Rapid dust color variations of comets beyond 3 au

Cometary dust, in detail, is a mixture of mineral and organic components (Guilbert-Lepoutre, et al., 2015; Prialnik, et al., 2004). As volatiles are released or sublimate, they drag out some dust particles. Together, they form a coma around the nucleus (Fulle, 2004). When we observe a comet, the light registered from the coma is a combination of solar light scattered by dust particles and gaseous emissions (Rosenbush, et al., 2020). However, at larger heliocentric distances, the latter can be negligible or absent (Ivanova, et al., 2019; Korsun, et al., 2008). It allows us to study the microphysical properties of dust particles using broadband photometry.

One of the basic photometric characteristics connected to dust is photometric color. The dust color is independent of the amount of dust particles in the coma and is defined by their physical and chemical characteristics, namely, size and chemical composition (Luk`yanyk et al., 2019; Betzler et al., 2017). Recent works of various authors show that the dust color may reveal significant variations, which sometimes can occur in a few days or weeks (Luk`yanyk, et al., 2019; Ivanova, et al., 2017). Still, changes in dust color remained poorly addressed. Predicting the color-change occurrence is impossible, so only continuous monitoring can be used for their study.

So, we aimed our work at searching for short-term (a few days or weeks) dust color variations of comets beyond 3 au from the Sun. The primary source of our data is the Skalnaté Pleso Observatory, with the 1.3-m Cassegrain-Nasmyth and 0.61-m Newtonian reflecting telescope. We used both archived and recently collected data. Additionally, we have obtained some observations at the Terskol Observatory using the 0.6-m Zeiss telescope and 6-m BTA SAO.

We have obtained data for 5 long-period comets, 5 hyperbolic comets, and 1 short-period comet 29P/Schwassmann-Wachmann 1. The latter is known for its continuous activity with outbursts, which provoke color changes. We have registered dust color variations using observations during two outbursts in October 2018 (Voitko, et al., 2022) and November 2020. Among other comets, we have found dust color variations from red to neutral or blue (or vice versa) for 3 long-period comets C/2016 M1 (PanSTARRS) (Voitko, et al., 2024), C/2017 T2 (PanSTARRS), and C/2016 N4 (MASTER) and hyperbolic comet C/2020 V2 (ZTF). Interestingly, the activity of these objects was relatively stable, with no outbursts. Hyperbolic comet C/2020 S4 (PanSTARRS) deserves a separate mention as it revealed some changes in dust color during its perihelion passage, but they were not as quick as we are looking for. It appears that 5 of 11 considered objects reveal dust color variations. To estimate dust properties, we used the model of agglomerated debris particles. It showed that changes in dust chemical composition are the main cause of color variations. In particular, a higher abundance of water-ice or Mg-rich silicate particles in the coma can induce a blue or neutral dust color. In contrast, Fe-Mg silicates and organics usually cause red color. However, we do not exclude the dependence of color from the size of dust particles. Our statistics are currently relatively sparse. Nevertheless, they show that dust color variations can be found in monitoring data. We also supplement our findings with the results from the literature to better understand the frequency of occurrence and possible mechanisms.

Acknowledgments

Our work was supported by the Slovak Grant Agency APVV no. APVV-19-0072, the Slovak Grant Agency VEGA 2/0059/22, and Doktogrant no. APP0363.

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