Archived Spitzer Observations of (3200) Phaethon: an aqueously altered asteroid.
Abstract
(3200) Phaethon is a perplexing object. It is the progenitor of the Geminids meteor stream, yet activity observed at perihelion is not sufficient to be the sole source of the meteors observed at Earth (e.g.,[1], [2]). It has been argued that Phaethon has a cometary origin but has been degassed due to many close solar passages (e.g., [3]). Phaethon has also been suggested to be related to asteroids based on near-infrared spectral similarity (e.g., [4]) and dynamical association between Phaethon's orbit and (2) Pallas (e.g., [5]). Here we present archived, mid-infrared Spitzer spectroscopy of (3200) Phaethon to investigate if Phaethon is spectrally similar to cometary nuclei or asteroids. Phaethon was observed by Spitzer with the IRS instrument on January 14, 2005 using the short-low and long-low modes of the IRS instrument with wavelength ranges of 5.5-13.2-μm and 14.0-38-μm, respectively. The integration time was 58.7 s. Its heliocentric distance was 1.1 AU and distance from Earth was 0.7 AU with a phase angle of 60.3 degrees. We first fit Phaethon's infrared spectrum with the Near Earth Asteroid Thermal Model [6] finding a best fit diameter of 7.4 +/-0.79 km, an albedo of 0.041(+0.003, -0.004) and an eta parameter of 2.30 (+0.05, -0.04). Cometary nuclei are known to have distinct spectral signatures from asteroids in the mid-infrared (especially between 5.5-13.2-mm; e.g., [7]). Phaethon appears to be asteroid-like in this wavelength region, lacking the characteristic cometary spectral signature. Furthermore, we find that Phaethon appears to be most similar to intermediately altered carbonaceous chondrites in the CM and CI chemical groups. This indicates that Phaethon has 75-80% hydrated minerals on its surface, despite frequent close passages to the Sun. References: [1] Li, J., & Jewitt, D. (2013). AJ, 145(6), 154. [2] Hui, M. T., & Li, J. (2016). AJ,153(1), 23. [3] Fox, K., et al (1984). MNRAS, 208(1), 11P-15P. [4] Licandro, J. et al., (2007). A&A, 461(2), 751-757. [5] de León, J., et al., (2010). A&A 513, A26. [6] Harris, A. W. (1998). Icarus, 131(2), 291-301. [7] Kelley, M. S., (2017). Icarus, 284, 344-358.
- Publication:
-
AAS/Division for Planetary Sciences Meeting Abstracts #50
- Pub Date:
- October 2018
- Bibcode:
- 2018DPS....5031201M