Thermal properties of (4) Vesta derived fromHerschelmeasurements

Abstract

Aims. We report in this paper the results provided by new infrared observations of the asteroid (4) Vesta obtained between April 2010 and June 2010 with the Herschel space observatory (ESA) in the wavelength range 70–500 μm when the asteroid was seen from the Northern hemisphere. Over this period, the thermal flux of Vesta was observed at different rotational phases and a large fraction of the surface was visible. Methods. A thermophysical model was used to derive thermal properties such as infrared emissivity, thermal inertia and surface roughness. Results. We found an average thermal inertia Γ= 20 +20 −10 Jm −2 K −1 s −1/2 . The surface roughness on the Northern hemisphere, while poorly constrained, appears to be low on Vesta with a mean slope angle ¯ θ ∼ 23 ◦ . Surface temperatures range from 40 K to 248 K. We cannot confirm any longitudinal variation of the thermal flux as it remains within the error bars. More observations are needed to confirm if the eastern hemisphere could have either a slightly higher thermal inertia suggesting a rockier surface or a smooth surface with a higher beaming factor. We emphasize an important decrease of the infrared emissivity of Vesta with the wavelength, ranging from 0.9 at 70 μm to 0.70 at 500 μm.