The "dark side" of Comet 67P Churyumov-Gerasimenko has come into view of the Rosetta spacecraft for the first time. Specifically, it has examined a region near the comet’s south pole that experiences a long and cold winter, which had previously been in darkness for five years.
Comet 67P takes 12.4 hours to rotate, while it takes 6.5 years to orbit the Sun, but not all of the comet receives equal amounts of sunlight owing to its irregular shape. The northern hemisphere experiences a summer lasting 5.5 years, while the southern hemisphere has only a short and brief summer when the comet is closest to the Sun, which will last until early 2016.
In August this year, the comet reached its closest point to the Sun – known as its perihelion – and Rosetta was there for the ride. This meant the spacecraft was able to study the southern hemisphere using all its instruments. Only the Microwave Instrument for Rosetta Orbiter (MIRO) instrument had investigated it before, with initial results from those observations published in the journal Astronomy and Astrophysics.
The results suggest it has an intriguing and different composition compared to the rest of the comet, and now that it has been illuminated by sunlight, Rosetta will be able to study it in even greater detail using the rest of its instruments.
"We observed the 'dark side' of the comet with MIRO on many occasions after Rosetta’s arrival at 67P/C-G, and these unique data are telling us something very intriguing about the material just below its surface," said Mathieu Choukroun from NASA's Jet Propulsion Laboratory (JPL), Pasadena, California, lead author of the study, in a statement.
For example, the team thinks there might be large amounts of ice in the first few tens of centimeters beneath the surface in this region, being mostly composed of water ice or carbon dioxide ice. "Surprisingly, the thermal and electrical properties around the comet’s south pole are quite different than what is found elsewhere on the nucleus," said Choukroun.
The exact processes at play in this region aren’t fully known, though. One possibility is that water and gases released during the previous perihelion condensed on the surface, remaining in place until this subsequent "summer." Data taken during this seasonal change should shed more light on the processes taking place.
Rosetta is now being placed in a distant orbit about 1,500 kilometers (930 miles) from the comet, to observe its surrounding coma (atmosphere) as a whole, before later being brought into close orbits to study its surface in more detail.
Image in text: A view of the south polar region taken on 26 August 2015. ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0.