Electrical properties and porosity of the first meter of the nucleus of 67P/Churyumov-Gerasimenko

Abstract

Context. Comets are primitive objects, remnants of the volatile-rich planetesimals from which the solar system condensed. Knowing their structure and composition is thus crucial for the understanding of our origins. After the successful landing of Philae on the nucleus of 67P/Churyumov-Gerasimenko in November 2014, for the first time, the Rosetta mission provided the opportunity to measure the low frequency electrical properties of a cometary mantle with the permittivity probe SESAME-PP (Surface Electric Sounding and Acoustic Monitoring Experiment−Permittivity Probe). Aims. In this paper, we conduct an in-depth analysis of the data from active measurements collected by SESAME-PP at Abydos, which is the final landing site of Philae, to constrain the porosity and, to a lesser extent, the composition of the surface material down to a depth of about 1 m. Methods. SESAME-PP observations on the surface are then analyzed by comparison with data acquired during the descent toward the nucleus and with numerical simulations that explore different possible attitudes and environments of Philae at Abydos using a method called the Capacity-Influence Matrix Method. Results. Reasonably assuming that the two receiving electrode channels have not drifted with respect to each other during the ten-year journey of the Rosetta probe to the comet, we constrain the dielectric constant of the first meter below the surface at Abydos to be >2.45 ± 0.20, which is consistent with a porosity <50% if the dust phase is analogous to carbonaceous chondrites and <75% in the case of less primitive ordinary chondrites. This indicates that the near surface of the nucleus of 67P/Churyumov-Gerasimenko is more compacted than its interior and suggests that it could consist of a sintered dust-ice layer.

The Philae lander mission and science overview

“…The relative electric permittivity of the first metre into the ground at Abydos was found to be above 2.45 ± 0.2 as measured by the PP permittivity probe of the SESAME experiment [51]. This is much higher than the average value for the cometary interior found by the CONSERT radio sounding experiment (1.27 [21]).…”

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

“…Taking into account the temperature dependence, the values predicted with our model for temperatures below 200 K validate the range of considered for the CONSERT simulations (from 1.025 to 1.45) and encompass the value of derived by Kofman et al [], which is equal to 1.27 ± 0.05. 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 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.…”

Dust-to-Gas and Refractory-to-Ice Mass Ratios of Comet 67P/Churyumov-Gerasimenko from Rosetta Observations

“…Rosetta instruments that inform the parameters needed to derive the dust-to-gas and refractory-to-ice mass ratios of 67P 2015). At Abydos, Philae's final resting place on 67P, the SESAME package (Seidensticker et al 2007) made observations compatible with a meter-scale layer of compact materials in the subsurface via SESAME-PP (Lethuillier et al 2016) and SESAME-CASSE (Knapmeyer et al 2018). The locally high thermal inertia of 85 ± 35 J m −2 K −1 s −0.5 ("high" by cometary standards only) measured around Abydos by the MUPUS-TM sensors (Spohn et al 2015) suggests that this layer could be formed by sublimation and recondensation of water ice at depth within the pores, similarly to observations made in the KOmet-SImulationen (KOSI) laboratory experiments (Kochan et al 1989;Grün et al 1991).…”