Low Water outgassing from (24) Themis and (65) Cybele – Implications on 3.1 µm water absorption spectra understanding

Asteroids (24) Themis and (65) Cybele have an absorption feature at 3.1 µm reported to be directly linked to surface water ice. We searched for water vapour escaping from these asteroids with the Herschel Space Observatory Heterodyne Instrument for the Far Infrared (HIFI). While no H₂O line emission was detected, we obtain sensitive 3σ water production rate upper limits of Q(H₂O) < 4.1 × 10²⁶ mol. s⁻¹ for Themis and Q(H₂O) < 7.6 × 10²⁶ mol. s⁻¹ for Cybele. Using a Thermophysical Model (TPM), we merge data from Subaru/Comics and Herschel/SPIRE with the contents of a multi-observatory database to derive new radiometric properties for these two asteroids. For Themis, we find a thermal inertia Γ = 20⁺²⁵_-10 J m^-2 s^-1/2 K^-1, a diameter 192 ⁺¹⁰_-7 km and a geometric V-band albedo p_V = 0.07 ±0.01. For Cybele we obtain a thermal inertia Γ = 25 ⁺²⁸_-19 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 (& sub-surface) water ice may exist in small localized amounts on both asteroids, it is not the reason for the observed 3.1µm absorption feature.