Jean-Marie Perrin

Dust measurements in the coma of comet 67P/Churyumov-Gerasimenko inbound to the Sun

Critical measurements for understanding accretion and the dust/gas ratio in the solar nebula, where planets were forming 4.5 billion years ago, are being obtained by the GIADA (Grain Impact Analyser and Dust Accumulator) experiment on the European Space Agency’s Rosetta spacecraft orbiting comet 67P/Churyumov-Gerasimenko. Between 3.6 and 3.4 astronomical units inbound, GIADA and OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) detected 35 outflowing grains of mass 10−10 to 10−7 kilograms, and 48 grains of mass 10−5 to 10−2 kilograms, respectively. Combined with gas data from the MIRO (Microwave Instrument for the Rosetta Orbiter) and ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) instruments, we find a dust/gas mass ratio of 4 ± 2 averaged over the sunlit nucleus surface. A cloud of larger grains also encircles the nucleus in bound orbits from the previous perihelion. The largest orbiting clumps are meter-sized, confirming the dust/gas ratio of 3 inferred at perihelion from models of dust comae and trails.

Evolution of the Dust Size Distribution of Comet 67P/Churyumov–Gerasimenko from 2.2 au to Perihelion

The Rosetta probe, orbiting Jupiter-family comet 67P/Churyumov–Gerasimenko, has been detecting individual dust particles of mass larger than 10−10 kg by means of the GIADA dust collector and the OSIRIS Wide Angle Camera and Narrow Angle Camera since 2014 August and will continue until 2016 September. Detections of single dust particles allow us to estimate the anisotropic dust flux from 67P, infer the dust loss rate and size distribution at the surface of the sunlit nucleus, and see whether the dust size distribution of 67P evolves in time. The velocity of the Rosetta orbiter, relative to 67P, is much lower than the dust velocity measured by GIADA, thus dust counts when GIADA is nadir-pointing will directly provide the dust flux. In OSIRIS observations, the dust flux is derived from the measurement of the dust space density close to the spacecraft. Under the assumption of radial expansion of the dust, observations in the nadir direction provide the distance of the particles by measuring their trail length, with a parallax baseline determined by the motion of the spacecraft. The dust size distribution at sizes >1 mm observed by OSIRIS is consistent with a differential power index of −4, which was derived from models of 67Ps trail. At sizes <1 mm, the size distribution observed by GIADA shows a strong time evolution, with a differential power index drifting from −2 beyond 2 au to −3.7 at perihelion, in agreement with the evolution derived from coma and tail models based on ground-based data. The refractory-to-water mass ratio of the nucleus is close to six during the entire inbound orbit and at perihelion.

GIADA: shining a light on the monitoring of the comet dust production from the nucleus of 67P/Churyumov-Gerasimenko

During the period between 15 September 2014 and 4 February 2015, the Rosetta spacecraft accomplished the circular orbit phase around the nucleus of comet 67P/Churyumov-Gerasimenko (67P). The Grain Impact Analyzer and Dust Accumulator (GIADA) onboard Rosetta moni- tored the 67P coma dust environment for the entire period. Aims. We aim to describe the dust spatial distribution in the coma of comet 67P by means of in situ measurements. We determine dynamical and physical properties of cometary dust particles to support the study of the production process and dust environment modification. Methods. We analyzed GIADA data with respect to the observation geometry and heliocentric distance to describe the coma dust spatial distribu- tion of 67P, to monitor its activity, and to retrieve information on active areas present on its nucleus. We combined GIADA detection information with calibration activity to distinguish different types of particles that populate the coma of 67P: compact particles and fluffy porous aggregates. By means of particle dynamical parameters measured by GIADA, we studied the dust acceleration region. Results. GIADA was able to distinguish different types of particles populating the coma of 67P: compact particles and fluffy porous aggregates. Most of the compact particle detections occurred at latitudes and longitudes where the spacecraft was in view of the comet’s neck region of the nucleus, the so-called Hapi region. This resulted in an oscillation of the compact particle abundance with respect to the spacecraft position and a global increase as the comet moved from 3.36 to 2.43 AU heliocentric distance. The speed of these particles, having masses from 10−10 to 10−7 kg, ranged from 0.3 to 12.2 m s−1 . The variation of particle mass and speed distribution with respect to the distance from the nucleus gave indications of the dust acceleration region. The influence of solar radiation pressure on micron and submicron particles was studied. The integrated dust mass flux collected from the Sun direction, that is, particles reflected by solar radiation pressure, was three times higher than the flux coming directly from the comet nucleus. The awakening 67P comet shows a strong dust flux anisotropy, confirming what was suggested by on-ground dust coma observations performed in 2008.

GIADA: ITS STATUS AFTER THE ROSETTA CRUISE PHASE AND ON-GROUND ACTIVITY IN SUPPORT OF THE ENCOUNTER WITH COMET 67P/CHURYUMOV-GERASIMENKO

GIADA (Grain Impact Analyser and Dust Accumulator) on-board the Rosetta mission to comet 67P/Churyumov-Gerasimenko was designed to study the physical and dynamical properties of dust particles ejected by the comet during the encounter. In this paper we report the results of the analysis of data collected by GIADA during the past seven years of the cruise phase. During this period the GIADA detection subsystems were switched on for periodic in-flight payload checkouts to monitor their state-of-health including potential changes in its performance that could affect its data collection. Only slight variations in sensitivity and dynamical range were identified that will not affect the GIADA measurement capability during the Rosetta comet encounter and rendezvous phase. The GIADA microbalance system detected the presence of low-volatility material over a period of about 169 days when the GIADA cover remained partially opened. It is highly probable that this material originated from the spacecraft itself, as a spacecrafts outgassing was observed by the ROSINA mass spectrometer (on-board Rosetta) during the cruise phase. The identification of the low-volatility mass deposited on the microbalances as self-contamination will allow us to evaluate the mass rate background to be subtracted from the GIADA science data. These results obtained from GIADA cruise data analysis coupled with laboratory calibration data obtained from measurements using the GIADA spare model for selected cometary dust analogs will be the basis for the interpretation of the GIADA scientific data.

Performance of micro-balances for dust flux measurement

Abstract Micro-balances have been used in the past for volatile deposition monitoring in laboratory and in space environment. In order to determine their suitability to measure mass deposition in the form of solid particles, some topical aspects must be characterised, such as the sensitivity versus temperature and grain mass and the sticking efficiency versus grain speed. These parameters have been retrieved for different sensor configurations, i.e. with and without an adhesive coating, used in the perspective of improving the sensor particle collection efficiency. Our studies show that the adhesive coating improves the sensor sticking efficiency only for fast (100–400 m s −1 ) grains. However, the stability of the output signal with temperature is worse in the coated configuration by a factor of about ten. These results provide important inputs in the view of using micro-balances for dust monitoring. In particular, they have been carefully considered for the selection of the configuration of micro-balances, included as sub-systems of the GIADA experiment onboard the ESA ROSETTA mission and aimed at studying flux and dynamic properties of cometary grains.

The grain detection system for the GIADA instrument: design and expected performances

Abstract The Grain Detection System (GDS) is part of the instrument GIADA (Grain Impact Analyser and Dust Accumulator), included in the scientific payload of the ESA Rosetta mission towards comet 46P/Wirtanen. GIADA is aimed at analysing the grain dynamic properties and the dust flux evolution in the cometary environment. The GDS is devoted to the detection of single grains entering the instrument, to measure their velocity and to obtain information about their morphology and possibly composition. In this paper we describe the design of the GDS instrument and give some predictions on GDS performances at the comet. In particular, the simulation shows that the GDS will be able to count a total of 4.7 × 10 5 particle “events” during all the GIADA operating phase and so satisfy scientific requirements for a statistically reliable analysis of cometary grains.

The GIADA experiment for ROSETTA mission to comet 46P/Wirtanen: Design and performances

Abstract Rosetta is one of the most ambitious missions planned by ESA for the beginning of the next millennium. It will explore from very close a comet nucleus along its trajectory up to perihelion. In the instrument complex forming the scientific payload, the GIADA (Grain Impact Analyser and Dust Accumulator) experiment is devoted to study the cometary dust flux evolution and grain dynamic properties. To achieve the required performances and the expected scientific return, GIADA has been designed as a multi-sensor instrument. It is able to detect grain passage by laser light scattering measurement, particle momentum through piezoelectric transducers and mass flux by means of quartz crystal microbalances. In this paper we describe the technical solutions and performances which have been reached on the development models of GIADA.

The sticking efficiency of quartz crystals for cosmic sub-micron grain collection

Abstract Mass flux monitoring of cosmic dust grains is possible by means of microbalance sensors, such as those used in the GIADA-MBS system, onboard the Rosetta mission. An important parameter, which determines the efficient collection of grains with time, is the sticking efficiency of the electrode-coated quartz crystals forming the sensor of the microbalance. In principle, an improving of performances should be obtained by applying a thin coating of adhesive material onto the sensor surface. In order to evaluate the sticking properties for coated and uncoated microbalance crystals, we bombarded them with 1.2 μm silica spheres and 0.64 μm irregular SiC grains, having velocities of up to 55 m s −1 . We studied individual impacts obtaining the sticking efficiency as a function of the grain velocity. For silica sphere projectiles, the presence of the adhesive coating slightly increases the sticking, whereas for irregular SiC particles it is noticeably high even for the uncoated crystal (0.7–0.8). We conclude that, for micrometer irregular grains, in the studied velocity range, the microbalance sensor without any sticking coating guarantees rather high collection performances.

Tentative detection of clear-air turbulence using a ground-based Rayleigh lidar

Atmospheric gravity waves and turbulence generate small-scale fluctuations of wind, pressure, density, and temperature in the atmosphere. These fluctuations represent a real hazard for commercial aircraft and are known by the generic name of clear-air turbulence (CAT). Numerical weather prediction models do not resolve CAT and therefore provide only a probability of occurrence. A ground-based Rayleigh lidar was designed and implemented to remotely detect and characterize the atmospheric variability induced by turbulence in vertical scales between 40 m and a few hundred meters. Field measurements were performed at Observatoire de Haute-Provence (OHP, France) on 8 December 2008 and 23 June 2009. The estimate of the mean squared amplitude of bidimensional fluctuations of lidar signal showed excess compared to the estimated contribution of the instrumental noise. This excess can be attributed to atmospheric turbulence with a 95% confidence level. During the first night, data from collocated stratosphere-troposphere (ST) radar were available. Altitudes of the turbulent layers detected by the lidar were roughly consistent with those of layers with enhanced radar echo. The derived values of turbulence parameters Cn2 or CT2 were in the range of those published in the literature using ST radar data. However, the detection was at the limit of the instrumental noise and additional measurement campaigns are highly desirable to confirm these initial results. This is to our knowledge the first successful attempt to detect CAT in the free troposphere using an incoherent Rayleigh lidar system. The built lidar device may serve as a test bed for the definition of embarked CAT detection lidar systems aboard airliners.

Local enhanced solar irradiance on the ground generated by cirrus: measurements and interpretation

Abstract This study concerns the influence of thin cirrus on the solar irradiance budget on the ground, using data obtained by remote sensing from a set of instruments operated at the Observatoire de Haute Provence (France). Comparisons between solar irradiance ground-based measurements obtained during clear and cloudy days usually show a lower irradiance in the latter case as expected. However, there are cases lasting a maximum of 30 min for which the solar irradiance is greater in the presence of cumulus than in clear-sky conditions, and this phenomenon was named enhanced solar irradiance. Our pyranometer measurements reveal the same phenomenon associated with the presence of thin clouds alone, as revealed by a daytime wide-field camera and occurring both in total and UVB* (280 to 320 nm) solar irradiance. In the case of thin cirrus, this phenomenon can last up to several hours. To understand this phenomenon, we have developed a model of solar light scattering within thin cirrus clouds that takes into account the presence of the atmosphere. The model reproduces the enhanced solar irradiance phenomenon in total and UBV* spectral range. Monoscattering and multiscattering processes inside the cirrus clouds also will be discussed.