Polarimetric observations of comet 67P/Churyumov-Gerasimenko during its 2008–2009 apparition

Abstract

Context. Remote observations of the light scattered by comet 67P/Churyumov-Gerasimenko dust coma are of major importance for determining the physical properties of the particles and prepare the rendezvous with the ESA/Rosetta spacecraft in 2014. Aims. Light scattering and especially linear polarization observations allow comparison between different coma regions and different comets, including comets that have been studied by space probes. Our aim is to retrieve physical properties of the dust particles and to characterize their evolution around perihelion passage. Results. The decrease in intensity as a function of the distance to nucleus in log-log scale is on average close to −1, although important variations with values down to −1.5 are noticed, in agreement with previous observations in 1982−83 and 1995−96. The intensity along the tailward direction decreases with a slope between −1.2 two months before perihelion (2009 February 28) to −1.0 two months after perihelion, and the decrease is more pronounced in the sunward direction. Before perihelion, aperture polarization values are comparable to polarization values measured on other comets at similar phase angles. The sharp decrease in intensity and the feature in the tailward direction, without any difference in polarization in the coma before perihelion, could suggest the presence of large dark particles. The post-perihelion increase in intensity and in polarization suggests that an outburst has occurred. The freshly ejected dust polarizes more the scattered light and is more sensitive to the solar radiation pressure, suggesting small micron-or submicron-sized grains. Conclusions. Polarization and intensity variations in the coma of 67P/Churyumov-Gerasimenko are reminiscent of those noticed for some comets such as comet 81P/Wild 2 and comet 9P/Tempel 1. The presence of rather large particles can thus be suggested before and just after perihelion and the ejection of post-perihelion smaller grains, eventually in fluffy aggregates. A strong seasonal effect related to the obliquity of the comet suggests that the different grains originate in different hemispheres of the nucleus.

Morphology and dynamics of the jets of comet 67P/Churyumov-Gerasimenko: Early-phase development

“…5 are within the range of values determined for the coma (∼1.35-1.92) within ρ < 100 000 km in February and March, 2009 near perihelion (Lara et al 2011). Hadamcik et al (2010 found that the inner coma of 67P for ρ between 2000 and 8500 km in December 2008 at a heliocentric distance of 1.35 AU had a brightness slope of 1.5, which might be explained in terms of the presence of large and fluffy particles.…”

“…Hadamcik et al (2010 found that the inner coma of 67P for ρ between 2000 and 8500 km in December 2008 at a heliocentric distance of 1.35 AU had a brightness slope of 1.5, which might be explained in terms of the presence of large and fluffy particles. The similarity to the brightness slopes discussed here might be deceptive because both ground-based results (Lara et al 2011;Hadamcik et al 2010) are for a dust coma much larger than the near-nucleus region and under outgassing conditions very different from the distant coma activity at about 3.5 AU from the Sun (Snodgrass et al 2013). The comparative study of the coma activities of 67P observed by Rosetta at different heliocentric distances (from 3.5 AU to 1.2 AU) in the months to come will probably be the next step for the global investigations of the jet morphology and dynamics.…”

Imaging polarimetry of comet 73P/Schwassmann–Wachmann 3 main fragments during its 2006 apparition

“…This high polarization in the inner part can be interpreted by large dark particles, which move slowly. Some comets with an important seasonal effect, as comet 67P/Churyumov-Gerasimenko, may behave like low-P max comets before perihelion and present more jets (with smaller grains) at perihelion or after it (Hadamcik et al, 2010). The presence of large particles (100 m or more) before 2015 perihelion has been confirmed by the first results from COSIMA and GIADA experiments onboard Rosetta spacecraft (e.g.…”

Pre-perihelion activity of comet 67P/Churyumov-Gerasimenko

“…When active, the morphology and composition of the coma was constrained by Vincent et al (2013) and Schleicher (2006), respectively; the dust trail was studied by Ishiguro (2008), Kelley et al (2008), Based on observations obtained at the ESO/VLT in the framework of programme 281.C-5004. ; and the polarisation properties of the comet dust were studied by Hadamcik et al (2010). The overall dust environment of the comet coma and tail was studied by Fulle et al (2004), Moreno et al (2004), andFulle et al (2010).…”