ESA’s Rosetta spacecraft has spent the last year orbiting and studying a comet up close. Its target: Comet 67P/Churyumov-Gerasimenko (Comet 67P for short). Comets, just like any other body in the Solar System, have a day-night cycle. On Earth, there are 24 hours in one day, but on Comet 67P the same day-night cycle is only 12 hours. After analyzing data from Rosetta’s Visible, InfraRed and Thermal Imaging Spectrometer (VIRTIS), Rosetta scientists discovered evidence of something they had long suspected – comets have an active water-ice cycle that syncs with its rotation period. The findings were published today in the journal Nature.
“We found a mechanism that replenishes the surface of the comet with fresh ice at every rotation: this keeps the comet ‘alive,’” Maria Cristina De Sanctis of INAF-IAPS in Rome, Italy, and lead author of the study said in a statement. “We saw the tell-tale signature of water ice in the spectra of the study region but only when certain portions were cast in shadow. Conversely, when the Sun was shining on these regions, the ice was gone. This indicates a cyclical behavior of water ice during each comet rotation.”
Studying VIRTIS data collected in September 2014, the team concentrated on a one square kilometer region on the comet’s neck. At the time, the neck was one of the most active areas. The data indicated that water ice both on and just below the surface sublimated (turned straight from solid to gas) during the day, with the gas escaping into space. As nighttime fell, the surface cooled rapidly. However, the underlying layers had retained enough heat during the daytime that any subsurface water ice continued to sublimate, with the gas traveling through the comet’s porous interior to the surface. Upon reaching the surface, the gases immediately freeze, forming a thin layer of ice. When the Sun “rises” the next day, the cycle begins again.
“We suspected such a water ice cycle might be at play at comets, on the basis of theoretical models and previous observations of other comets but now, thanks to Rosetta’s extensive monitoring at 67P/Churyumov-Gerasimenko, we finally have observational proof,” says Fabrizio Capaccioni, VIRTIS principal investigator at INAF-IAPS in Rome, Italy.
Based on the data, the scientists were able to estimate the relative abundance of water ice with respect to other material. Water ice accounts for 10-15% of the material over the region (down to a few centimeters deep) surveyed by VIRTIS and appears to be well mixed with other materials. The team used Rosetta’s microwave sensor (MIRO) to calculate that the same region was emitting 3% of the total water vapor emitted by the comet at that time.
“It is possible that many patches across the surface were undergoing the same diurnal cycle, thus providing additional contributions to the overall outgassing of the comet,” adds Dr. Capaccioni.
Rosetta is capable of tracking small-scale and large-scale changes over time, and the team is currently analyzing the data collected in the months around the closest approach to see what effects the increased activity had on the water-ice cycle. While these initial findings provide only a glimpse into the comet’s inner workings, with Rosetta’s help scientists hope to better understand the processes involved.
