Using polarimetry to evaluate carbon depletion found by the Stardust mission in Comet 81P/Wild 2

When observed near opposition, the regions of the coma near a comet's nucleus reveal a strong negative polarization with amplitudes approaching 10%. Other regions of the coma display significantly lower negative polarizations or are positively polarized. Cometary jets, for instance, have revealed only positive polarization through all phase angles. We model single scattering from agglomerate particles of different size, structure, and refractive index. In all cases, the presence of absorbing materials significantly reduce the negative polarization. The circumnucleus halo, as interpreted by polarimetry would represent a carbon-depleted region of comets. One of the surprises of Stardust was the lack of carbonaceous particles captured, since high-carbon content has been a characterizing feature in all cometary dust samplings prior to the Stardust mission. Our polarimetry modeling suggests that the anomalous results are due to Stardust sampling in this carbon-depleted region. Sampling in this anomalous region may also explain why the crystalline structure of silicates found is different from the chondritic porous interplanetary dust that was expected to be present. While the knowledge of the origin and evolution of comets developed before the Stardust mission may not require an immediate revision, the physical mechanisms that produce a circumnucleus halo are currently not well understood. The small size of the halo suggests that it consists of particles that were not accelerated by an expanded gas, like the cometary jets. Its shape may indicate that its dust originated from a relatively large area of the nucleus or that it persists for long periods. Therefore, one can hypothesize that the circumnucleus halo may be caused by near-surface processes directly stimulated by sunlight or through long-term settling of particles within the coma. The preferential selection of non-absorbing particles and the abundance of material resembling chondritic meteorites may provide clues to the halo's origin.