Thermal decomposition as an activity driver of near-Earth asteroid (3200) Phaethon

Abstract

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 [1][2][3]. The mechanism causing the activity is the subject of many investigations, and the suggested mechanisms lack clear supporting evidence [4][5][6]. Phaethon has been identified as the likely parent body of the annual Geminid meteor shower [7-10], 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 [11][12][13][14][15][16]. Methods & Results: 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 latestage thermal metamorphism [17][18][19]. 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 S2 and CO2 gas upon heating to ∼700 • C [20][21][22]. 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. Conclusion: All of these lines of evidence are strong indicators that gas release from thermal decomposition reactions is responsible for Phaethon's