As Venus made its transit the sun, spacecraft and ground-based telescopes ahave been looking for a phenomenon that's only recently been explained: the Arc of Venus.
"I was flabbergasted when I first saw it during the 2004 transit," says astronomy professor Jay Pasachoff of Williams College. "A bright, glowing rim appeared around the edge of Venus soon after it began to move into the sun."
Researchers now believe the arc of light is created when, backlit by the sun, Venus's atmosphere refracts sunlight passing through layers of air above the clouds.
And, it turns out, researchers can learn a lot about Venus's atmosphere by observing the arc.
"We do not understand why our sister planet's atmosphere evolved to be so different than Earth's," explains planetary scientist Thomas Widemann of the Observatoire de Paris.
While the two planets are of similar size and distance from the sun, and are made of the same basic materials, they have stunningly dissimilar atmospheres. Venus's is almost 100 times more massive than Earth's, and consists mainly of CO2, raising the surface temperature to almost 900°F. Clouds of sulfuric acid reach 14 miles high and travel as fast as 220 mph.
One of the biggest mysteries of Venus is super-rotation. The whole atmosphere circles the planet in just four Earth days, much faster than the planet's spin period of 243 days.
And, scientists hope, the Arc of Venus could help provide the answer. The brightness of the arc reveals the temperature and density structure of Venus's middle atmosphere, or mesosphere, where the sunlight is refracted.
According to some models, the mesosphere is key to the physics of super-rotation - and, by analyzing the lightcurve of the arc, its temperature and density can be determined from pole to pole.
Pasachoff and Widemann organized a worldwide effort to monitor the phenomenon, using nine coronagraphs spaced around the world, and now plan to analyze the data.