Volcanic eruptions may be the explanation for large changes in the sulfur dioxide content of Venus’s atmosphere, and one intriguing possible explanation is volcanic eruptions.
The thick atmosphere of Venus contains over a million times as much sulfur dioxide as Earth’s. Most is hidden below the planet’s dense upper cloud deck, as it's readily destroyed by sunlight, and is therefore coming from below.
Venus is covered in hundreds of volcanoes - but it's not at all clear whether they remain active today. In 2010, though, NASA scientists found some evidence of recent eruptions.
And now, ESA's analysis of sulfur dioxide concentration in the upper atmosphere over six years has provided another clue. Immediately after arriving at Venus in 2006, the spacecraft recorded a significant increase in the average density of sulfur dioxide in the upper atmosphere, followed by a sharp decrease which means values are roughly ten times lower today.
A similar fall was also seen during NASA’s Pioneer Venus mission, which orbited the planet from 1978 to 1992.
"If you see a sulfur dioxide increase in the upper atmosphere, you know that something has brought it up recently, because individual molecules are destroyed there by sunlight after just a couple of days," says Dr Emmanuel Marcq of Laboratoire Atmosphères, Milieux, Observations Spatiales in France.
A volcanic eruption could act like a piston to blast sulfur dioxide up to these levels. But as-yet-understood peculiarities in the circulation of the planet could also mix the gas to reproduce the same result, says the team.
If volcanism was responsible for the initial increase, then it could come from a relatively gentle increased output of several active volcanoes rather than one dramatic eruption.
"By following clues left by trace gases in the atmosphere, we are uncovering the way Venus works, which could point us to the smoking gun of active volcanism," says Håkan Svedhem, ESA’s project scientist for Venus Express.