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Context. Comets are primitive objects, remnants of the volatile-richplanetesimals from which the solar system condensed. Knowing their structureand composition is thus crucial for the understanding of our origins. After thesuccessful landing of Philae on the nucleus of 67P/Churyumov-Gerasimenko inNovember 2014, for the first time, the Rosetta mission provided the opportunityto measure the low frequency electrical properties of a cometary mantle withthe permittivity probe SESAME-PP (Surface Electric Sounding and AcousticMonitoring Experiment - Permittivity Probe).  Aims. In this paper, we conduct an in-depth analysis of the data from activemeasurements collected by SESAME-PP at Abydos, which is the final landing siteof Philae, to constrain the porosity and, to a lesser extent, the compositionof the surface material down to a depth of about 1 meter.  Methods. SESAME-PP observations on the surface are then analyzed bycomparison with data acquired during the descent toward the nucleus and withnumerical simulations that explore different possible attitudes andenvironments of Philae at Abydos using a method called the Capacity-InfluenceMatrix Method.  Results. Reasonably assuming that the two receiving electrode channels havenot drifted with respect to each other during the ten-year journey of theRosetta probe to the comet, we constrain the dielectric constant of the firstmeter below the surface at Abydos to be > 2.45 p/m 0.20, which is consistentwith a porosity < 50% if the dust phase is analogous to carbonaceous chondritesand < 75% in the case of less primitive ordinary chondrites. This indicatesthat the near surface of the nucleus of 67P/Churyumov-Gerasimenko nucleus ismore compacted than its interior and suggests that it could consist of asintered dust-ice layer.

astronomy and astrophysics

Electrical properties and porosity of the first meter of the nucleus of  67P/Churyumov-Gerasimenko. As constrained by the Permittivity Probe  SESAME-PP/Philae/Rosetta