A Simple Model for Understanding the DIM Dust Measurement at Comet 67P/Churyumov-Gerasimenko

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 (Krueger et al. 2015) We show how model results when applied to the GIADA measurements (Rotundi et al. 2015; Della Corte 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.Comment: 15 pages, 4 figures. Accepted for publication in Planetary and Space Scienc

Dust Impact Monitor DIM Onboard Rosetta / Philae: Comparison of experimental Results and the Theory behind the Experiment

The Rosetta lander spacecraft Philae will land on the nucleus surface of comet 67P/Churyumov-Gerasimenko in November 2014. Philae is equipped with the Dust Impact Monitor (DIM). DIM is part of the SESAME instrument package onboard Philae [Seidensticker et al., 2007] and employs piezoelectric PZT sensors to detect dust particle impacts. The sensors are mounted on the outer side of a cube, facing in orthogonal directions, this way allowing for the detection of grains approaching normal to the nucleus surface and from two horizontal directions. DIM’s total sensitive area is approximately 70 cm**2. It will measure impacts of sub-millimeter and millimeter sized ice and dust particles that are emitted from the nucleus and transported into the cometary coma by the escaping gas flow. A grain-size dependent fraction of the emitted grains is expected to fall back to the nucleus surface due to gravity. DIM will be able to detect both these components, the backfalling particles as well as the grains hitting the detector on direct trajectories from the surface. With DIM we will be able to measure fluxes, impact directions as well as the speed and size of the impacting cometary particles

Dust Impact Monitor (SESAME-DIM) Measurements at Comet 67P/Churyumov-Gerasimenko

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

Brightness of Saturn's rings with decreasing solar elevation

International audienceEarly ground-based and spacecraft observations suggested that the temperature of Saturn's main rings (A, B and C) varied with the solar elevation angle, B'. Data from the composite infrared spectrometer (CIRS) on board Cassini, which has been in orbit around Saturn for more than five years, confirm this variation and have been used to derive the temperature of the main rings from a wide variety of geometries while B' varied from near -24o to 0o (Saturn's equinox). Still, an unresolved issue in fully explaining this variation relates to how the ring particles are organized and whether even a simple mono-layer or multi-layer approximation describes this best. We present a set of temperature data of the main rings of Saturn that cover the ˜23o - range of B' angles obtained with CIRS at low ( alpha˜30o) and high ( alpha>=120o) phase angles. We focus on particular regions of each ring with a radial exten

PZT Dust Impact Monitor (DIM) Onboard Rosetta/Philae: Experimental Results and Theoretical Background

The Dust Impact Monitor (DIM) experiment on board the Rosetta spacecraft's lander, Philae, is a cube with three sides covered with piezo-electric sensors (total sensitive area: ~ 70 ccm), aimed at measuring the physical properties of millimetric and sub-millimetric dust particles that move near the surface of comet 67P/Churyumov-Gerasimenko. After being launched in March 2004, the Rosetta spacecraft encounters after a 10.5 years flight the comet 67P in mid 2014 and a landing of Philae is planned for mid-November 2014. In this work we study the performance of DIM based on impact experiments and compare the measurements with the sensor's expected theoretical behavior as derived from Hertz' theory of elastic impact. We present the results of impact experiments performed with spherical particles of different densities and elastic properties. We performed three types of experiment: (a) we analyze the performance of the different sensor sides under identical impacts, (b) we investigate the performance of DIM under impacts of different materials and different impact speeds, and (c) we report on the behavior of the DIM sensor with varying impact angle. We discuss the influence of the microstructure of the PZT sensor on the signal strength and its variation with position of the impacting particles. Our results show that the signal strength and the contact times measured with the DIM PZT sensors can be well approximated by Hertz' contact mechanics