Venus may have an unexpectedly youthful face, and that's good news for scientists interested in the planet's more modern blemishes. A new look at Venus' craters suggests that the surface might be up to 620 million years younger than previous estimates, a discovery that has implications for signs of relatively recent volcanic activity.
Although it is commonly called Earth's twin due to its similar size and bulk composition, Venus looks nothing like our watery world. It is a hot, desolate place, with over 1,600 major volcanoes or volcanic features—more than any other planet in the solar system. However, it's long been assumed that all its volcanic activity occurred in the past, either in one large burst or several smaller, episodic spasms.
Then last year, scientists studying data taken by the European Space Agency's Venus Express orbiter found four bright spots in a relatively young region known as Ganiki Chasma. The spots seemed to indicate that volcanic activity hadn't quite called it quits on the planet.
According to geologist Patrick McGovern of the Lunar and Planetary Institute in Texas, the surfaces around the volcanic mountains of Venus are thought to be younger than the overall surface of the planet. But with an estimated surface age of up to 800 million years, it wasn't clear whether those volcanoes were spouting lava millions of years ago or a few months ago.
That's where the new work by Bill Bottke, of the Southwest Research Institute in Colorado, comes in.
On Earth, a consistent deposit of iridium sandwiched between geologic layers around the globe led scientists to conclude that the Chicxulub impact in the Gulf of Mexico played a role in wiping out large dinosaurs 65 million years ago. That's because meteors have much higher levels of iridium than our planet's average amounts, so the catastrophic impact must have delivered the extra metal.
But when Bottke spoke with impact specialists, he found out that the iridium deposits in various craters on Earth weren't large enough to account for the estimated sizes of the asteroids involved.
"Somehow, we're missing all this mass," Bottke said in a presentation at the American Astronomical Society's Division of Planetary Sciences meeting last month in Oxen Hill, Maryland.
Instead of searching Earth for the missing material, Bottke decided to probe the computer code responsible for the predictions. Current models suggest that an asteroid hurtling toward Earth should make a crater about 10 to 20 times as large. In his re-examination, Bottke found that craters should be closer to 24 times as large as the object that created them.
The Chicxulub asteroid, for instance, would only have been about 4 miles long, instead of the 6 miles normally cited.
This matters for Venus because scientists use crater sizes and numbers to estimate the ages of planetary surfaces. Just as trees form rings as they grow, planets without active surface geology accumulate craters over millennia of impacts. By matching a planet's blemishes to the known population of space rocks, scientists can work backward to trace its impact history and get a rough estimate of its surface age.
Previous estimates for Venus relied on the handful of objects known to orbit the inner solar system a few decades ago. Since 1998, NASA's Spaceguard effort has been working to identify and monitor over 90 percent of these near-Earth objects larger than 0.6 miles. When combined with the revised scaling between asteroids and craters, Bottke found that the estimated age for Venus' skin would change significantly: down to about 180 million years old.
"If you have on average a younger surface age for the planet, it means overall that volcanoes are going to be younger than that," McGovern says. "It's an exciting result because it gets us closer to having a more active planet."
McGovern also pointed to research from 2011 that suggested crater floors on Venus are filled with basaltic lavas, leading the authors to estimate a planetary surface age of about 150 million years—close to Bottke's range. According to McGovern, this research is still being debated, which makes the new results even more significant.
"That's vital, coming at it in from an interdisciplinary angle," he says. "Things are converging for a younger Venus."
The result, which Bottke is now submitting for publication, is very surprising, adds comet and asteroid researcher Paul Weissman, a senior scientist at the Planetary Science Institute in California. He was particularly intrigued to learn that the scaling remained constant across the solar system, from airless bodies like the moon to worlds with thicker atmospheres like Venus.
"It was surprising to find that all craters are about 24 times [as large]," he says, adding that he looks forward to seeing more work on the topic.
But Venus hasn't garnered as much attention as planets such as Mars, despite suggestions that it could help researchers understand how life emerged on Earth. That's partly due to the planet's punishing heat and extreme surface pressure, which makes exploration a challenge even for the toughest robots we can fling at it.
Aside from the Russian Venera missions of the late 1970s, most of the recent spacecraft studying the planet have only captured a brief glimpse on their way to other worlds. As a result, planetary scientists have a relatively small collection of images to use for surface studies. Still, a search is underway in images from NASA's Magellan spacecraft for other hot spots similar to those identified last year.
That could change in the near future. In September NASA selected two proposed missions for further study that would investigate Venus. One of these, the Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy mission (VERITAS), would provide high-resolution imaging of the planet's surface for geologic study.
"Perhaps things are going on that we could see if we send more spacecraft," McGovern says. "Maybe there is something to look for."