AGU: Geophysical Research Letters

 

Editors' Highlight

Sodium atoms illuminate the extent of Mercury's comet-like tail

Mercury's 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. (2008) studied this tail by observing the bright yellow-orange light emitted by the sodium atoms in the tail. The authors found 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 away from the planet. Further analysis showed 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 tail's maximum observed extent, is approximately 15 hours. The authors noted 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.

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