spacecraft has measured concentrations of water vapour in the planet’s atmosphere and found proof that the solar wind has stripped away Venus’ once plentiful oceans. The new results are presented for the first time today at the European Planetary Science Congress (EPSC) in Potsdam, Germany.

Measurements of water vapour in the Venusian atmosphere by the SPICAV (Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus) and VIRTIS (Visible and Infrared Thermal Imaging Spectrometer) instruments on the European Space Agency mission have shown that the ratio of heavy water (which contains the isotope deuterium instead of hydrogen) to normal water is nearly twice as high above the cloud tops compared to its value in the lower atmosphere. This suggests that the heavier water has not been able to escape Venus’ gravity as easily as normal water.

A global view of Venus and its thick atmosphere, imaged by the VIRTIS instrument. The night-side is glowing in infrared light. Image: ESA/VIRTIS and VMC teams.

It is believed that Venus was much more like Earth’s twin in the past, but as it warmed up and the runaway greenhouse effect took hold the oceans began to evaporate. Today there is very little water left on Venus. If in liquid form now, the current abundance of water vapour would equate to a surface depth of just a few centimetres.

“This enrichment of heavy water provides strong evidence that water loss is occurring in the upper atmosphere and that Venus was probably more humid and Earth-like in the distant past,” says Emmanuel Marcq of the LATMOS laboratory in France.

SPICAV was also able to measure wind speeds at different altitudes above Venus. Such measurements are crucial to understanding super-rotation – the weird phenomenon whereby Venus’ atmosphere rotates every four days, but the planet itself rotates on its axis every 243 days. SPICAV saw practically no winds blowing north to south in the lower atmosphere between tropical and sub-polar latitudes, compared to the high wind speeds in the upper atmosphere, and yet cloud structures were seen moving north and south with velocities up to 40 kilometres per hour.

“This is one of the features in the meteorology of Venus that is difficult to understand,” says Dr Ricardo Hueso of the Universidad País Vasco in Spain, who led the wind-speed observations. “The average of all these turbulent and chaotic motions is very close to zero but rarely some structures can travel at these high speeds of 40 kilometres per hour to the north or south. When we have been able to analyse further these turbulent motions in the lower clouds, we might discover important hints to the origin of the atmospheric super-rotation.”