Captured Small Solar System Bodies in the Ice Giant Region

This white paper advocates for the inclusion of small, captured Outer Solar system objects, found in the Ice Giant region in the next Decadal Survey. These objects include the Trojans and Irregular satellite populations of Uranus and Neptune. The captured small bodies provide vital clues as to the formation of our Solar system. They have unique dynamical situations, which any model of Solar system formation needs to explain. The major issue is that so few of these objects have been discovered, with very little information known about them. The purpose of this document is to prioritize further discovery and characterization of these objects. This will require the use of NASA and NSF facilities over the 2023-2032 decade, including additional support for analysis. This is in preparation for potential future in-situ missions in the following decades

The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets

This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sun’s centre, equal to half of Mercury’s perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics

CO2-driven surface changes in the Hapi region on Comet 67P/Churyumov-Gerasimenko

Between 2014 December 31 and 2015 March 17, the OSIRIS cameras on Rosetta documented the growth of a 140 m wide and 0.5 m deep depression in the Hapi region on Comet 67P/Churyumov-Gerasimenko. This shallow pit is one of several that later formed elsewhere on the comet, all in smooth terrain that primarily is the result of airfall of coma particles. We have compiled observations of this region in Hapi by the microwave instrument MIRO on Rosetta, acquired during October and November 2014. We use thermophysical and radiative transfer models in order to reproduce the MIRO observations. This allows us to place constraints on the thermal inertia, diffusivity, chemical composition, stratification, extinction coefficients, and scattering properties of the surface material, and how they evolved during the months prior to pit formation. The results are placed in context through long-term comet nucleus evolution modelling. We propose that: 1) MIRO observes signatures that are consistent with a solid-state greenhouse effect in airfall material; 2) CO2 ice is sufficiently close to the surface to have a measurable effect on MIRO antenna temperatures, and likely is responsible for the pit formation in Hapi observed by OSIRIS; 3) the pressure at the CO2 sublimation front is sufficiently strong to expel dust and water ice outwards, and to compress comet material inwards, thereby causing the near-surface compaction observed by CONSERT, SESAME, and groundbased radar, manifested as the 'consolidated terrain' texture observed by OSIRIS

Origin and Evolution of Saturn's Ring System

The origin and long-term evolution of Saturn's rings is still an unsolved problem in modern planetary science. In this chapter we review the current state of our knowledge on this long-standing question for the main rings (A, Cassini Division, B, C), the F Ring, and the diffuse rings (E and G). During the Voyager era, models of evolutionary processes affecting the rings on long time scales (erosion, viscous spreading, accretion, ballistic transport, etc.) had suggested that Saturn's rings are not older than 100 My. In addition, Saturn's large system of diffuse rings has been thought to be the result of material loss from one or more of Saturn's satellites. In the Cassini era, high spatial and spectral resolution data have allowed progress to be made on some of these questions. Discoveries such as the ''propellers'' in the A ring, the shape of ring-embedded moonlets, the clumps in the F Ring, and Enceladus' plume provide new constraints on evolutionary processes in Saturn's rings. At the same time, advances in numerical simulations over the last 20 years have opened the way to realistic models of the rings's fine scale structure, and progress in our understanding of the formation of the Solar System provides a better-defined historical context in which to understand ring formation. All these elements have important implications for the origin and long-term evolution of Saturn's rings. They strengthen the idea that Saturn's rings are very dynamical and rapidly evolving, while new arguments suggest that the rings could be older than previously believed, provided that they are regularly renewed. Key evolutionary processes, timescales and possible scenarios for the rings's origin are reviewed in the light of tComment: Chapter 17 of the book ''Saturn After Cassini-Huygens'' Saturn from Cassini-Huygens, Dougherty, M.K.; Esposito, L.W.; Krimigis, S.M. (Ed.) (2009) 537-57

On the origin and evolution of the material in 67P/Churyumov-Gerasimenko

International audiencePrimitive objects like comets hold important information on the material that formed our solar system. Several comets have been visited by spacecraft and many more have been observed through Earth- and space-based telescopes. Still our understanding remains limited. Molecular abundances in comets have been shown to be similar to interstellar ices and thus indicate that common processes and conditions were involved in their formation. The samples returned by the Stardust mission to comet Wild 2 showed that the bulk refractory material was processed by high temperatures in the vicinity of the early sun. The recent Rosetta mission acquired a wealth of new data on the composition of comet 67P/Churyumov-Gerasimenko (hereafter 67P/C-G) and complemented earlier observations of other comets. The isotopic, elemental, and molecular abundances of the volatile, semi-volatile, and refractory phases brought many new insights into the origin and processing of the incorporated material. The emerging picture after Rosetta is that at least part of the volatile material was formed before the solar system and that cometary nuclei agglomerated over a wide range of heliocentric distances, different from where they are found today. Deviations from bulk solar system abundances indicate that the material was not fully homogenized at the location of comet formation, despite the radial mixing implied by the Stardust results. Post-formation evolution of the material might play an important role, which further complicates the picture. This paper discusses these major findings of the Rosetta mission with respect to the origin of the material and puts them in the context of what we know from other comets and solar system objects

Editorial to the Topical Collection: Comets: Post 67P/Churyumov-Gerasimenko Perspectives

descripción no proporcionada por scopusWith funding from the Spanish government through the "María de Maeztu Unit of Excellence" accreditation (MDM-2017-0737

Rosetta/OSIRIS observations of the 67P nucleus during the April 2016 flyby: high-resolution spectrophotometry

From August 2014 to September 2016, the Rosetta spacecraft followed comet 67P/Churyumov-Gerasimenko along its orbit. After the comet passed perihelion, Rosetta performed a flyby manoeuvre over the Imhotep-Khepry transition in April 2016. The OSIRIS/Narrow-Angle-Camera (NAC) acquired 112 observations with mainly three broadband filters (centered at 480, 649, and 743 nm) at a resolution of up to 0.53 m/px and for phase angles between 0.095 degrees and 62 degrees. Aims. We have investigated the morphological and spectrophotometrical properties of this area using the OSIRIS/NAC high-resolution observations. Methods. We assembled the observations into coregistered color cubes. Using a 3D shape model, we produced the illumination conditions and georeference for each observation. We mapped the observations of the transition to investigate its geomorphology. Observations were photometrically corrected using the Lommel-Seeliger disk law. Spectrophotometric analyses were performed on the coregistered color cubes. These data were used to estimate the local phase reddening. Results. The Imhotep-Khepry transition hosts numerous and varied types of terrains and features. We observe an association between a feature's nature, its reflectance, and its spectral slopes. Fine material deposits exhibit an average reflectance and spectral slope, while terrains with diamictons, consolidated material, degraded outcrops, or features such as somber boulders present a lower-than-average reflectance and higher-than-average spectral slope. Bright surfaces present here a spectral behavior consistent with terrains enriched in water-ice. We find a phase-reddening slope of 0.064 +/- 0.001%/100 nm/degrees at 2.7 au outbound, similar to the one obtained at 2.3 au inbound during the February 2015 flyby. Conclusions. Identified as the source region of multiple jets and a host of water-ice material, the Imhotep-Khepry transition appeared in April 2016, close to the frost line, to further harbor several potential locations with exposed water-ice material among its numerous different morphological terrain units.© ESO 2019OSIRIS was built by a consortium of the Max-Planck-Institut fur Sonnensystemforschung, Gottingen, Germany, CISAS-University of Padova, Italy, Laboratoire d'Astrophysique de Marseille, France, Instituto de Astrofisica de Andalucia, CSIC, Granada, Spain, Research and Scientific Support Department of the European Space Agency, Noordwijk, The Netherlands, Instituto Nacional de Tecnica Aeroespacial, Madrid, Spain, Universidad Politechnica de Madrid, Spain, Department of Physics and Astronomy of Uppsala University, Sweden, and Institut fur Datentechnik und Kommunikationsnetze der Technischen Universitat Braunschweig, Germany. The support of the national funding agencies of Germany (DLR), France (CNES), Italy (ASI), Spain (MEC), Sweden (SNSB), and the ESA Technical Directorate is gratefully acknowledged. Rosetta is an ESA mission with contributions from its member states and NASA. Rosetta's Philae lander is provided by a consortium led by DLR, MPS, CNES, and ASI. The SPICE libraries and PDS resources are developed and maintained by NASA. The authors thank the referee and editors for their questions, remarks, and advices for the improvement of this article

Opposition effect on comet 67P/Churyumov-Gerasimenko using Rosetta-OSIRIS images

Aims. We aim to explore the behavior of the opposition effect as an important tool in optical remote sensing on the nucleus of comet 67P/Churyumov-Gerasimenko (67P), using Rosetta-OSIRIS images acquired in different filters during the approach phase, July-August 2014 and the close flyby images on 14 of February 2015, which contain the spacecraft shadow. Methods. We based our investigation on the global and local brightness from the surface of 67P with respect to the phase angle, also known as phase curve. The local phase curve corresponds to a region that is located at the Imhotep-Ash boundary of 67P. Assuming that the region at the Imhotep-Ash boundary and the entire nucleus have similar albedo, we combined the global and local phase curves to study the opposition-surge morphology and constrain the structure and properties of 67P. The model parameters were furthermore compared with other bodies in the solar system and existing laboratory study. Results. We found that the morphological parameters of the opposition surge decrease monotonically with wavelength, whereas in the case of coherent backscattering this behavior should be the reverse. The results from comparative analysis place 67P in the same category as the two Mars satellites, Phobos and Deimos, which are notably different from all airless bodies in the solar system. The similarity between the surface phase function of 67P and a carbon soot sample at extremely small angles is identified, introducing regolith at the boundary of the Imhotep-Ash region of 67P as a very dark and fluffy layer.© ESO, 2017.The support of the national funding agencies of Germany (DLR), France (CNES), Italy (ASI), Spain (MEC), Sweden (SNSB), and the ESA Technical Directorate is gratefully acknowledgedPeer Reviewe