Tracing Asymmetries in the 67P’s Dust Coma Brightness Distribution Using Rosetta’s OSIRIS Observations

Comet 67P/Churyumov–Gerasimenko provides a rare opportunity to explore the interplay between illumination geometry, seasonal activity, and dust coma structure, thanks to ESA’s Rosetta mission. In this study, we examine directional asymmetries in the inner coma brightness using OSIRIS Wide-Angle Camera (WAC) images. We focus on azimuthal profiles at a fixed radial distance from the nucleus, allowing us to quantify day-night brightness ratios and assess whether peak intensities align with the projected subsolar direction.

A Python-based processing pipeline was developed to enable the automatic selection, calibration, and analysis of OSIRIS WAC images. This tool supports large-scale, reproducible analysis of the dust coma by systematically extracting azimuthal intensity distributions along circular profiles at a fixed impact parameter (typically 10–12 km from the nucleus center). This radius is commonly used in cometary coma studies as it approximates the force-free radial flow regime, where column densities scale as 1/r, enabling a robust comparison of brightness profiles across time and observational geometry (Gerig et al. 2018, Zakharov et al. 2018).

First stages of our research included reproducing the work of Gerig et al. (2021), who tracked the dayside-to-nightside (DS:NS) brightness ratio over the mission timeline. Indeed, the ratio grows from 2.5 to 4 as the comet moves from 1.88 AU to perihelium at 1.25 AU (perihelium). One might expect a much greater contrast in brightness under the assumption that solar illumination is the primary driver of gas and dust emission, particularly considering that the nightside outgassing was estimated to contribute only around 2-10% of the total production (Bieler et al. 2015). 

The key observation from this work is that the primary direction of the dust emission  almost never lies precisely in the subsolar direction, and the minimum is not strictly antipodal as would be expected in a purely illumination-driven model for a spherical nucleus. 

This misalignment suggests that the comet's rotation, orientation, and shape, as well as possibly thermal inertia effects, significantly influence the spatial distribution of dust emission. By quantifying the angular offset between the peak brightness and the subsolar direction over time, we aim to better understand the role of topographically shaded regions, thermal lag, and non-uniform subsurface volatile distribution in shaping the dust coma.

The underlying cause of such asymmetries is likely linked to 67P’s seasonal illumination cycle, driven by its extreme axial tilt (52°). This results in a variation of the sub-solar latitude between +52° and -52° over its 6.45-year orbit. The obliquity causes prolonged northern summer near aphelion, with the southern hemisphere (e.g., Anhur region) in darkness. Around perihelion (1.24 AU), the subsolar point shifts rapidly southward, causing intense but brief southern summer activity. These cycles together with the non-spherical shape create strong asymmetries in outgassing, which in turn shape the spatial dust distribution.

Figure 1 shows how the peak of the intensity distribution shifts relatively to the subsolar direction (0°) during the comet's inbound journey. Positive values indicate that peak lies towards the afternoon, while negative values correspond to shifts towards the morning. In Figure 2 we see the comparison of two images: one (25.07.2015) with a calibrated morning peak associated with the neck region, other (27.06.2015) with multiple peaks.

In future work, these results could be compared with existing models or used to guide targeted simulations of localized activity. While further analysis is needed to fully interpret the observed offsets, our method offers a reproducible way to characterize asymmetries in the dust coma and supports a deeper understanding of cometary dust dynamics.

Figure 1: Angular difference between sub-solar direction and direction of the maximum intensity changing with heliocentric distance.

Figure 2: Circular histograms of azimuthal brightness extracted at 12 km impact parameter from 2 OSIRIS/WAC images (27.06.2015 on the left, 25.07.2015 on the right). Green bars indicate normalized brightness as a function of angle. The yellow arrow marks the solar direction. The brightness maximum is offset from the subsolar point, demonstrating non-radial symmetry in coma structure.