The sodium tail of Mercury

Mercury is difficult to observe because it is so close to the Sun. However, when the angle of the ecliptic is near maximum in the northern hemisphere, and Mercury is near its greatest eastern elongation, it can be seen against the western sky for about a half hour after sunset. During these times, we were able to map sodium D 2 emission streaming from the planet, forming a long comet‐like tail. On 2001 May 26 (U.T.) we mapped the tail downstream to a distance of ˜40 000 km. Sodium velocities in the tail increased to ˜11 km s −1 at 40 000 km as the result of radiation pressure acceleration. On 2000 June 5 (U.T.) we mapped the cross‐sectional extent of the tail at a distance of ˜17 500 km downstream. At this distance, the half‐power full‐width of the emission was ˜20 000 km. We estimated the transverse velocity of sodium in the tail to range from 2 to 4 km s −1 . The velocities we observed imply source velocities from the planet surface of the order of 5 km s −1 , or 4 eV. Particle sputtering is a likely candidate for production of sodium atoms at these velocities. The total flux of sodium in the tail was ˜1 times 10 23 atoms s −1 , which corresponds to 1 to 10% of the estimated total production rate of sodium on the planet.