The Spatial Distribution and Temperature of Mercury's Potassium Exosphere

Aside from the well-studied sodium doublet, the potassium D lines are the only optical emissions in Mercury's exosphere that are amply bright to contrast with the dayside disk. Measurements of the K exosphere are limited compared to Na, but the K regolith abundance is better constrained, so new insights may help to understand surface–exosphere coupling. We use imaging spectroscopy to map the K brightness over Mercury's evening hemisphere, which shows an enhancement at low to midlatitudes, well equatorward of the Na peak. Both Na and K are brighter in the south, but the ratio between northern and southern hemisphere K emission appears less symmetric than that of Na. The disk-averaged Na/K column density ratio is between 70 and 130. During the same night, the dayside emission was mapped, we used a high-resolution spectrograph to attempt to resolve the Na and K line widths on the nightside. Forward-modeling the alkaline line profiles with hyperfine structure gives Na D1 and D2 line widths of 1114 ± 50 K and 1211 ± 45 K, respectively. D2 may appear hotter solely because its higher opacity adds preferentially to the profile wings. The K line width is surprisingly cold and cannot be easily distinguished from the instrumental line width, even at R = 137,500. Line widths roughly constrain K gas between the surface temperature and 1000 K, making it the coldest metallic constituent of Mercury's exosphere. Although Na and K are chemical analogs and often assumed to have similar properties, the results herein illustrate quite different characteristics between these elements in Mercury's exosphere.