Low Water Outgassing from (24) Themis and (65) Cybele: 3.1 μm Near-IR Spectral Implications

• Published in: Astrophysical journal. Letters Vol. 898; no. 2; p. L45
• Main Authors: O’Rourke, L., Müller, T. G., Biver, N., Bockelée-Morvan, D., Hasegawa, S., Valtchanov, I., Küppers, M., Fornasier, S., Campins, H., Fujiwara, H., Teyssier, D., Lim, T.
• Format: Journal Article
• Language: English
• Published: Austin IOP Publishing 01.08.2020; Bristol : IOP Publishing
• Subjects: Absorption; Albedo; Asteroids; Far infrared radiation; Ice; Inertia; Infrared cameras; Infrared instruments; Infrared photometry; Infrared spectrometers; Observatories; Outgassing; Sciences of the Universe; Space telescopes; Specific heat; Surface chemistry; Surface water; Thermal inertia; Thermophysical models; Water ice; Water vapor; Comets; Main-belt comets; 1469; Asteroids; 2036; Main belt asteroids; 2131; 280; 72; Astronomy data modeling; 1859; Small solar system bodies
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/aba62b

Abstract Asteroids (24) Themis and (65) Cybele have an absorption feature at 3.1 μ m reported to be directly linked to surface water ice. We searched for water vapor escaping from these asteroids with the Herschel Space Observatory Heterodyne Instrument for the Far Infrared. While no H 2 O line emission was detected, we obtain sensitive 3 σ water production rate upper limits of Q (H 2 O) < 4.1 × 10 26 molecules s −1 for Themis and Q (H 2 O) < 7.6 × 10 26 molecules s −1 for Cybele. Using a thermophysical model, we merge data from the Subaru/Cooled Mid-Infrared Camera and Spectrometer and the Herschel/Spectral and Photometric Imaging Receiver with the contents of a multi-observatory database to derive new radiometric properties for these two asteroids. For Themis, we find a thermal inertia J m −2 s −1/2 K −1 , a diameter km, and a geometric V -band albedo p V  = 0.07 ± 0.01. For Cybele, we obtain a thermal inertia J m −2 s −1/2 K −1 , a diameter 282 ± 9 km, and an albedo p V  = 0.042 ± 0.005. Using all inputs, we estimate that water ice intimately mixed with the asteroids’ dark surface material would cover <0.0017% (for Themis) and <0.0033% (for Cybele) of their surfaces, while an areal mixture with very clean ice (Bond albedo 0.8 for Themis and 0.7 for Cybele) would cover <2.2% (for Themis) and <1.5% (for Cybele) of their surfaces. While surface (and subsurface) water ice may exist in small localized amounts on both asteroids, it is not the reason for the observed 3.1 μ m absorption feature.