Dust color variations of comet C/2016 M1 (PanSTARRS)

Our research aims to study the dust environment of comets that are active beyond the snow line. The current work considers dust color variations in the coma of long-period comet C/2016 M1 (PanSTARRS). We observed the comet from July to October 2017 (12 epochs) using a 0.61-m telescope at the Skalnaté Pleso Observatory. The activity of the comet was stable and typical for other long-period comets. Our data show a slight bluing of the comet S'BR = −0.5 ± 0.9% per 1000 Å on August 26 and reddening to S'BR = 6.0 ± 1.4% per 1000 Å on August 28, while during the rest of the observation period, the comet mostly had red color that is typical of other comets; i.e., the average S'BR is equal to 9.5 ± 1.1% per 1000 Å. We have performed modeling of the observed color variations using model agglomerated debris particles. We could reproduce the dust color variations through changes in the dust composition; however, photometric data on their own do not allow us to make a precise conclusion. For instance, the bluest color could be induced by either water-ice, Mg-rich silicate, amorphous carbon, or their mixture. The reddest color is consistent with Mg-Fe silicates and organics. Nevertheless, further study of the comet in terms of polarimetry would yield a more robust conclusion.