  Near-Earth asteroid (3200) Phaethon is an active asteroid with a dust tail
repeatedly observed over the past decade for 3 days during each perihelion
passage down to a heliocentric distance of 0.14 au. The mechanism causing the
activity is still debated, and the suggested mechanisms lack clear supporting
evidence. Phaethon has been identified as the likely parent body of the annual
Geminid meteor shower, making it one of the few active asteroids associated
with a meteoroid stream. Its low albedo and B-type reflectance spectrum
indicates that Phaethon's composition is similar to carbonaceous chondrite
meteorites, but a connection to a specific meteorite group is ambiguous due to
the lack of diagnostic absorption features. In this study, we analyze the
mid-infrared emissivity spectrum of Phaethon and find that it is closely
associated with the Yamato-group (CY) of carbonaceous chondrites. The CY
chondrites represent primitive carbonaceous material that experienced early
aqueous alteration and subsequent late-stage thermal metamorphism. Minerals in
these meteorites, some of which we identify in Phaethon's spectrum, show
evidence of thermal decomposition; notably, the dehydroxylation and
transformation of phyllosilicates into poorly crystalline olivine.
Additionally, sulfides and carbonates in CYs are known release S2and CO2 gas
upon heating to ~700oC. We show that Phaethon's surface temperature during its
observed window of activity is consistent with the thermal decomposition
temperatures of several components in CY meteorites. All of these lines of
evidence are strong indicators that gas release from thermal decomposition
reactions is responsible for Phaethon's activity. The results of this study
have implications for the formation of the Geminid meteoroid stream, the
origins of thermally-altered primitive meteorites, and the destruction of
low-perihelion asteroids.
