Venus Unveiled

Spacecraft finds Earthy features on sister planet

Like many siblings, Venus and Earth bear a familial resemblance. Venus is similar to Earth in size, composition and gravitational pull. But some peculiar quirks, from sulfuric acid clouds to swirling polar vortices, make Venus a twisted sister indeed.

Venus Express has mapped heat patterns on the planet’s Idunn Mons peak, depicted here. Red is warmest. ESA, JPL, NASA

Venus’ south pole hosts swirling clouds, shown here rotating over a period of 16 hours. VIRTIS-VenusX/ESA, INAF-IASF, LESIA-Obs.Paris, D. Luz/Univ. Lisbon, D. Berry/Univ. Evora

New results from the European Space Agency’s Venus Express spacecraft, which has been orbiting the cloud-shrouded planet since 2006, suggest that beneath the acidic cloak lies an extreme world that provides an important point of comparison for understanding Earth’s evolution.

“The more we actually get results from Venus, the more we see how important it is to study Venus if we want to learn more about the Earth and planetary systems in general,” says Venus Express project scientist Håkan Svedhem, who works for the European Space Agency out of Noordwijk, the Netherlands.

Venus, like Earth, has an atmosphere. But while Earth’s mainly comprises nitrogen and oxygen, Venusian air is mostly carbon dioxide. A runaway greenhouse effect has heated the planet’s surface to a smoldering 470Ë Celsius, perhaps evaporating ancient oceans. Venus also appears to have lightning and ozone, new Venus Express observations suggest. And the planet shows signs of recent geological activity, with hot spots resembling those beneath Hawaii.

Hot release

Scientists know that planets with warm interiors need some way to cool off. Earth is covered with a patchwork of tectonic plates that release heat as they jostle and slide, yet Venus appears to be insulated by one large, solid lid.

“That’s always been a puzzle,” says Suzanne Smrekar of the Jet Propulsion Laboratory in Pasadena, Calif. “How is that heat coming out?”

It has long been suspected that volcanic processes might be releasing heat accumulating in the planet’s interior, because water vapor and sulfur dioxide in the atmosphere are probably products of volcanic outgassing. Crater dating has revealed resurfacing, possibly from volcanoes, within the last billion years. But until Smrekar used a spectrometer aboard Venus Express to look at the planet’s sweltering surface, there was little evidence for recent volcanism.

Some regions in the southern hemisphere, she found, are emitting more heat than others. The warm areas, which appear to be younger, resembled volcanic terrains on Earth, complete with basaltic lava flows. Venus hosts at least nine volcanic hot spots, or mantle plumes, similar to the plumes thought to power the Hawaiian island chain, Smrekar and colleagues suggested in Science last year. She estimates that the present flows are between 250,000 and 2.5 million years old.

“It’s very likely that there has been volcanic activity recently,” says Svedhem, who wasn’t involved in the new analysis. “This is really something that we have been expecting to see, but it’s important to see it.”

Though alternative explanations for the Venusian veneer point toward cata­clysmic processes, Svedhem says the data support gradual resurfacing similar to that on Earth, suggesting that the two sisters may share some internal geology.

Recent Venus Express measurements, reported in the November Icarus, also confirm a thin layer of ozone on Venus’ nightside. “We already knew we had oxygen on the planet; it was quite natural that you would get some concentration of ozone,” Svedhem says. Tracking these ozone molecules will help scientists study the steaming planet’s atmospheric circulation and the chemistry at work in its clouds, as well as the conditions under which ozone can exist. Ozone often flags a planet as potentially life-bearing, but on Venus the molecule’s concentration is only one-thousandth the amount on Earth — far too low to tantalize alien hunters.

Striking signal

While scientists are coming to accept that Venus and Earth share similar volcanic and atmospheric processes, the presence of Earthlike lightning in the Venusian clouds remains disputed. Christopher Russell of the University of California, Los Angeles is convinced that Venus has stormy skies, complete with electric flashes similar to those seen on his home planet.

Russell studies lightning using a magnetometer aboard Venus Express that detects low-frequency electro­magnetic waves in the Venusian ionosphere, the charged region of the upper atmosphere. “Some days, there are a lot of signals coming into the ionosphere,” Russell said in October at a planetary sciences meeting in Nantes, France. “Basically, those are our stormy days.” The signals observed by Venus Express depend on the density of charged particles in the ionosphere, as a scientist studying Earth would expect for electrical activity, Russell says. The flashes, which resemble those seen by previous probes, also have an Earthlike frequency and intensity.

On Earth, lightning is presumed to have helped life form from the mix of molecules swimming in the primordial stew. Seeing lightning on Venus suggests that electrical discharges might be common wherever the right atmospheric ingredients are found. Planets elsewhere in the cosmos, then, might possess a potential catalyst for life, too.

But skeptics point out that the Cassini spacecraft, which swung by Venus in the late 1990s, failed to detect anything resembling the flashes that occur on Earth. “Terrestrial lightning is always occurring somewhere on Earth,” says planetary physicist Don Gurnett of the University of Iowa in Iowa City. “If it were terrestrial-like, we would have detected it.”

Russell and Gurnett would both like evidence of lightning in the form of an optical flash within the planet’s acid storm clouds — something Venus Express can’t image. But the Japanese Akatsuki spacecraft, launched in May 2010, might have made such an observation this year. Unfortunately, in December of 2010 the spacecraft’s engine failed during orbit insertion. Akatsuki is still wandering the inner solar system, with a second attempted rendezvous planned for 2015.

Swirling shroud

If Akatsuki does enter Venus orbit, it will begin imaging the Venusian clouds to produce data that will help scientists peer more closely at a thick shroud that reaches 70 kilometers above the planet.

At the planet’s poles, that shroud’s height shrinks to 65 kilometers, and it is embellished by mysterious vortices — enormous, shape-shifting masses of swirling clouds. Whipped into a frenzy by some unknown mechanism, the vortices swirl around a central 2- to 3-kilometer-wide hole, a tunnel that plunges through the atmosphere. And they look like Earth’s hurricanes, except that they are about the size of Europe, says Dmitry Titov of the European Space Agency and the Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany. “The morphological similarity is really striking,” he says. “But the physical mechanisms, I think they should be different.”

Earth’s hurricanes are maintained by moist air condensing and releasing energy. On Venus, Titov says, the vortices appear to be the result of a global pattern of atmospheric circulation known as “superrotation.” The clouds on Venus fly around the planet at speeds up to 150 meters per second, completing a round trip in just four Earth days, even though a single Venusian day lasts for 243 Earth days. Whatever is cranking up the clouds might also drive the vortices.

In 2006, Venus Express glimpsed a pair of gargantuan, spooky snake eyes peering out from the planet’s south pole: a double vortex with two dark spots, double trouble for anything caught in the acidic cloud. Triple vortices, as well as weird ones shaped like triangles, have also been observed. Svedhem speculates that atmospheric instabilities produce the varying shapes — “just like if you have your bathtub and the water goes out.”

Studying the churning Venusian clouds may improve scientists’ understanding of planetary atmospheres, including how Earth’s might behave in the future if current warming trends continue. While a number of plausible scenarios have been developed to describe Earth’s atmospheric circulation, they all “fail completely” when applied to Venus, Svedhem says. If the scenarios are true representations, they should describe even Venus’ more exotic conditions. “Where, and why do they fail? That’s a very important point to study,” he says.

Venus Express will continue collecting data to answer such questions through the end of 2014. Sometime in mid-2015, the spacecraft will fall into the toxic clouds. “We’ll get a lot of nice measurements when it goes into the atmosphere, and very interesting data during the last orbits,” Svedhem says. “It will be very hot, of course.”