Dust Impact Monitor (SESAME-DIM) on board Rosetta/Philae: Millimetric particle flux at comet 67P/Churyumov-Gerasimenko

Abstract

Context. The Philae lander of the Rosetta mission, aimed at the in situ investigation of comet 67P/Churyumov-Gerasimenko, was deployed to the surface of the comet nucleus on 12 November 2014 at 2.99 AU heliocentric distance. The Dust Impact Monitor (DIM) as part of the Surface Electric Sounding and Acoustic Monitoring Experiment (SESAME) on the lander employed piezoelectric detectors to detect the submillimetre-and millimetre-sized dust and ice particles emitted from the nucleus. Aims. We determine the upper limit of the ambient flux of particles in the measurement range of DIM based on the measurements performed with the instrument during Philae's descent to its nominal landing site Agilkia at distances of about 22 km, 18 km, and 5 km from the nucleus barycentre and at the final landing site Abydos. Methods. The geometric factor of the DIM sensor was calculated assuming an isotropic ambient flux of the submillimetre-and millimetre-sized particles. For the measurement intervals when no particles were detected the maximum true impact rate was calculated by assuming Poisson distribution of the impacts, and it was given as the detection limit at a 95% confidence level. The shading by the comet environment at Abydos was estimated by simulating the pattern of illumination on Philae and consequently the topography around the lander. Results. Based on measurements performed with DIM, the upper limit of the flux of particles in the measurement range of the instrument was of the order of 10 −8 −10 −7 m −2 s −1 sr −1 during descent. The upper limit of the ambient flux of the submillimetreand millimetre-sized dust and ice particles at Abydos was estimated to be 1.6 × 10 −9 m −2 s −1 sr −1 on 13 and 14 November 2014. A correction factor of roughly 1/3 for the field of view of the sensors was calculated based on an analysis of the pattern of illumination on Philae. Conclusions. Considering particle speeds below escape velocity, the upper limit for the volume density of particles in the measurement range of DIM was constrained to 10 −11 m −3 −10 −12 m −3 . Results of the calculations performed with the GIPSI tool on the expected particle fluxes during the descent of Philae were compatible with the non-detection of compact particles by the DIM instrument.

The Philae lander mission and science overview

“…Alternatively, though considered less likely, one may also interpret the regolith-depleted surface region at Abydos as being due to an enhanced clean-up of the surface layer from granular material driven by recent gas activity. In this respect, it is noteworthy to mention that signs of local activity, for instance dust grains released from the local surface, are not reported from (direction-sensitive) SESAME-DIM dust impact monitor measurements at Abydos [47,48] despite significantly longer measurement times and a very close surface distance with respect to the SESAME dust detection during the lander descent. The comparison of the surface texture of Agilkia and Abydos suggests that different degrees of physical processing have shaped the surface landscape at both sites, with the material at Abydos being more representative of the less processed nucleus surface.…”

Helios spacecraft data revisited: detection of cometary meteoroid trails by following in situ dust impacts

“…Our IMEX trail simulations show that up to four of the seven candidate cometary trail particles detected by Helios are compatible with an origin from comet 45P/H-M-P or 72P/D-F. Based on these trail identifications we attempt to constrain the dust fluxes in these trails by combining measurements and model results. The case of a single particle detection with an in situ dust detector was considered by Hirn et al (2016). The authors applied Poisson statistics to the measurements performed by the Dust Impact Monitor (DIM) on board the Rosetta lander Philae at comet 67P/Churyumov-Gerasimenko.…”

Synthesis of the morphological description of cometary dust at comet 67P/Churyumov-Gerasimenko

“…The cube had three active sensor sides, and each side had a total sensitive area of 24 cm 2 (Seidensticker et al 2007). During the descent of Philae to the surface of 67P, DIM recorded an impact of a cometary dust particle (among many other impact signals identified as false impacts) at 2.4 km from the comet surface (Hirn et al 2016;Krüger et al 2015;Flandes et al 2018). Experiments support the identification of this particle (aerogel was used as a comet analogue material to characterise the properties of this particle).…”