Venus transit 2012: a chance to test Earth-hunting techniques (+video)

The group plans to use a spectrometer aboard Hubble to track changes in the spectra of light reflecting off the moon before, during, and after the transit. Before and after the transit, the light will carry only the fingerprints of chemical elements in the sun. During the transit, however, sunlight passing through Venus's atmosphere will add chemical signatures from the planet's envelope of gases to the mix. By subtracting the "during" from the "before and after," the team expects to glean information about Venus's atmospheric recipe.

The results aren't likely to provide any new insight into Venus's atmosphere itself, Dr. Desert says. The composition of its atmosphere is already well known. The team, however, will use them to evaluate new techniques to discern the atmospheres of Earth-like planets in other solar systems. 

As close as Venus and the sun are, pulling information on Venus's atmosphere out of sunlight reflected off the moon is just as challenging as trying to spot the chemical fingerprints of an atmosphere on a planet orbiting a star light-years away, Desert explains.

NASA's Solar Dynamics Observatory also will be observing the transit directly and will use the sun's backlighting to analyze Venus's atmosphere as well as to use the transit itself as a means of calibrating the observatory's telescope.

Super-rotation observations

Meanwhile, another team is gathering data from solar telescopes around the world relating to the bright, thin, halo-like arc of light tracing the planet's disc as it begins and ends its pass across the sun. Scientists spotted the arc as they observed Venus's transit eight years ago. It was sunlight reflected through layers of the atmosphere above the cloud tops, forming an illuminated arc similar to those astronauts see tracing Earth's curvature at sunrise and sunset as they orbit.

By making careful measurements of this limb of light and teasing from it information about how temperatures and densities in that part of the atmosphere vary with latitude, the US-French team conducting the research hopes to provide an additional test of models that researchers have constructed to explain super rotation, says Jay Paschoff, an astronomy professor at Williams College in Williamstown, Mass. He and astronomer Thomas Widemann of the Paris Observatory are coordinating the effort, which involves nine Earth-based solar observatories, as well as two sun-watching spacecraft.

Such high-speed atmospheric circulation relative to a planet's or moon's rotation rate also has been observed on Jupiter, Saturn, and Saturn's moon Titan.

The transit is set to begin at 6:03 p.m. Eastern Daylight Time as Venus first appears to touch the sun's disk. The transit will end around 12:50 a.m. EDT June 6. As a starting point for viewing tips and other information about the transit, visit NASA's Venus-transit site.

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