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June 03, 2013

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“Ring of fire” to offer clues about Venus, our hellish twin planet

June 5, 2012
Courtesy of NASA
and World Science staff

When Ve­nus passes in front of the Sun on June 5 and 6, an ar­ma­da of tele­scopes will be on the look­out for some­thing elu­sive and, un­til re­cent­ly, un­ex­pected: the Arc of Ve­nus.

“I was flab­ber­gasted when I first saw it dur­ing the 2004 tran­sit,” that is, when Ve­nus last crossed in front of the Sun, re­calls as­tron­o­mer Jay Pasa­choff of Wil­liams Col­lege in Wil­liamstown, Mass. “A bright, glow­ing rim ap­peared around the edge of Ve­nus soon af­ter it be­gan to move in­to the sun.”

For an in­stant, the plan­et had be­come a “ring of fire.”

Three pho­tos from the Arc of Ve­nus tak­en dur­ing the plan­et's 2004 trans­it by am­a­teur as­tron­o­mer near Tou­louse, France. (Im­age cour­te­sy An­dré Rondi)


Re­search­ers say they now un­der­stand what hap­pened. Back­lit by the sun, Ve­nus’s at­mos­phere bent, or re­fracted, sun­light pass­ing through lay­ers of air above the plan­et’s cloud­tops. This cre­at­ed an arc of light. Sci­en­tists now be­lieve can learn a lot about Ve­nus by ob­serv­ing the arc. In­deed, it touches on some of the deep­est mys­ter­ies of the sec­ond plan­et from the Sun.

“We do not un­der­stand why our sis­ter plan­et’s at­mos­phere evolved to be so dif­fer­ent than Earth­’s,” said plan­etary sci­ent­ist Thom­as Wide­mann of the Ob­ser­va­toire de Par­is.

Earth and Ve­nus are si­m­i­lar dis­tances from the Sun, con­sist of the same bas­ic ma­te­ri­als, and are al­most the same size. Yet they’re wrapped in very dif­fer­ent blan­kets of air. Ve­nus’s at­mos­phere is al­most 100 times heav­i­er than Earth’s and con­sists mainly of car­bon di­ox­ide, a heat-trapping gas that raises the sur­face tem­pe­r­a­ture to al­most 900 de­grees Fahr­en­heit (500 Cel­sius). Clouds of sul­fu­ric ac­id al­so tow­er 14 miles (23 km) high and whip around the plan­et as fast as 220 miles (350 km) per hour. 

A pe­rson trans­port­ed to this hell­ish en­vi­ron­ment would be crushed, suf­fo­cate, dry out, and pos­sibly ig­nite.

Plan­e­tary sci­ent­ists have lit­tle 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,” said 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,” said Wide­mann.

This is where the Arc of Ve­nus comes in. Its bright­ness 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.

When the arc ap­peared in 2004, as­tron­o­mers were caught off guard, and un­pre­pared to make the best of the fast-changing da­ta com­ing at their tele­scopes. This time, they say, they’re ready. Pasa­choff and Wide­mann have or­gan­ized a world­wide ef­fort to mon­i­tor the phe­nom­e­non. “We’re go­ing to ob­serve the arc us­ing nine coro­n­a­graphs spaced around the world,” said Pasa­choff. A co­ro­na­graph is an at­tach­ment to a tel­e­scope that blocks di­rect sun­light so that the sur­round­ings of the Sun can be ob­served with­out prob­lems.

“Ob­serv­ing sites in­clude Ha­le­a­ka­la [in Hawaii], Big Bear, and Sac­ra­men­to Peak,” both in Cal­i­for­nia, Pasa­choff added. “Japan’s Hin­ode space­craft and NASA’s So­lar Dy­nam­ics Ob­serv­a­to­ry will al­so be gath­er­ing da­ta.”

Skygaz­ers are be­ing urged to take pro­per pre­cau­tions when view­ing the event. For those who have tele­scopes, Pasa­choff has this ad­vice. “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 [U­ni­ver­sal Time] on June 5. Egress oc­curs be­tween 4:32 and 4:50 [U­ni­ver­sal Time]. Be sure your tel­e­scope is safely fil­tered. Both white light and H-alpha fil­ters might pos­sibly show the arc.”


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When Venus passes in front of the Sun on June 5 and 6, an armada of telescopes will be on the lookout for something elusive and, until recently, unexpected: the Arc of Venus. “I was flabbergasted when I first saw it during the 2004 transit,” that is, when Venus last crossed in front of the Sun, recalls astronomer Jay Pasachoff of Williams College in Williamstown, Mass. “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 become a “ring of fire.” Researchers say they now understand what happened. Backlit by the sun, Venus’s atmosphere bent, or refracted, sunlight passing through layers of air above the planet’s cloudtops. This created an arc of light. Scientists now believe can learn a lot about Venus by observing the arc. Indeed, it touches on some of the deepest mysteries of the second planet from the Sun. “We do not understand why our sister planet’s atmosphere evolved to be so different than Earth’s,” said planetary scientist Thomas Widemann of the Observatoire de Paris. Earth and Venus are similar distances from the Sun, consist of the same basic materials, and are almost the same size. Yet they’re wrapped in very different blankets of air. Venus’s atmosphere is almost 100 times heavier than Earth’s and consists mainly of carbon dioxide, a heat-trapping gas that raises the surface temperature to almost 900 degrees Fahrenheit (500 Celsius). Clouds of sulfuric acid also tower 14 miles (23 km) high and whip around the planet as fast as 220 miles (350 km) per hour. A person transported to this hellish environment would be crushed, suffocate, dry out, and possibly ignite. Planetary scientists have little 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,” said 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,” said Widemann. This is where the Arc of Venus comes in. Its brightness 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. When the arc appeared in 2004, astronomers were caught off guard, and unprepared to make the best of the fast-changing data coming at their telescopes. This time, they say, they’re ready. Pasachoff and Widemann have organized a worldwide effort to monitor the phenomenon. “We’re going to observe the arc using nine coronagraphs spaced around the world,” said Pasachoff. A coronagraph is an attachment to a telescope that blocks direct sunlight so that the surroundings of the Sun can be observed without problems. “Observing sites include Haleakala [in Hawaii], Big Bear, and Sacramento Peak,” both in California, Pasachoff added. “Japan’s Hinode spacecraft and NASA’s Solar Dynamics Observatory will also be gathering data.” Skygazers are being urged to take proper precautions when viewing the event. For those who have telescopes, Pasachoff has this advice. “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 [Universal Time] on June 5. Egress occurs between 04:32 and 04:50 [Universal Time]. Be sure your telescope is safely filtered. Both white light and H-alpha filters might possibly show the arc.”