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

Asteroids (24) Themis and (65) Cybele have an absorption feature at 3.1 mu 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 H2O line emission was detected, we obtain sensitive 3 sigma water production rate upper limits of Q(H2O) < 4.1 x 10(26) molecules s(-1) for Themis and Q(H2O) < 7.6 x 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 Gamma = 20(-10)(+25) J m(-2) s(-1/2) K-1, a diameter 192(-7)(+10) km, and a geometric V-band albedo p(V) = 0.07 +/- 0.01. For Cybele, we obtain a thermal inertia Gamma = 25(-19)(+28) 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 mu m absorption feature.