Sub-mm/mm optical properties of real protoplanetary matter derived from Rosetta/MIRO observations of comet 67P

Abstract

Optical properties are required for the correct understanding and modelling of protoplanetary and debris discs. By assuming that comets are the most pristine bodies in the solar system, our goal is to derive optical constants of real protoplanetary material. We determine the complex index of refraction of the near-surface material of comet 67P/Churyumov-Gerasimenko by fitting the sub-millimetre/millimetre observations of the thermal emission of the comet’s sub-surface made by the Microwave Instrument for the Rosetta Orbiter (MIRO) with synthetic temperatures derived from a thermophysical model and radiative-transfer models. According to the two major formation scenarios of comets, we model the sub-surface layers to consist of pebbles as well as of homogeneously packed dust grains. In the case of a homogeneous dusty surface material, we find a solution for the length-absorption coefficient of α ≈ 0.22 cm−1 for a wavelength of 1.594 mm and α ≥ 3.84 cm−1 for a wavelength of 0.533 mm and a constant thermal conductivity of 0.006 Wm−1K−1. For the pebble scenario, we find for the pebbles and a wavelength of 1.594 mm a complex refractive index of n = (1.074 − 1.256) + i (2.580 − 7.431) · 10−3 for pebble radii between 1 mm and 6 mm. Taking into account other constraints, our results point towards a pebble makeup of the cometary sub-surface with pebble radii between 3 mm and 6 mm. The derived real part of the refractive index is used to constrain the composition of the pebbles and their volume filling factor. The optical and physical properties are discussed in the context of protoplanetary and debris disc observations.

A Microphysical Thermal Model for the Lunar Regolith: Investigating the Latitudinal Dependence of Regolith Properties

“…The data could also uniquely constrain the parameter set investigated in this study and solve the degeneracy problem. However, for the interpretation of the microwave data, a radiative transfer model is needed and therefore the optical properties of the regolith must be known (Bürger, Glißmann, et al., 2023; Feng et al., 2020). The interpretation of the Diviner measurements is based on the assumption that only the uppermost regolith particle emits the measured radiation.…”

Porous Dust Particles in Protoplanetary Disks: Application to the HL Tau Disk

“…In addition, porous particles are detected (Bentley et al 2016;Mannel et al 2016Mannel et al , 2019 and account for more volume fractions than the compact particles among the collected samples (Merouane et al 2016). While it is not clear whether the microporosity solely contributes to the total porosity, voids should be no larger than ∼9 m from radar measurements (Ciarletti et al 2017), and models (Blum et al 2017;Bürger et al 2023) with a combination of particle porosity (microporosity; Weidling et al 2009) and random packing of the particles (Skorov & Blum 2012;Fulle & Blum 2017) can closely explain the observations from the Rosetta mission. 8 Theory and experiments support the existence of porous particles in protoplanetary disks as micron-sized dust particles coagulate to form fluffy agglomerates (Ossenkopf 1993;Weidenschilling & Cuzzi 1993;Wurm & Blum 1998;Kempf et al 1999;Krause & Blum 2004;Okuzumi et al 2012;Kataoka et al 2013;Krijt et al 2015;Estrada et al 2016.…”

“…review by Güttler et al 2019). Measurements on the bulk density (Jorda et al 2016;Pätzold et al 2016) and the particle's permittivity (Kofman et al 2015;Herique et al 2016;Bürger et al 2023) indicate the nucleus has porosity from 70%-85%. This is consistent with our constraint on the porosity in HL Tau from millimeter dust self-scattering polarization observations.…”

“…However, macro-porosity (space between particles, pebbles, or boulders) might also contribute to the bulk porosity, even though the void should be no larger than 9 m (Ciarletti et al 2017). Blum et al (2017) and Bürger et al (2023) explained the bulk porosity as the combination of 60% microporosity as the result of compaction at the bouncing barrier (Weidling et al 2009;Zsom et al 2010) and 80% macro-porosity due to the random packing of these dust particles (Skorov & Blum 2012;Fulle & Blum 2017). If the porosity ∼60% due to compaction is applicable to the whole disk, the porosity of 70%-97% in HL Tau might indicate the dust is still growing and has not finished the compaction.…”

“…In other words, the approximation is better for smaller f. It is also worth noting that for radio wavelengths, n-1 ?k. 23 A recent analysis on Rosetta/MIRO data constrains the refractive index of millimeter-to centimeter-sized pebbles from the subsurface of comet 67P to be n = 1.074-1.256 and k = (2.580-7.431) ×10 −3 (Bürger et al 2023) at 1.594 mm. This is similar to the DSHARP refractive index at 1 mm with f ∼ 0.1.…”