OSIRIS coma dust phase function I. Bertini, F. La Forgia, C. Tubiana, C. Guttler, S. Fornasier, V. Da Deppo, JB, Vincent…et al. (very provisional author list..more people are getting interested and involved…)

Scientific Aim Derive information on the intimate nature (shape, composition, size, and size distribution) of the dust grains in the coma solving the inverse scattering problem (model-dependent solution). Main assumption: we observe dust with similar properties at the different pointing stations. Relationship and comparison with in-situ instruments results (GIADA, COSIMA) and with Monte-Carlo simulations output  obtaining a more detailed view of the dust environment.

The Observational Geometry Phase Angle Sun-Comet-S/C always close to 90°. 3 WAC Series taken so far: MTP011: 9 Jan 2015 @ 28 km MTP012: 29 Jan 2015 @ 28 km MTP014P: 29 Mar 2015 @ 56-74 km Sun S/C Pointing Stations

Pointing Stations S/C Sun

Sun S/C Pointing Stations

The Measurements in Reflectance: MTP011 Results obtained measuring the median calibrated intensity in a 60X60 px square at the center of the images.

The Measurements in Reflectance: MTP012 Results obtained measuring the median calibrated intensity in a 60X60 px square at the center of the images.

The Measurements in Reflectance: MTP014 Results obtained measuring the median calibrated intensity in a 60X60 px square at the center of the images.

Considering the Entire Frames Once we move the photometric aperture through the frame ( covering a large sampling in phase angles) we detect intensity gradients within the frames. Intensity gradients at large phase angles ( small solar elongations) may be due to the presence of Sun straylight. Straylight+Coma

Straylight from the Sun etc. Large phase angle phase function data Dust on baffle? Not comet dust… 2004 Straylight Analysis by Sonia Grains trails Solar Straylight Ongoing Action: Straylight removal from phase function data (coma+straylight) with old cruise data (straylight only) obtained in similar observational geometries.

Optically Thicker Regions Sampled? I.e. Jets and Gas contribution removal FoV FoV FoV Ongoing Action: Application of the JB’s 3D jets model to remove the possible contribution of thicker regions.

The purpose of the aforementioned actions is providing measurements where only the background coma behavior is depicted, removing straylight and thick jets contributions (extending this way also the phase angle coverage by using entire frames). Another point to discuss is the gas contribution at different phase angles. Action on ‘gas removal’ is TBD.

Once the ‘clean’ background coma phase function is derived, the following step is comparing the measurements with a data cube of sinthetic phase functions and, solving the inverse scattering problem, derive info on the dust intimate nature. Light scattering codes for theoretical results: Mie. Pro: large particles up to geometric optic limit. Con: only spherical particles. DDSCAT. Pro: irregular particles (including Marco/Stavro ellipsoids), fractals complex structures. Con: high precision only for size parameters < 25 (i.e. with radius lower that 4 micron at the OSIRIS larger wavelengths) and refractive index not large compared to unity. T-Matrix. Pro: irregular particles with size parameter up to 100. Con: Less shapes available wrt DDSCAT.

Future Measurements Improvement (MTP017) Next Phase Function sequence in MTP017 - cancelled. Better phase angle coverage if possible, otherwise cluster few observations around low, intermediate, and large phase angles to sample the significative points of the phase curve. Using also NAC images to improve the dust/gas S/N ratio. Contemporaneous IR VIRTIS data  Phase function = F_Visible/F_IR to remove the influence of column density (amount of dust along the FoV)

Interaction with VIRTIS - I Contemporaneous IR VIRTIS data  since dust particles are reacting in different ways to different incident wavelengths according to their size, we can use VIS and IR data to constrain particles size. Ex: r = Scattering from 1 µm silicate spherical particle Ex: r = Scattering from 10 µm silicate spherical particle

Interaction with VIRTIS - II Ex: r = Scattering from 1 µm silicate spherical particle Contemporaneous IR VIRTIS data  Phase function = F_Visible/F_IR to remove the influence of column density (amount of dust along the FoV): F ̴ N*Phase_function(particle’s properties-depending on alpha) In the IR we should have lower dependance on alpha  VIS/IR we get rid of N factor but we maintain almost same dependance on alpha as in the VIS. No IR data  we have to assume we sampled same N at different alpha, or we have to introduce corrections using complex DSMC calculations to model the 3D dust field

Ongoing Action after OSIRIS/VIRTIS meeting: check VR-H infrared ride along possibility with SR phase function.

First Comparison with Theoretical Models

First Comparison with Theoretical Models

Bigger Picture COSIMA: Large grains (300-500 micron) as fluffy collection of smaller entities, no ice inside Na-rich as IDPs Coming from nucleus dust layer More than 12,000 grains collected --> size distribution GIADA: (i) compact particles (ranging in size from 0.03 to 1 mm), witnessing the presence of materials that underwent processing within the solar nebula (ii) fluffy aggregates (ranging in size from 0.2 to 2.5 mm) of sub-micron grains that may be a record of a primitive component, probably linked to interstellar dust. The dynamics of the fluffy aggregates constrain their equivalent bulk density to < 1 kg m−3 The density of such optically thick aggregates is consistent with the low bulk density of the nucleus. The mass contribution of the fluffy aggregates to the refractory component of the nucleus is negligible and their coma brightness contribution is less than 15%.