Dust Impact Monitor (SESAME-DIM) measurements at comet 67P/Churyumov-Gerasimenko

Abstract

Context. The Rosetta lander Philae successfully landed on the nucleus of comet 67P/Churyumov-Gerasimenko on 12 November 2014. Philae carries the Dust Impact Monitor (DIM) on board, which is part of the Surface Electric Sounding and Acoustic Monitoring Experiment (SESAME). DIM employs piezoelectric PZT sensors to detect impacts by submillimeter-and millimeter-sized ice and dust particles that are emitted from the nucleus and transported into the cometary coma. Aims. The DIM sensor measures dynamical data such as flux and the directionality of the impacting particles. Mass and speed of the particles can be constrained assuming density and elastic particle properties. Methods. DIM was operated during three mission phases of Philae at the comet: (1) before the separation of Philae from Rosetta at distances of about 9.6 km, 11.8 km, and 25.3 km from the nucleus barycenter. In this mission phase particles released from the nucleus on radial trajectories remained undetectable because of significant obscuration by the structures of Rosetta, and no dust particles were indeed detected; (2) during Philae's descent to its nominal landing site Agilkia, DIM detected one approximately millimeter-sized particle at a distance of 5.0 km from the nucleus' barycenter, corresponding to an altitude of 2.4 km from the surface. This is the closest ever dust detection at a cometary nucleus by a dedicated in situ dust detector; and (3) at Philae's final landing site, Abydos, DIM detected no dust impact which may be due to low cometary activity in the vicinity of Philae or due to shading by obstacles close to Philae, or both. Results. Laboratory calibration experiments showed that the material properties of the detected particle are compatible with a porous particle having a bulk density of approximately 250 kg m −3 . The particle could have been lifted off the comet's surface by sublimating water ice.

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.…”

The Rosetta mission orbiter science overview: the comet phase

“…The approximate 7 h descent and subsequent multiple touchdowns on the comet surface facilitated unprecedented dual-spacecraft magnetic field measurements of the local environment and indicated the nucleus to have no intrinsic magnetic field on length scales more than 1 m [ 22 ]. A single 1 mm dust particle was detected approximately 2.4 km from the surface, with comparisons to laboratory experiments suggesting a bulk density of 250 kg m −3 , probably being a porous conglomerate [ 23 ]. Subsequent detections were hampered by detector obscuration and operation times, although upper limits on millimetric particle flux (1.6 × 10 −9 m −2 sr −1 ) and volume density 10 −11 –10 −12 m −3 on and near the surface have been provided [ 24 ].…”

A simple model for understanding the DIM dust measurement at comet 67P/Churyumov–Gerasimenko

“…During the Philae descent to the surface of comet 67P/Churyumov-Gerasimenko, the Dust Impact Monitor (DIM) -aboard the Philae lander-recorded the impact of a cometary grain (Krüger et al 2015).…”

“…The detected particle produced an impact signal with U m =2.45 mV and T c = 61 µs (Krüger et al 2015). In Krüger et al (2015), it is assumed that this particle had similar mechanical properties as those of a particular aerogel sample (ρ = 0.25 g/cm 3 and Y = 15 M P a) used for calibration purposes, given that, it was possible to match the detected U m and T c values with those U m and T c values produced by the impact of aerogel particles in the laboratory ( (Krüger et al 2015;Flandes 2015). A summary of the properties of the detected particle is shown in Table 1.…”

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We present a simple model for gas and dust flow from 67P/Churyumov-Gerasimenko that can be used to understand the grain impact observed by the DIM instrument on Philae (Krüger et al 2015). We show how model results when applied to the GIADA measurements (Rotundi et al 2015;Della Corte, V. et al 2015) can be used, in conjunction with the results found by the MIRO (Schloerb et al 2015) and VIRTIS (De Sanctis et al 2015) instruments to infer surface properties such as surface temperature and surface ice fraction.

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