E. Bussoletti

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.

Temperature Dependence of the Absorption Coefficient of Cosmic Analog Grains in the Wavelength Range 20 Microns to 2 Millimeters

We have measured the absorption coefficient per unit mass of cosmic dust analog grains, crystalline fayalite and forsterite, amorphous fayalite, and two kinds of disordered carbon grains, between 20 μm and 2 mm over the temperature range 295-24 K. The results provide evidence of a significant dependence on temperature. The opacity systematically decreases with decreasing temperature; at 1 mm, it varies by a factor of between 1.9 and 5.8, depending on the material, from room temperature to 24 K. The variations are more marked for the amorphous grains. The wavelength dependence of the absorption coefficient is well fitted by a power law with exponent β that varies with temperature. For the two amorphous carbons, β(24 K) ~1.2 with increases of 24% and 50% with respect to the room-temperature values. A 50% increase is found for amorphous fayalite, characterized by β(24 K) = 2. A less pronounced change of β with temperature, 14% and 10%, is observed for crystalline forsterite, β(24 K) = 2.2, and fayalite, β(24 K) = 2.3, respectively. For amorphous fayalite grains, the millimeter opacity at 24 K is larger by a factor of ~4 than that of the crystalline counterpart. In addition, a temperature dependence of the infrared bands present in the spectrum of the two crystalline silicates is found. The features become more intense, sharpen, and shift to slightly higher frequencies with decreasing temperature. The results are discussed in terms of intrinsic far-infrared-millimeter absorption mechanisms. The linear dependence of the millimeter absorption on temperature suggests that two-phonon difference processes play a dominant role. The absorption coefficients reported in this work can be useful in obtaining a more realistic simulation of a variety of astronomical data concerning dust at low temperatures and give hints to better identify its actual properties. In particular, they are used to discuss the origin of the diffuse far-infrared-millimeter interstellar dust emission spectrum. It is proposed that composite particles formed of silicate and amorphous carbon grains can reproduce the observations. The presence of these particles in the diffuse medium is consistent with the recent interstellar extinction model by Mathis.

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%.

A New Approach to the Puzzle of the Ultraviolet Interstellar Extinction Bump

We present a model that is able to shed light on the long-standing problem of the attribution of the UV interstellar extinction band at 4.6 μm. The model relies on a basic physical description of the electronic structure of carbon materials and is supported by laboratory simulations of UV processing of interstellar grains. The UV bump is attributed to a population of nano-sized, UV-processed hydrogenated amorphous carbon grains: the bump carrier carbons (BCCs). Specifically, we model the feature with a linear combination of absorption from different BCC populations present in interstellar regions sampled along a line of sight. The observed bump width variations are the result of different contributions of BCC grains along different lines of sight. The absorption from less processed particles prevails for wider bumps (denser regions), while more processed grains dominate in the case of sharper features (diffuse medium).

Raman and infrared spectra of polycyclic aromatic hydrocarbon molecules of possible astrophysical interest

We present the results of a laboratory analysis performed by means of Raman and infrared spectroscopy on six polycyclic aromatic hydrocarbons (PAHs): chrysene, triphenylene, perylen, benzo[α]pyrene, pentacene, and coronene. These are among the most stable and the most abundant PAHs suggested as an important constituent of interstellar matter. The Raman spectrum of PAHs exhibits intense peaks around 1350 and 1600 cm −1 , falling close to the two broad bands observed from interplanetary dust particles (IDPs)

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.

Amorphous carbon and the unidentified infrared bands

Absorption spectra of submicron amorphous carbon (AC) particles taken at room temperature and at 100 C, 260 C, and 400 C in vacuum are presented. The data show the presence of six bands which fit well six of the unidentified infrared emission bands discovered in the spectrum of the planetary nebula NGC 7027 and in a wide variety of galactic and extragalactic regions rich in UV flux: 3.4, 3.51, 5.78, 6.29, 6.85, and 11.3 microns. A comparison between these data and the bands expected from polycyclic aromatic hydrocarbons (PAHs) suggests that UIR bands may be produced by a collection of both AC and PAHs coexisting together in space. 27 references.

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.

VIRTIS: An imaging spectrometer for the ROSETTA mission

The VIRTIS (Visual IR Thermal Imaging Spectrometer) experiment has been one of the most successful experiments built in Europe for Planetary Exploration. VIRTIS, developed in cooperation among Italy, France and Germany, has been already selected as a key experiment for 3 planetary missions: the ESA-Rosetta and Venus Express and NASA-Dawn. VIRTIS on board Rosetta and Venus Express are already producing high quality data: as far as Rosetta is concerned, the Earth-Moon system has been successfully observed during the Earth Swing-By manouver (March 2005) and furthermore, VIRTIS will collect data when Rosetta flies by Mars in February 2007 at a distance of about 200 kilometres from the planet. Data from the Rosetta mission will result in a comparison – using the same combination of sophisticated experiments – of targets that are poorly differentiated and are representative of the composition of different environment of the primordial solar system. Comets and asteroids, in fact, are in close relationship with the planetesimals, which formed from the solar nebula 4.6 billion years ago. The Rosetta mission payload is designed to obtain this information combining in situ analysis of comet material, obtained by the small lander Philae, and by a long lasting and detailed remote sensing of the comet, obtained by instrument on board the orbiting Spacecraft. The combination of remote sensing and in situ measurements will increase the scientific return of the mission. In fact, the “in situ” measurements will provide “ground-truth” for the remote sensing information, and, in turn, the locally collected data will be interpreted in the appropriate context provided by the remote sensing investigation. VIRTIS is part of the scientific payload of the Rosetta Orbiter and will detect and characterise the evolution of specific signatures – such as the typical spectral bands of minerals and molecules – arising from surface components and from materials dispersed in the coma. The identification of spectral features is a primary goal of the Rosetta mission as it will allow identification of the nature of the main constituent of the comets. Moreover, the surface thermal evolution during comet approach to sun will be also studied.

Activation of an Ultraviolet Resonance in Hydrogenated Amorphous Carbon Grains by Exposure to Ultraviolet Radiation

The results of an experiment aimed at simulating the UV processing of hydrogenated amorphous carbon grains that occurs in the interstellar medium are presented. UV exposure of these grains induces significant changes in the UV-visible spectrum and, in particular, activates a resonance at 215 ± 2 nm, very close to the position of the interstellar extinction bump. This is the first reported laboratory observation of the activation of a UV resonance in hydrogenated carbon grains irradiated by UV photons. The spectral variations depend on the UV dose deposited in the samples; as the dose increases, the band becomes more intense while its peak position remains stable. We attribute the band to π-π* electronic transitions in sp2 ringed clusters that form the grains and interpret the spectral variations in terms of structural changes of the grains. This interpretation is confirmed by the behavior of the optical gap, which indicates an increase of the sp2 clustering degree as a function of the grain processing. The results of the present experiment suggest that it is unlikely that hydrogenated amorphous carbon grains can be transformed into pure graphite grains by UV processing in a typical diffuse-cloud timescale. The possibility that the interstellar bump is due to π-π* transitions in graphitic clusters that form small carbon grains is analyzed. This hypothesis requires a definite internal structure for the bump carrier. A process able to determine the grain structure is briefly discussed.