Academic literature on the topic 'Thermal Evolution Comets'

Questions regarding how primordial or pristine the comets of the solar system are have been an ongoing controversy. In this review, we describe comets’ physical evolution from dust and ice grains in the solar nebula to the contemporary small bodies in the outer solar system. This includes the phases of dust agglomeration, the formation of planetesimals, their thermal evolution and the outcomes of collisional processes. We use empirical evidence about comets, in particular from the Rosetta Mission to comet 67P/Churyumov–Gerasimenko, to draw conclusions about the possible thermal and collisional evolution of comets.

Abstract Evidence for cometary activity beyond Jupiter’s and Saturn’s orbits—such as that observed for Centaurs and long-period comets—suggests that the thermal processing of comet nuclei starts long before they enter the inner solar system, where they are typically observed and monitored. Such observations raise questions as to the depth of unprocessed material and whether the activity of Jupiter-family comets (JFCs) can be representative of any primitive material. Here we model the coupled thermal and dynamical evolution of JFCs, from the moment they leave their outer solar system reservoirs until their ejection into interstellar space. We apply a thermal evolution model to a sample of simulated JFCs obtained from dynamical simulations that successfully reproduce the orbital distribution of observed JFCs. We show that due to the stochastic nature of comet trajectories toward the inner solar system, all simulated JFCs undergo multiple heating episodes resulting in significant modifications of their initial volatile contents. A statistical analysis constrains the extent of such processing. We suggest that primordial condensed hypervolatile ices should be entirely lost from the layers that contribute to cometary activity observed today. Our results demonstrate that understanding the orbital (and thus, heating) history of JFCs is essential when putting observations in a broader context.

ABSTRACT Models of the zodiacal cloud’s thermal emission and sporadic meteoroids suggest Jupiter-family comets (JFCs) as the dominant source of interplanetary dust. However, comet sublimation is insufficient to sustain the quantity of dust presently in the inner Solar system, suggesting that spontaneous disruptions of JFCs may supply the zodiacal cloud. We present a model for the dust produced in comet fragmentations and its evolution. Using results from dynamical simulations, the model follows individual comets drawn from a size distribution as they evolve and undergo recurrent splitting events. The resulting dust is followed with a kinetic model which accounts for the effects of collisional evolution, Poynting–Robertson drag, and radiation pressure. This allows to model the evolution of both the size distribution and radial profile of dust, and we demonstrate the importance of including collisions (both as a source and sink of dust) in zodiacal cloud models. With physically motivated free parameters this model provides a good fit to zodiacal cloud observables, supporting comet fragmentation as the plausibly dominant dust source. The model implies that dust in the present zodiacal cloud likely originated primarily from disruptions of ∼50-km comets, since larger comets are ejected before losing all their mass. Thus much of the dust seen today was likely deposited as larger grains ∼0.1 Myr in the past. The model also finds the dust level to vary stochastically; e.g. every ∼50 Myr large (>100 km) comets with long dynamical lifetimes inside Jupiter cause dust spikes with order of magnitude increases in zodiacal light brightness lasting ∼1 Myr. If exozodiacal dust is cometary in origin, our model suggests it should be similarly variable.

AbstractThe birth-endowed organic fraction of the newly formed (hot) Earth was destroyed by thermal decomposition during the cooling epoch. After Earth cooled sufficiently, an early organic inventory was likely replenished by the impact of comets and asteroids — a process which continues even today. The present organic composition of comets and asteroids can provide information relevant to this secondary organic seeding of the planets, for comparison with scenarios leading to self-replicating organic entities. Although impacts no longer deliver organics in significant quantities, compared with the existing terrestrial inventory, small bodies can deliver their organics intact to Earth‘s surface while giant impacts may cause punctuated extinction of living species (and create opportunities for renewed speciation). Hence, the exogenous organic flux has great importance for life’s origins and terminations. Current knowledge of the organic composition of comets is reviewed, and recent progress in our understanding of the chemical evolution of organic material from its formation to its incorporation into comets and later into planets is outlined. The need for detailed quantitative chemical analysis of material obtained directly from the cometary nucleus is indicated.

Abstract One of the common approximations in long-term evolution studies of small bodies is the use of circular orbits averaging the actual eccentric ones, facilitating the coupling of processes with very different timescales, such as the orbital changes and the thermal processing. Here we test a number of averaging schemes for elliptic orbits in the context of the long-term evolution of comets, aiming to identify the one that best reproduces the elliptic orbits’ heating patterns and the surface and subsurface temperature distributions. We use a simplified thermal evolution model applied on simulated comets both on elliptic and on their equivalent averaged circular orbits, in a range of orbital parameter space relevant to the inner solar system. We find that time-averaging schemes are more adequate than spatial-averaging ones. Circular orbits created by means of a time average of the equilibrium temperature approximate efficiently the subsurface temperature distributions of elliptic orbits in a large area of the orbital parameter space, rendering them a powerful tool for averaging elliptic orbits.

Abstract It was recently proposed that there exists a “gateway” in the orbital parameter space through which Centaurs transition to Jupiter-family comets (JFCs). Further studies have implied that the majority of objects that eventually evolve into JFCs should leave the Centaur population through this gateway. This may be naively interpreted as gateway Centaurs being pristine progenitors of JFCs. This is the point we want to address in this work. We show that the opposite is true: gateway Centaurs are, on average, more thermally processed than the rest of the population of Centaurs crossing Jupiter’s orbit. Using a dynamically validated JFC population, we find that only ∼20% of Centaurs pass through the gateway prior to becoming JFCs, in accordance with previous studies. We show that more than half of JFC dynamical clones entering the gateway for the first time have already been JFCs—they simply avoided the gateway on their first pass into the inner solar system. By coupling a thermal evolution model to the orbital evolution of JFC dynamical clones, we find a higher than 50% chance that the layer currently contributing to the observed activity of gateway objects has been physically and chemically altered, due to previously sustained thermal processing. We further illustrate this effect by examining dynamical clones that match the present-day orbits of 29P/Schwassmann-Wachmann 1, P/2019 LD2 (ATLAS), and P/2008 CL94 (Lemmon).

Context. Comets are conglomerates of ice and dust particles, the latter of which encode information on changes in the radiative and thermal environments. Dust displays distinctive scattered and thermal radiation in the visible and mid-infrared (MIR) wavelengths, respectively, based on its inherent characteristics. Aims. We aim to identify a possible correlation between the properties of scattered and thermal radiation from dust and the principal dust characteristics responsible for this relationship, and therefrom gain insights into comet evolution. Methods. We use the NASA/PDS archival polarimetric data on cometary dust in the red (0.62−0.73 μm) and K (2.00−2.39 μm) domains to leverage the relative excess of the polarisation degree of a comet to the average trend at the given phase angle (Pexcess) as a metric of the dust’s scattered light characteristics. The flux excess of silicate emissions to the continuum around 10 μm (FSi/Fcont) is adopted from previous studies as a metric of the dust’s MIR feature. Results. The two observables – Pexcess and FSi/Fcont – show a positive correlation when Pexcess is measured in the K domain (Spearman’s rank correlation coefficient ρ = 0.71−0.19+0.10). No significant correlation was identified in the red domain (ρ = 0.13−0.15+0.16). The gas-rich comets have systematically weaker FSi/Fcont than the dust-rich ones, and yet both groups retain the same overall tendency with different slope values. Conclusions. The observed positive correlation between the two metrics indicates that composition is a peripheral factor in characterising the dust’s polarimetric and silicate emission properties. The systematic difference in FSi/Fcont for gas-rich versus dust-rich comets would instead correspond to the difference in their dust size distribution. Hence, our results suggest that the current MIR spectral models of cometary dust, which search for a minimum χ2 fit by considering various dust properties simultaneously, should prioritise the dust size and porosity over the composition. With light scattering being sensitive to different size scales in two wavebands, we expect the K-domain polarimetry to be sensitive to the properties of dust aggregates, such as size and porosity, which might have been influenced by evolutionary processes. On the other hand, the red-domain polarimetry reflects the characteristics of sub-micrometre constituents in the aggregate.

AbstractThe structural and thermodynamical properties of water ice and ice mixtures containing CO, CO2, CH4, and NH3 are thought to be important for the evolution of cometary nuclei. Based on recent laboratory studies performed by several groups, an overview is given of the properties of various ices condensed at low temperatures and of their evolution during heating up to a temperature of about 200 K, typical of the perihelion temperature of a comet such as P/Halley. It is shown that the porous surface of amorphous water ice plays an important role in the retention of other volatiles. The kinetics of formation and of decomposition of clathrate hydrates are discussed. The molecular hydrates formed by NH3 are briefly presented, and the possibility of their occurrence in comet nuclei is discussed. With special attention drawn to amorphous ices and clathrate hydrates, a qualitative discussion of the influence of the physical properties of various types of ices on the thermal behavior of comet nuclei and on gas production rates of comets is presented.

ABSTRACT Radar observations provide crucial insights into the formation and dynamical evolution of comets. This ability is constrained by our knowledge of the dielectric and textural properties of these small-bodies. Using several observations by Rosetta as well as results from the Earth-based Arecibo radio telescope, we provide an updated and comprehensive dielectric and roughness description of Comet 67P/CG, which can provide new constraints on the radar properties of other nuclei. Furthermore, contrary to previous assumptions of cometary surfaces being dielectrically homogeneous and smooth, we find that cometary surfaces are dielectrically heterogeneous ( εr′≈1.6–3.2), and are rough at X- and S-band frequencies, which are widely used in characterization of small-bodies. We also investigate the lack of signal broadening in CONSERT observations through the comet head. Our results suggest that primordial building blocks in the subsurface are either absent, smaller than the radar wavelength, or have a weak dielectric contrast (Δ εr′). To constrain this ambiguity, we use optical albedo measurements by the OSIRIS camera of the freshly exposed subsurface after the Aswan cliff collapse. We find that the hypothetical subsurface blocks should have |Δ εr′|≳0.15, setting an upper limit of ∼ 1 m on the size of 67P/CG's primordial building blocks if they exist. Our analysis is consistent with a purely thermal origin for the ∼ 3 m surface bumps on pit walls and cliff-faces, hypothesized to be high-centred polygons formed from fracturing of the sintered shallow ice-bearing subsurface due to seasonal thermal expansion and contraction. Potential changes in 67P/CG's radar reflectivity at these at X- and S-bands can be associated with large-scale structural changes of the nucleus rather than small-scale textural ones. Monitoring changes in 67P/CG's radar properties during repeated close-approaches via Earth-based observations can constrain the dynamical evolution of its cometary nucleus.