A porosity gradient in 67P/C-G nucleus suggested from CONSERT and SESAME-PP results: an interpretation based on new laboratory permittivity measurements of porous icy analogues

Brouet, Yann; Levasseur-Regourd, A. C.; Sabouroux, Pierre; Neves, Luísa A.; Encrenaz, P.; Poch, Olivier; Pommerol, Antoine; Thomas, N.; Kofman, W.; Gall, Alice Le; Ciarletti, Valérie; Hérique, Alain; Lethuillier, Anthony; Carrasco, Nathalie; Szopa, Cyril

doi:10.1093/mnras/stw2151

Cited by 33 publications

(25 citation statements)

References 62 publications

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“…Marschall et al 2020a). This could suggests that the dust layer has a high porosity, possibly similar to the macroscopic porosity of the nucleus Kofman et al 2015;Brouet et al 2016). The low thermal inertia of the surface layer is also consistent with this conclusion.…”

Section: The Dust and Ice Boundary Layersupporting

confidence: 70%

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Cometary Comae-Surface Links

et al. 2020

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A comet is a highly dynamic object, undergoing a permanent state of change. These changes have to be carefully classified and considered according to their intrinsic temporal and spatial scales. The Rosetta mission has, through its contiguous in-situ and remote sensing coverage of comet 67P/Churyumov-Gerasimenko (hereafter 67P) over the time span of August 2014 to September 2016, monitored the emergence, culmination, and winding down of the gas and dust comae. This provided an unprecedented data set and has spurred a large effort to connect in-situ and remote sensing measurements to the surface. In this review, we address our current understanding of cometary activity and the challenges involved when linking comae data to the surface. We give the current state of research by describing what we know about the physical processes involved from the surface to a few tens of kilometres above it with respect to the gas and dust emission from cometary nuclei. Further, we describe how complex multidimensional cometary gas and dust models have developed from the Halley encounter of 1986 to today. This includes the study of inhomogeneous outgassing and determination of the gas and dust production rates. Additionally, the different approaches used and results obtained to link coma data to the surface will be discussed. We discuss forward and inversion models and we describe the limitations of the respective approaches. The current literature suggests that there does not seem to be a single uniform process behind cometary activity. Rather, activity seems to be the consequence of a variety of erosion processes, including the sublimation of both water ice and more volatile material, but possibly also more exotic processes such as fracture and cliff erosion under thermal and mechanical stress, sub-surface heat storage, and a complex interplay of these processes. Seasons and the nucleus shape are key factors for the distribution and temporal evolution of activity and imply that the heliocentric evolution of activity can be highly individual for every comet, and generalisations can be misleading.

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Section: The Dust and Ice Boundary Layersupporting

confidence: 70%

“… 2015 ; Brouet et al. 2016 ). The low thermal inertia of the surface layer is also consistent with this conclusion.…”

Section: The Physics Of Developing Cometary Comaementioning

confidence: 99%

Cometary Comae-Surface Links

et al. 2020

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“…Furthermore, from data collected by the permittivity probe (hereafter PP) of the SESAME package (Surface Electric Sounding and Acoustic Monitoring Experiment) [ Seidensticker et al , ] during the FSS, Lethuillier et al [] have determined a lower limit of

ε^{'}

equal to 2.45 for the near surface of the small lobe, at the final landing site of Philae, in the 400–800 Hz range of frequency. Based on the results presented in the current paper and the results obtained by the CONSERT and SESAME‐PP instruments, and taking into account the temperature conditions during the FSS, Brouet et al [] show that the porosity increases with depth in the small lobe of the nucleus.…”

Section: Discussionmentioning

confidence: 78%

Characterization of the permittivity of controlled porous water ice-dust mixtures to support the radar exploration of icy bodies

Brouet

Neves

Sabouroux

et al. 2016

J. Geophys. Res. Planets

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The internal properties of porous and icy bodies in the solar system can be investigated by ground‐penetrating radars (GPRs), like the COmet Nucleus Sounding Experiment by Radiowave Transmission instrument on board the Rosetta spacecraft which has sounded the interior of the nucleus of comet 67P/Churyumov‐Gerasimenko. Accurate constraints on the permittivity of icy media are needed for the interpretation of the data. We report novel permittivity measurements performed on water ice samples and icy mixtures with porosities in the 31–91% range. The measurements have been performed between 50 MHz and 2 GHz with a coaxial cell on a total of 38 samples with a good reproducibility. We used controlled procedures to produce fine‐grained and coarse‐grained ice samples with a mean diameter of 4.5 μm and 67 μm, respectively, and to prepare icy mixtures. The JSC‐1A lunar regolith simulant was used as the dust component in the mixtures. The results are focused on the real‐part ε′ of the permittivity, which constrains the phase velocity of the radio waves in low‐loss media. The values of ε′ show a nondispersive behavior and are within the range of 1.1 to 2.7. They decrease with the increasing porosity Φ according to E(1 − Φ), with E equal to about 3.13 for pure water ice, and in the 3.8–7.5 range for ice‐dust mixtures with a dust‐to‐ice volumetric ratio in the 0.1–2.8 range, respectively. These measurements are also relevant for radiometers operating in the millimeter‐submillimeter domains, as suggested by the nondispersive behavior of the mixtures and of the pure components.

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“…While the internal structure of the comet beneath the visible surface remains largely unknown (see below), some measurements have increased our knowledge. Brouet et al (2016) showed that CONSERT and SESAME-PP data indicate that the porosity increases with the increasing depth in the small lobe of the nucleus (see also Ciarletti et al 2017).…”

Section: Some Knowledge Of Depth Structurementioning

confidence: 95%

“…Consequently, only part of the nucleus's small lobe has been sounded and characterised. On the detected signals, only the propagation delays and the shape of the received pulses have been analysed and provided an average value for the real part of the permittivity and some hints about the spatial variations (Brouet et al 2016; and the limited heterogeneity inside the sounded area (Ciarletti et al 2017). The interpretation of the received amplitudes would have brought further constraints on the nucleus composition that have already been obtained ) but would require a better knowledge of the Philae's antenna radiation pattern in the actual local environment.…”

Section: Deep Interiormentioning

confidence: 99%

Towards New Comet Missions

et al. 2019

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The Rosetta observations have greatly advanced our knowledge of the cometary nucleus and its immediate environment. However, constraints on the mission (both planned and unplanned), the only partially successful Philae lander, and other instrumental issues have inevitably resulted in open questions. Surprising results from the many successful Rosetta observations have also opened new questions, unimagined when Rosetta was first planned. We discuss these and introduce several mission concepts that might address these issues. It is apparent that a sample return mission as originally conceived in the 1980s during the genesis of Rosetta would provide many answers but it is arguable whether it is technically feasible even with today's technology and knowledge. Less ambitious mission concepts are described to address the suggested main outstanding scientific goals.

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A porosity gradient in 67P/C-G nucleus suggested from CONSERT and SESAME-PP results: an interpretation based on new laboratory permittivity measurements of porous icy analogues

Cited by 33 publications

References 62 publications

Cometary Comae-Surface Links

Cometary Comae-Surface Links

Characterization of the permittivity of controlled porous water ice-dust mixtures to support the radar exploration of icy bodies

Towards New Comet Missions

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