Earth isn't the only place with seasons. Other planets and even very small celestial bodies can have them, too, as ESA's Rosetta probe has shown in its explorations of comet 67P/Churyumov-Gerasimenko. When the unmanned spacecraft went into orbit about the comet, it revealed that the southern hemisphere of the dumbbell-shaped nucleus is shrouded in a dark winter that lasts over five years and, according to data collected by the Rosettas's onboard spectrometer, hides ice in larger amounts than the rest of the comet.

When Rosetta arrived at 67/P in August 2014, it was able to make detailed observations of the northern hemisphere, but much of the southern hemisphere near the south pole was shrouded in darkness. This is because 67/P's peculiar double-lobed shape and the inclination of its rotation axis causes some unusual seasons during its 6.5-year-long orbit. The northern hemisphere as a very long summer, lasting over 5.5 years, while the southern hemisphere has a long, dark, cold winter

Without direct sunlight, the much of the south polar regions was invisible to Rosetta's Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) camera and its Visible, InfraRed, and Thermal Imaging Spectrometer (VIRTIS). The only instrument that could take a look until perihelion was the Microwave Instrument for Rosetta Orbiter (MIRO), which is a spectrometer designed to map the concentration, temperature, and velocity of the molecules that make up the comet's emissions. It can also take temperature readings in the nucleus down to a depth of 1 in (3 cm).

Subsurface temperature maps of 67P/C-G, showing the southern hemisphere of the comet(Credit: ESA/Rosetta/NASA/JPL-Caltech)

Between August and October 2014, scientists from ESA and NASA's Jet Propulsion Laboratory studied millimeter and sub-millimeter wavelength channels and discovered a discrepancy between the two, indicating that there were large amounts of ice present.

"Surprisingly, the thermal and electrical properties around the comet’s south pole are quite different than what is found elsewhere on the nucleus," says Mathieu Choukroun of JPL. "It appears that either the surface material or the material that’s a few tens of centimeters below it is extremely transparent, and could consist mostly of water ice or carbon-dioxide ice."

The scientists believe that the ice might be the result of the long, cold winter at the south polar regions. As the comet approaches perihelion, water and gases are released from other parts of the comet, and when they travel south, the cold causes them to condense and resolidify.

ESA stresses that this is only a preliminary conclusion and will need to be tested against later measurements of the polar region's geography and other data revealed by observations made during the southern summer, which began in May as the comet approached its perihelion, and is expected to last until early next year.

It's hoped that this will determine whether the region has a different composition from the rest of the comet nucleus, or if this is just seasonal, like snow in winter on Earth.

"In the past few months, Rosetta has flown over the southern polar regions on several occasions, starting to collect data from this part of the comet after summer began there,” says Matt Taylor, ESA Rosetta project scientist. "At the beginning of the southern summer, we had a paucity of observations in these regions as Rosetta's trajectory focused on the northern hemisphere due to ongoing communication with the lander, Philae. However, closer to perihelion we were able to begin observing the south."

Rosetta is currently at a distance of 930 mi (1,500 km) from 67/P, but will move in closer to again orbit the comet for a better look at the northern and southern regions with the full suite of instruments.

The latest findings on the dark side of 67P/Churyumov-Gerasimenko are published in Astronomy and Astrophysics.

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