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| Transit of VENUS |
The Mysterious Arc of Venus
ScienceDaily (June 4, 2012) —
When Venus transits the sun on June 5th and 6th, an armada of
spacecraft and ground-based telescopes will be on the lookout for
something elusive and, until recently, unexpected: The Arc of Venus.
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"I was flabbergasted when I first saw it during the 2004 transit,"
recalls 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."
For a brief instant, the planet had turned into a "ring of fire."
Researchers now understand what happened. Backlit by the sun, Venus's
atmosphere refracted sunlight passing through layers of air above the
planet's cloudtops, creating an arc of light that was visible in
backyard telescopes and spacecraft alike.
It turns out, researchers can learn a lot about Venus by observing
the arc. Indeed, it touches on some of the deepest mysteries of the
second planet.
› View larger The arc of Venus photographed in 2004 by Riccardo
Robitschek and Giovanni Maria Caglieris of Milan, Italy. "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.
Earth and Venus are similar distances from the sun, are made of the
same basic materials, and are almost perfect twins in terms of size. Yet
the two planets are wrapped in stunningly dissimilar blankets of air.
Venus's atmosphere is almost 100 times more massive than Earth's and
consists mainly of CO2, a greenhouse gas that raises the surface
temperature to almost 900°F. Clouds of sulfuric acid tower 14 miles high
and whip around the planet as fast as 220 mph. A human being
transported to this hellish environment would be crushed, suffocate,
desiccate, and possibly ignite.
For the most part, planetary scientists have no idea how Venus turned out this way.
"Our models and tools cannot fully explain Venus, which means we lack
the tools for understanding our own planet," points out Widemann.
"Caring about Venus is caring about ourselves."
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. "The dynamics of super-rotation
are still a puzzle despite a wealth of data from landmark missions such
as NASA's Pioneer Venus, Russia's Venera and VEGA missions, NASA's
Magellan and more recently ESA's Venus Express."
The arc of Venus as seen by NASA's TRACE spacecraft in 2004. Credit:
NASA/Trace/LMSAL This is where the Arc of Venus comes in. 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. By analyzing the lightcurve of the arc, researchers can
figure out the temperature and density of this critical layer from pole
to pole.
When the arc appeared in 2004, the apparition took astronomers by
surprise; as a result, their observations were not optimized to capture
and analyze the fast-changing ring of light.
This time, however, they are ready. Together, Pasachoff and Widemann
have organized a worldwide effort to monitor the phenomenon on June 5th,
2012. "We're going to observe the arc using 9 coronagraphs spaced
around the world," says Pasachoff. "Observing sites include Haleakala,
Big Bear, and Sacramento Peak. Japan's Hinode spacecraft and NASA's
Solar Dynamics Observatory will also be gathering data."
Pasachoff has some advice for amateur astronomers who wish to observe
the arc. "The best times to look are ingress and egress--that is, when
the disk of Venus is entering and exiting the sun. Ingress is between
22:09 and 22:27 UT on June 5th; egress occurs between 04:32 and 04:50
UT. Be sure your telescope is safely filtered. Both white light and
H-alpha filters might possibly show the arc."
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