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.

The second assumed nucleus fitted to P/W is an inhomogeneous sphere of dusty ice, most of the gas and dust production A new step of development of the 3-D circumnuclear coma being due to four identical active areas separated by a back-(''CNC'') model described in Crifo et al. (1995) is presented:ground area, and covering a fraction g ‫؍‬ 0.43 of the surface. now the gas and dust production are computed from an explicit It is found that, up to at least 2 AU, the structure of the CNC dusty-ice sublimation model and a realistic dust mass spectrum is, in such a case complex. (1) Owing to the difference in solar is used. For the first time, (1) a clear distinction is made between zenith angle, identical active areas produce differing CNC gas nucleus active area fraction g and active surface icy area fraction and dust distributions: therefore, in an inhomogeneous rotating f ; (2) the dependence of the dusty ice sublimation rate on f nucleus, an active region is not characterized by a fixed corotatand that of f on the heliocentric distance r h due to the evolution ing CNC pattern. (2) As with more productive comets, the of the dust cover are explicitly taken into account; (3) the interaction between gas issuing from different active regions 3-D structure of the CNC is described thoroughly instead of leads to the formation of three-dimensional quasi stationary only in one symmetry plane; (4) the 3-D CNC model is quantitashock structures. (3) In particular, the weaker active areas tively fitted to observational data; (5) a detailed comparison is cannot expand freely in the sunward hemisphere, but are surmade between the CNC structures resulting from two alternarounded by a low-altitude concave sheath of shocked gas and tive assumptions concerning the nucleus. deflected dust. (4) Near to the shock surfaces, the dust density The first assumed nucleus is a homogeneous sphere of dusty distribution is characterized, as in more productive comets, by ice ( g ‫؍‬ 1). We show that, in this case: (1) The maximum density patterns that mimic dust jets. (5) Over the background ejectable mass of spherical dust grains has a ؔ f (r h ) cos z/r 2 h areas, and close to the surface, the gas flows transverse to the dependence upon solar zenith angle z and upon r h . (2) The vertical, and exhibits steep increases in density outwards. (6) terminal ejected dust velocities have an approximate The results reveal a fast smoothing-out of the CNC near-surface ؔ ͙cos z dependence on z and a strong ؔ ͙f (r h )/r h depenpatterns with increasing distance to the surface. dence on r h . Fitting-for definiteness-the model to the lightThe implications of the present results for (1) the general curve of the weak Comet P/Wirtanen (P/W), target of the physical characterization of comet nuclei on the basis of their future Rosetta mission, we find, assuming an upper limit nucoma-averaged properties, and (2) the assessment of the envicleus radius of 1.4 km, that: (1) f decreases from about 14% at ronmental parameters of future cometary close encounter misperiheli...

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 1 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 67P's 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.