1,721,315 research outputs found
How Comets Work
No full textTwo major questions regarding comets have been up to now far from any solution. (i) How is it possible that water-ice sublimation from the nucleus surface does not lead to an insulating crust, stopping every gas and dust ejection within a few days? (ii) How is it possible that the gas flow crossing the refractory surface crust ejects dust particles bonded by tensile strengths larger than tens of Pa when the perihelion gas pressure at the nucleus-coma interface is less than one Pa? We have developed a simple but rigorous analytical model, assuming that the cometary nucleus consists of agglomerates of ice and dust ("clusters"). As soon as the clusters become exposed to sunlight, gas diffusion from their inside leads to their dehydration. We find that (i) the gas diffusing from the interior to the surface of a sunlit cluster has a steep density gradient at the cluster surface, which blasts the cluster into particles of sizes larger than or equal to those actually observed by Rosetta dust instruments; (ii) the heat-conduction and diffusion timescales are much shorter than the dehydration timescale, ensuring that the described process prevents any dumping of the nucleus activity driven by water-ice sublimation from 4 au inbound to 4 au outbound; and (iii) the clusters are in fact cm-sized pebbles, so that a cometary nucleus made of pebbles is confirmed to be the only one consistent with cometary gas and dust activity, a process unexplained until now
Là dove il fuoco sfida il cielo
No full textArticolo su rivista divulgativa "Newton", edita da RCS - Rizzoli Corriere della Ser
CO-driven activity constrains the origin of comets
No full textContext. An open question in the study of comets is the so-called cohesion bottleneck, that is, how dust particles detach from the nucleus. Aims. We test whether the CO pressure buildup inside the pebbles of which cometary nuclei consist can overcome this cohesion bottleneck. Methods. A recently developed pebble-diffusion model was applied here to comet C/2017K2 PANSTARRS, assuming a CO-driven activity. Results. (i) The CO-gas pressure inside the pebbles erodes the nucleus into the observed dust, which is composed of refractories, H2O ice and CO2 ice. (ii) The CO-driven activity onset occurs up to heliocentric distances of 85 au, depending on the spin orientation of the comet nucleus. (iii) The activity onset observed at ≈26 au suggests a low obliquity of the nucleus spin axis with activity in a polar summer. (iv) At 14 au, the smallest size of the ejected dust is ≈0:1 mm, consistent with observations. (v) The observed dust-loss rate of ≈200 kg s-1 implies a fallout ≥30%, a nucleus surface active area ≥10 km2, a CO-gas loss rate ≥10 kg s-1, and a dust-to-gas ratio ≤20. (vi) The CO-driven activity never stops if the average refractory-to-all-ices mass ratio in the nucleus is ≤4:5 for a nucleus all-ices-to-CO mass ratio ≈4, as observed in comets Hale-Bopp and Hyakutake. These results make comet C/2017K2 similar to the Rosetta target comet 67P/Churyumov-Gerasimenko. (vii) The erosion lifetime of cometary planetesimals is a factor 103 shorter than the timescale of catastrophic collisions. This means that the comets we observe today cannot be products of catastrophic collisions
Contributions about Etna activity - Report relative to the period: December 1999 - January 2000
No full textThe sensitivity of the size distribution to the grain dynamics: Simulation of the dust flux measured by GIOTTO at P/Halley
No full textA cometary dust emission model, based on rigorous keplerian dynamics, is developed and, for the first time, the dependence of fluence on the probability distribution of the dust ejection velocity vector is demonstrated. The results are compared with the fluences measured by the DIDSY experiment on board the GIOTTO spacecraft during the Halley's fly-by in 1986. A fit of the total fluence is obtained and an interpretation of the observed differences, between pre and post fly-by, is proposed. From the best-fitting process we conclude that the dust ejection from P/Halley was strongly anisotropic and mainly Sun-ward oriented with an angular dispersion of 18.4°, for the adopted Gaussian distribution. The most probable velocity at the fly-by is 50 ± 5 m s-1 for 1 mm sized grains and the power index of the velocity size-dependence is -0.5. Both these results agree with those of dust-gas drag models. Moreover, the dust velocity presents a wide dispersion (35 ±5 m s-1), which explains the velocity size-dependence derived by Neck-Line photometry. For grains larger than 20 fim, the power index of the differential size distribution is constant (a = -3.5 ± 0.2). Since a > -4, most of the dust mass is released in the form of large grains. The dust to gas ratio is x = 4 ± 1. The last two conclusions agree with the output of previous DIDSY fitting processes and are compatible with inverse dust tail models; they must be considered the best constrained results coming from the DIDSY experiment. Our results imply that future in-situ cometary experiments will have to measure both mass and velocity vector for each grain, in order to determine the dust size distribution
Orbital evolution of meteoroids from short period comets
No full textWe perform an accurate modelling of orbital evolution of dust grains taking into account both the ejection parameters derived from the analysis of the dust tail of each considered parent comet (Fulle, 1989A&A...217..283F), and the integration of the Newton equations in the context of a nine-body problem (Sun, seven major planets and the dust particle) plus solar radiation and wind forces. Among Short Period Comets (SPC) we have selected P/Schwassmann-Wachmann 1 (P/SW1) and P/Griegg-Skjellerup (P/GS), which represent two significantly different objects from a dynamical point of view. Dust from P/SW1 is dominated by Jupiter perturbations: after 2x10^4^-years, about 7% of the grains are ejected in hyperbolic orbits, 80% of the grains have the perihelion out of 4AU from the Sun, and only 1% of them reaches the Sun distance of 1AU, thus contributing to the inner zodiacal cloud. Dust from P/GS is dominated by the P-R drag, although large grains, due to their longer collapse lifetime, are sensitive to Jupiter perturbations. Therefore the Tisserand criterion represents a useful tool both to estimate the orbital evolution of grains larger than 100μm (i.e. the most likely canditates to replenish the zodiacal dust cloud, Gruen et al. 1985Icar...62..244G), and in distinguishing the parent sources of meteoroids collected with near Earth space experiments able to measure the impact velocity vectors. Jupiter perturbations oppose to the P-R drag forces and reduce significantly the contribution of SPC to the inner zodiacal dust: the simple sum of the dust mass contribution from each SPC may be an overestimate of their actual supply
The radial brightness dependence in the dust coma of comet P/Grigg-Skjellerup
No full textIn a previous paper (Fülle et al. 1993a) we have obtained a three-dimensional coma model of the short period comet P/Grigg-Skjellerup, by fitting ground-based images taken in coincidence with the fly-by of the GIOTTO spacecraft (GEM mission). Concerning the cometary dust environment, we confirmed the excess of large grains measured by the in-situ experiment GEM-DIDSY (McDonnell et al. 1993). In this letter we extend the comparison with space-born data by considering the results provided by the GEM-OPE experiment (Levasseur-Regourd 1993). The same model, once projected along the line of sight of GEM-OPE, is able to produce radial brightness slopes in quite good agreement with those obtained from the in-situ experiment. This result confirms the physical reliability of the proposed dust environment
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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