Water ice and surface temperature at Hapi
Maps of water ice abundance (left) and surface temperature (right) focusing on the Hapi 'neck' region of Comet 67P/Churyumov-Gerasimenko. These maps are based on images and spectra collected with Rosetta's Visible, InfraRed and Thermal Imaging Spectrometer, VIRTIS on 12 (top), 13 (middle) and 14 September (bottom) 2014.
The ice abundance map is based on images taken with VIRTIS in the optical band at 0.7 micron and VIRTIS infrared spectra, combined with models of the surface and near-surface material. The infrared spectra taken in low illumination conditions show a strong dip at wavelengths between 2.7 and 3.6 micron, showing the characteristic shape of an absorption feature caused by water ice on the surface. In the water ice abundance maps, white indicates higher abundance of ice on the surface (over 5%) while blue hues indicate lower abundances: the darker blue, the lower abundance of ice, down to 0%.
The surface temperature maps are based on VIRTIS spectra at wavelengths above 4.5 micron. White and brighter hues indicate higher temperatures, with the highest temperature shown reaching -63°C; darker and redder hues indicate lower temperatures, with the darkest hues indicating surface temperatures around -133°C.
The 12 and 13 September maps are separated by about one comet rotation, while the 13 and 14 September maps are separated by three rotations. Due to the complex topography of the comet, the illumination conditions were different in each of the three occasions.
By comparing the two series of maps, the scientists have found that, especially on the left side of each frame, water ice is more abundant on colder patches (white areas in the water ice abundance maps, corresponding to darker areas in the surface temperature maps), while it is less abundant or absent on warmer patches (dark blue areas in the water ice abundance maps, corresponding to brighter areas in the surface temperature maps). In addition, water ice was only detected on patches of the surface when they were cast in shadow.
This indicates a cyclical behaviour of water ice during each comet rotation.