The fourth species detected in Mercury's exosphere was sodium (Na). It was discovered in 1985 by Drew Potter and Tom Morgan, who observed its Fraunhofer emission lines at 589 and 589.6 nm. The average column density of this element is about 1 cm−2. Sodium is observed to concentrate near the poles, forming bright spots. Its abundance is also enhanced near the dawn terminator as compared to the dusk terminator. Some research has claimed a correlation of the sodium abundance with certain surface features such as Caloris or radio bright spots; however these results remain controversial. A year after the sodium discovery, Potter and Morgan reported that potassium (K) is also present in the exosphere of Mercury, though with a column density two orders of magnitude lower than that of sodium. The properties and spatial distribution of these two elements are otherwise very similar. In 1998 another element, calcium (Ca), was detected with column density three orders of magnitude below that of sodium. Observations by the "MESSENGER" probe in 2009 showed that calcium is concentrated mainly near the equator—opposite to what is observed for sodium and potassium. Further observations by Messenger reported in 2014 note the atmosphere is supplemented by materials vaporized off the surface by meteors both sporadic and in a meteor shower associated with Comet Encke.