In-situ Properties of Comet 67P/Churyumov-Gerasimenko measured with SESAME

Abstract

The SESAME experiment (Surface Electric Sounding and Acoustic Monitoring Experiment) is part of the lander Philae of the ESA Rosetta mission that landed on the nucleus of comet 67P/Churyumov-Gerasimenko on 12 November 2014. SESAME is a suite of the three instruments CASSE, DIM and PP that use different measurement techniques in order to determine physical properties of the surface and the activity of 67P. SESAME was operated successfully during the descent until the end of the First Science Sequence (FSS). The Comet Acoustic Surface Sounding Experiment (CASSE) can record and generate acoustic vibrations with two kinds of sensors in the range from 50 Hz to 10 kHz. It was operated during the descent and measured completely Philae's first touchdown at Agilkia at 15:34:04 UTC with three triaxial accelerometers mounted in Philae's feet. These data, which are influenced by the properties of Philae's landing gear and by the complex surface layer at Agilkia with a regolith ranging from dust to boulders, describe the sequence of Philae's surface contacts. At the final landing site Abydos, CASSE recorded many signals generated by the hammering of the Philae experiment MUPUS-Pen, in order to determine sound velocities of the cometary surface layer and related elastic properties. Finally, CASSE conducted sounding measurements from foot to foot. The Dust Impact Monitor (DIM) used three piezoelectric sensor plates orthogonally mounted on a cube to detect impacts of sub-millimeter and millimeter-sized ice and dust particles that are emitted from the cometary surface. DIM measurements during the 7 h descent to the cometary surface were hampered by the fact that none of the sensor plates pointed to the cometary surface. Nevertheless, one millimeter-sized particle was detected at an altitude of 2.4 km. Comparison with laboratory data showed that the particle properties are compatible with a porous grain having a bulk density of approximately 250 kg m-3. At Abydos, DIM recorded no particle impacts, which may be due either to low cometary activity at this place or shading by obstacles close to Philae or both. The SESAME Permittivity Probe (PP) is a combined quadrupole permittivity detector and plasma wave sensor. The unplanned landing in Abydos required many modifications to the FSS operation plan. Thus, PP could study the surface layer with only a three-electrode configuration reducing the resolution significantly. Nevertheless, PP's data constrain the relative permittivity of the surface material to about 2.5 at a surface temperature of about -160 °C for the position of Philae's foot –Y, which was in permanent shadow. This result suggests that the first meters of the surface of Abydos are more compact and/or richer in dust than its interior as probed by the orbiter/lander experiment CONSERT. Additionally, PP detected weak plasma wave activities at Abydos two hours after local sunset. By combining data from different FSS operation periods, we were able to determine temperature profiles of the CASSE sensors in each Philae foot over 9.2 h of a 12.4 h comet day. With proper modeling, these data could give the surface temperature variation at Abydos during FSS.