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The brief review summarizes data on the chemical and mineral composition, as well as on the physical properties, of the first extrasolar comet 2I/Borisov, obtained from observations that were carried out from September 2019 to the end of March 2020. It is noted that the qualitative chemical composition of the volatile and mineral components comet 2I/Borisov is similar to the composition of comets in the Solar System, but there are differences that indicate the specific conditions for the formation of its nucleus in a circumstellar gas and dust disk. Different release rates of CO and H2O molecules in the vicinity of perihelion indicate the possible heterogeneity of the comet's nucleus, which was formed from more homogeneous ice blocks, but differing in composition. These constituent blocks could have formed over a wide range of radial distances: from the snow line of H2O to the CO snow line. Their accumulation in the comet's nucleus indicates large-scale mixing of protocometary bodies in the circumstellar disk. No spectra of finely crystalline magnesium silicates were found in cometary coma of 2I/Borisov, which can be interpreted as the absence of a significant amount of gas and dust transfer from the inner hot regions of the disk to the outside, into the zone of formation of protocometary bodies. [ABSTRACT FROM AUTHOR]

In this paper, we develop previous studies that considered a free molecular gas flow through a dust porous mantle of a cometary nucleus. Before, we considered various types of both homogeneous and heterogeneous layers built of nonintersecting spheres, including the layers containing microscopic cracks and inner cavities. At the same time, data from the Rosetta space mission provide convincing evidence that the near-surface layer is composed of porous aggregates rather than homogeneous solids. In this study, we propose models, in which the layer is constructed of porous aggregates formed by ballistic agglomeration. The effective porosity of the model layers is in a range of the values resulting from the analysis of observations of comet Churyumov–Gerasimenko. With the test-particle method, we quantitatively estimated the distribution functions for the free paths, the layer's permeability, and the other effective kinetic characteristics of sublimation products that passed through a nonisothermal porous layer. In addition, we estimated the volume adsorption of the visible solar light in the near-surface absorbing layer. For all of the considered transport characteristics, we present approximating expressions that may effectively be used in nonstationary thermophysical models of the physics of a cometary nucleus. [ABSTRACT FROM AUTHOR]

Cometary dust is refractory particles lifted and ejected by sublimation of volatiles from the surface of comets with typical diameters from sub-micrometers to centimeters. These particles distribute around and recede from cometary nuclei and are illuminated by the Sun, forming the observable dust comae and tails. The fundamental characteristics of cometary dust, such as their size and size distribution, ejection velocity from the nuclei, etc., are of great significance to understanding the formation and evolution of comets. This paper presents a method, the Machine Learning Based Dynamic Parameter Fitting method (MLDPF method), for deriving the fundamental properties of cometary dust based on dynamical simulation and machine learning. In this process, the Monte Carlo method is used to generate dust particles in the assumed parameter space and solve for the spatial distributions of dust at the times of observations according to dynamical models. Then, we use these simulations to train a CNN (Convolutional Neural Networks) model, and finally fit the observed photos to derive the parameters of the cometary dust. Using this approach, we analyzed the ground-based images of Comet 103P/Hartley collected in the visible-band. The dust ejected from 103P is dominated by micro-particle is 2.11 ± 0.41 μm in radius, and follows an assumed exponential dust size distribution with a coefficient of −4.26 ± 1.41. The initial ejection velocity of dust is 87 ± 13.4 m/s. The dust producing rate is about 11.35 × 1012 s−1 according to the Afρ parameter obtained from the optical photos and the best-fit parameters. The dust emission is likely to be solar insolation dependent. The machine learning and the fitting process were able to converge to a set of solutions that are in good agreement with the previous analyses in the literature. The MLDPF method can establish a cometary dust parameter-image model by machine learning, in which training and test set of the model were calculated based on simulated images; and then the model can be applied to real images. It has a wide range of applicability to different comets and can be used to predict comet morphology. This method also has the scalability of parameters and can be used for the study of cometary dust under more complex parameters and dynamics. [ABSTRACT FROM AUTHOR]

A wide range of evidence for pointing to our cosmic origins is close to the point of being overwhelming. Yet the long-entrenched paradigm of Earth-centered biology appears to prevail in scientific culture. A matter of crucial importance is to carry out a decisive experiment that is long overdue—establishing empirically beyond any doubt that extraterrestrial microbiota reaches the surface of the Earth at the present day. Such an experiment may of course happen naturally by the appearance of pandemics of new disease as discussed in an earlier chapter. [ABSTRACT FROM AUTHOR]

During ESA's Rosetta science mission, the COSIMA instrument collected dust particles in the coma of Comet 67P/Churyumov-Gerasimenko during two years near the comet's nucleus. The largest particles are about 1 m m in size. The collection process involved a low velocity impact on porous gold-black surfaces, often resulting in breakup, from which information on structural properties has previously been derived (Langevin et al., 2016). However, some of the particles were collected with little damage, but fragmented due to charging during subsequent secondary ion mass spectrometry. This report shows that the details of this electrical fragmentation support the concept of the existence of stable units with sizes of tens of μ m within the incoming cometary dust particles prior to collection, possibly representing remnants of the early accretion processes. [Display omitted] • We collected dust in the near coma of Comet 67P/Churyumov-Gerasimenko. • Dust particles become charged when exposed to the ion beam of the COSIMA secondary ion mass spectrometer and are fragmented by electrical forces. • The results confirm the existence of stable sub-units of several tens of μ m in size within the cometary matter. • Consequently, 67/P refractory cometary matter can be considered pristine on size scales of tens of μ m and smaller. • These sub-units are possible remnants of the early accretion processes. [ABSTRACT FROM AUTHOR]

We consider the estimates of the main forces acting on dust particles near a cometary nucleus. On the basis of these estimates, the motion of dust particles of different structure and mass is analyzed. We consider the following forces: (1) the cometary nucleus gravity, (2) the solar radiation pressure, and (3) the drag on dust particles by a flow of gas produced in the sublimation of cometary ice. These forces are important for modeling the motion of dust particles relative to the cometary nucleus and may substantially influence the dust transfer over its surface. In the simulations, solid silicate spheres and homogeneous ballistic aggregates are used as model particles. Moreover, we propose a technique to build hierarchic aggregates—a new model of quasi-spherical porous particles. A hierarchic type of aggregates makes it possible to model rather large dust particles, up to a millimeter in size and larger, while no important requirements for computer resources are imposed. We have shown that the properties of such particles differ from those of classical porous ballistic aggregates, which are usually considered in the cometary physics problems, and considering the microscopic structure of particles is of crucial significance for the analysis of the observational data. With the described models, we study the dust dynamics near the nucleus of comet 67P/Churyumov-Gerasimenko at an early stage of the Rosetta probe observations when the comet was approximately at 3.2 AU from the Sun. The interrelations between the main forces acting on dust aggregates at difference distances from the nucleus have been obtained. The dependence of the velocity of dust aggregates on their mass has been found. The numerical modeling results and the data of spaceborne observations with the Grain Impact Analyzer and Dust Accumulator (GIADA) and the Cometary Secondary Ion Mass Analyzer (COSIMA) onboard the Rosetta probe are compared at a quantitative level. [ABSTRACT FROM AUTHOR]

Recent in-situ studies of the environment of comet 67P/Churyumov–Gerasimenko by the dust instruments onboard the Rosetta spacecraft have indicated a complex structure of cometary dust particles. The majority of those particles appeared to be large aggregates of hierarchical structure, i.e. aggregates of particles, which, in turn, were aggregates of smaller particles. This confirmed an earlier hypothesis that dust particles in protoplanetary disks grow under hierarchical growth process. Thus, it is very likely that hierarchical aggregates are common type of natural dust particles. In this paper, we present results of computer simulations of light scattering by a variety of hierarchical aggregates to determine how their structure affects their brightness and polarization phase curves as well as photometric and polarimetric color, and albedo. The computations were done using Multi-Sphere T-Matrix method. Our results show that the type of hierarchical structure of aggregates, specified by the number of particles at each level of hierarchy, causes variations in their light-scattering characteristics, which noticeably exceed the variations caused by different configuration of monomers in the aggregates of the same hierarchical structure. Although we could not find any regularities in the brightness and polarization dependence on the structure of the aggregates, our results clearly show that not only composition or size of aggregates, but also their specific structure should be carefully examined when light scattering by cometary or any other type of dust is modeled. Specifically, we may need to reconsider modeling the cometary dust particles using simple ballistic particle-cluster and cluster-cluster aggregates. [ABSTRACT FROM AUTHOR]

Numerical analogues of non-fractal porous cometary dust particles are generated using a recently introduced model of open-pore solid particles. The refractive index of amorphous silicate is considered and the particle sizes follow a Sekanina-Hanner distribution within the size-range (0.01 μ m- 1 μ m). Using the Discrete Dipole Approximation (DDA) method, we calculate the thermal re-emission spectrum of a population of porous particles in the spectral region 0.2–39.8 μ m and the equilibrium temperature of the particles (when placed at 1 AU distance from Sun). Porosity tends to reduce the temperature of the particles because porous particles radiate heat more effectively than dense particles. A characteristic particle size is determined for which the particle temperature is maximum regardless of the porosity. The temperature of a cloud of polydisperse particles is more homogeneous for highly porous particles. These results are compared with those generated by Mie theory where the Bruggeman mixing rule is applied (EMT method). When agreement between the DDA and the EMT results is achieved, simple approximations are derived from the EMT to analyze and make clear the various behaviors of the spectral thermal re-emission revealed in the numerical data. In particular, the reason why the emission peak around 10 μ m is depressed by porosity and shifted to the smaller wavelength by a steeper particle-size distribution is explained. • a model of small (radius <1 ​μm) non-fractal porous particles made of amorphous silicate is studied in the context of cometary dust. • The profiles of the infrared silicate bands are discussed according to the porosity of the particles. • The temperature of a cloud of polydisperse porous particles is more homogeneous if the particles are porous. • Steeper decrease of the porous-particle size distribution shifts the infrared silicate peaks to shorter wavelengths. • The theory of the effective medium makes it possible to explain qualitatively the various behaviors due to porosity, and approximate formulas are given. [ABSTRACT FROM AUTHOR]

Soon after the arrival of the ROSETTA spacecraft at Comet 67/P Churyumov-Gerasimenko the onboard instrument COSIMA (“Cometary Secondary Ion Mass Analyzer”) collected a large number of cometary dust particles on targets from gold black of thickness between 10 and 30 μm. Inspection by its camera subsystem revealed that many of them consist of smaller units of typically some tens of micrometers in size. The collection process left the smaller dust particles in an essentially unaltered state whereas most particles larger than about 100 μm got fragmented into smaller pieces. Using the observed fragment size distributions, the present paper includes a first assessment of the strength for those dust particles that were disrupted upon impact. [ABSTRACT FROM AUTHOR]

The Micro-Imaging Dust Analysis System (MIDAS) atomic force microscope (AFM) onboard the Rosetta orbiter was the first such instrument launched into space in 2004. Designed only a few years after the technique was invented, MIDAS is currently orbiting comet 67P Churyumov–Gerasimenko and producing the highest resolution 3D images of cometary dust ever made in situ . After more than a year of continuous operation much experience has been gained with this novel instrument. Coupled with operations of the Flight Spare and advances in terrestrial AFM a set of “lessons learned” has been produced, cumulating in recommendations for future spaceborne atomic force microscopes. The majority of the design could be reused as-is, or with incremental upgrades to include more modern components (e.g. the processor). Key additional recommendations are to incorporate an optical microscope to aid the search for particles and image registration, to include a variety of cantilevers (with different spring constants) and a variety of tip geometries. [ABSTRACT FROM AUTHOR]

We introduce an inhomogeneous irregular-particle model for reproducing the typical light-scattering features of cometary dust such as the negative polarization near the backscattering direction, and the weak increase of the backscattering intensity. The model is based on the hierarchical Voronoi-partitioning and the algorithm provides fast generation of irregular particles with a flexible control of inhomogeneity. The input parameters of the model are refractive indices, their volumetric abundances, and the number of constituents on each level. The light-scattering properties of these particles with parameters relevant to cometary dust are solved by the volume-integral-equation method. The light-scattering features of inhomogeneous particles are compared with the mixtures of homogeneous particles, and particles with the refractive index obtained by the effective-medium approximation. We show that with the inhomogeneity size of order 0.2 μm, the different models produce qualitatively similar scattering features while some quantitative differences are observed which have an effect on the retrieved material composition of dust. [ABSTRACT FROM AUTHOR]

We present the results of a global coma morphology campaign for comet C/2012 S1 (ISON), which was organized to involve both professional and amateur observers. In response to the campaign, many hundreds of images, from nearly two dozen groups were collected. Images were taken primarily in the continuum, which help to characterize the behavior of dust in the coma of comet ISON. The campaign received images from January 12 through November 22, 2013 (an interval over which the heliocentric distance decreased from 5.1 AU to 0.35 AU), allowing monitoring of the long-term evolution of coma morphology during comet ISON׳s pre-perihelion leg. Data were contributed by observers spread around the world, resulting in particularly good temporal coverage during November when comet ISON was brightest but its visibility was limited from any one location due to the small solar elongation. We analyze the northwestern sunward continuum coma feature observed in comet ISON during the first half of 2013, finding that it was likely present from at least February through May and did not show variations on diurnal time scales. From these images we constrain the grain velocities to ~10 m s −1 , and we find that the grains spent 2–4 weeks in the sunward side prior to merging with the dust tail. We present a rationale for the lack of continuum coma features from September until mid-November 2013, determining that if the feature from the first half of 2013 was present, it was likely too small to be clearly detected. We also analyze the continuum coma morphology observed subsequent to the November 12 outburst, and constrain the first appearance of new features in the continuum to later than November 13.99 UT. [ABSTRACT FROM AUTHOR]

Comets C/2007 N3 (Lulin), C/2011 L4 (PANSTARRS) and 290P/Jager were observed at phase angles < 40 ○ ; between 2009 and 2014; with the 0.8 m Telescope at Haute-Provence Observatory (OHP) in France. The observations were polarimetric and mainly carried out in the red and near infra-red wavelength domains to reduce gaseous contaminations. The overall shape of the coma of comet Lulin is about circular without evident jet structure. The coma of comet C/2011 L4 is enlarged in the antisolar direction. Jets in a fan-like structure have been noticed between position angles 135°–245° in the treated intensity images. A correlated higher polarization region is noted on the corresponding polarization map. The shape of the coma of 290P/Jager is slightly elongated in the antisolar direction. Deeper negative polarization is found in the inner coma. The whole coma polarization values of these three comets correspond to the synthetic phase curve for comets at similar phase angles and wavelength. [ABSTRACT FROM AUTHOR]