Seasonal exposure of carbon dioxide ice on the nucleus of comet 67P/Churyumov-Gerasimenko.

Carbon dioxide (CO) is one of the most abundant species in cometary nuclei, but because of its high volatility, CO ice is generally only found beneath the surface. We report the infrared spectroscopic identification of a CO ice-rich surface area located in the Anhur region of comet 67P/Churyumov-Gerasimenko. Spectral modeling shows that about 0.1% of the 80- by 60-meter area is CO ice. This exposed ice was observed a short time after the comet exited local winter; following the increased illumination, the CO ice completely disappeared over about 3 weeks. We estimate the mass of the sublimated CO ice and the depth of the eroded surface layer. We interpret the presence of CO ice as the result of the extreme seasonal changes induced by the rotation and orbit of the comet.