CONSERT suggests a change in local properties of 67P/Churyumov-Gerasimenko’s nucleus at depth

Abstract

Context. After the successful landing of Philae on the nucleus of 67P/Churyumov-Gerasimenko, the Rosetta mission provided the first opportunity of performing measurements with the CONSERT tomographic radar in November 2014. CONSERT data were acquired during this first science sequence. They unambiguously showed that propagation through the smaller lobe of the nucleus was achieved. Aims. While the ultimate objective of the CONSERT radar is to perform the tomography of the nucleus, this paper focuses on the local characterization of the shallow subsurface in the area of Philae's final landing site, specifically determining the possible presence of a permittivity gradient below the nucleus surface. Methods. A number of electromagnetic simulations were made with a ray-tracing code to parametrically study how the gradient of the dielectric constant in the near-subsurface affects the ability of CONSERT to receive signals. Results. At the 90 MHz frequency of CONSERT, the dielectric constant is a function of porosity, composition, and temperature. The dielectric constant values considered for the study are based on observations made by the other instruments of the Rosetta mission, which indicate a possible near-surface gradient in physical properties and on laboratory measurements made on analog samples. Conclusions. The obtained simulated data clearly show that if the dielectric constant were increasing with depth, it would have prevented the reception of signal at the CONSERT location during the first science sequence. We conclude from our simulations that the dielectric constant most probably decreases with depth.

The primordial nucleus of comet 67P/Churyumov-Gerasimenko

“…Yet, the detection of 2-3 well-defined propagation paths show the potential presence of large ( 10 m) structures within the small lobe (Kofman et al 2015). Further analysis of the same data by Ciarletti et al (2015) shows that the dielectric constant decreases by ∼40% with depth within ∼150 m of the surface. This is interpreted by Ciarletti et al (2015) as a ∼15% increase in porosity over that depth, or that the dust-to-ice mass ratio drops from ∼4 to ∼ 0.1.…”

“…Further analysis of the same data by Ciarletti et al (2015) shows that the dielectric constant decreases by ∼40% with depth within ∼150 m of the surface. This is interpreted by Ciarletti et al (2015) as a ∼15% increase in porosity over that depth, or that the dust-to-ice mass ratio drops from ∼4 to ∼ 0.1. In either case, the density near the surface is higher than ∼150 m below.…”

“…However, Pätzold et al (2016) do not discuss the implications of their C 11 , S 11 , C 21 , and S 21 values and error bars not overlapping those of the Preusker et al (2015) and Jorda et al (2016) shape models with constant density, for example, whether these discrepancies are consistent with the heterogeneity discussed by Ciarletti et al (2015). Also, they do not quantify the size of voids they can exclude or the upper limit on global density variations.…”

Permittivity measurements of porous matter in support of investigations of the surface and interior of 67P/Churyumov-Gerasimenko

“…Moreover, permittivity estimates of the interior are required to produce inversion models dedicated to the interpretation of the data (Herique et al 1999). After the Philae landing, CONSERT data have been obtained through the first hundreds of meter of the subsurface and through the small lobe of the nucleus (Ciarletti et al 2015;Kofman et al 2015). Variations in are expected to arise from (unknown) variations in density or porosity, in composition, and in temperature.…”

“…The composition and the porosity of the nucleus are likely to be refined by the first determination of the bulk permittivity of the small lobe of the nucleus by CONSERT (Kofman et al 2015). Moreover, analysis of CONSERT data at grazing angles by Ciarletti et al (2015) suggests a possible near-surface decrease of the real part of the permittivity below the area of the Philae's final landing site, which could be interpreted as an increase of the porosity with depth or by a drop of the dust-to-ice volumetric ratio. The data presented in Tables 4 and 5 are directly relevant to these analyses, which are based essentially on the behavior of the real part of the permittivity along the electrical path of the radar wave.…”