Nicolas Altobelli

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.

Comet 67P/Churyumov–Gerasimenko: Close-up on Dust Particle Fragments

The COmetary Secondary Ion Mass Analyser instrument on board ESAs Rosetta mission has collected dust particles in the coma of comet 67P/Churyumov-Gerasimenko. During the early-orbit phase of the Rosetta mission, particles and particle agglomerates have been imaged and analyzed in the inner coma at distances between 100 km and 10 km off the cometary nucleus and at more than 3 AU from the Sun. We identified 585 particles of more than 14 μm in size. The particles are collected at low impact speeds and constitute a sample of the dust particles in the inner coma impacting and fragmenting on the targets. The sizes of the particles range from 14 μm up to sub-millimeter sizes and the differential dust flux size distribution is fitted with a power law exponent of -3.1. After impact, the larger particles tend to stick together, spread out or consist of single or a group of clumps, and the flocculent morphology of the fragmented particles is revealed. The elemental composition of the dust particles is heterogeneous and the particles could contain typical silicates like olivine and pyroxenes, as well as iron sulfides. The sodium to iron elemental ratio is enriched with regard to abundances in CI carbonaceous chondrites by a factor from ˜1.5 to ˜15. No clear evidence for organic matter has been identified. The composition and morphology of the collected dust particles appear to be similar to that of interplanetary dust particles.

Rosetta begins its COMET TALE

Comets are the best sample of primitive solar nebula material presently available to us, dating back 4.57 billion years to the origin of our planetary system. Past missions to comets have all been “fast flybys”: They provided only a snapshot view of the dust and ice nucleus, the nebulous coma surrounding it, and how the solar wind interacts with both of these components. Such space-based investigations of comets began in the 1980s with a flotilla of spacecraft: the European Space Agencys (ESAs) first deep space mission, Giotto, which pursued comet 1P/Halley; Deep Space 1 at 19P/Borrelly; Stardust at 81P/Wild 2; Deep Impact and Stardust NeXT at 9P/Tempel; and EPOXI at 103P/Hartley 2. ![Figure][1] PHOTO: ESA/ROSETTA/PHILAE/CIVA Rosetta is now taking a more prolonged look. The spacecraft is an ESA mission, with contributions from member states and from NASA, and it currently orbits the Jupiter family comet 67P/Churyumov-Gerasimenko (67P). Rosetta met the comet nucleus on 6 August 2014, at 3.7 astronomical units (AU) from the Sun, and delivered the Philae lander to the nucleus surface on 12 November 2014, when the comet was 3.0 AU from the Sun. Rosetta is uniquely positioned to further the understanding of these primitive bodies, having revealed an unusual and fascinating object. After rendezvous, the Rosetta spacecraft moved from 100 km above the comet to a bound orbit only ~10 km away. This early period of the mission has revealed previously unseen details of a comet nucleus, as Rosettas instruments recorded measurements that were once impossible. This issue of Science contains the first published scientific results from Rosetta at comet 67P. The surface of the comet shows evidence of many active processes and is highly complex. The solid nucleus is an object for which neither horizontal nor vertical variations are modest (Thomas et al. , this issue). The current comet shape model suggests that the mass is 1013 kg (about 100 million times the mass of the international space station), with a bulk density of ~470 kg/m3 (similar to cork, wood, or aerogel). The low mass and density values strongly constrain the composition and internal structure of the nucleus, implying a relatively fluffy nature, with a porosity of 70 to 80% (Sierks et al. , this issue). The nucleus surface itself appears rich in organic materials, with little sign of water ice (Capaccioni et al. , this issue). The coma produced by ices sublimating from the nucleus is highly variable, displaying large diurnal and possibly seasonal changes. For example, both atomic H and O have been detected close to the nucleus and vary with time, probably stemming from electron impact dissociation of venting H2O vapor. The total H2O gas production rates varied from 1 × 1025 molecules per second in early June 2014 to 4 × 1025 molecules per second in early August, broadly consistent with predictions. In August, water outflow from the surface varied by a factor of at least 5, owing to the effects of terrain, comet shape, and daily illumination changes and possibly other factors (Gulkis et al. , this issue). The science team reports the detection of several molecules, including H2 17O, H2 18O, CO, and CO2, and assessed their time variability and heterogeneous distribution (Hassig et al. , this issue). A high D/H ratio in water, 5.3 × 10−4, was measured, which precludes the idea that Jupiter family comets contain solely Earth ocean–like water (Altwegg et al. , this issue). As observed at 3.6 AU from the Sun, a cloud of about 105 grains (larger than 5 cm) surrounds the nucleus in bound orbits, likely from the previous perihelion passage. The nucleus currently emits dust grains up to 2 cm in size, giving a dust/gas mass ratio of 4 ± 2 averaged over the sunlit nucleus surface (Rotundi et al. , this issue). This is higher than generally accepted for comets. In a progressive series of observations, Rosetta observed the emergence of an energetic ion environment from a low-activity comet nucleus under the influence of the solar wind (Nilsson et al. , this issue). The data presented here allow us to build a detailed portrait of comet 67P. These initial observations provide a reference description of the global shape, the surface morphology and composition, and the bulk physical properties of the nucleus. Subsequent measurements with the orbiter and with the Philae lander will further describe the comet over time. Rosetta will follow the comet at close range through its closest approach to the Sun, perihelion, in August 2015, and then as the comet moves away from the Sun. The spacecraft will perform many flybys that will allow the onboard instruments to measure the evolution of the nucleus and coma with respect to the comets initial state, defined by the data presented here. The Rosetta mission has begun to explore our origins, thanks to the efforts of thousands of people at ESA, NASA, industrial partners, and space agencies and to engineers and scientists from around the world. For more than 25 years, they dreamed of these moments when they designed, developed, and launched the Rosetta spacecraft and then followed its interplanetary journey, watched over its long sleep, and woke it from hibernation. These first papers are dedicated to all of them. [1]: pending:yes

A new look into the Helios dust experiment data: presence of interstellar dust inside the Earth's orbit

An analysis of the Helios in situ dust data for interstellar dust (ISD) is presented in this work. Recent in situ dust measurements with impact ionization detectors on-board various spacecraft (Ulysses, Galileo ,a ndCassini) showed the deep penetration of an ISD stream into the Solar System. The Helios dust data provide a unique opportunity to monitor and study the ISD stream alteration at very close heliocentric distances. This work completes therefore the comprehensive picture of the ISD stream properties within the heliosphere. In particular, we show that gravitation focusing facilitates the detection of big ISD grains (micrometer-size), while radiation pressure prevents smaller grains from penetrating into the innermost regions of the Solar System. A flux value of about 2.6±0.3×10 −6 m −2 s −1 is derived for micrometer-size grains. A mean radiation pressure-to-gravitation ratio (so-called β ratio) value of 0.4 is derived for the grains, assuming spheres of astronomical silicates to modelize the grains surface optical properties. From the ISD flux measured on the Helios trajectory, we infer a lower limit of 3 ± 3 × 10 −25 kg m −3 to the spatial mass density of micron-sized grains in the Local Interstellar Cloud (LIC). In addition, compositional clues for ISD grains are obtained from the data provided by the time-of-flight mass spectrometer subsystem of the Helios instrument. No clustering of single minerals is observed but rather a varying mixture of various minerals and carbonaceous compounds.

Interstellar dust flux measurements by the Galileo dust instrument between the orbits of Venus and Mars

[i] We present an analysis of the data obtained by the Galileo in situ dust instrument for interstellar dust (ISD). Between December 1989 and the end of 1993, three orbit segments were favorable for the detection of ISD at less than 3 AU heliocentric distance. After removing background events from the data sample, which were mostly due to interplanetary dust impactors, we infer that the flux of ISD grains between 0.7 AU and 3 AU is about 4 x 10 -5 m -2 s -1 . This result is compatible with the ISD flux of 3 × 10 -5 m -2 s -1 (grain size 0.4 μm) derived from the Cassini measurements at about 1 AU. Using a new concept called ISD β spectroscopy, we are able to estimate at different locations in the inner solar system the ISD flux alteration resulting from the competing effects of radiation pressure and gravitation. In particular, we confirm results of previous Ulysses dust data analysis showing that radiation pressure prevents smaller ISD grains (radius smaller than 0.3 μm) from reaching the innermost region of the solar system. Furthermore, our analysis shows the relevance of gravitational focusing in the dynamics of bigger ISD grains (micron-sized grains). The Galileo measurements were performed 10 years before the Cassini measurements. Thus the available ISD data now cover almost a full 11-year solar cycle. Nonetheless, the flux of ISD grains with radius bigger than 0.4 μm shows no significant temporal variation. This suggests that the dynamics of these ISD grains is not influenced much by the interaction with the time-dependent solar magnetic field.

Dissolution on Titan and on Earth: Toward the age of Titan's karstic landscapes

Titans polar surface is dotted with hundreds of lacustrine depressions. Based on the hypothesis that they are karstic in origin, we aim at determining the efficiency of surface dissolution as a landshaping process on Titan, in a comparative planetology perspective with the Earth as reference. Our approach is based on the calculation of solutional denudation rates and allow inference of formation timescales for topographic depressions developed by chemical erosion on both planetary bodies. The model depends on the solubility of solids in liquids, the density of solids and liquids, and the average annual net rainfall rates. We compute and compare the denudation rates of pure solid organics in liquid hydrocarbons and of minerals in liquid water over Titan and Earth timescales. We then investigate the denudation rates of a superficial organic layer in liquid methane over one Titan year. At this timescale, such a layer on Titan would behave like salts or carbonates on Earth depending on its composition, which means that dissolution processes would likely occur but would be 30 times slower on Titan compared to the Earth due to the seasonality of precipitation. Assuming an average depth of 100 m for Titans lacustrine depressions, these could have developed in a few tens of millions of years at polar latitudes higher than 70°N and S, and a few hundreds of million years at lower polar latitudes. The ages determined are consistent with the youth of the surface (<1 Gyr) and the repartition of dissolution-related landforms on Titan.

The flow of interstellar dust into the solar system

Context. Interstellar dust (ISD) is a major component in the formation and evolution of stars, stellar systems, and planets. Astronomical observations of interstellar extinction and polarization, and of the infrared emission of the dust, are the most commonly used technique for characterizing interstellar dust. Besides this, the interstellar dust from the local interstellar cloud enters the solar system owing to the relative motion of the Sun with respect to this cloud. Once in the solar system, in-situ observations can be made by spacecraft using impact ionization detectors and time-of-flight spectrometers like the ones flown on the Cassini, Ulysses, and Galileo, spacecrafts. Also a sample return can be done, as in the Stardust mission. Once in the solar system, the trajectories of these dust grains are shaped by gravitational, solar radiation pressure, and Lorentz forces. The Lorentz forces result from the interaction of the charged dust particles with the interplanetary magnetic field. The ISD densities in the solar system thus depend both on the location in the solar system and on time, correlated to the solar cycle. Aims. This paper aims at giving the reader insight into the flow patterns of ISD when it moves through the solar system. This is useful for designing future in-situ or sample return missions or for knowing whether for specific missions, simplified assumptions can be used for the dust flux and direction, or whether full simulations are needed. Methods. We characterize the flow of ISD through the solar system using simulations of the dust trajectories. We start from the simple case without Lorentz forces and expand to the full simulation. We pay attention to the different ways of modeling the interplanetary magnetic field and discuss the influence of the dust parameters on the resulting flow patterns. Dust densities, fluxes, and directionalities are derived from the trajectory simulations. Different graphics representations are used to gain insight into the flow patterns. As an illustration of how the model can be used, we predict the fluxes and directionalities of the ISD for the Cassini mission. Results. The characteristics of the flow of ISD through the solar system have been investigated to gain insight in the patterns of the flow. The modeling can also be used for predicting dust fluxes for different space missions or planets, and for understanding spacecraft measurements, such as those from Ulysses, Cassini, and Stardust.