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How Phaethon Got the Blues

Presentation #202.07 in the session Asteroid Dynamics.

Published onOct 20, 2022
How Phaethon Got the Blues

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 interacts 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, vaporizing any dark red refractory organics, red nanophase (microcrystalline) iron, and darkish pyroxenes, leaving behind relatively bright bluish surface material while creating mineral sublimation-driven “rock comet” activity with gas production rate (Qgas) of ~ 1022 molecules/sec that can help supply the Geminid stream. The same mechanism should make any initially C-type body with a comparable orbit very blue and active, and indeed the nuclei of sungrazing 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 low-perihelion orbits, and by in situ measurements of Phaethon’s surface and coma composition with the upcoming JAXA DESTINY+ flyby mission of Phaethon (Kawakatsu & Itawa 2013, Arai+2018).

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