Short-term color changes in comets at large heliocentric distances

We present the results of our search for rapid changes in cometary dust color and the investigation of their frequency of occurrence, possible mechanisms, and triggers. Basically, we collected and processed the observational data and explored the previously published reports in order to find and analyse variations in the dust color from red to blue or neutral or vice versa, that occurred in a few days. As the subject of the research, we selected distant comets observed beyond 3 au from the Sun. Contrary to comets orbiting at shorter heliocentric distances, namely Jupiter-family members, the activity of these objects is driven by different mechanisms than water-ice sublimation. There can be annealing or crystallization of water ice, or sublimation of volatiles, even though the contribution of water ice cannot be entirely excluded (1; 2). Another reason for choosing distant comets is that, typically, for research on them, sporadic observations are used, whereas we employed monitoring data (3).

Despite significant progress in the in situ techniques, the remote sensing methods still account for the majority of the gathered knowledge. Among the available techniques, an effective approach is to study the light-scattering response from comets. It enables us to reproduce the properties of coma dust particles that are inherently linked to the nucleus.  One of the key light-scattering characteristics is the color, which reflects the difference in scattering efficiency by dust particles at two different wavelengths or filters. This parameter depends on the size distribution and chemical composition of dust; however, it is insensitive to the number of particles within the aperture (4). Consequently, variations in the dust color may indicate the ongoing processes causing the alternation of particles emanating from the nucleus or in the coma, or generally reflect specific characteristics of the nucleus.

The main part of the work was performed based on the archival observations from the Skalnaté Pleso Observatory (IAU code 056) supplemented by recent observations from this and three other observatories. We managed to collect data for 11 various comets. Among these, 5 revealed some changes in their color during periods of heightened activity (Afρ ≥ 1000 cm) or at the perihelion when their activity peaked. In some cases, the strong activity also supported the appearance of morphological features within the coma. The application of the model of agglomerated debris particles (5) allowed us to estimate specifically how the microphysical properties of dust probably changed. Furthermore, the identified morphological features were reproduced using the geometrical model (6), which allowed for the estimation of the rotation and surface properties of several nuclei.

To extend the research, we have performed a review of the literature published since 1970. It allowed us to compile a database of dust color variations containing more than 200 records, accompanied by multiple parameters. The analysis showed that the majority of color variations were reported for Jupiter-family comets within 3 au. It can be attributed to the higher frequency of observations of these objects and the fact that they maintain relatively high activity levels at such heliocentric distances. Conversely, at larger distances from the Sun, changes in color were primarily reported for long-period and hyperbolic comets. Various authors considered the observed color changes as the result of some alteration of dust. In particular, our estimations of the properties of dust particles are generally in agreement with their findings.

In conclusion, our study showed that dust color variations and the formation of some morphological structures in comae can be triggered by strong activity. The bluer dust color is associated with smaller particles with a higher abundance of water-ice, Mg-rich, or pure Mg silicates, while the redder color can be induced by larger particles or those containing a larger amount of organics, dirty ices, or Mg-Fe silicates.

This work establishes a foundation for future studies. Particularly, there is a necessity for a more detailed investigation of the mechanisms causing the dust color variations. However, they should be addressed through dedicated modeling and further observational campaigns.

References

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