AGU journal highlights -- Feb. 26, 2008( 1. Sodium glow reveals Mercurys com...)

Mercurys gravity is too weak to hold a permanent atmosphere. Thus, when atoms are evaporated from the surface by solar photons or other energetic processes, some of these atoms are accelerated to escape velocity by solar radiation pressure. The neutral components of this escaped gas form a comet-like tail that points away from the Sun. Baumgardner et al. study this tail by observing the bright yellow-orange light emitted by the sodium atoms in the tail. The authors find that the tail, previously detected out to 15 times the radius of Mercury, actually extends more than 100 times that distance, or 2.5 million kilometers (1.6 million miles) away from the planet. Further analysis shows that the time of flight for the sodium atoms in the tail, from the time they are sputtered from the surface to when they reach the tails maximum observed extent, is approximately 15 hours. The authors note that although sodium atoms compose only a small fraction of the atoms sputtered from the surface, the bright emission from the sodium can serve as a tracer for other constituents invisible to ground-based observers.

Title: Imaging the sources and full extent of the sodium tail of the planet Mercury

Authors: Jeffrey Baumgardner, Jody Wilson, and Michael Mendillo: Center for Space Physics, Boston University, Boston, Massachusetts, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032337, 2008; http://dx.doi.org/10.1029/2007GL032337

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