Thermal Alteration and Differential Sublimation Can Create 3200 Phaethon's "Rock Comet" Activity and Blue Color

Asteroid 3200 Phaethon is a small (~6 km diameter) asteroid that comes extremely close to the Sun (0.16 au, or more than twice as close as Mercury) every 1.4 years. It is also one of the bluest asteroids known (there are only ~20 blue asteroids known versus ~200,000 red/grey ones). It is also the source of the Gemini's meteor stream and shower the Earth intersects with each December. In Lisse & Steckloff 2022 (Icarus 381:114995), we showed how these are all connected, in that the ~1100K noontime temperatures achieved at perihelion wander across its surface, boiling off any dark red refractory organics, red nanophase (microcrystalline) iron, and darkish pyroxenes, leaving behind relatively bright bluish surface material while creating "rock comet" activity, with gas production rate Qgas ~ 1022 molecules/sec that can help supply the yearly Geminids. The same mechanism should make any initially C-type body very blue and active, and indeed the nuclei of comets 96P/Machholz and 322P/SOHO are also anomalously blue and highly active. These predictions are testable by searching for signs of spectral bluening of the surfaces of other small bodies in Phaethon-like small perihelion orbits, and by in situ measurements of Phaethon's surface and coma composition near perihelion with the upcoming DESTINY+ mission (Kawakatsu & Itawa 2013, Arai et al. 2018) to Phaethon by JAXA.