For decades, scientists thought the planet Venus was geologically dead. But it turns out it still has plenty of life: at least 37 regions show signs of geologically recent volcanism – volcanism that, as far as anyone knows, might even be happening today.
In a paper in the journal Nature Geoscience, a team led by Anna Gülcher from the Institute of Geophysics, ETH Zürich, Switzerland, re-examined three-decade-old radar images of 133 Venusian volcanic provinces called coronae.
Most appear to be inactive, but 37 showed signs of activity within at least the last few million years.
Coronae are large, more or less circular structures that show signs of lava flows and/or crustal deformation. (The term is derived from the Latin word for crowns.)
Often, Gülcher says, they have a central highland (though there can also be a crater-like depression) surrounded by a ring of cracks and trenches, possibly with additional cracks extending like spokes outward from the center. “You can best think of coronae as a field of lava flows and major faults that cover the circular area,” she says.
Some are only a few tens of kilometres across, but many are huge. “The ones we are considering are larger than 300 kilometres,” she says.
They are created, she adds, by plumes of interior heat rising from deep in the Venusian interior. These force columns of hot, slightly plastic, mantle rock to move upward through the Venusian mantle, sometimes rising high enough to interact with the crust or even punch through it.
Coronae don’t exist on Earth, a fact, she says, “that makes these structures even more intriguing”.
But Earth does have mantle plumes, says -co-author Laurent Montési, from the University of Maryland, US. The most famous are the heat sources that build volcanic island chains like Hawaii or the Canary Islands, or provide geothermal heat for the geysers at America’s Yellowstone National Park.
Ours, however, are wimpier than Venus’s because the Earth has other ways of bringing interior heat to the surface, such as plate tectonics, a process that appears to be absent on Venus.
Coronae were well known to be signs of volcanic activity. But many scientists had thought the plumes that fed them were long dead.
In the new study, Gülcher and Montési’s team began by modelling how an active mantle plume would affect the Venusian surface – and how that would change if a plume fell dormant for millions of years.
One of the biggest differences between the effects of active and inactive plumes, Montési says, turns out to involve the depth of the cracks and trenches around a corona’s outside edges.
When a plume is active, hot, buoyant rock is slowly rising in the center of the corona. At the edges, however, cooler, denser rock is descending back into the interior, dragging parts of the crust down with it – the process that creates the cracks and trenches.
When plume activity ceases, the downward pull ends, and the crust bounces back up and levels off. The cracks remain. But, Montési says, “the trenches vanish in just a couple million years. So, if we see them, that tells us this place is being pulled down today”.
All of this produces an important change in our understanding of Venus.
Previously, the planet was believed to have undergone a global convulsion of volcanism about 500 million to 700 million years ago and been inactive ever since – presumably slowly building up internal heat until its crust again reaches the breaking point and the entire planet again erupts in lava.
But if dozens of plumes are still active, that means Venus’s present relatively quiet phase isn’t as quiet as previously imagined.
That, Montési says, has major implications for our understanding of not only the Venusian surface, but also the process by which volcanic gases have been introduced into its air to produce the runaway greenhouse effect that led to its present superheated state. (Venus’s surface temperature is estimated to be 470 degrees Celsius, more than hot enough to melt lead.)
“This could be a way of pumping gases into the Venusian atmosphere,” Montési says – a mechanism that would progressively replenish them rather than introduce them in widely spaced cataclysms such as the one that appears to have happened 500 to 700 million years ago.
“We like to study the Venusian atmosphere because it’s so hot,” he says, “and we don’t want Earth to become like that. But it’s still a mystery why the two planets came to such different outcomes.”
Meanwhile, he says, the new study should add impetus to rising interest in both Europe and the US in a new mission to Venus.
The data used in his study came from NASA’s Magellan mission, launched in 1989. “The technology we have today would give us much more detailed maps,” he says. “We would learn a huge amount.”
And, he notes, if one of the space agencies wants to send down a lander, “I know where I would want to go. Here are 37 spots you want to check out”.
Richard A Lovett
Richard A Lovett is a Portland, Oregon-based science writer and science fiction author. He is a frequent contributor to Cosmos.
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