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Three Micron Phase Curves of Main Belt Asteroids from NEOWISE Photometry

Presentation #503.05D in the session Asteroids: Main Belt (Oral Presentation)

Published onOct 23, 2023
Three Micron Phase Curves of Main Belt Asteroids from NEOWISE Photometry

The shape of an asteroid phase curve is influenced by the physical properties of the surface of the asteroid such as, grain size distribution, surface roughness, porosity, and composition. While phase curves are most often constructed using visible photometric data, infrared phase curves can also provide useful insight into the surface properties of an asteroid given that spectral absorption features associated with hydrated and organic materials occur at infrared wavelengths. Here we present thermally and rotationally corrected 3 μm phase curves for a sample of ~100 main belt asteroids. These phase curves were constructed using photometric observations centered near 3.4 μm from the W1 band of the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) mission. Our sample includes asteroids observed by the AKARI satellite, which collected spectroscopic data from 2.5 to 5 μm in search of absorption features associated with hydrated materials, as well as a subset of the Themis dynamical family, which has several members on which volatiles have been detected. Studying the W1 phase curves of these two populations provides a unique opportunity to investigate asteroids that may harbor substances important to the development of life.

In this work, we determine the distribution of linear W1 phase slopes (solar phase angles > 10°) for a subset of AKARI asteroids and Themis family asteroids. We compare AKARI objects to 2.7 μm and 3.1 μm band depths, while also including comparisons to visible and W1 geometric albedos from NEOWISE. We find little correlation between the depth of 3 μm absorption features with the W1 phase slope among the AKARI sample, but we do observe increasing W1 phase slope vs. 2.7 μm band depth among C-complex asteroids in the sample. We also find that lower albedo taxonomies (C-types) have steeper average W1 phase slopes than higher albedo taxonomies (M and S-types), following the expectation at visible wavelengths. For Themis family asteroids, we compare their W1 phase slopes to their W1 albedos and find similarities to trends observed among Jupiter Family Comets, which showed steepening R-band phase slopes with increasing R-band albedo. We also observe steeper W1 phase slopes with decreasing object diameter. We suggest that the surface properties associated with larger and darker objects (smaller particle sizes, smoother surfaces, more pronounced shadow hiding and single particle scattering) lead to more uniform scattering and result in the observed flatter linear phase slopes for larger objects.

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