A. C. Lopez-Jimenez

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.

DENSITY AND CHARGE of PRISTINE FLUFFY PARTICLES FROM COMET 67P/CHURYUMOV-GERASIMENKO

The Grain Impact Analyzer and Dust Accumulator (GIADA) instrument on board ESA’s Rosetta mission is constraining the origin of the dust particles detected within the coma of comet 67 P/Churyumov–Gerasimenko (67P). The collected particles belong to two families: (i) compact particles (ranging in size from 0.03 to 1 mm), witnessing the presence of materials that underwent processing within the solar nebula and (ii) fluffy aggregates (ranging in size from 0.2 to 2.5 mm) of sub-micron grains that may be a record of a primitive component, probably linked to interstellar dust. The dynamics of the fluffy aggregates constrain their equivalent bulk density to <1 kg m-3. These aggregates are charged, fragmented, and decelerated by the spacecraft negative potential and enter GIADA in showers of fragments at speeds <1 m s-1. The density of such optically thick aggregates is consistent with the low bulk density of the nucleus. The mass contribution of the fluffy aggregates to the refractory component of the nucleus is negligible and their coma brightness contribution is less than 15%.

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.

Physical properties of the aerosol debris generated by the impact of fragment H of comet P/Shoemaker-Levy 9 on Jupiter

The physical properties of the aerosol debris coming from the impact of fragment H of comet P/Shoemaker-Levy on Jupiter are analyzed. Based on a high spatial resolution image set obtained at the 4.2m William Herschel Telescope at La Palma (Spain), we derived detailed limb darkening curves at three wavelengths (3600, 8920 and 9480 A), which we used to retrieve the aerosol physical properties and spatial distribution in the central core region of the impact site. We derived these properties combining the observations at two dates, that corresponding to the night of July 18th, 1994, just after the impact time, and that of July 21st, 1994. Radiative transfer models in combination with a Mie scattering program were used to retrieve the possible values of aerosol properties and spatial distribution compatible with these observations. The main results of the model computations are that the particles are rather dark, with imaginary refractive indexes varying between 0.02 at 3600 A and 0.006 in the 9000 A region, have a mean radius of about 0.15 µm, and are distributed between 1 and 450 mbar with a total column concentration of 8.2 × 109 cm−2, giving a total optical thickness of 3.6 at 9480 A. A lower limit to the volume of the impact debris aerosol is estimated to be that corresponding to a sphere of about 230 m in diameter.

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 Giada Experiment for the Rosetta Mission

The Grain Impact Analyser and Dust Accumulator (GIADA) instrument, on board the ESA Rosetta mission, shall analyse the physical and dynamical properties of grains ejected by the target comet and monitor the coma evolution in terms of dust flux and spatial distribution vs. time. The mission, formerly planned to visit comet 46P/Wirtanen, is now targeted to a rendezvous with comet 67P/Churyumov-Gerasimenko. The present operative mission plan foresees that Rosetta will follow the comet from about 4 AU pre-perihelion to about 2 AU post-perihelion. This will allow us to study, for the first time, the onset and evolution of activity of a comet nucleus and its environment. GIADA is composed by different sub-systems designed to measure mass, momentum and speed of single grains larger than about 30 μm in size and to monitor the cumulative flux of smaller grains coming from different directions. GIADA technical characteristics and scientific performances will guarantee a full monitoring of the dust environment and the achievement of unprecedented scientific results about cometary dust physics.