Mercury's silhouette to reveal its tenuous atmosphere

Mercury's silhouette to reveal its tenuous atmosphere

08 November 2006
NewScientist.com news service
David Shiga
http://space.newscientist.com/article/dn10489-mercurys-silhouette-to-reveal-its-tenuous-atmosphere.html

Observers around the world watched Mercury make a rare passage in front of the Sun on Wednesday, in an event that will not be repeated until 2016. The event may help shed light on the planet's tenuous atmosphere.

Mercury appeared as a tiny black circle just 1/200th the width of the Sun. It began moving across the Sun's face at 1912 GMT (1112 PST).

In order for transits to occur, Mercury has to come exactly between Earth and the Sun. If Mercury and Earth orbited the Sun in the same plane, this would happen every 116 days – when Mercury, which orbits the Sun every 88 days, catches up to Earth in its orbit.

But because the orbits of the two planets are slightly tilted with respect to each other, Mercury transits occur only about 13 times per century.

Mercury's 2006 transit was visible from a broad swath of Earth including the Americas, Australia and eastern Asia. The Sun set before the end of the transit for observers in the Americas, except for those on the west coast.
Ejected particles

Observing the transit requires a telescope or binoculars fitted with special filters – looking directly at the Sun can cause permanent vision damage.

You can also view the transit live online until Mercury leaves the face of the Sun at 0010 GMT on Thursday (1610 PST on Wednesday), or watch an animation made from a series of transit images.

But beyond the pure spectacle of the event, scientists are also studying the transit to try to learn more about Mercury's tenuous atmosphere. It appears to be made mostly of sodium atoms, which may have been ejected from the planet's surface by particles in the solar wind.

Andrew Potter, an astronomer at the National Solar Observatory (NSO) in Tucson, Arizona, US, who co-discovered the atmosphere in 1985 with Thomas Morgan, is observing the transit with the NSO's McMath-Pierce Solar Telescope at Kitt Peak in Arizona.

He will be looking for variations in the density of the atmosphere above different parts of the planet. "We can see how much sunlight is absorbed to find out how much sodium there is," he says.