Rosetta's 'rubber ducky' comet changed color as it neared the sun.

The Rosetta spacecraft's rubber ducky comet has slowly changed color as it moved through space, from red to blueish and then red again.

According to a new paper published Feb. 5 in the journal Nature, the color change is a signal of a water cycle on the first comet ever visited by a human probe. As comet 67P/Churyumov–Gerasimenko (Rosetta's comet's full name) crossed a boundary in its orbit around the sun, known as the frost line, ice began to turn to gas on its surface, sublimating away into space. When that happened, an outer layer of dirty ice on the comet's surface, full of reddish dust, blew away into the vacuum, revealing the bluer, cleaner ice underneath.

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(Image: ESA/Rosetta/NAVCAM)

A single frame Rosetta navigation camera image of Comet 67P/Churyumov-Gerasimenko.

Source / Image Courtesy

(Image credit: European Space Agency)

A diagram shows how the comet changed from redder to bluer and back to red again as it passed the sun.

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Compilation of images taken by Rosetta’s high resolution OSIRIS camera during the mission’s final hours at Comet 67P/Churyumov-Gerasimenko, including its final resting place and one of Philae’s three legs.