The scattering phase function of comet 67P/Churyumov–Gerasimenko coma as seen from the Rosetta/OSIRIS instrument

Abstract

The study of dust, the most abundant material in cometary nuclei, is pivotal in understanding the original materials forming the Solar system. Measuring the coma phase function provides a tool to investigate the nature of cometary dust. Rosetta/OSIRIS sampled the coma phase function of comet 67P/Churyumov-Gerasimenko, covering a large phase angle range in a small amount of time. Twelve series were acquired in the period from 2015 March to 2016 February for this scientific purpose. These data allowed, after stray light removal, measuring the phase function shape, its reddening, and phase reddening while varying heliocentric and nucleocentric distances. Despite small dissimilarities within different series, we found a constant overall shape. The reflectance has a u-shape with minimum at intermediate phase angles, reaching similar values at the smallest and largest phase angle sampled. The comparison with cometary phase functions in literature indicates OSIRIS curves being consistent with the ones found in many other single comets. The dust has a negligible phase reddening at α < 90 • , indicating a coma dominated by single scattering. We measured a reddening of [11-14] %/100 nm between 376 and 744 nm. No trend with heliocentric or nucleocentric distance was found, indicating the coma doesn't change its spectrum with time. These results are consistent with single coma grains and close-nucleus coma photometric results. Comparison with nucleus photometry indicates a different backscattering phase function shape and similar reddening values only at α < 30 • . At larger phase angles, the nucleus becomes significantly redder than the coma.

The Dust-to-Gas Ratio, Size Distribution, and Dust Fall-Back Fraction of Comet 67P/Churyumov-Gerasimenko: Inferences From Linking the Optical and Dynamical Properties of the Inner Comae

“…This should indeed be the case because larger particles should behave more and more like "small comets" themselves and thus be representative of the nucleus scattering properties. For small particles, the best agreement of a single dust size with the coma phases function [38] is between 10 and 100 µm. The numerical method of Markkanen et al [34] is not applicable to particles smaller than 1µm.…”

Linking surface morphology, composition, and activity on the nucleus of 67P/Churyumov-Gerasimenko

“…where w λ is the single-scattering albedo from Fornasier et al (2015), estimated from Hapke modeling (Hapke, 2012) of the surface scattering curve, p (g) is the particle phase function from Bertini et al (2017), and p(0) is the extrapolation of this phase function for phase g = 0. Assuming a differential grain size distribution n (r) , described by a power law ∝ r h+1 , and assuming spherical grains, the expression for the integrated dust mass is M = 2ρ · f · πR 2 n (r) r 2 dr n (r) rdr , Table 2); squares show the double type-C outbursts on 30 August 2015 at 12h21 (jets 17 and 18 in Table 2), and the large square corresponds to the brighter of the two jets (jet 18); and the diamond indicates a transient event on 30 August 2015 at 8h09 (jet 13 in Table 2) where R is the aperture radius in meters and r is the grain radius.…”

Linking studies of tiny meteoroids, zodiacal dust, cometary dust and circumstellar disks

“…When dust scattering is accounted for, it is modeled either by an isotropic phase function or by an anisotropic phase function that only includes forward scattering. Neither phase function is actually consistent with that of the dust coma of comet 67P/C-G, which features both moderate backward and forward scattering at optical and near-infrared wavelengths (Bertini et al 2017;Bockelée-Morvan et al 2019). Adopting this observed phase function to the MWC 758 disk model of Baruteau et al (2019) leads to a lower level of flux and an enhancement of the disk emission between the two asymmetric bright rings (Fig.…”

“…Left. Phase curves of two debris disks around HD35841 (measured in the H-band with the Gemini Planet Imager,Esposito et al 2018) and HD191089 (measured in the optical with HST/ STIS and in the J band with HST/NICMOS,Ren et al 2019), presenting quite fair an agreement with those of 67P/C-G extracted from OSIRIS on 28 August 2015, soon after perihelion passage(Bertini et al 2017). Right.…”

“…Finally, in the inner coma, the dust-brightness phase curves reveal a flattened u-shape that may rule out a sharp surge in the forward-scattering region (Bertini et al 2017;Fulle et al 2018). Numerical as well as experimental simulations suggest that such trends also imply a significant amount of organic compounds and of fluffy aggregates (Moreno et al 2018;Markkanen et al 2018;.…”