Located far beyond the orbit of Mars, Comet PanSTARRS (C/2016 R2) is gliding through a cold and distant region of interplanetary space where things ordinarily change slowly, if at all. But PanSTARRS is no ordinary comet. On Jan. 10th, Austrian astrophotographer Michael Jäger recorded dramatic changes in only two hours:

Jäger is a veteran observer of comets, and he says he has never seen one quite like this. "Over the years, I have photographed 850 comets. But there was no other comet that changed its appearance so quickly at such great distances from the sun. I remember a few comets like 2015 Comet Lovejoy with significant changes from day to day, but they were much closer to the sun [where hot sunlight could drive activity]," says Jäger.
Only a few days ago, Comet PanSTARRS looked substantially different. Jäger took this color photo on Jan. 6th:

Since November 2017, Comet PanSTARRS has changed its appearance almost every time astronomers look at it. Images show jets waving wildly around the comet's core and clouds of gas billowing through the comet's tail.
What's happening? Blue is the telltale clue. That's the color of ionized carbon monoxide (CO+) fluorescing in the 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 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 another 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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