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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.

During the formation of terrestrial planets, volatile loss may occur through nebular processing, planetesimal differentiation, and planetary accretion. We investigate iron meteorites as an archive of volatile loss during planetesimal processing. The carbon contents of the parent bodies of magmatic iron meteorites are reconstructed by thermodynamic modeling. Calculated solid/molten alloy partitioning of C increases greatly with liquid S concentration, and inferred parent body C concentrations range from 0.0004 to 0.11 wt%. Parent bodies fall into two compositional clusters characterized by cores with medium and low C/S. Both of these require significant planetesimal degassing, as metamorphic devolatilization on chondrite-like precursors is insufficient to account for their C depletions. Planetesimal core formation models, ranging from closed-system extraction to degassing of a wholly molten body, show that significant open-system silicate melting and volatile loss are required to match medium and low C/S parent body core compositions. Greater depletion in C relative to S is the hallmark of silicate degassing, indicating that parent body core compositions record processes that affect composite silicate/iron planetesimals. Degassing of bare cores stripped of their silicate mantles would deplete S with negligible C loss and could not account for inferred parent body core compositions. Devolatilization during small-body differentiation is thus a key process in shaping the volatile inventory of terrestrial planets derived from planetesimals and planetary embryos.

After 25 years of laboratory research on protoplanetary dust agglomeration, a consistent picture of the various processes that involve colliding dust aggregates has emerged. Besides sticking, bouncing and fragmentation, other effects, like, e.g., erosion or mass transfer, have now been extensively studied. Coagulation simulations consistently show that μ m -sized dust grains can grow to mm- to cm-sized aggregates before they encounter the bouncing barrier, whereas sub- μ m -sized water-ice particles can directly grow to planetesimal sizes. For siliceous materials, other processes have to be responsible for turning the dust aggregates into planetesimals. In this article, these processes are discussed, the physical properties of the emerging dusty or icy planetesimals are presented and compared to empirical evidence from within and without the Solar System. In conclusion, the formation of planetesimals by a gravitational collapse of dust “pebbles” seems the most likely.

Many stars are surrounded by disks of dusty debris formed in the collisions of asteroids, comets, and dwarf planets, but is gas also released in such events? Observations at submillimeter wavelengths of the archetypal debris disk around β Pictoris show that 0.3% of a Moon mass of carbon monoxide orbits in its debris belt. The gas distribution is highly asymmetric, with 30% found in a single clump 85 astronomical units from the star, in a plane closely aligned with the orbit of the inner planet, β Pictoris b. This gas clump delineates a region of enhanced collisions, either from a mean motion resonance with an unseen giant planet or from the remnants of a collision of Mars-mass planets.

The existence of water in extrasolar planetary systems is of great interest because it constrains the potential for habitable planets and life. We have identified a circumstellar disk that resulted from the destruction of a water-rich and rocky extrasolar minor planet. The parent body formed and evolved around a star somewhat more massive than the Sun, and the debris now closely orbits the white dwarf remnant of the star. The stellar atmosphere is polluted with metals accreted from the disk, including oxygen in excess of that expected for oxide minerals, indicating that the parent body was originally composed of 26% water by mass. This finding demonstrates that water-bearing planetesimals exist around A-and F-type stars that end their lives as white dwarfs.

The textures of chondritic meteorites demonstrate that they are not the products of planetary melting processes. This has long been interpreted as evidence that chondrite parent bodies never experienced large-scale melting. As a result, the paleomagnetism of the CV carbonaceous chondrite Allende, most of which was acquired after accretion of the parent body, has been a long-standing mystery. The possibility of a core dynamo like that known for achondrite parent bodies has been discounted because chondrite parent bodies are assumed to be undifferentiated. Resolution of this conundrum requires a determination of the age and timescale over which Allende acquired its magnetization. Here, we report that Allende's magnetization was acquired over several million years (Ma) during metasomatism on the parent planetesimal in a >

Earth's inventory of principle volatiles C, H, N, and S is a legacy of its early stages of accretion and differentiation. Elemental ratios (C/H, C/N, C/S) are powerful tools for understanding early processing of Earth's volatiles, as they monitor relative fractionations through important processes even when absolute concentrations are less well defined. The C/H ratio of the bulk silicate Earth (BSE), defined from surface reservoirs and minimally degassed oceanic basalts is 1.3 ± 0.3, which is 5-15 times lower than the C/H ratio of carbonaceous and enstatite chondrites and 2-5 times lower than ordinary chondrites. The BSE C/N ratio is superchondritic (40 ± 8; Bergin et al. 2015) while the C/S ratio (0.49 ± 0.14) is nearly chondritic. Successful models of volatile acquisition and processing must account for the effects of accretion, core formation, and atmospheric loss on all three of these ratios.Simple models of equilibration between a magma ocean, the overlying atmosphere, and alloy destined for the core are used to explore the influence of core formation and atmospheric loss on major volatile concentrations and ratios. Among major volatile elements, C is most siderophile, and consequently core formation leaves behind a non-metallic Earth with low C/H, C/N, and C/S ratios compared to originally accreted materials and compared to the BSE. Compared to the predicted effect of early differentiation, the relatively high C/X ratios of the BSE argue in part that significant volatile replenishment occurred after core formation ceased, possibly in the form of a late veneer. However, a late veneer with chondritic composition is insufficient to explain the pattern of major volatile enrichments and depletions because BSE C/H and C/N ratios are non-chondritic. The C/H ratio is best explained if an appreciable fraction of H in the BSE predates delivery in the late veneer. Although atmospheric blow-off is an attractive explanation for the high C/N ratio, available data for C and N solubility and metal/silicate partitioning suggest that atmospheric blow-off cannot counter core formation to produce subchondritic C/N. Thus, unless virtually all core-forming metal segregated prior to volatile accretion (or relative C and N solubilities are appreciably different from those assumed here), the BSE C/N ratio suggests that accreting materials had elevated ratios compared to carbonaceous chondrites. One possibility is that a fraction of Earth's volatiles accreted from differentiated C-rich planetesimals similar to the ureilite parent body. Reconciling C/H, C/N, and C/S ratios of the BSE simultaneously presents a major challenge that almost certainly involves a combination of parent body processing, core formation, catastrophic atmospheric loss, and partial replenishment by a late veneer. The chondritic C/S ratio of the BSE and relatively low S content of the BSE constrains the BSE C concentration, but a potential complicating factor in interpreting the BSE C/S ratio is the possible effect of segregation of an S-rich matte to the core during the later parts of core-mantle differentiation.

The objective of the Psyche Magnetometry Investigation is to test the hypothesis that asteroid (16) Psyche formed from the core of a differentiated planetesimal. To address this, the Psyche Magnetometer will measure the magnetic field around the asteroid to search for evidence of remanent magnetization. Paleomagnetic measurements of meteorites and dynamo theory indicate that a diversity of planetesimals once generated dynamo magnetic fields in their metallic cores. Likewise, the detection of a strong magnetic moment ( > 2 × 10 14 Am 2 ) at Psyche would likely indicate that the body once generated a core dynamo, implying that it formed by igneous differentiation. The Psyche Magnetometer consists of two three-axis fluxgate Sensor Units (SUs) mounted 0.7 m apart along a 2.15-m long boom and connected to two Electronics Units (EUs) located within the spacecraft bus. The Magnetometer samples at up to 50 Hz, has a range of ± 80 , 000 nT , and an instrument noise of 39 pT axis − 1 3 σ integrated over 0.1 to 1 Hz. The two pairs of SUs and EUs provide redundancy and enable gradiometry measurements to suppress noise from flight system magnetic fields. The Magnetometer will be powered on soon after launch and acquire data for the full duration of the mission. The ground data system processes the Magnetometer measurements to obtain an estimate of Psyche’s dipole moment.