Cometary Activity beyond the Planets

Recent observations show activity in long-period comet C/2017 K2 at heliocentric distances beyond the orbit of Uranus. With this as motivation, we constructed a simple model that takes a detailed account of gas transport modes and simulates the time-dependent sublimation of supervolatile ice from beneath a porous mantle on an incoming cometary nucleus. The model reveals a localized increase in carbon monoxide (CO) sublimation close to heliocentric distance r H = 150 au (local blackbody temperature ∼23 K), followed by a plateau and then a slow increase in activity toward smaller distances. This localized increase occurs as heat transport in the nucleus transitions between two regimes characterized by the rising temperature of the CO front at larger distances and nearly isothermal CO at smaller distances. As this transition is a general property of sublimation through a porous mantle, we predict that future observations of sufficient sensitivity will show that inbound comets (and interstellar interlopers) will exhibit activity at distances far beyond the planetary region of the solar system.

Structural analysis of the comet 45P/Honda based on isophote modeling

The work focuses on using the isophote method to construct a 45P/Honda comet model. At the same time, important problems were solved for modeling the physical surface of a comet and studying the structure of the cometary nucleus. This is due to the fact that, on the basis of modern studies of meteoroids, complex internal processes and dynamic phenomena on their surface have been discovered. The study of comet nuclei is of great importance, since, according to the theory of their formation, they were formed from the matter of the protoplanetary disk. Thus, modeling and analysis of the structure of various comets make it possible to create a more accurate theory of their evolution. This made it possible to evaluate the structural parameters more accurately and reliably. This allowed for the evaluation of the structural parameters more accurately and reliably. Isophotes of the nucleus, coma and tail of comet 45P/Honda were determined. Depending on the point where the comet is located on the trajectory of its orbit, one can see structural changes in the comet’s brightness from the nucleus to the peripheral region. Near the cometary nucleus, the isophotes are circular in shape. If in the center of the model the isophotes have a shape close to narrow rings, then elongations in the direction of the cometary tail and thickening of their structure appear towards the peripheral regions. Large and small tail rays can be distinguished, and the nucleus is well marked. In the future, the author’s method for modeling isophotes, developed in this work, will allow studying the structure of various cometary objects, and, based on the results, determine the degree of comet activity. On the other hand, about the development of the theory of dynamic processes and the evolution of the Solar system, one can use the data on changes in cometary activity in the process of its movement around the Sun.

AMBITION – comet nucleus cryogenic sample return

We describe the AMBITION project, a mission to return the first-ever cryogenically-stored sample of a cometary nucleus, that has been proposed for the ESA Science Programme Voyage 2050. Comets are the leftover building blocks of giant planet cores and other planetary bodies, and fingerprints of Solar System’s formation processes. We summarise some of the most important questions still open in cometary science and Solar System formation after the successful Rosetta mission. We show that many of these scientific questions require sample analysis using techniques that are only possible in laboratories on Earth. We summarize measurements, instrumentation and mission scenarios that can address these questions. We emphasize the need for returning a sample collected at depth or, still more challenging, at cryogenic temperatures while preserving the stratigraphy of the comet nucleus surface layers. We provide requirements for the next generation of landers, for cryogenic sample acquisition and storage during the return to Earth. Rendezvous missions to the main belt comets and Centaurs, expanding our knowledge by exploring new classes of comets, are also discussed. The AMBITION project is discussed in the international context of comet and asteroid space exploration.

The asteroid 162173 Ryugu: a cometary origin

The Japanese Hayabusa2 mission has revealed in detail the physical characteristics of the C-type asteroid 162173 Ryugu, in particular, its spinning top-shaped rubble pile structure [1] and the potentially extremely high organic content [2,3]. A widely-accepted formation scenario for Ryugu is catastrophic collision between larger asteroids and the subsequent slow gravitational accumulation of collisional debris [4,5]. However, the collisional re-accumulation scenario does not explain the origin of the abundant organic matter. An alternative scenario is that Ryugu is an extinct comet, which lost its icy components [2,6,3]. Here, the sublimation of water ice from a uniform porous cometary nucleus was numerically simulated until the refractory components, such as silicate rocks and organic matter were left behind as evaporative residues. Such a process represents the transformation from a comet to an asteroid. The spin-up related to the shrinking nucleus, associated with the water ice sublimation, was also calculated. The result of the calculation indicates that the cometary origin scenario can quantitatively account for all the features of Ryugu discussed above. We conclude that organic-rich spinning top-shaped rubble pile asteroids, such as Ryugu, are comet-asteroid transition objects or extinct comets.

The effect of varying porosity and inhomogeneities of the surface dust layer on the modelling of comet gas production.

We study how uncertainties in knowledge of the microstructure of the porous dust surface layer affect our estimates of effective gas production of comet 67P/Churyumov-Gerasimenko. We analyse the transport properties (distribution of free paths, mean pore size, permeability) for random densely packed porous layers. Bimodal layers, as well as various cases of inhomogeneous layers having cavities and cracks, are considered. Modelling is performed for various values of effective porosity and various particle sizes and thicknesses of the dust layer. The results of computer simulations are embedded in thermal models of the surface region of the cometary nucleus, including explicitly a radiation thermal conductivity and a resistance of the dust layer for the gas flow is used. The simulations show that the porosity variations within the range of values expected for the 67P can change the effective gas production by tens of per cent for thin layers, and can vary by several times with increasing layer thickness. These effects are manifested at all heliocentric distances where sublimation of water ice makes a significant contribution to the energy budget of the nucleus. Modelling also shows that in all considered cases the microscopic cavities and cracks have a minor effect on the effective gas production: changes in the presence and number of inhomogeneities in comparison with a uniform layer of the same effective porosity are negligible.

Constraining the orientation of the spin axes of extrasolar minor bodies 1I/2017 U1 (‘Oumuamua) and 2I/Borisov

Context. The orientation of the spin axis of a comet is defined by the values of its equatorial obliquity and its cometocentric longitude of the Sun at perihelion. These parameters can be computed from the components of the nongravitational force caused by outgassing if the cometary activity is well characterized. The trajectories of known interstellar bodies passing through the Solar System show nongravitational accelerations. Aims. The spin-axis orientation of 1I/2017 U1 (‘Oumuamua) remains to be determined; for 2I/Borisov, the already released results are mutually exclusive. In both cases, the values of the components of the nongravitational force are relatively well constrained. Here, we investigate – within the framework of the forced precession model of a nonspherical cometary nucleus – the orientation of the spin axes of ‘Oumuamua and 2I/Borisov using public orbit determinations that consider outgassing. Methods. We applied a Monte Carlo simulation using the covariance matrix method together with Monte Carlo random search techniques to compute the distributions of equatorial obliquities and cometocentric longitudes of the Sun at perihelion of ‘Oumuamua and 2I/Borisov from the values of the nongravitational parameters. Results. We find that the equatorial obliquity of ‘Oumuamua could be about 93°, if it has a very prolate (fusiform) shape, or close to 16°, if it is very oblate (disk-like). Different orbit determinations of 2I/Borisov gave obliquity values of 59° and 90°. The distributions of cometocentric longitudes were in general multimodal. Conclusions. Our calculations suggest that the most probable spin-axis direction of ‘Oumuamua in equatorial coordinates is (280°, +46°) if very prolate or (312°, −50°) if very oblate. Our analysis favors a prolate shape. For the orbit determinations of 2I/Borisov used here, we find most probable poles pointing near (275°, +65°) and (231°, +30°), respectively. Although our analysis favors an oblate shape for 2I/Borisov, a prolate one cannot be ruled out.

An Analysis of Regional H2O and CH3OH Production rates of Comet 67P from the MIRO Measurements

<p>The coma structure and gas production rates of H<sub>2</sub>O, CO, CH<sub>3</sub>OH, and NH<sub>3</sub> of comet 67P/Churyumov-Gerasimenko were investigated in detail by different instruments onboard the Rosetta spacecraft between August 2014 and September 2016. We analyzed the nadir-pointing microwave spectroscopic data from the MIRO experiment before and during the perihelion passage in June, July, and August 2015 in order to examine the solar zenith angle dependence of the gas production rates (Q) of H<sub>2</sub>O and CH<sub>3</sub>OH, respectively. From a detailed analysis of the spectral shapes of H<sub>2<sup>18</sup></sub>O at a frequency of 547.676 GHz and those of CH<sub>3</sub>OH at 553.146 GHz, the surface distributions of H<sub>2</sub>O and CH<sub>3</sub>OH can be mapped. We found that the Q-values generally increased with solar insolation. However, there also existed large variations that might be related to chemical heterogeneity of the cometary nucleus surface (or subsurface in individual geomorphological areas and at different spatial scales). A comparison of the CH<sub>3</sub>OH/H<sub>2</sub>O mixing ratio in the two lobes will also be attempted.</p>