Skywatchers around the world will get a once-in-a-lifetime chance to see Venus cross in front of the sun today (June 5). The so-called transit of Venus is more than just the last such event for more than a century — it might help shed light on some of the enduring mysteries of our planetary neighbor.
Bizarre stripes on Venus
Strange stripes in the upper clouds of Venus are called "blue absorbers" or "UV absorbers" because they absorb blue and ultraviolet wavelengths of light. These soak up a dramatic amount of energy — nearly half of the total solar energy the world takes in. As such, they seem to play a key role in keeping Venus as hot as it is, with surface temperatures of more than 860 degrees Fahrenheit (460 degrees Celsius).
The Venus Express spacecraft the European Space Agency launched in 2005 is armed with what's called a solar occultation spectrometer "to help us pinpoint the altitude and latitude distribution for these UV absorbers and understand their behavior a bit more now," said David Grinspoon, curator of astrobiology at the Denver Museum of Nature and Science. "However, we have not yet definitively identified it. We think it's a sulfur compound, or it could be a form of elemental sulfur."
The sunlight streaming around Venus during its transit "may help probe Venus' atmosphere," Grinspoon told SPACE.com. "We could ask interesting questions about its concentration of sulfur gas."
The secret of lightning on Venus
Evidence of lightning on the planet was confirmed by Venus Express, even though such weather displays should be impossible there.
"You need rainfall for lightning, and we're not sure if we actually get rainfall on Venus," said Grinspoon, an interdisciplinary scientist on the Venus Express mission. "But in some ways, we don't really understand lightning on Earth very well, so by cracking how lightning works on Venus, we may understand it better here. With Venus Express, we're getting more information at the latitudes lightning's distributed in, which we hope to link with activity in the atmosphere."
The Venus Climate Orbiter Akatsuki, which means "Dawn" in Japanese, was supposed to help capture vital clues about Venusian lightning with a camera dedicated to photographing it. Unfortunately, the Japanese space probe overshot the planet in 2010, although there's a chance it may still get to Venus. "It's a shame Akatsuki didn't make it into orbit around Venus — it was primed to address many questions, this matter of lightning in particular," Grinspoon said.
One of the biggest mysteries of Venus is the "super-rotation" of its atmosphere. Violent winds drive storms and clouds around that world at speeds of more than 220 mph (360 kph), some 60 times faster than the planet rotates.
"Super-rotation is still an unsolved mystery, but we're building more sophisticated 3-D models of cloud motion to simulate it," Grinspoon said. "They could end up helping us better understand how climate works on other planets, including exoplanets and Earth."
While these questions are perplexing, there is one unsolved mystery about the planet that takes the cake.
"The greatest mystery of Venus to me is what happened to its oceans," Grinspoon said.
The long-vanished oceans of Venus are thought to have been culprits of what is called a runaway greenhouse effect. Scientists think that planet's close proximity to the sun heated its water, causing it to build up in Venus' atmosphere as steam. Water is a greenhouse gas, trapping heat from the sun that would have vaporized still more water, a vicious cycle that heated Venus enough to boiled away its oceans.
Ultraviolet light would have then eventually split this atmospheric water into hydrogen and oxygen. The hydrogen escaped into space, the oxygen became trapped in the rocks of the planet, and the end-result was a bone-dry Venus.
Still, the specifics of this event remain uncertain. "How long did the oceans take to dissipate? How did Venus evolve?" Grinspoon asked. "Those are hard questions to answer, and we don't have a time machine to go back and see."
Venus Express is helping answer these questions by monitoring gases in the planet's atmosphere and witnessing hydrogen and oxygen escaping from Venus. "Given that data, we can hopefully extrapolate backward in time and get an idea of what the past was like," Grinspoon said.