Dust Mantle Research Articles

The chemical composition of CO-rich comet C/2023 H2 (Lemmon)

We report on the composition of comet C/2023 H2 (Lemmon) (hereafter C/2023 H2) as measured with CRIRES$^+$ at VLT/ESO, under the Director Discretionary Time program: 2112.C-5015. We observed C/2023 H2 between November 24 and 27, 2023, after perihelion and during its close approach to Earth. We used three settings sampling the spectral region from approx 2.5 to 5 mu m, to search for fluorescence emission lines of H$_2$O, HCN, C$_2$H$_2$, NH$_3$, C$_2$H$_6$, H$_2$CO, CH$_3$OH, CH$_4$, and CO. C/2023 H2 spectra are dominated by signatures from hyper-volatile species, namely CO, C$_2$H$_6$, and CH$_4$, while it is particularly difficult to identify lines from less volatile species such as water or methanol. When compared to other comets, C/2023 H2 has an overall typical-to-enriched composition, with CO showing one of the highest infrared values reported so far in Solar System comets within 2 au from the Sun. In this respect, C/2023 H2 shares many similarities with C/2013 R1 (Lovejoy), C/2009 P1 (Garrad), and C/1999 T1 (McNaught-Hartley), although still being rather unique. Results from the analysis of optical spectra (approx 300 to 650 nm) obtained on November 16, 2023, using the Intermediate Dispersion Spectrograph at the Isaac Newton Telescopes are consistent with the infrared ones. Assuming that the composition of C/2023 H2 is original, this comet most likely formed in a region of the disc where CO was particularly enriched, or it could have been captured from other planetary systems forming in the Sun's birth cluster. Similarities with a few other comets suggest the existence of a sub-class of CO-enriched comets, which may be currently under-sampled. Alternatively, water sublimation may have been ineffective due to the presence of a thick dust mantle covering the nucleus surface.

Phobos photometric properties from Mars Express HRSC observations

Aims. This study aims to analyze Phobos’ photometric properties using Mars Express mission observations to support the Martian Moons exploration mission (MMX) devoted to the investigation of the Martian system and to the return of Phobos samples. Methods. We analyzed resolved images of Phobos acquired between 2004 and 2022 by the High Resolution Stereo Camera (HRSC) on board the Mars Express spacecraft at a resolution ranging from ~30 m px−1 to 330 m px−1. We used data acquired with the blue, green, red, and IR filters of HRSC and the panchromatic data of the Super Resolution Channel (SRC). The SRC data are unique because they cover small phase angles (0.2–10°), permitting the investigation of the Phobos opposition effect. We simulated illumination and geometric conditions for the different observations using the Marx Express and the camera spice kernels provided by the HRSC team. We performed photometric analysis using the Hapke model for both integrated and disk-resolved data. Results. The Phobos phase function is characterized by a strong opposition effect due to shadow hiding, with an amplitude and a half-width of the opposition surge of 2.28±0.03 and 0.0573±0.0001, respectively. Overall, the surface of Phobos is dark, with a geometric albedo of 6.8% in the green filter and backscattering. Its single-scattering albedo (SSA) value (7.2% in the green filter) is much higher than what has been found for primitive asteroids and cometary nuclei and is close to the values reported in the literature for Ceres. We also found a surface porosity of 87%, indicating the presence of a thick dust mantle or of fractal aggregates on the top surface. The SSA maps revealed high reflectance variability, with the blue unit area in the northeast Stickney rim being up to 65% brighter than average, while the Stickney floor is among the darkest regions, with reflectance 10 to 20% lower than average. Photometric modeling of the regions of interest selected in the red and blue units indicates that red unit terrains have a stronger opposition effect and a smaller SSA value than the blue ones, but they have similar porosity and backscattering properties. Conclusions. The HRSC data provide a unique investigation of the Phobos phase function and opposition surge, which is valuable information for the MMX observational planning. The Phobos opposition surge, surface porosity, phase integral, and spectral slope are very similar to the values observed for the comet 67P and for Jupiter family comets in general. Based on these similarities, we formulate a hypothesis that the Mars satellites might be the results of a binary or bilobated comet captured by Mars.

Localised sublimation under cometary dust mantles

Observations of cometary surfaces have indicated that the amount of exposed surficial water ice is insufficient to supply the measured water production rates within the inner solar system. The measured water production rates are also a small fraction of that which would arise from freely subliming pure water ice bodies of similar surface area. A plausible mechanism, given the now well-established, high porosity of comets is that water ice is subliming below the visible surface with the produced gas escaping through the pores in a non-subliming hot dust matrix. If this process leads to an additional heating of the evolving gas, this may provide an elegant explanation for the temperature of the gas required to match measured terminal velocities. We investigate this process using a series of numerical models based on the Direct Simulation Monte Carlo (DSMC) method for rarified gas dynamics. We show that inhomogeneities in the source are not evident in the flow field once the thickness of the porous layer reaches a specific size and that lateral flow through the porous layer (parallel to the surface) can broaden the apparent source size when the source is small and isolated. The temperatures of the gas flow at the visible surface can be significantly increased and will respond to the temperature distribution within the non-volatile porous layer. The influence of the structural properties of the porous layer (how the porous layer is mechanically constructed) is shown to have only limited influence on the final gas parameters at the surface.

Activity Analysis on 68P/Klemola and 78P/Gehrels 2 in 2018–2020 Perihelion Passage

We performed secular monitoring broadband photometric observations on Jupiter Family Comets (JFCs) 68P/Klemola and 78P/Gehrels 2 from 2018 November to 2020 March with the Yaoan High Precision Telescope. Our main purpose is to study the dust activity, coma properties, and dynamical history of the two comets and analyze the activity evolution of 78P/Gehrels 2 in the recent past. We use aperture photometry to obtain the magnitude and the A(0)f ρ values from the R band observations. The maximum A(0)f ρ values we recorded for 68P/Klemola and 78P/Gehrels 2 are 339.7 ± 4.4 cm and 1028.1 ± 13.3 cm, respectively, showing that the activity of 68P/Klemola is of middle level while 78P/Gehrels 2 is one of the most active JFCs. The mean color of 78P/Gehrels 2 is (B − V) = 0.88 ± 0.02 and (V − R) = 0.27 ± 0.02. Dynamical history analysis suggests that 78P/Gehrels 2 could have actually resided in this region for a long time in the past 1 Myr, though it recently migrated into the inner solar system. The high activity of 78P/Gehrels 2 reported in the past three perihelion passages could be attributed to the perihelion distance decl. from 2.3 to 2.0 au before 1997 that boosted the water-ice sublimation rate and formed new active regions. The activity decl. over recent apparitions could be attributed to the reformation of the dust mantle.

Discovery of a Dust Sorting Process on Boulders Near the Reiner Gamma Swirl on the Moon

AbstractIn a database of lunar fractured boulders (Rüsch & Bickel, 2023, https://doi.org/10.3847/psj/acd1ef), we found boulders with reflectance features dissimilar to previously known morphologies. We performed a photo‐geologic investigation and determined that the features correspond to a dust mantling on top of boulders with a unique photometric behavior. We next performed a photometric model inversion on the dust mantling using Bayesian inference sampling. Modeling indicates that the dust photometric anomaly is most likely due to a reduced opposition effect, whereas the single scattering albedo is not significantly different from that of the nearby background regolith. This implies a different structure of the dust mantling relative to the normal regolith. We identified and discussed several potential processes to explain the development of such soil. None of these mechanisms can entirely explain the multitude of observational constraints unless evoking anomalous boulder properties. Further study of these boulders can shed light on the workings of a natural dust sorting process potentially involving dust dynamics, a magnetic field, and electrostatic dust transport. The presence of these boulders appears to be limited to the Reiner K crater near the Reiner Gamma magnetic and photometric anomaly. This close spatial relationship further highlights that poorly understood processes occur in this specific region of the Moon.

Cliff collapse on Comet 67P/Churyumov–Gerasimenko – I. Aswan

ABSTRACT The Aswan cliff on Comet 67P/Churyumov–Gerasimenko collapsed on 2015 July 10. Thereby, relatively pristine comet material from a depth of $\sim 12\, \mathrm{m}$ was exposed at the surface. Observations of the collapse site by the microwave instrument Rosetta/Microwave Instrument for Rosetta Orbiter (MIRO) have been retrieved from 8 months prior to collapse, as well as from 5, 7, and 11 months post-collapse. The MIRO data are analysed with thermophysical and radiative transfer models. The pre-collapse observations are consistent with a 30 MKS thermal inertia dust mantle with a thickness of at least $3\, \mathrm{cm}$. The post-collapse data are consistent with (1) a dust/water–ice mass ratio of 0.9 ± 0.5 and a molar CO2 abundance of ∼30 per cent relative to water; (2) formation of a dust mantle after ∼7 months, having a thickness of a few millimetres or a fraction thereof; (3) a CO2 ice sublimation front at 0.4 cm that withdrew to 2.0 cm and later to $20\pm 6\, \mathrm{cm}$; (4) a thermal inertia ranging 10–45 MKS; (5) a gas diffusivity that decreased from $0.1$ to $0.001\, \mathrm{m^2\, s^{-1}}$; and (6) presence of a solid-state greenhouse effect parts of the time. The data and the analysis provide a first empirical glimpse of how ice-rich cometary material ages and evolves when exposed to solar heating.

Interaction of H2S with H atoms on grain surfaces under molecular cloud conditions

Context. Hydrogen sulfide (H2S) is thought to be efficiently formed on grain surfaces through the successive hydrogenation of sulfur atoms. Its non-detection so far in astronomical observations of icy dust mantles thus indicates that effective destruction pathways must play a significant role in its interstellar abundance. While chemical desorption has been shown to remove H2S very efficiently from the solid phase, in line with H2S gas-phase detections, possible ice chemistry triggered by the related HS radical have been largely disregarded so far, despite it being an essential intermediate in the H2S + H reaction scheme. Aims. We aim to thoroughly investigate the fate of H2S upon H-atom impact under molecular cloud conditions, providing a comprehensive analysis combined with detailed quantification of both the chemical desorption and ice chemistry that ensues. Methods. We performed experiments in an ultrahigh vacuum chamber at temperatures between 10 and 16 K in order to investigate the reactions between H2S molecules and H atoms on interstellar ice analogs. The changes in the solid phase during H-atom bombardment were monitored in situ by means of reflection absorption infrared spectroscopy (RAIRS), and desorbed species were complementarily measured with a quadrupole mass spectrometer (QMS). Results. We confirmed the formation of H2S2 via reactions involving H2S + H and quantified its formation cross section under the employed experimental conditions. Additionally, we directly assessed the chemical desorption of H2S by measuring the gas-phase desorption signals with the QMS, providing unambiguous desorption cross sections. Chemical desorption of H2S2 was not observed. The relative decrease of H2S ices by chemical desorption changed from ~85% to ~74% between temperatures of 10 and 16 K, while the decrease as the result of H2S2 formation was enhanced from ~15% to ~26%, suggesting an increasingly relevant sulfur chemistry induced by HS radicals at warmer environments. The astronomical implications are further discussed.

GTasb3D: A Novel 3D Framework for Modeling Thermal Evolution and Rarefied Flows in Porous Active Small Bodies with Various Shapes

Volatiles in small bodies provide important clues to solar system evolution and are of in-situ-resource-utilization interest. Explicit modeling of small bodies’ global thermophysical process is essential to assess volatiles’ evolution and abundance. Previous numerical studies commonly use a finite difference/volume method, which has limited capability in simulating the interior thermal dynamics of small bodies with realistic shapes. Here we developed a novel 3D framework using the generalized finite difference method for modeling thermal evolution of active small bodies (GTasb3D). By fully solving the energy and mass conservation equations using a mesh-free, Cartesian-coordinate-based method, this framework can evaluate the heat and mass transport in a porous cometary body of various shapes. Several tests and comparisons with previous studies have been carried out to verify this framework's accuracy and efficiency. We show that the timescale to achieve thermal equilibrium and the global temperature distribution are in good agreement with previous theoretical and numerical estimates. The GTasb3D simulations show that ice sublimation mainly occurs near the ice front, and parts of the resulting vapor recondense beneath the ice front. The surface gas density dramatically decreases as the ice retreats. For a 1 km radius object located at 3 au with initially homogeneous dust-ice distribution, the depth to ice at the equator is >∼2 cm after ∼10 yr, assuming that a dust mantle is left behind after ice depletion. At this stage, the global gas production rate is below the gas emission detection capability but is capable of lifting submillimeter-sized dust from the nucleus’s near-subsurface.

TNO or Comet? The Search for Activity and Characterization of Distant Object 418993 (2009 MS9)

The perihelion of the trans-Neptunian object (TNO) 2009 MS9 brings it close to the distance where some long-period comets are seen to become active. Knowing this, and the fact that this object appears to brighten in excess of its predicted nucleus brightness, suggests that 2009 MS9 has a delayed onset of activity brought on by the sublimation of a species more volatile than water. In this paper, we characterize 2009 MS9's physical properties and investigate potential outgassing through composite images, sublimation models, and measurements of spectral reflectivity. We find that deep composite images of the object at various epochs along its orbit show no evidence of dust yet place sensitive limits to the dust production. We estimate the nucleus radius to be 11.5 ± 3.5 km using thermal IR modeling from NEOWISE data and use this and data pre-perihelion to estimate a geometric albedo of 0.25. We compare a CO sublimation activity model to its post-perihelion heliocentric light curve and find that these data support an active fractional area of 5 × 10−6 assuming 2 μm–sized grains and other typical comet parameters. The spectral reflectivity of the surface materials obtained with the Gemini Observatory and CFHT at different epochs shows a reddening spectral slope. We compare the physical properties of 2009 MS9 to both TNO and comet populations and speculate that 2009 MS9's reddening may be due to the buildup of a dust mantle on the surface and could be an explanation of why TNOs exhibit a color bimodality.

New Insights Into Subsurface Stratigraphy Northwest of Ascraeus Mons, Mars, Using the SHARAD and MARSIS Radar Sounders

AbstractThe Tharsis Montes volcanoes on Mars are the source of laterally extensive lava flows and other volcanic deposits generating a complex stratigraphy throughout the Tharsis Volcanic Province. We use SHAllow RADar (SHARAD) and Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) observations in a region northwest of Ascraeus Mons to determine the composition, density, thickness, and spatial distribution of these emplaced volcanic materials. We identified subsurface reflectors along 43 SHARAD and five MARSIS observations. Reflectors in the volcanic plains are interpreted to be sequences of basaltic lava flows with interspersed pyroclastic material, dust, or regolith during a hiatus in activity. Others correspond to the base of three major flow fields. Several plain reflectors were detected by both MARSIS and SHARAD. Other notable reflectors were identified near Ascraeus' flank where lava buried glacially derived sediment. We derived thickness and other material properties for flows using their distinct topographic boundary visible in the radar images. Permittivity ranged from 7.0 to 11.2 corresponding to lava flow densities of 3.20–3.52 g/cm3. Flow thicknesses ranged from 19.8 to 60.2 m. Loss tangents were low for the flow fields ranging from 0.024 to 0.043. Loss tangents in the plains ranged from 0.010 to 0.076. Higher loss tangents correspond to lossier regions that may have higher concentrations of radar absorbing minerals like hematite. Surface roughness controls where reflectors are detected. SHARAD detects the base of three out of the four flow fields in this region with muted surface roughness from dust mantling and erosion.

Probable ice-rich deposits on north-facing slopes in Alba Patera, Mars

We examine unconformable lobate deposits along the north-facing slopes of Alba Patera (40°N, 250°E with an elevation of ~6 km), Mars, using data from the Mars Reconnaissance Orbiter Context Camera, High-Resolution Imaging Science Experiment, and the Mars Orbiter Laser Altimeter. We interpret the lobate north-facing slope deposits (NSFDs) to be small (<3 km from source to toe) lobate debris aprons (LDAs), making them the highest-elevation LDAs yet identified on Mars. The total volume of the deposits along the caldera walls is ~5–11 km3. Deposits filling impact craters around the Alba Mons summit appear to be similar to the NFSDs. These NFSDs bear two distinct textures either singly or together: a hummocky surface of decameter-scale hills and a smooth surface. Smooth-textured NFSDs have lobate margins and convex-up topographic profiles, and show evidence of having flowed; hummocky NSFDs have concave or linear topographic profiles and are generally found on steep slopes. These deposits likely formed due to reduced insolation on north-facing slopes, which allowed for the preferential accumulation or preservation of water ice. The presence of small LDAs restricted to north-facing slopes could indicate that Alba Patera was only a marginal environment for glaciation, possibly because little water vapor was available at such a high elevation. The hummocky material is interpreted to be the eroded remnants of a mantle of ice-cemented dust that was superposed on the NFSDs. This mantle has been almost completely removed in smooth-textured areas. This difference in rates of ice removal was the result of either lower insolation on steep north-facing slopes than on shallow slopes or the greater mobility of the thicker, lobate portions of NFSDs producing a more densely fractured mantle.

The Central 1000 au of a Prestellar Core Revealed with ALMA. II. Almost Complete Freeze-out

Abstract Prestellar cores represent the initial conditions in the process of star and planet formation. Their low temperatures (&lt;10 K) allow the formation of thick icy dust mantles, which will be partially preserved in future protoplanetary disks, ultimately affecting the chemical composition of planetary systems. Previous observations have shown that carbon- and oxygen-bearing species, in particular CO, are heavily depleted in prestellar cores due to the efficient molecular freeze-out onto the surface of cold dust grains. However, N-bearing species such as NH3 and, in particular, its deuterated isotopologues appear to maintain high abundances where CO molecules are mainly in the solid phase. Thanks to ALMA, we present here the first clear observational evidence of NH2D freeze-out toward the L1544 prestellar core, suggestive of the presence of a “complete depletion zone” within a ≃1800 au radius, in agreement with astrochemical prestellar core model predictions. Our state-of-the-art chemical model coupled with a non-LTE radiative transfer code demonstrates that NH2D becomes mainly incorporated in icy mantles in the central 2000 au and starts freezing out already at ≃7000 au. Radiative transfer effects within the prestellar core cause the NH2D(111 − 101) emission to appear centrally concentrated, with a flattened distribution within the central ≃3000 au, unlike the 1.3 mm dust continuum emission, which shows a clear peak within the central ≃1800 au. This prevented NH2D freeze-out from being detected in previous observations, where the central 1000 au cannot be spatially resolved.

Modelling the water and carbon dioxide production rates of Comet 67P/Churyumov–Gerasimenko

ABSTRACT The European Space Agency Rosetta/Philae mission to Comet 67P/Churyumov–Gerasimenko in 2014–2016 is the most complete and diverse investigation of a comet carried out thus far. Yet, many physical and chemical properties of the comet remain uncertain or unknown, and cometary activity is still not a well-understood phenomenon. We here attempt to place constraints on the nucleus abundances and sublimation front depths of H2O and CO2 ice, and to reconstruct how the nucleus evolved throughout the perihelion passage. We employ the thermophysical modelling code ‘Numerical Icy Minor Body evolUtion Simulator’, or nimbus, to search for conditions under which the observed H2O and CO2 production rates are simultaneously reproduced before and after perihelion. We find that the refractories to water–ice mass ratio of relatively pristine nucleus material is μ ≈ 1, that airfall material has μ ≈ 2, and that the molar abundance of CO2 relative H2O is near 30 per cent. The dust mantle thickness is typically $\lesssim 2\, \mathrm{cm}$. The average CO2 sublimation front depths near aphelion were $\sim 3.8\, \mathrm{m}$ and $\sim 1.9\, \mathrm{m}$ on the northern and southern hemispheres, respectively, but varied substantially with time. We propose that airfall material is subjected to substantial fragmentation and pulverization due to thermal fatigue during the aphelion passage. Sub-surface compaction of material due to CO2 activity near perihelion seems to have reduced the diffusivity in a measurable way.

New constraints on the chemical composition and outgassing of 67P/Churyumov-Gerasimenko

Strong heterogeneities in the composition of the volatile species have been detected in the coma of comet 67P/Churyumov-Gerasimenko (67P/C-G) by the ROSINA instrument onboard the ESA’s Rosetta spacecraft. However, it is not clear if these heterogeneities are indicative of heterogeneities in the near-surface nucleus composition or if the coma composition is mainly insolation-driven. In order to clarify the link between the composition of the nucleus and the composition of the coma we have performed numerical simulations and compare our results with measurements acquired by ROSINA/DFMS for three major volatile species namely, H2O, CO2, and CO. We use a previously published thermo-physical numerical model designed to study cometary nucleus evolution, including volatile outgassing and internal stratigraphy, as the comet orbits the Sun. The model follows schemes used for much of the past three decades to model cometary outgassing. Our results match well the experimental volatiles density measurements of ROSINA/DFMS for most of the Rosetta mission. They suggest that the outgassing pattern is mainly insolation-driven and the variations are caused by the tilt of rotation axis and eccentricity of the nucleus. The nucleus shows to 1st order a homogeneous composition and therefore we can provide constraints on the bulk volatiles composition of 67P/C-G nucleus which is dominated by H2O (91.4% ​± ​4.5%), then CO2 (6.7% ​± ​3.5%) and CO in small amount (1.9% ​± ​1.2%). However, in details, a dichotomy in composition between the northern and southern hemispheres of the comet is revealed. A CO/CO2 bulk composition ratio of about 0.6 ​± ​0.1 is required to reproduce the measurements from the northern hemisphere and about 0.2 ​± ​0.1 for the southern hemisphere. To match the data, the thermal properties of the nucleus surface must be modified by adding a thin desiccated dust mantle (~5 ​mm) for northern latitudes while this appears not to be necessary for southern latitudes. This may be related to the observed dichotomy in putative airfall deposits. We suspect that, because of thermal inertia, seasonally non-illuminated areas continue to outgas and influence the ROSINA measurements. This effect cannot be reproduced with the model. Therefore during some periods of the mission, the fits are not ideal. Finally, the outgassing of the different ices leads to a layered internal structure defined by the sublimation front of each ice and formation of harder layers close to the surface due to sublimation/condensation processes. However, the thermo-physical model overestimates the absolute volatiles production (mainly in the southern hemisphere) leading to an overestimation of the erosion rates. Further investigations will be performed to improve the thermo-physical model and the sensitivity analysis.

Bubbles to Chondrites-II. Chemical fractionations in chondrites

We attempt to develop a possible theory of chemical fractionations in chondrites, that is consistent with various features of chondritic components and current observation of protoplanetary disks (PPD). Combining the 3+2 component fitting calculation that simulates chondrule formation process proposed in paper (I) with additional mixing procedures, we investigate essential causes that made various types of chondrites evolve from the uniform solar system composition, the CI-chondritic composition. Seven chemical types of chondrites (CM, CV, CO, E, LL, L and H) are examined, for which reliable chemical compositions for both bulk chondrites and chondrules therein are known. High vaporization degree of the primordial dust aggregates (dustons) required by the calculation vindicates that the chondrule formation was the driving force for the chemical fractionations in all chondrites examined. Various initial redox states in dustons and different timings of CAIs’ invasion to the chondrule formation zone are identified for different chondrite types. These results, together with a good correlation with the D/H ratios of chondrites measured previously, lead us to the notion that PPD evolved from reducing to oxidizing. We explore the heating mechanism for the chondrule formation and the place it occurred. Only heat source being consistent with our chondrule formation model is lightning discharge. We postulate that large vortices encompassing the snow-line are ideal places for large charge separation to occur between dustons and small ice particles, and that direct strikes on dustons should make them boil for ten seconds and longer and allow a swarm of chondrules released from their surfaces. Chemical fractionations are completed by an aerodynamic separation of dustons from chondrules inside the vortex, in such a way that the dustons fall fast into the vortex center and form a planetesimal immediately, while chondrules with dust mantles fall slow and form a thin veneer on the planetesimal surface. During collisional episodes, the veneers are preferentially fragmented and reassemble themselves by a weak self-gravity to form a rubble-piled chondritic asteroid, i.e. chondrite.

Seeds of Life in Space (SOLIS)

Context.The interstellar complex organic molecules (iCOMs) are C-bearing molecules containing at least six atoms; two main proposals for their formation are suggested: a direct formation in the icy mantle of the dust grains and formation through the reaction in gas phase of released grain mantle species. The shocked gas along outflows driven by low-mass protostars is a unique environment to study how the iCOMs can be formed as the composition of the dust mantles is sputtered into the gas phase.Aims.The chemical richness in shocked material associated with low-mass protostellar outflows has been so far studied in the prototypical L1157 blue-shifted outflow to investigate the iCOM formation routes. To understand whether the case of L1157-B1 is unique, we imaged and studied the IRAS 4A outflows in the NGC 1333 star forming region.Methods.We used the NOrthern Extended Millimeter Array interferometer as part of the IRAM Seeds Of Life in Space (SOLIS) Large Program to image the large-scale bipolar outflows driven by the IRAS 4A system in the 3 mm band, and we compared the observation with the GRAINOBLE+ astrochemical model.Results.We report the first detection, in the IRAS 4A outflows, of several iCOMs: six lines of methanol (CH3OH), eight of acetaldehyde (CH3CHO), one of formamide (NH2CHO), and four of dimethyl ether (CH3OCH3), all sampling upper excitation energy up to ~30 K. We found a significant chemical differentiation between the southeast outflow driven by the IRAS 4A1 protostar, showing a richer molecular content, and the north–southwest one driven by the IRAS 4A2 hot corino. The CH3OH/CH3CHO abundance ratio is lower by a factor of ~4 in the former; furthermore, the ratio in the IRAS 4A outflows is lower by a factor of ~10 with respect to the values found in different hot corinos.Conclusions.After L1157-B1, the IRAS 4A outflow is now the second outflow to show an evident chemical complexity. Given that CH3OH is a grain surface species, the astrochemical gas-phase model run with GRAINOBLE+ reproduced our observation assuming that acetaldehyde is formed mainly through the gas-phase reaction of the ethyl radical (CH3CH2) and atomic oxygen. Furthermore, the chemical differentiation between the two outflows suggests that the IRAS 4A1 outflow is likely younger than that of the IRAS 4A2. Further investigation is needed to constrain the age of the outflow. In addition, observation of even younger shocks are necessary. In order to provide strong constraints on the CH3CHO formation mechanisms it would be interesting to observe CH3CH2, but given that its frequencies are not known, future spectroscopic studies on this species are needed.

Comparison of InSight Homestead Hollow to Hollows at the Spirit Landing Site

AbstractThe InSight spacecraft landed within an ~27‐m diameter highly degraded impact crater, informally called Homestead hollow, that was disturbed during landing by pulsed retrorockets that blew out dust and scoured loose sand around the landing site. In order to provide insight into what the surface of Homestead hollow originally looked like before landing and to further characterize hollow physical properties, we examined images of similar hollows taken by the Spirit rover at the Gusev landing site. Hollows at both sites are characterized by a quasi‐circular appearance with little or no crater rim still visible and a bright interior with fewer and smaller rock sizes relative to the surrounding plains. Resolvable clast lengths (&gt;2 mm) measured in Laguna hollow at the Spirit site and Homestead hollow are comparable with most clasts between 3 and 7 mm in length. Measurements of clast shapes show that those in Laguna hollow are slightly more elongate relative to those in Homestead hollow, although this may be an artifact of the differing viewing geometry and (or) a thicker dust mantle obscuring the full shape of clasts at Laguna hollow. The soils at both hollows show evidence for cohesion and a duricrust, with a trench dug at Laguna hollow and pits exposed at Homestead hollow exhibiting steep slopes, overhanging layers, and clods of soils. The similarities in morphology and physical properties of hollows at two different landing sites suggest recent environmental conditions that degrade and infill impact craters are comparable and pervasive for equatorial volcanic plains on Mars.

Investigating the Rosetta/RTOF observations of comet 67P/Churyumov-Gerasimenko using a comet nucleus model: influence of dust mantle and trapped CO

Context.Cometary outgassing is induced by the sublimation of ices and the ejection of dust originating from the nucleus. Therefore measuring the composition and dynamics of the cometary gas provides information concerning the interior composition of the body. Nevertheless, the bulk composition differs from the coma composition, and numerical models are required to simulate the main physical processes induced by the illumination of the icy body.Aims.The objectives of this study are to bring new constraints on the interior composition of the nucleus of comet 67P/Churyumov-Gerasimenko (hereafter 67P) by comparing the results of a thermophysical model applied to the nucleus of 67P and the coma measurements made by the Reflectron-type Time-Of-Flight (RTOF) mass spectrometer. This last is one of the three instruments of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA), used during the Rosetta mission.Methods.Using a thermophysical model of the comet nucleus, we studied the evolution of the stratigraphy (position of the sublimation and crystallisation fronts), the temperature of the surface and subsurface, and the dynamics and spatial distribution of the volatiles (H2O, CO2and CO). We compared them with the in situ measurements from ROSINA/RTOF and an inverse coma model.Results.We observed the evolution of the surface and near surface temperature, and the deepening of sublimation fronts. The thickness of the dust layer covering the surface strongly influences the H2O outgassing but not the more volatiles species. The CO outgassing is highly sensitive to the initial CO/H2O ratio, as well as to the presence of trapped CO in the amorphous ice.Conclusions.The study of the influence of the initial parameters on the computed volatile fluxes and the comparison with ROSINA/RTOF measurements provide a range of values for an initial dust mantle thickness and a range of values for the volatile ratio. These imply the presence of trapped CO. Nevertheless, further studies are required to reproduce the strong change of behaviour observed in RTOF measurements between September 2014 and February 2015.

Seeds of Life in Space (SOLIS)

Context.It is nowadays clear that a rich organic chemistry takes place in protostellar regions. However, the processes responsible for it, that is, the dominant formation routes to interstellar complex organic molecules, are still a source of debate. Two paradigms have been evoked: the formation of these molecules on interstellar dust mantles and their formation in the gas phase from simpler species previously synthesised on the dust mantles.Aims.In the past, observations of protostellar shocks have been used to set constraints on the formation route of formamide (NH2CHO), exploiting its observed spatial distribution and comparison with astrochemical model predictions. In this work, we follow the same strategy to study the case of acetaldehyde (CH3CHO).Methods.To this end, we used the data obtained with the IRAM-NOEMA interferometer in the framework of the Large Program SOLIS to image the B0 and B1 shocks along the L1157 blueshifted outflow in methanol (CH3OH) and acetaldehyde line emission.Results.We imaged six CH3OH and eight CH3CHO lines which cover upper-level energies up to ~30 K. Both species trace the B0 molecular cavity as well as the northern B1 portion, that is, the regions where the youngest shocks (~1000 yr) occurred. The CH3OH and CH3CHO emission peaks towards the B1b clump, where we measured the following column densities and relative abundances: 1.3 × 1016cm−2and 6.5 × 10−6(methanol), and 7 × 1013cm−2and 3.5 × 10−8(acetaldehyde). We carried out a non-LTE (non-Local Thermodinamic Equilibrium) Large Velocity Gradient (LVG) analysis of the observed CH3OH line: the average kinetic temperature and density of the emitting gas areTkin~ 90 K andnH2~ 4 × 105cm−3, respectively. The CH3OH and CH3CHO abundance ratio towards B1b is 190, varying by less than a factor three throughout the whole B0–B1 structure.Conclusions.Comparison of astrochemical model predictions with the observed methanol and acetaldehyde spatial distribution does not allow us to distinguish whether acetaldehyde is formed on the grain mantles or in the gas phase, as its gas-phase formation, which is dominated by the reaction of ethyl radical (CH3CH2) with atomic oxygen, is very fast. Observations of acetaldehyde in younger shocks, for example those of ~102yr old, and/or of the ethyl radical, whose frequencies are not presently available, are necessary to settle the issue.

The effect of internal gravity waves on cloud evolution in sub-stellar atmospheres

Context. Sub-stellar objects exhibit photometric variability, which is believed to be caused by a number of processes, such as magnetically-driven spots or inhomogeneous cloud coverage. Recent sub-stellar models have shown that turbulent flows and waves, including internal gravity waves, may play an important role in cloud evolution. Aims. The aim of this paper is to investigate the effect of internal gravity waves on dust nucleation and dust growth, and whether observations of the resulting cloud structures could be used to recover atmospheric density information. Methods. For a simplified atmosphere in two dimensions, we numerically solved the governing fluid equations to simulate the effect on dust nucleation and mantle growth as a result of the passage of an internal gravity wave. Furthermore, we derived an expression that relates the properties of the wave-induced cloud structures to observable parameters in order to deduce the atmospheric density. Results. Numerical simulations show that the density, pressure, and temperature variations caused by gravity waves lead to an increase of the dust nucleation rate by up to a factor 20, and an increase of the dust mantle growth rate by up to a factor 1.6, compared to their equilibrium values. Through an exploration of the wider sub-stellar parameter space, we show that in absolute terms, the increase in dust nucleation due to internal gravity waves is stronger in cooler (T dwarfs) and TiO2-rich sub-stellar atmospheres. The relative increase, however, is greater in warm (L dwarf) and TiO2-poor atmospheres due to conditions that are less suited for efficient nucleation at equilibrium. These variations lead to banded areas in which dust formation is much more pronounced, similar to the cloud structures observed on Earth. Conclusions. We show that internal gravity waves propagating in the atmosphere of sub-stellar objects can produce banded clouds structures similar to that observed on Earth. We propose a method with which potential observations of banded clouds could be used to estimate the atmospheric density of sub-stellar objects.