E. Mazzotta Epifani

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%.

Comet 67P/Churyumov-Gerasimenko: the GIADA dust environment model of the Rosetta mission target

Context. The ESA Rosetta spacecraft will reach the short-period comet 67P/Churyumov-Gerasimenko in 2014. Orbiting strategy, orbiter safety conditions, landing scenarios and expected results from dust collectors depend on models of the 67P dust environment. Many papers already tackled this matter, analysing a limited set of observations, and therefore often reaching conflicting conclusions. Aims. We consider a set of observations representative of all ground-based and IR (thermal infrared) Spitzer data collected over the last three perihelion passages, to determine the 67P dust environment after the end of the gas drag on dust (at about 20 nucleus radii) consistent with available 67P gas and dust coma photometry, images of the dust coma, tail and trail, at optical and IR wavelengths. Methods. In order to obtain the best fit to 67P data, we consider three independent tail and trail simulation codes (developed by three independent groups), which parametrise cometary dust by the quantity β, the ratio between solar radiation pressure and gravity forces. GIADA, the dust monitor instrument of the Rosetta orbiter, will provide an experimental determination of the β-dust mass relation. Results. A 67P environment model based on a perihelion-symmetric dust velocity and on a perihelion-asymmetric dust size distribution, is consistent with all available data. During most Rosetta operations, the dust cross-section is dominated by mm to cm-sized grains, while the ejected dust mass is dominated by grains larger than a few mm, with a dust-to-gas ratio of 3 around perihelion. Conclusions. 67P onsets its activity at Sun-distances rh ≥ 3.4 AU; the dust geometric albedo is 0.04 ± 0.02; at 3.0 AU, 10 g grains escape the nucleus gravity field (10 kg grains at perihelion) with a dust mass-loss rate of 10−40 kg s −1 (500 kg s −1 at perihelion); 67P’s activity depends on seasons, with the northern heminucleus (rich in large grains and CN depleted) active before perihelion.

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.

The distant activity of short-period comets – I

A surprisingly large number of short-period comets have been observed with significant activity (a coma and even a well-developed dust tail) at heliocentric distances greater than 3 au, where the water sublimation rate is low and thus the sublimation of other volatiles, such as for example CO, could drive the presence of a coma. As CO is not the main ice in the comet nucleus, the dust release from the nucleus surface as a result of CO drag is expected to be very different from that caused by water. In order to investigate the complexity of the transition between bare nucleus and the presence of a well-developed coma, to compare activity levels and to obtain information concerning evolutionary differences resulting from distinct dynamical histories, we started a long-term programme of CCD imaging of distant short-period comets. In this paper we present the results of the first observing run, performed at the 3.5-m Telescopio Nazionale Galileo on La Palma on 2004 December 17. Five comets were imaged in the R band (three numbered short-period comets and the two fragments of an unnumbered short-period comet): 36P/Whipple, 111P/Helin–Roman–Crockett, 159P/LONEOS, and P/2004 V5 (LINEAR–Hill) A and B. The heliocentric distance of targets was 3.47 ≤Rh≤ 4.43 au. The targets presented several levels of activity, ranging from a stellar appearance to a well-developed coma and tail. The stellar appearance of 111P/allowed a range to be derived for the nuclear radius rnucleus from 0.46 to 1.39 km (assuming a ‘classical’ albedo value of 0.04), depending on the presence of an unresolved coma. For the active comets, we measured dust production levels in terms of the quantity Afρ, which was in the range 11 ≤Afρ≤ 224 cm. The distant activity of the target comets is analysed in relation to the target dynamical history (in terms of the perihelion heliocentric distance). A possible preliminary conclusion can be obtained, namely that the hypothesis of distant activity ‘induced’ by higher temperature owing to perihelion lowering cannot be univocally invoked for short-period comets.

The distant activity of the Long Period Comets C/2003 O1 (LINEAR) and C/2004 K1 (Catalina)

Aims. We study the distant dust environment of two Long Period Comets: C/2003 O1 (LINEAR) (observed at rh = 7.39 AU) and C/2004 K1 (Catalina) (observed at rh = 3.43 AU). The case of C/2003 O1 is particularly interesting since the comet has a quite large perihelion heliocentric distance (rh = 6.85 AU). Methods. We analysed R-band images taken at the CAHA 2.2 m telescope to characterise the properties of the cometary dust coma. The images of both comets were also used as input into an inverse numerical model, to derive information about the dynamical parameters of coma dust grains until a time ∼500 days before the observation. Results. Both the comets appeared to be active, and of similar shape and size despite the great difference in their heliocentric distance of observation. C/2003 O1 exhibited a tail extending to at least 3.7 × 10 5 km. It is a very active object, since its Afρ is 552 ± 36 cm within the inner 5 �� .C /2004 K1 showed a similar long tail, extending to 2.6 × 10 5 km. Its Afρ is 539 ± 35 cm within the inner 5 �� .A n almost constant dust grain ejection velocity between 0.5 and 0.9 m/s at 1 cm dust size has been derived for C/2003 O1 and an increase from 0.5 to 2 m/s at 1 cm dust size for C/2004 K1, reflecting the different volatile dragging the dust environment (probably CO for C/2003 O1 and water for C/2004 K1). Model results allow some speculations about the comet nucleus size of C/2003 O1 (LINEAR) and its CO content: for a QCO similar in value to those observed for other distant cometary objects, a comet radius Rn from 13 to 17 km can be inferred.

The activity of comet C/2007 D1 (LINEAR) at 9.7 AU from the Sun

Aims. We report on the unusual distant activity of the dynamically new comet C/2007 D1 (LINEAR), observed at rh = 9.7 AU postperihelion. This comet is particularly interesting because of its quite large perihelion distance (rq = 8. 8A U). Methods. We analyse V and R band images taken at the TNG telescope to characterise the dust coma of the comet and investigate its photometry, colours, and dust production. Results. The colour of the dust is redder than the Sun (V − R = 0.505 ± 0.093 at the photometric aperture of radius 2.2 �� ). The Afρ value is measured to be 1983 ± 81 cm for an aperture of radius = 2.2 �� . An upper limit to the cometary nucleus radius of aN ≤ 18 km is derived. A dust mass-loss rate of dM d dt = 5.3 × 10 2 kg s −1 is measured, which implies that C/2007 D1 (LINEAR) is a very active

Photometry and taxonomy of trans-Neptunian objects and Centaurs in support of a Herschel key program

Context. The investigation of Centaurs and trans-Neptunian objects (TNOs) provides essential information about the early conditions and evolution of the outer solar system. The radiometric technique combines measurements in the visible and thermal infrared; with these one can estimate the size and albedo of Centaurs and TNOs. Aims. Our aim is to obtain visible photometry of a sample of Centaurs and TNOs, a subset of the targets of the “TNOs are cool” key program at the Herschel Space Observatory. Methods. We carried out visible photometry of Centaurs and TNOs, making use of the DOLORES instrument at the Telescopio Nazionale Galileo (TNG, La Palma, Spain). Results. We report photometric observations of 20 objects and present the computed absolute magnitudes. We derive the taxonomy of our targets (nine are classified for the first time, the results for five objects agree with the literature, the other targets are tentatively classified based on incomplete datasets) and combine the results with the literature, searching for correlations between taxonomy and dynamics. We look for comet-like activity in our Centaur sample, including (248835) 2006 SX368, which was previously described as active. Conclusions. We provide an accurate determination of the absolute magnitude and of the relative error for each of our targets. These values can be readily used in combination with thermal infrared data. The surface of TNO (65489) Ceto seems to be heterogeneous. Our results seem to support an evolutionary origin for the color dichotomy of Centaurs, and the occurrence of a strong mixing after the TNO formation. No evident cometary activity is detected around the five Centaurs in our sample; assuming that an unresolved coma is present around (248835) 2006 SX368, we use the “photometric model” to derive the possible dust production rate, finding that Qdust is in the range 1–31 kg/s.

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.