Feb. 28, 2008 -- The first rock from the sun has a glowing dragon tail of sodium atoms that is more than seven times longer than ever suspected.
New measurements of Mercury's yellow-orange tail, which streams in the solar wind like the long tail of a kite, put it at more than 100 times the radius of the planet itself.
The neutral sodium atoms that make up the 1.6-million-mile-long streamer are thought to be blasted off the surface by the sun and micro-meteor impacts. These impart enough energy to launch the atoms into space.
Other elements are also in the tail, but it's the sodium which lights up and can be detected.
"This (sodium) ion is the 'little atom that could,'" said Mercury researcher Ann Sprague of the Lunar and Planetary Laboratory at the University of Arizona in Tucson. It scatters photons like crazy, making it a great clue to various processes at work on and around the planet.
Mercury's tail had been spotted before, but its great length was missed because previous attempts were looking at too small a piece of the sky, said researcher Jeffrey Baumgardner of Boston University. Baumgardner is the lead author of a report on Mercury's sodium tail in the current issue of Geophysical Research Letters.
"Our forte was wide-angle imaging," Baumgardner told Discovery News.
The 8 degree-wide image his team created using a telescope at McDonald Observatory in Texas is about 16 full-moons wide. The actual 2 degree sodium tail is as long as four full moons. In their image Mercury is blocked out because its brightness outshines the sodium tail. If Mercury were shown to scale in the image, it would be less than a pixel in diameter.
"But even so, you are limited by the sodium in Earth's atmosphere," said Baumgardner. Earth's own sodium glow is itself created by the steady supply of meteors that burn up in the atmosphere, he explained.
To better understand how Mercury's tail is created, Baumgardner's team also made close-up sodium-glow images of Mercury. This revealed that the planet has two sodium hot spots, both at high latitudes.
These could be the product of the planet's mineralogy, topography or have something to do with how the planet's magnetic field channels in particles from the sun--similar to how Earth does the same thing and creates aurora light shows near the poles.
The secrets of these hotspots are likely to be revealed by the Messenger spacecraft, said Sprague. Messenger made a close flyby of the planet in January and is scheduled to settle into orbit in the year March, 2011. A large part of Messenger's mission will be to map out the mineral composition of the swiftly moving planet.
Mercury is not the only heavenly body with a sodium tail, Baumgardner points out. Neutral sodium is also seen streaming from the moon and forming a haze around Jupiter from the sodium blasted off of its tiny and hyper-volcanic moon, Io. It's also seen blowing from comets.
Because of its association with rocky bodies in our solar system, it's conceivable that someday such tails could help planet hunters identify rocky worlds orbiting other stars, said Sprague.
"It's a stepping stone to understanding other planets," Sprague said.
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