Near-infrared spectroscopic characterization of the Pallas family

¹ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Santiago, Chile
² Planetary Science Institute, 1700 E Fort Lowell Rd STE 106, Tucson, AZ 85719, USA
³ Charles University, Faculty of Mathematics and Physics, Institute of Astronomy, V Holešovičkách 2, 18000 Prague, Czech Republic

^★ Corresponding author; prachi.chavan@mail.udp.cl

Received: 18 December 2024
Accepted: 24 February 2025

Abstract

Context. Asteroid families hold clues to the collisional processes that shaped the Solar System over billions of years. The Pallas collisional family, named after (2) Pallas, is notable for its high orbital inclination and the distinct blue color of Pallas and a few larger B-type family members. While Pallas itself, as one of the largest asteroids, has been studied in detail, most of its smaller family members still remain unexplored.

Aims. This study aims to characterize the physical properties of medium- to small-sized Pallas family asteroids to investigate the origin of their unusual blueness. We seek to establish connections between asteroid spectra and meteorite analogs. Additionally, we explore the relationship between the Pallas family and the near-Earth object (NEO) (3200) Phaethon.

Methods. We conducted near-infrared (NIR) spectroscopy with the NASA Infrared Telescope Facility (IRTF) to collect reflectance spectra for 22 asteroids, including one from the IRTF Legacy Archive. Spectroscopic and dynamical analyses were carried out to identify outliers, while additional data from NEOWISE and Gaia were incorporated to examine potential correlations among their physical properties. Meteorite analogs were identified through χ² matching using samples from the RELAB database.

Results. The observed Pallas family asteroids exhibit nearly identical spectral profiles, suggesting a homogeneous composition of ejected material. Small variations in spectral slopes are observed, which may result from different levels of alteration experienced by individual asteroids, with some influence from variations in grain size. Most of the observed spectra of the Pallas asteroids, from 0.8 to 2.2 µm, closely resemble those of the CY and CI meteorites. The blueness of asteroid surfaces is likely due to the presence of magnetite, troilite, or phyllosilicates, which are products of aqueous alteration. The striking spectral similarity between (3200) Phaethon and Pallas family members of comparable sizes suggests a potential common origin.