Venus may be covered in active volcanoes, new evidence suggests.

It has long be debated whether the second planet from our Sun is volcanically active. But after six years of observations by the European Space Agency's Venus Express, it now seems more likely than ever that the surface is dominated by massive, toxic volcanoes.

The new research focuses on a sudden rise in the amount of sulphur dioxide in the planet's atmosphere, followed by an equally sudden decrease.

Venus already has a thick, toxic atmosphere containing millions of times more sulphur dioxide than we have on Earth. When sulphur dioxide does occur naturally here, it is usually the result of volcanic activity.

The Venus Express, which arrived in 2006, noted the changes over time, and scientists think the spike might only be explained by a giant volcanic eruption.

That's largely because sulphur dioxide does not last long in the upper atmosphere, where sunlight easily breaks it down.

Venus also has an unusually violent weather system, which spins around the rocky planet in about four Earth days - faster than the 243 Earth days it takes to spin around its own axis. That makes it hard to pimpoint where the sulphur may be coming from.

"If you see a sulphur 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," said Emmanuel Marcq of French research institute Latmos.

"A volcanic eruption could act like a piston to blast sulphur dioxide up to these levels."

It is still debated whether Venus really does have volcanoes, however, and ESA said it would continue to study the planet and search for new clues.