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Both heliophysics and planetary physics seek to understand the complex nature of the solar wind's interaction with solar system obstacles like Earth's magnetosphere, the ionospheres of Venus and Mars, and comets. Studies with this objective are frequently conducted with the help of single or multipoint in situ electromagnetic field and particle observations, guided by the predictions of both local and global numerical simulations, and placed in context by observations from far and extreme ultraviolet (FUV, EUV), hard X-ray, and energetic neutral atom imagers (ENA). Each proposed interaction mechanism (e.g., steady or transient magnetic reconnection, local or global magnetic reconnection, ion pick-up, or the Kelvin- Helmholtz instability) generates diagnostic plasma density structures. The significance of each mechanism to the overall interaction (as measured in terms of atmospheric/ionospheric loss at comets, Venus, and Mars or global magnetospheric/ionospheric convection at Earth) remains to be determined but can be evaluated on the basis of how often the density signatures that it generates are observed as a function of solar wind conditions. This paper reviews efforts to image the diagnostic plasma density structures in the soft (low energy, 0.1-2.0 keV) X-rays produced when high charge state solar wind ions exchange electrons with the exospheric neutrals surrounding solar system obstacles.

Interstellar comets offer direct samples of volatiles from distant protoplanetary disks. 2I/Borisov is the first notably active interstellar comet discovered in our Solar System 1 . Comets are condensed samples of the gas, ice and dust that were in a star’s protoplanetary disk during the formation of its planets, and inform our understanding on how chemical compositions and abundances vary with distance from the central star. Their orbital migration distributes volatiles 2 , organic material and prebiotic chemicals around their host system 3 . In our Solar System, hundreds of comets have been observed remotely, and a few have been studied up close by space missions 4 . However, knowledge of extrasolar comets has been limited to what could be gleaned from distant, unresolved observations of cometary regions around other stars, with only one detection of carbon monoxide 5 . Here we report that the coma of 2I/Borisov contains substantially more CO than H 2 O gas, with abundances of at least 173%, more than three times higher than previously measured for any comet in the inner (<2.5 au) Solar System 4 . Our ultraviolet Hubble Space Telescope observations of 2I/Borisov provide the first glimpse into the ice content and chemical composition of the protoplanetary disk of another star that is substantially different from our own. Hubble Space Telescope data show that interstellar comet 2I/Borisov has an unusually high CO/H 2 O ratio—higher than any other comet that has been seen in the inner regions of our Solar System. This allows us to constrain the nature and location of the circumstellar region from which 2I/Borisov originated.

Mars provides our local analogue for unmagnetized terrestrial planets and is thus key to understanding the habitability of exoplanets. The lack of a global magnetic field means that the atmosphere interacts directly with the solar wind, causing significant loss of the atmosphere. While in situ measurements provide a wealth of detailed local information, they are limited in deriving the global picture. In contrast, remote X-ray observations can provide important global instantaneous coverage over multiple seasons and sampling different solar wind. Previous XMM–Newton observations have detected significant flux via the solar wind charge exchange emission (SWCX) mechanism from an extended planetary halo, and from atmospheric fluorescence. In contrast, Chandra observations only detected a low-luminosity disc and a faint halo. It is postulated that these observational differences are due to transient solar wind with increased heavy ion fractions. Here, we present simulated spectra for the proposed NASA mission Line Emission Mapper, of both halo and disc regions, under quiet and transient solar wind. We show that even under moderate solar wind conditions, both SWCX and fluorescence emission lines are readily detected above the background, providing new insights into the loss of planetary atmospheres and the molecular composition of less well-characterized atmospheric abundances.

Bilobate comets—small icy bodies with two distinct lobes—are a common configuration among comets, but the factors shaping these bodies are largely unknown. Cometary nuclei, the solid centres of comets, erode by ice sublimation when they are sufficiently close to the Sun, but the importance of a comet’s internal structure on its erosion is unclear. Here we present three-dimensional analyses of images from the Rosetta mission to illuminate the process that shaped the Jupiter-family bilobate comet 67P/Churyumov–Gerasimenko over billions of years. We show that the comet’s surface and interior exhibit shear-fracture and fault networks, on spatial scales of tens to hundreds of metres. Fractures propagate up to 500 m below the surface through a mechanically homogeneous material. Through fracture network analysis and stress modelling, we show that shear deformation generates fracture networks that control mechanical surface erosion, particularly in the strongly marked neck trough of 67P/Churyumov–Gerasimenko, exposing its interior. We conclude that shear deformation shapes and structures the surface and interior of bilobate comets, particularly in the outer Solar System where water ice sublimation is negligible.The shape and internal structure of bilobate comet 67P is controlled by shear deformation inducing mechanically driven erosion along shear fracture networks, according to a 3D analysis of images from the Rosetta mission.

While the structural complexity of cometary comae is already recognizable from telescopic observations 1 , the innermost region, within a few radii of the nucleus, was not resolved until spacecraft exploration became a reality 2 , 3 . The dust coma displays jet-like features of enhanced brightness superposed on a diffuse background 1 , 4 , 5 . Some features can be traced to specific areas on the nucleus, and result conceivably from locally enhanced outgassing and/or dust emission 6 – 8 . However, diffuse or even uniform activity over topographic concavity can converge to produce jet-like features 9 , 10 . Therefore, linking observed coma morphology to the distribution of activity on the nucleus is difficult 11 , 12 . Here, we study the emergence of dust activity at sunrise on comet 67P/Churyumov–Gerasimenko using high-resolution, stereo images from the OSIRIS camera onboard the Rosetta spacecraft, where the sources and formation of the jet-like features are resolved. We perform numerical simulations to show that the ambient dust coma is driven by pervasive but non-uniform water outgassing from the homogeneous surface layer. Physical collimations of gas and dust flows occur at local maxima of insolation and also via topographic focusing. Coma structures are projected to exhibit jet-like features that vary with the perspective of the observer. For an irregular comet such as 67P/Churyumov–Gerasimenko, near-nucleus coma structures can be concealed in the shadow of the nucleus, which further complicates the picture. Images of 67P's nucleus from the Rosetta spacecraft, together with numerical simulations, show that the jet-like features of cometary comae can be produced by diffuse activity focused by the nucleus topography as well as non-uniform insolation over the surface.

We studied dissociation reactions of electron impact on water vapor for several fragment species at optical and near ultraviolet wavelengths (200 - 850 nm). The resulting spectrum is dominated by the Hydrogen Balmer series, by the OH (A \\(^2\\Sigma^+\\) - X \\(^2\\Pi\\)) band, and by the emission of ionic H\\(_2\\)O\\(^+\\) (A \\(^2\\)A\\(_1\\) - X \\(^2\\)B\\(_1\\)) and OH\\(^+\\) (A \\(^3\\Pi\\) - X \\(^3\\Sigma^-\\)) band systems. Emission cross sections and reaction channel thresholds were determined for energies between 5 - 100 eV. We find that electron impact dissociation of H\\(_2\\)O results in an emission spectrum of the OH (A \\(^2\\Sigma^+\\) - X \\(^2\\Pi\\)) band that is distinctly different than the emission spectra from other excitation mechanisms seen in planetary astronomy. We attribute the change to a strongly non-thermal population of rotational states seen in planetary astronomy. This difference can be utilized for remote probing of the contribution of different physical reactions in astrophysical environments.

Hyperactive comets are a small group of comets whose activity are higher than expected. They seem to emit more water than they should based on the size of their nucleus and comet 46P/Wirtanen is one of them. Investigating its activity and composition evolution could provide clues about its origins and formation region in the Solar nebulae. Given the exceptional close approach in 2018 of comet 46P to the Earth, we aim to study the evolution of its activity and composition as a function of heliocentric distances before and after perihelion. We used both TRAPPIST telescopes to monitor the comet for almost a year with broad-band and narrow-band filters. We derived the production rates of five gaseous species, e.g. OH, NH, CN, C\\(_3\\) and C\\(_2\\), using a Haser model as well as the A(\\(\\theta\\))f\\(\\rho\\), dust proxy parameter. The comet was also observed with two optical high resolution spectrographs UVES and ESPRESSO mounted on the 8-m ESO VLT to measure the isotopic ratios of C and N, the oxygen forbidden lines ratios and the NH\\(_2\\) ortho-to-para ratios. We followed during almost a year the rise and decline of the production rates of different species as well as the dust activity of 46P on both pre- and post-perihelion. Relative abundances with respect to CN and OH along the orbit of the comet show constant and symmetric abundance ratios and a typical coma composition. We determined the rotation period of the nucleus using high cadence observations and long series of CN images on several nights, and we obtained a value of (9.18\\(\\pm\\)0.05) hr at perihelion. Using high resolution spectra of 46P coma, we derived C and N isotopic ratios of 100\\(\\pm\\)20 and 150\\(\\pm\\)30 and a green-to-red forbidden oxygen [OI] lines ratio of 0.23\\(\\pm\\)0.02. We measured a NH\\(_2\\) ortho-to-para ratio of 3.31\\(\\pm\\)0.03 and derived an ammonia ratio of 1.19\\(\\pm\\)0.03 corresponding to a spin temperature of 27\\(\\pm\\)1 K.

We here study the transfer process of material from one hemisphere to the other (deposition of airfall material) on an active comet nucleus, specifically 67P/Churyumov-Gerasimenko. Our goals are to: 1) quantify the thickness of the airfall debris layers and how it depends on the location of the target area, 2) determine the amount of \\(\\mathrm{H_2O}\\) and \\(\\mathrm{CO_2}\\) ice that are lost from icy dust assemblages of different sizes during transfer through the coma, and 3) estimate the relative amount of vapor loss in airfall material after deposition in order to understand what locations are expected to be more active than others on the following perihelion approach. We use various numerical simulations, that include orbit dynamics, thermophysics of the nucleus and of individual coma aggregates, coma gas kinetics and hydrodynamics, as well as dust dynamics due to gas drag, to address these questions. We find that the thickness of accumulated airfall material varies substantially with location, and typically is of the order \\(0.1\\)-\\(1\\,\\mathrm{m}\\). The airfall material preserves substantial amounts of water ice even in relatively small (cm-sized) coma aggregates after a rather long (\\(12\\,\\mathrm{h}\\)) residence in the coma. However, \\(\\mathrm{CO_2}\\) is lost within a couple of hours even in relatively large (dm-sized) aggregates, and is not expected to be an important component in airfall deposits. We introduce reachability and survivability indices to measure the relative capacity of different regions to simultaneously collect airfall and to preserve its water ice until the next perihelion passage, thereby grading their potential of contributing to comet activity during the next perihelion passage.

In this chapter, we provide a review of radiative processes in cometary atmospheres spanning a broad range of wavelengths, from radio to X-rays. We focus on spectral modeling, observational opportunities, and anticipated challenges in the interpretation of new observations, based on our current understanding of the atomic and molecular processes occurring in the atmospheres of small, icy bodies. Close to the surface, comets possess a thermalized atmosphere that traces the irregular shape of the nucleus. Gravity is too low to retain the gas, which flows out to form a large, collisionless exosphere (coma) that interacts with the heliospheric radiation environment. As such, cometary comae represent conditions that are familiar in the context of planetary atmosphere studies. However, the outer comae are tenuous, with densities lower than those found in vacuum chambers on Earth. Comets, therefore, provide us with unique natural laboratories that can be understood using state-of-the-art theoretical treatments of the relevant microphysical processes. Radiative processes offer direct diagnostics of the local physical conditions, as well as the macroscopic coma properties.These can be used to improve our understanding of comets and other astrophysical environments such as icy moons and the interstellar medium.

Asteroid collisions are one of the main processes responsible for the evolution of bodies in the main belt. Using observations of the Dimorphos impact by the DART spacecraft, we estimate how asteroid collisions in the main belt may look in the first hours after the impact. If the DART event is representative of asteroid collisions with a ~1m size impactor, then the light curves of these collisions will rise on time scales of about >100s and will remain bright for about one hour. Next, the light curve will decay on a few hours time scale to an intermediate luminosity level in which it will remain for several weeks, before slowly returning to its baseline magnitude. This estimate suffers from several uncertainties due to, e.g., the diversity of asteroid composition, their material strength, and spread in collision velocities. We estimate that the rate of collisions in the main belt with energy similar or larger than the DART impact is of the order of 7000 per year (+/-1dex). The large range is due to the uncertainty in the abundance of ~1-m size asteroids. We estimate the magnitude distribution of such events in the main belt, and we show that ~6% of these events may peak at magnitudes brighter than 21. The detection of these events requires a survey with <1hr cadence and may contribute to our understanding of the asteroids' size distribution, collisional physics, and dust production. With an adequate survey strategy, new survey telescopes may regularly detect asteroid collisions.