Beyond the orbit of Mars, blue Comet PanSTARRS (C/2016 R2) is behaving strangley. Active jets of gaseous material are spewing from the comet's core, feeding a chaotic tail marked by rough swirls and an unusual rectangular "knee." Michael Jäger photographed it from St. Oswald, Austria, on Jan. 6th:

"I took 30 shots in 90 minutes, which allowed me to make a nice animation," says Jäger.

What's happening to this comet? Blue is the telltale clue. That's the color of ionized carbon monoxide (CO+) fluorescing in the near-vacuum of interplanetary space.

Comet PanSTARRS has abundant stores of CO. Last month, astronomers K. Wierzchos and M. Womack of the University of South Florida used the Arizona Radio Observatory's 10-m Submillimeter Telescope at Mount Graham to detect as many as 4.7 x 1028 CO molecules emerging from the comet's core every second. "This comet appears to be very CO-rich," they wrote in International Astronomical Union telegram CBET 4464.
Carbon monoxide can make a comet behave strangely because it is extremely volatile. CO can sublimate (change suddenly from solid to gas) at temperatures as low as -248 C (25 K). Only a little bit of sunlight is therefore required to turn deposits of frozen CO into wild jets and billowing clouds.

"The last notable comet with high CO was Comet Humason in 1962, so this is quite a rare sight," notes veteran comet observer Michael Mattiazzo of Australia. "It will be very interesting to watch Comet PanSTARRS as it makes its closest approach to the sun (2.6 AU) in May 2018." Stay tuned!

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