Issue Date: November 21, 2016
Solid CO2 spotted on a comet
Before it ended its mission in September, the European Space Agency’s Rosetta spacecraft, which began orbiting comet 67P/Churyumov-Gerasimenko in 2014, made the first observation of solid CO2 on a comet. The craft also unveiled complex sublimating and freezing patterns taking place in the comet’s water ice and dust. These phenomena result from seasonal and diurnal extremes that a comet experiences during its elliptical orbit around the sun. An international team led by Gianrico Filacchione of the Institute for Space Astrophysics & Planetology examined data from Rosetta’s infrared spectrometer and found a patch of solid CO2 that existed on the comet’s surface when it was in its deep winter. This patch of CO2 ice disappeared within three weeks after reexposure to the sun. CO2 is so volatile that its ice sublimation temperature is a very low –190 °C; therefore, scientists expected it to exist only below the comet’s surface. The Filacchione team’s findings suggest that even more volatile species, such as carbon monoxide and methane, could also exist as surface ice during the comet’s winter (Science 2016, DOI: 10.1126/science.aag3161). Another team, led by Sonia Fornasier of the Paris Observatory, found that exposed water ice survives on 67P’s surface for a short time and that it is well-mixed with dust, which may help explain why comet cores appear dark even if they are rich in water ice (Science 2016, DOI: 10.1126/science.aag2671).
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