DIX Planetary Science Seminar

Since 1882, comets have routinely been observed near the Sun to exhibit intense 589.0/589.6-nm D-line fluorescence emission by sodium atoms volatilized in large part from ejected dust grains. Since 1995, the LASCO coronagraphs onboard the SOHO spacecraft have observed thousands of comets in close proximity to the Sun and have found their sodium D emission to be exceptionally brilliant, often outshining their dust and all other gas emission combined. Meanwhile, (3200) Phaethon—a uniquely blue, 5–6 km diameter asteroid seemingly responsible for the Geminids meteoroid stream—has been repeatedly seen by the STEREO-A spacecraft's HI1 imager to suddenly brighten and develop a tail while near the Sun—phenomena previously attributed to the recurrent ejection of micron-sized dust grains. In this talk, I will present a more careful analysis of both new and archival SOHO/LASCO and STEREO/HI1 observations to show that Phaethon's observed activity instead captures the fluorescence of thermally desorbed sodium drawn into a golden orange tail by solar radiation pressure. This finding directly connects Phaethon to the large population of similarly behaving SOHO/LASCO comets, a subset of which has been suggested to also represent otherwise ordinary asteroids now thermally disrupting after being driven into Sun-approaching orbits.