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Register a new userComet formation in collapsing pebble clouds. What cometary bulk density implies for the cloud mass and dust-to-ice ratio
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Comets are remnants of the icy planetesimals that formed beyond the ice line in the Solar Nebula. Growing from micrometre-sized dust and ice particles to km-sized objects is, however, difficult because of growth barriers and time scale constraints. The gravitational collapse of pebble clouds that formed through the streaming instability may provide a suitable mechanism for comet formation. We study the collisional compression of cm-sized porous ice/dust-mixed pebbles in collapsing pebble clouds. For this, we developed a collision model for pebbles consisting of a mixture of ice and dust, characterised by their dust-to-ice mass ratio. Using the final compression of the pebbles, we constrain combinations of initial cloud mass, initial pepple porosity, and dust-to-ice ratio that lead to cometesimals which are consistent with observed bulk properties of cometary nuclei. We find that observed high porosity and low density of ~0.5 g/cc of comet nuclei can only be explained if comets formed in clouds with mass approximately M>1e18 g. Lower mass clouds would only work if the pebbles were initially very compact. Furthermore, the dust-to-ice ratio must be in the range of between 3 and 9 to match the observed bulk properties of comet nuclei. (abridged version)
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Planetesimals are compact astrophysical objects roughly 1-1000 km in size, massive enough to be held together by gravity. They can grow by accreting material to become full-size planets. Planetesimals themselves are thought to form by complex physical processes from small grains in protoplanetary disks. The streaming instability (SI) model states that mm/cm-size particles (pebbles) are aerodynamically collected into self-gravitating clouds which then directly collapse into planetesimals. Here we analyze ATHENA simulations of the SI to characterize the initial properties (e.g., rotation) of pebble clouds. Their gravitational collapse is followed with the PKDGRAV N-body code, which has been modified to realistically account for pebble collisions. We find that pebble clouds rapidly collapse into short-lived disk structures from which planetesimals form. The planetesimal properties depend on the clouds scaled angular momentum, l=L/(M R_H^2 Omega, where L and M are the angular momentum and mass, R_H is the Hill radius, and Omega is the orbital frequency. Low-l pebble clouds produce tight (or contact) binaries and single planetesimals. Compact high-l clouds give birth to binary planetesimals with attributes that closely resemble the equal-size binaries found in the Kuiper belt. Significantly, the SI-triggered gravitational collapse can explain the angular momentum distribution of known equal-size binaries -- a result pending verification from studies with improved resolution. About 10% of collapse simulations produce hierarchical systems with two or more large moons. These systems should be found in the Kuiper belt when observations reach the threshold sensitivity.
To date, at least three comets -- 2I/Borisov, C/2016 R2 (PanSTARRS), and C/2009 P1 (Garradd) -- have been observed to have unusually high CO concentrations compared to water. We attempt to explain these observations by modeling the effect of drifting solid (ice and dust) material on the ice compositions in protoplanetary disks. We find that, independent of the exact disk model parameters, we always obtain a region of enhanced ice-phase CO/H2O that spreads out in radius over time. The inner edge of this feature coincides with the CO snowline. Almost every model achieves at least CO/H2O of unity, and one model reaches a CO/H2O ratio > 10. After running our simulations for 1 Myr, an average of 40% of the disk ice mass contains more CO than H2O ice. In light of this, a population of CO ice enhanced planetesimals are likely to generally form in the outer regions of disks, and we speculate that the aforementioned CO-rich comets may be more common, both in our own Solar System and in extrasolar systems, than previously expected.
Before Rosetta, the space missions Giotto and Stardust shaped our view on cometary dust, supported by plentiful data from Earth based observations and interplanetary dust particles collected in the Earths atmosphere. The Rosetta mission at comet 67P/Churyumov-Gerasimenko was equipped with a multitude of instruments designed to study cometary dust. While an abundant amount of data was presented in several individual papers, many focused on a dedicated measurement or topic. Different instruments, methods, and data sources provide different measurement parameters and potentially introduce different biases. This can be an advantage if the complementary aspect of such a complex data set can be exploited. However, it also poses a challenge in the comparison of results in the first place. The aim of this work therefore is to summarise dust results from Rosetta and before. We establish a simple classification as a common framework for inter-comparison. This classification is based on a dust particles structure, porosity, and strength as well as its size. Depending on the instrumentation, these are not direct measurement parameters but we chose them as they were the most reliable to derive our model. The proposed classification already proved helpful in the Rosetta dust community and we propose to take it into consideration also beyond. In this manner we hope to better identify synergies between different instruments and methods in the future.
We model the infrared emission from zodiacal dust detected by the IRAS and COBE missions, with the aim of estimating the relative contributions of asteroidal, cometary and interstellar dust to the zodiacal cloud. Our most important result is the detection of an isotropic component of foreground radiation due to interstellar dust. The dust in the inner solar system is known to have a fan-like distribution. If this is assumed to extend to the orbit of Mars, we find that cometary, asteroidal and interstellar dust account for 70%, 22% and 7.5% of the dust in the fan. We find a worse fit if the fan is assumed to extend to the orbit of Jupiter. Our model is broadly consistent with the analysis by Divine (1993) of interplanetary dust detected by Ulysses and other spacecraft. Our estimate of the mass-density of interstellar dust in the inner solar system is consistent with estimates from Ulysses at 1.5 au, but is an order of magnitude higher than Ulysses estimates at r > 4 au. Only 1% of the zodiacal dust arriving at the earth would be interstellar, in our model. Our models can be further tested by ground-based kinematical studies of the zodiacal cloud, which need to extend over a period of years to monitor solar cycle variations in interstellar dust, by dynamical simulations, and by in situ measurements from spacecraft.
We use the gravitational instability formation scenario of cometesimals to derive the aggregate size that can be released by the gas pressure from the nucleus of comet 67P/Churyumov-Gerasimenko for different heliocentric distances and different volatile ices. To derive the ejected aggregate sizes, we developed a model based on the assumption that the entire heat absorbed by the surface is consumed by the sublimation process of one volatile species. The calculations were performed for the three most prominent volatile materials in comets, namely, H_20 ice, CO_2 ice, and CO ice. We find that the size range of the dust aggregates able to escape from the nucleus into space widens when the comet approaches the Sun and narrows with increasing heliocentric distance, because the tensile strength of the aggregates decreases with increasing aggregate size. The activity of CO ice in comparison to H_20 ice is capable to detach aggregates smaller by approximately one order of magnitude from the surface. As a result of the higher sublimation rate of CO ice, larger aggregates are additionally able to escape from the gravity field of the nucleus. Our model can explain the large grains (ranging from 2 cm to 1 m in radius) in the inner coma of comet 67P/Churyumov-Gerasimenko that have been observed by the OSIRIS camera at heliocentric distances between 3.4 AU and 3.7 AU. Furthermore, the model predicts the release of decimeter-sized aggregates (trail particles) close to the heliocentric distance at which the gas-driven dust activity vanishes. However, the gas-driven dust activity cannot explain the presence of particles smaller than ~1 mm in the coma because the high tensile strength required to detach these particles from the surface cannot be provided by evaporation of volatile ices. These smaller particles can be produced for instance by spin-up and centrifugal mass loss of ejected larger aggregates.
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