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A fully kinetic perspective on weakly active comets: Asymmetric outgassing
No full textInternational audienceThe European Space Agency's Rosetta mission measured the complex plasma environment surrounding comet 67P/Churyumov-Gerasimenko for more than two years. In this work, the collisionless dynamics of the plasma interaction during the comet's weakly outgassing phases is investigated through a fully kinetic semi-implicit particle-in-cell approach. The effects of an asymmetric outgassing profile with respect to the upstream plasma conditions are compared with a spherically symmetric Haser model. The three-dimensional shape of the plasma density and the parallel acceleration potential are used as primary measures. It is found that the four-fluid coupled system is not majorly distorted. The different components of the potential structure can be associated with the large-scale behavior and density profiles of the four simulated plasma species. The implications for the acceleration and cooling of electrons within the cometary plasma environment are identified by contrasting the differences in the shape of the acceleration potential between the distinct asymmetric outgassing models. The analysis provides a detailed overview that can help interpret past Rosetta plasma measurements and could be key to help disentangle the physical drivers active in the plasma environment of comets visited by future exploration missions
Design of the VenSpec-H instrument on ESA’s EnVision mission: development of critical elements, highlighting the wavefront corrector and grating
No full textInternational audienceEnVision is the European Space Agency’s upcoming mission to Venus with a launch scheduled in 2031. One of the payloads on board is the Venus Spectrometers (VenSpec) suite, containing three spectrometer channels, one of which is Venus Spectrometer with high resolution (VenSpec-H). VenSpec-H performs absorption measurements in the atmosphere of Venus in four near-infrared spectral bands. VenSpec-H is developed under Belgian management and builds on heritage from instruments on Venus Express and Trace Gas Orbiter. The operating wavelength range (1.15 to 2.5μm) imposes stringent temperature requirements on the instrument to make nightside measurements below the Venus clouds possible. Most importantly, the spectrometer’s optical components are held in a separate cold section inside the instrument, cooled down to −45°C, to remove the thermal background from the signal. Some passive optical elements in the cold spectrometer had low technological readiness at the start of the project. One of them is a wavefront corrector: the freeform corrector plate, used to compensate for aberrations introduced in the system by a parabolic mirror. This device is developed by the Brussels Photonics lab of Vrije Universiteit Brussel using a supply chain with shape-adaptive corrective polishing and dedicated metrology. Another is the echelle grating, used to disperse the incoming light into its spectral components, which is built by Advanced Mechanical and Optical Systems. We highlight the manufacturing and metrology processes of both devices. Besides that, some mechanisms, placed in the warmer part of the instrument, had to be developed: a turn window unit to protect the interior of the instrument during the aerobraking phase of the mission, a filter wheel mechanism to select the spectral bands of interest, and an integrated detector cooler assembly to register the spectra
Relief inversion and denudation dynamics in a semi-arid landscape (Araripe Plateau, NE Brazil): Insights from cosmogenic nuclides and geomorphic surfaces
No full textInternational audienc
Macroscopic quantum-like behavior of a turbulent jet
No full textWe apply the scale-relativity theory of turbulence to the turbulent round jet problem. In a previous work [1], on the basis of Kolmogorov scaling δv2 ∼ δt which is equivalent to fractal dimension 2 of trajectories in velocity space, the time derivative of the Navier-Stokes equations has been integrated under the form of a macroscopic v-Schrödinger equation. However, for a flow reaching scales far larger than the integral scale, which is the case in jets, one can show that a new transition occurs around the integral scales toward fractal dimension 2 of trajectories now in position space. The Navier-Stokes equations can then be integrated under the form of a macroscopic x-Schrödinger-type equation in which the constant which replaces Planck's constant is now macroscopic. This equation is solved in terms of a wave function whose modulus squared yields the concentration profile. The solution obtained is in excellent agreement with the experimentally observed turbulent jet concentration. Then we have performed a new experiment, in order to put to the test such a macroquantum-like behavior. The free turbulent jet has a natural conic self-similar structure with a transition from the inner turbulent regime to the outer laminar flow around an angle αS ≈ 0.235. The PrOJET experiment consists of confining a jet inside its own conic shape: this is, according to the scale-relativity approach, a macroscopic analog of a quantum particle in a cylindrical box. Thanks to the infinite potential well thus achieved, excited states can be reached. In the first excited state, one predicts from the Schrödinger equation a vanishing concentration on the centerline of the jet, in complete opposition with the free jet. This new theoretical prediction is validated with high statistical significance in our jet experiment, which therefore brings strong support to the existence of macroscopic quantum-like phenomena driven by (effective) nondifferentiability of a medium and its induced fractality, as theoretically predicted more than 30 years ago [2]
Carbonate sources and dynamics in coastal environments: application off the coast of Saint-Valéry-en-Caux (Seine-Maritime)
No full textInternational audienceThe Seine-Maritime coastline, shaped by constantly eroding Mesozoic chalk cliffs, experiences significant and poorly understood sandy siltation, particularly to the east of Saint-Valéry-en-Caux. To better determine the origin of this siltation and to follow the sedimentary dynamics, 45 surface sediment samples over a 10 km² area off the coast of Saint-Valéry-en-Caux were collected. Elemental chemical analyses (Ca, Sr, Si, BrOrg) were carried out: (i) on the bulk sediment, and (ii) on separate fractions. The GSTA (Geochemical Sediment Trend Analysis) method was applied to the bulk sediment, while the COVA (COmposition Variability Analysis) method was used for separate fractions. Carbonates produced in situ and likely to be fragmented during transport are excellent indicators of local dynamics. Their stock dynamics were monitored using an innovative approach (PASTA: PArticulate Stock Trend Analysis). The result is a complete picture of sediment dynamics. Three zones can be distinguished: (i) a steeply sloping western zone, in dynamic swell/tide equilibrium, where shells make little cross-shore transport towards the coast, (ii) a gently sloping eastern zone, also in dynamic swell/tide equilibrium, where shells and quartz migrate coastward through the minimum energy zone located at -15 m, and (iii) an intermediate zone, which appears to be much more mixed, with disrupted cross-shore exchange
Cosmic topology. Part IIIb. Eigenmodes and correlation matrices of spin-2 perturbations in orientable Euclidean manifolds
No full textInternational audienceWe study the eigenmodes of the spin-2 Laplacian in orientable Euclidean manifolds and their implications for the tensor-induced part of the cosmic microwave background (CMB) temperature and polarization anisotropies. We provide analytic expressions for the correlation matrices of Fourier-mode amplitudes and of spherical harmonic coefficients. We demonstrate that non-trivial spatial topology alters the statistical properties of CMB tensor anisotropies, inducing correlations between harmonic coefficients of differing and and across every possible pair of temperature and - and -modes of polarization. This includes normally forbidden and correlations. We compute the Kullback-Leibler (KL) divergence between the pure tensor-induced CMB fluctuations in the usual infinite covering space and those in each of the non-trivial manifolds under consideration, varying both the size of the manifolds and the location of the observer. We find that the amount of information about the topology of the Universe contained in tensor-induced anisotropies does not saturate as fast as its scalar counterpart; indeed, the KL divergence continues to grow with the inclusion of higher multipoles up to the largest we have computed. Our results suggest that CMB polarization measurements from upcoming experiments can provide new avenues for detecting signatures of cosmic topology, motivating a full analysis where scalar and tensor perturbations are combined and noise is included
A cometary Fluorescence Model for the ν3 Vibrational Band of Cyanogen
No full textInternational audienceCyanogen (C2N2) is suspected for a long time to be present in comets and to contribute to the creation of CN radical. So far no observations with ground-based facilities managed to detect this species but the Rosetta mission, thanks to in situ observations with the ROSINA mass spectrometer detected this species in the coma of 67P/Churyumov-Gerasimenko. To investigate its presence from infrared spectra in other comets, we developed a fluorescence model for the ν3 fundamental band. From new laboratory high-resolution infrared spectra of cyanogen, we analyzed the region of the ν3 band of C2N2, centered around 4.63 μm (2158 cm−1). In addition to line positions and intensities, molecular parameters for the ground and excited vibrational state were obtained. These parameters allowed us to develop a fluorescence model for cyanogen. Excitation rates of the ν3 band of cyanogen in cometary comae are presented. An attempt to detect cyanogen in a high-resolution spectrum of comet C/2022 E3 (ZTF) is discussed
Last-millennium volcanic forcing and climate response using SO<sub>2</sub> emissions
No full textInternational audienceClimate variability in the last millennium (past 1000 years) is dominated by the effects of large-magnitude volcanic eruptions; however, a long-standing mismatch exists between model-simulated and tree-ring-derived surface cooling. Accounting for the self-limiting effects of large sulfur dioxide (SO2) injections and the limitations in tree-ring records, such as lagged responses due to biological memory, reconciles some of the discrepancy, but uncertainties remain, particularly for the largest tropical eruptions. The representation of volcanic forcing in the latest generation of climate models has improved significantly, but most models prescribe the aerosol optical properties rather than using SO2 emissions directly and including interactions between the aerosol, chemistry, and dynamics. Here, we use the UK Earth System Model (UKESM) to simulate the climate of the last millennium (1250–1850 CE) using volcanic SO2 emissions. Averaged across all large-magnitude eruptions, we find similar Northern Hemisphere (NH) summer cooling compared with other last-millennium climate simulations from the Paleoclimate Modelling Intercomparison Project Phase 4 (PMIP4), run with both SO2 emissions and prescribed forcing, and a continued overestimation of surface cooling compared with tree-ring reconstructions. However, for the largest-magnitude tropical eruptions in 1257 (Mt. Samalas) and 1815 (Mt. Tambora), some models, including UKESM1, suggest a smaller NH summer cooling that is in better agreement with tree-ring records. In UKESM1, we find that the simulated volcanic forcing differs considerably from the PMIP4 dataset used in models without interactive aerosol schemes, with marked differences in the hemispheric spread of the aerosol, resulting in lower forcing in the NH when SO2 emissions are used. Our results suggest that, for the largest tropical eruptions, the spatial distribution of aerosol can account for some of the discrepancies between model-simulated and tree-ring-derived cooling. Further work should therefore focus on better resolving the spatial distribution of aerosol forcing for past eruptions
Seafloor evidence of structurally-controlled fluid expulsion from the upper Amazon deep-sea
No full textInternational audienceThe Amazon River culminates in one a deep-sea fan up to 10 km thick, a dynamic setting in which the rapid deposition of organic-rich sediment drives linked processes of methanogenesis, fluid migration and venting, gas hydrate formation, and large-scale slope instability. Growth of the fan over the last 8 Ma has been accompanied by its gravitational collapse on shale detachments to form extensional and compressional belts across the shelf and upper slope (<2250 m water depth), and by recurrent slope failure to form fan-wide megaslides. The upper slope compressional belt contains a ‘leaky’ gas hydrate system characterised by elongate bottom-simulating reflection (BSR) patches that are aligned with the crests of thrust-fold anticlines, and in places rise towards sub-circular seafloor fluid vents. Ongoing fluid venting from the fan is indicated by sea surface oil slicks reported on the shelf and upper slope, and water column gas flares observed on multibeam imagery obtained in 2016 across part of the thrust-fold belt. The extent of degassing across the vast fan area in water depths of 2500-4500 m is unknown due to a lack of water column data below the compressional front. The 2023 AMARYLLIS-AMAGAS I campaign acquired acoustic data (multibeam imagery, Chirp profiles) along multiple transects of the fan in water depths of 100-4200 m, and cores and heat flow data from sites in the thrust-fold belt. Here we present information on fluid expulsion from the Amazon fan based on seafloor data both from the campaign, and 3D seismic datasets on the upper slope (ANP Brazil). Multibeam imagery reveal hundreds of water column gas flares in water depths of 100-1900 m, with a peak in abundance near the upper limit of the MHSZ (565 ± 65 m water depth). Gas is observed to rise from areas of smooth seafloor in places, but mainly from sub-circular mounds and depressions. Bathymetric grids from multibeam and 3D seismic (4-50 m resolution) were used to capture sub-circular seafloor morphologies for morphometric analysis using a semi-automated training approach. Over 500 features were identified in water depths of 275-2265 m, identified as domes (59%), complex forms (28%) and depressions (13%); the vast majority (>96%) are <50 m in relief (mean 16 m) and <1 km wide (mean 500 m). Cores of alternating lighter hemipelagic and darker muds interpreted as mud extrusion were recovered both from domes and depressions; gas hydrates were cored in several domes with gas flares. Subbottom data reveal chaotic facies defining structures deeply-rooted in thrust-folds. We interpret the seafloor features as differing expressions of relatively small-scale mud volcanism, many actively venting gas. Our results indicate widespread fluid expulsion from the Amazon fan within the extensional and compressional belts, and a lack of evidence for venting in greater water depths. The primary control on degassing of the fan appears to be gravity tectonism, which provides pathways for fluid escape within and above the MHSZ. This is a contribution to studies of gas hydrate dynamics and slope stability in the context of the MEGA project (ANR-22-CE01-0031)