By Molly Bentley
Venus is an enigma, wrapped in a mystery, inside a dense cloud of carbon dioxide (CO2).
But a suite of orbiting instruments is proving its ability to penetrate the thick atmosphere and create a new and dynamic picture of Earth's sister planet.
Scientists at the Division of Planetary Sciences meeting in Pasadena, California, this week said that data streaming from the Venus Express probe had provided unprecedented detail of the Venusian atmosphere and the first-ever peek at its lower strata.
They hope the spacecraft will help answer fundamental questions about the planet's atmospheric composition and dynamics, as well as solve key Venus puzzles: what drives its "super-rotation"; are its volcanoes active; and just what is the strange ultraviolet-absorbing substance swirling at the cloud tops?
But for now, scientists are happy to report that all the instruments are in good working order and beaming back massive amounts of data.
"It's a treasure trove of information," said David Grinspoon, a participating scientist with the mission, "and we've barely opened the chest and looked in."
The seven instruments on the spacecraft, in obit around Venus since April, are examining the planet over a wide swath of the spectrum: from ultraviolet to visible, to infrared, and even radio wavelengths.
"Our main objective is to do a comprehensive study of the atmosphere," said Hakan Svedhem, Venus Express project scientist.
The instruments provide a look at Venus at different depths.
The Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS), for example, allows scientists to penetrate the otherwise opaque upper atmosphere of Venus and study the chemistry below it.
It takes advantage of "infrared windows", the few narrow wavelengths that carbon dioxide does not absorb.
"It's like looking through gaps in a picket fence for the first time and seeing through," said Dr Grinspoon.
Unlike Mars, Venus is a close planetary neighbour of Earth that remains relatively unexplored.
The dense atmosphere has shielded it from scrutiny, but scientists speculate that Venus once had vast oceans, similar to Earth. The oceans have since disappeared.
"The closest analogy to Venus today is the Achaean Earth, prior to life, when Earth's atmosphere was primarily CO2 and a lot of sulphur," said Frank Mills, a supporting investigator on the mission.
Scientists want to understand the evolutionary divergence of the sister planets - what happened to the oceans on Venus, and what triggered its runaway greenhouse?
Some clues may be found in understanding the role and amount of sulphur dioxide (SO2) in the atmosphere, said Dr Mills. When oxidised, it produces sulphuric acid, the main component of Venusian clouds and a tremendous greenhouse gas.
The gas is also a product of volcanoes. Previous measurements of Venus from ground-based instruments and the Pioneer spacecraft have showed a spike of sulphur dioxide followed by a gradual decrease, suggesting that the planet's volcanoes are not dormant.
But the last measurements of SO2 were 20 years ago. VIRTIS will provide a long-needed update.
Meanwhile, the Visible Monitoring Camera (VMC) may solve another Venusian mystery.
The VMC produces visible and ultraviolet images of the planet. In the visible range, the Venus surface remains opaque. But in the ultraviolet, a swirling weather pattern appears. It is unclear what it is made of. Some guess an aerosol or small crystals.
"It is some substance absorbing the ultraviolet, that's not CO2," said Dr Svedhem.
"People have tried to identify it and have failed, and it's really strange," added Dr Grinspoon; "it's dubbed the 'unknown ultraviolet absorber'. "
In the eye
As if the composition of the Venusian atmosphere is not intriguing enough, scientists hope Venus Express will also help explain why it is spinning faster than the planetary body beneath it.
While winds on Earth flow in Easterly and Westerly directions, on Venus they seem to flow only Westward, and at speeds faster than the planet's rotation.
Scientists don't know what's causing the super-rotation, or even how to describe it.
VIRTIS sees a peculiar double-eye vortex structure at the south pole
"Is it contributing to the planet's spin? Why doesn't it break down?" pondered Sanjay Limaye, a planetary scientist with the mission. "We just don't know."
He is pleased by the new images that VIRTIS has returned of the double vortex that sits like twin cyclones at the eye of the southern hemispheric circulation.
"Venus Express can help us define the depth and the structure of the vortex, and how the whole circulation is maintained," said Dr Limaye.
As Venus Express continues to beam back data over the next few years, scientists will create a new dynamic map of the planet's once impenetrable atmosphere.
"From what we have now, this will be like going from a 19th Century topographical map to the Weather Channel," said Dr Grinspoon.