Dust Mantle Research Articles

Infrared detection of gas phase formaldehyde towards the high mass protostar W33A

We report the detection of numerous absorption lines of the band of gaseous formaldehyde (H2CO) near 3.6 μm towards the high-mass protostar W33A. This is the first infrared detection of gaseous H2CO in an interstellar cloud. An upper limit toward RAFGL 7009S is also reported. The column density of H2CO detected at 3.6 μm toward W33A is much higher than that measured in large beam millimeter wave observations toward the same source, suggesting that it is concentrated close to the infrared continuum source. The mean temperature of the H2CO, derived from LTE spectral modelling, is ~100 K, which is close to the values found for most other gas phase species observed in absorption and is the expected temperature of the cloud at the dust mantle sublimation interface, implying a large jump in the abundance of gaseous H2CO from the surrounding cold cloud to the warm gas. The ratio of gaseous and solid H2CO column densities is about 0.03 for W33A and less than 0.02 for RAFGL 7009S. We discuss this detection in the context of the two most likely mechanisms for H2CO formation: grain mantle evaporation, and gas phase reactions between species produced by photodissociation of grain mantle evaporants.

Non-nebular origin of dark mantles around chondrules and inclusions in CM chondrites

Our examination of nine CM chondrites that span the aqueous alteration sequence leads us to conclude that compact dark fine mantles surrounding chondrules and inclusions in CM chondrites are not discrete fine-grained rims acquired in the solar nebula as modeled by Metzler et al. [Accretionary dust mantles in CM chondrites: evidence for solar nebula processes. Geochim. Cosmochim. Acta 56, 1992, 2873–2897]. Nebular processes that lead to agglomeration produce materials with porosities far higher than those in the dark mantles. We infer that the mantles were produced from porous nebular materials on the CM parent asteroid by impact-compaction (a process that produces the lowest porosity adjacent to chondrules and inclusions). Compaction was followed by aqueous alteration that formed tochilinite, serpentine, Ni-bearing sulfide, and other secondary products in voids in the interchondrule regions. Metzler et al. reported a correlation between mantle thickness and the radius of the enclosed object. In Yamato 791198 we find no correlation when all sizes of central objects and dark lumps are included but a significant correlation ( r 2 = 0.44) if we limit consideration to central objects with radii >35 μm; a moderate correlation is also found in QUE 97990. We suggest that impact-induced shear of a plum-pudding-like precursor produced the observed “mantles”; these were shielded from comminution during impact events by the adjacent stronger chondrules and inclusions. Some mantles in CM chondrites with low degrees of alteration show distinct layers that may largely reflect differences in porosity. Typically, a gray, uniform inner layer is surrounded by an outer layer consisting of darker silicates with BSE-bright speckles. The CM-chondrite objects characterized as “primary accretionary rocks” by Metzler et al. did not form in the nebula, but rather on the parent body. The absence of solar-flare particle tracks and solar-wind-implanted rare gases in these clasts reflect their lithified nature and low surface/volume ratios during the period when they resided in the regolith and were subject to irradiation by solar particles. The clasts are analogous to the light-colored metamorphosed clasts in ordinary-chondrite regolith breccias (which also lack solar-flare particle tracks and solar-wind gas).

67P/Churyumov–Gerasimenko nucleus model: Portrayal of the Rosetta target

Rosetta is a cornerstone ESA mission dedicated to the detailed study of a comet nucleus before and after the onset of activity. Comet 67P/Churyumov–Gerasimenko is the new Rosetta target. At the moment, little is known about this comet, but the successful design of the ROSETTA mission requires some preliminary knowledge of the comet’s physical parameters. In fact, surface and interior temperature, day–night variations, onset of activity, percentage of active surface, intensity of different gas and dust fluxes are needed to select safe orbits around the comet and to optimize the scientific return. These quantities cannot be determined only through ground-based observations, so predictive models of the thermal evolution and differentiation of a cometary nucleus are necessary. We have developed a comet nucleus thermal evolution model that helps in the estimations of the above quantities. The ideal nucleus is an initially homogeneous and porous sphere, composed by a mixture of different ices and dust particles. The results have been computed with a numerical code solving simultaneously the heat conduction and gas diffusion equations through an idealized spherical comet nucleus. We show the results obtained applying this model to a comet nucleus with the characteristics of Churyumov–Gerasimenko. The results indicate that the peculiar dynamical history has a strong effect on the comet with the formation of a dust mantle before the 1959 close encounter. After that event, the perihelion distance was reduced and the comet was probably “reactivated”. In our model, we see a slight increase of the activity passage after passage. We found a good agreement between the model results and the few data available.

Comet 67P/Churyumov–Gerasimenko: Modeling of orientation and structure

The comet 67P/Churyumov–Gerasimenko is the current target of the mission Rosetta, initially planned to investigate comet 46P/Wirtanen. These two comets have similar orbits, except the distance to the Sun at perihelion, but different orbital histories and different masses. Thus, structures of the nuclei can be significantly different. The evolution of comet Wirtanen was simulated by several authors, while comet Churyumov–Gerasimenko became an object of high interest only recently and is not well investigated. In the present work we simulate the evolution of the nucleus, down to tens of meters below the surface, using an extended version of the model previously applied for comet Wirtanen [Kossacki et al., 1999. Comet 46P/Wirtanen: evolution of the subsurface layer. Icarus 142, 202–218.]. The model includes strengthening of the nucleus due to sintering of the ice grains. Simulations are performed for different latitudes, accounting for the evolution of the orbit and for changes of the nucleus orientation, as well as diurnal and seasonal changes of insolation. The calculated loss of water vapor from the comet is integrated over the nucleus surface and is compared with the observational data. We have found, that the sublimation through the dust mantle can be large enough to reproduce the profile of the total water production as a function of time from perihelion. The required dependence of thickness of the dust layer on latitude qualitatively matches present distribution of the absorbed solar flux. The non-gravitational acceleration in the comet motion together with the simulated sublimation flux are used in order to estimate the mass and the bulk density of the nucleus.

The crater model of an active area as applied to comet 81P/WILD-2

Both temperature and sublimation regime of an active region on the comet Wild-2 nucleus, considered as a conical hole in the dust mantle, are calculated in the present paper. All the calculations are carried out for the case, when the season effects are distinctly expressed. It is shown that the additional heating by the infrared radiation from the hot side surface of the crater results in amplification of sublimation. The temperature and amplification factors are defined for different geometrical parameters of the active region. They were calculated separately for icy bottom and dusty walls. Practically there are no changes of the temperature of the icy bottom over the whole period of rotation. It is stabilized by sublimation and income of reradiated energy from the dust walls heated by the Sun. The obtained results show that a conical crater structure on the surface of the comet Wild-2 nucleus intensifies sublimation by a factor ⩽1.8.

Material Processing of Interstellar Dust in Comets

AbstractTo better understand the processing of pristine materials in comets, we compare the composition of cometary and interstellar dust. We suggest that the deficit of N in comet dust bears evidence for the processing of the organic refractory mantle of pre-solar interstellar dust, unless it arises from the protosolar disk chemistry. The amorphous silicate core, in contrast, seems to be protected against processing due to the presence of the organic refractory mantle.

Basaltic Rocks Analyzed by the Spirit Rover in Gusev Crater

The Spirit landing site in Gusev Crater on Mars contains dark, fine-grained, vesicular rocks interpreted as lavas. Pancam and Mini–Thermal Emission Spectrometer (Mini-TES) spectra suggest that all of these rocks are similar but have variable coatings and dust mantles. Magnified images of brushed and abraded rock surfaces show alteration rinds and veins. Rock interiors contain ≤25% megacrysts. Chemical analyses of rocks by the Alpha Particle X-ray Spectrometer are consistent with picritic basalts, containing normative olivine, pyroxenes, plagioclase, and accessory FeTi oxides. Mössbauer, Pancam, and Mini-TES spectra confirm the presence of olivine, magnetite, and probably pyroxene. These basalts extend the known range of rock compositions composing the martian crust.

Formation of accretionary dust mantles in the solar nebula: Evidence from preirradiated olivines in CM chondrites

Abstract— CM chondrites are regolith breccias consisting of lithic clasts embedded in a fine‐grained clastic matrix. The majority of these lithic clasts belongs to a texturally well‐defined rock type (primary rock) that can be described as an agglomerate of chondrules and other coarse‐grained components, most of which are surrounded by fine‐grained rims (dust mantles). Metzler et al. (1992) explain these textures as the result of accretionary processes in the solar nebula, while an alternative model explains them to be the result of regolith processes on the parent body (Sears et al. 1993).The main intention of the present study is to discern between both models by investigating the occurrence, frequency, spatial distribution, and textural setting of preirradiated (track‐rich) olivines in CM chondrites. Track‐rich olivines were studied in situ in six polished thin sections from 4 different CM chondrites (Cold Bokkeveld, Mighei, Murchison, Nogoya) by optical and scanning electron microscopy (SEM). It was found that their occurrence is restricted to the clastic matrix of these meteorites. The primary rock seems to have formed in an environment shielded from cosmic radiation, since fragments of this rock are free of track‐rich grains and solar noble gases. This finding supports the solar nebula model for the formation of dust mantles around chondrules and other coarse‐grained components, and points against a regolith origin.In Cold Bokkeveld, a small breccia‐in‐breccia clast was found, which has been irradiated as an entity within the uppermost millimeters to meters of its parent body for at least about 3 Ma. This clast seems to represent a compacted subsurface layer that was later excavated by impact and admixed to the host breccia.Furthermore, the results of this study may affect the interpretation of compaction ages obtained by fission track methods, since these ages may be mixtures of different contact ages between finegrained, U‐rich dust and U‐poor olivines. In some cases, they may date the formation of dust mantles in the solar nebula, while in other cases the lithification of the host breccias may be dated.

Basaltic rocks analyzed by the Spirit Rover in Gusev Crater.

The Spirit landing site in Gusev Crater on Mars contains dark, fine-grained, vesicular rocks interpreted as lavas. Pancam and Mini-Thermal Emission Spectrometer (Mini-TES) spectra suggest that all of these rocks are similar but have variable coatings and dust mantles. Magnified images of brushed and abraded rock surfaces show alteration rinds and veins. Rock interiors contain </=25% megacrysts. Chemical analyses of rocks by the Alpha Particle X-ray Spectrometer are consistent with picritic basalts, containing normative olivine, pyroxenes, plagioclase, and accessory FeTi oxides. Mössbauer, Pancam, and Mini-TES spectra confirm the presence of olivine, magnetite, and probably pyroxene. These basalts extend the known range of rock compositions composing the martian crust.

A quantitative model for comet nucleus topography

Topography of the surface of a comet nucleus is likely rough at all scales smaller than the mean effective radius. We present a flexible and easily scalable model for quantitative calculations simulating the effects of comet nucleus topography on gas release and dust mantle evolution. The topographic features we describe must be large enough (typically > 10 m) so that they will not erode in one orbit of the nucleus around the Sun. The maximum effective size of a hill is about 1 √2 times the effective radius of the nucleus. If it is larger, then an ellipsoidal shape of the nucleus is more appropriate. The procedure described here also permits for inhomogeneous composition of the topographic features, leading to locally different rates of gas production (e.g., jet-like features and filaments) or different thicknesses of the dust mantle. It also can give rise to different temperature patches, locally varying albedos and emissivities, and may explain the formation of permanent dust mantles.

Models of P/Borrelly: Activity and dust mantle formation

Comet 19P/Borrelly was observed by Deep Space One spacecraft on September 22, 2001 (Soderblom et al., 2002).The DS1 images show a very dark and elongate nucleus with a complex topography; the IR spectra show a strong red-ward slope consistent with a very hot and dry surface (345K to 300K). During DS1 encounter the comet coma was dominated by a prominent jet but most of the comet was inactive, confirming the Earth-based observations that <10% of the surface is actively sublimating. We have developed a thermal evolution model of comet PBorrelly, using a numerical code that is able to solve the heat conduction and gas diffusion equations at the same time across an idealized spherical nucleus ( De Sanctis et al., 1999, 2000; Capria et al., 2000; Coradini et al., 1997a,b). The comet nucleus is composed by water, volatiles ices and dust in different proportions. The refractory component is made by grains that are embedded in the icy matrix. The code is able to account for the dust release, contributing to the dust flux, and the formation of dust mantles on the comet surface. The model was applied to a cometary nucleus with the estimated physical and dynamical characteristics of P/Borrelly in order to infer the status and activity level of a body on such an orbit during the DS1 observation. The comet gas flux, differentiation and thermal behavior were simulated and reproduced. The model results are in good agreement with the DS1 flyby results and the ground based observations, in terms of activity, dust coverage and temperatures of the surface.

Resurfacing history of the northern plains of Mars based on geologic mapping of Mars Global Surveyor data

Geologic mapping of the northern plains of Mars, based on Mars Orbiter Laser Altimeter topography and Viking and Mars Orbiter Camera images, reveals new insights into geologic processes and events in this region during the Hesperian and Amazonian Periods. We propose four successive stages of lowland resurfacing likely related to the activity of near‐surface volatiles commencing at the highland‐lowland boundary (HLB) and progressing to lower topographic levels as follows (highest elevations indicated): Stage 1, upper boundary plains, Early Hesperian, &lt;−2.0 to −2.9 km; Stage 2, lower boundary plains and outflow channel dissection, Late Hesperian, &lt;−2.7 to −4.0 km; Stage 3, Vastitas Borealis Formation (VBF) surface, Late Hesperian to Early Amazonian, &lt;−3.1 to −4.1 km; and Stage 4, local chaos zones, Early Amazonian, &lt;−3.8 to −5.0 km. At Acidalia Mensa, Stage 2 and 3 levels may be lower (&lt;−4.4 and −4.8 km, respectively). Contractional ridges form the dominant structure in the plains and developed from near the end of the Early Hesperian to the Early Amazonian. Geomorphic evidence for a northern‐plains‐filling ocean during Stage 2 is absent because one did not form or its evidence was destroyed by Stage 3 resurfacing. Remnants of possible Amazonian dust mantles occur on top of the VBF. The north polar layered deposits appear to be made up of an up to kilometer‐thick lower sequence of sandy layers Early to Middle Amazonian in age overlain by Late Amazonian ice‐rich dust layers; both units appear to have outliers, suggesting that they once were more extensive.

An investigation of errors in estimates of the cometary nuclei active area fractions

Active area fractions of cometary nuclei are often estimated by comparing the observed water production rates with theoretical rates obtained by applying the fast rotator or subsolar point approximations to spherical model nuclei. Any discrepancy between observed and theoretical production rates is interpreted as a certain degree of dust mantling (or in some cases hyper activity) of the object. We here investigate the typical errors introduced in such active area fraction estimates by the usage of oversimplified spherical model nuclei. This is done by first calculating the production rates of slowly rotating irregular model bodies with dierent activity patterns on their surfaces and arbitrary spin axis orientations, for which solar illumination is treated properly. Next, the production rates of the spherical model objects under averaged insolation are compared to the production rates of the complex model objects in an attempt to recover the known active area fraction of the latter bodies. We then find that the fast rotator and subsolar point approximations generally yield large over- and underestimates of the active area fraction, depending on the characteristics of the simulated complex nuclei. Acceptable relative errors (<100%) only occur at small heliocentric distances, and the subsolar point approximation yields somewhat better results than the fast rotator approximation.

Near-infrared spectroscopy of the nucleus of comet 124P/Mrkos

We report near-infrared (0.9–2.3 μ m) spectral observations of the bare nucleus of comet 124P/Mrkos taken on two different nights using the Near Infrared Camera Spectrometer (NICS) attached to the 3.56 m Telescopio Nazionale Galileo (TNG). Both spectra are very similar, featureless and slightly redder than the Sun, resembling the spectrum of a D type asteroid. The spectrum and the low activity level of the nucleus of 124P suggest that it is covered by a large insulating dust mantle.

Ice concentration and distribution near the south pole of Mars: Synthesis of odyssey and global surveyor analyses

Mars Odyssey Gamma‐Ray Spectrometer (GRS) neutron spectrometer data are analyzed to determine the concentration and boundary of buried water ice near the south pole. The measurements are consistent with a circumpolar layer of average ice concentration from 35 to 100% by weight and superposed dust with thickness density product 30 to 40 g/cm2. The region of buried water ice extends from near the south pole to latitudes 48S to 58S. The equatorward boundary of the ice‐rich region compares favorably with the boundary of an ice‐rich dust mantle inferred from Mars Orbiter Laser Altimeter data and the locations of dissected terrain inferred from Mars Orbiter Camera images. The ice‐rich mantle is identified as the source of enhanced hydrogen sensed by the neutron spectrometer.

Recurring Outbursts and Nuclear Fragmentation of Comet C/2001 A2 (LINEAR)

Analysis of the visual light curve and fragmentation sequence of comet C/2001 A2 (LINEAR) shows a strong temporal correlation between the onset of outbursts and separation of companion nuclei. This scenario conforms to Sekanina's conceptual model for the release of sizable fragments of an inert dust mantle from the nucleus surface: an outburst is triggered as some of the mass rapidly disintegrates into fine dust.

Æolian dust contributions to soil of the Namoi Valley, northern NSW, Australia

Æolian processes have been regarded as having little relevance to pedogenesis in the subhumid regions of eastern Australia, due perhaps to a perceived paucity of clear evidence of dust deposition and to difficulties of measuring an æolian dust component within soils. Contrary to earlier opinions, an æolian soil component may occur in eastern Australia, as appreciable amounts of dust are deposited annually. We report dust deposition rates and some dust characteristics at Gunnedah, and measured granulometric, mineralogical and geochemical attributes of four soil profiles found in different physiographic areas of the Namoi Valley. Estimates of dust deposition rates indicate that a deposit at least 20 cm thick could have formed over the last 13,000 years. The soil profile which most clearly indicates the presence of dust overlies basalt at Bald Hill. Here, the topsoil contains a conspicuous population of particles with a modal diameter of 40–50 μm and has been enriched in Si by the addition of quartz, which cannot have originated from the basalt or from colluvial or alluvial processes. A clayey floodplain soil profile (Boonawa) also contained a large dust-particle population and also illite and moderate concentrations of K of possible æolian origin. Two sandy profiles at Round Swamp displayed the same conspicuous dust-particle populations, and one exhibited an interstratified illite–smectite component, suggesting dust additions. Local æolian processes were also indicated by the well-sorted, coarse sand-sized particles of the Round Swamp East profile, located on a low lunette adjacent to the swamp. These fine-grained, quartz-rich, æolian dust deposits differ considerably from the benchmark Australian æolian dust mantles, which form the clay-aggregated, calcareous parna soils of southern New South Wales.

A Model of Heat and Mass Transfer in a Porous Cometary Nucleus Based on a Kinetic Treatment of Mass Flow

The main aspect of this paper is to provide a synthesis between two major lines of development in the understanding of mass and heat transfer in a volatile porous medium. The first one is a macroscopic approach, where the medium is considered as a continuum, and heat and mass transfer equations are solved under appropriate boundary conditions for temperature and gas pressure (G. Steiner and N. I. Kömle 1991, Planet. Space Sci. 39, 507–513; Y. Mekler et al. 1990, Astrophys. J. 356, 682–686; S. J. Espinasse et al. 1991, Icarus 92, 350–365), while the second one is a kinetic model, calculating gas flow in tubes under the assumption of a known temperature distribution (N. I. Kömle and G. Dettleff 1991, Icarus 89, 73–84; Yu. V. Skorov et al. 1999, Icarus 140, 173–188). We review briefly the main aspects of this previous work, and subsequently present a combined consistent model, which uses a macroscopic heat transfer equation, but kinetic solutions for the gas flow. This new model was implemented as a numerical code and its performance is demonstrated by a couple of example calculations. The main advantage of the new model in comparison to the macroscopic approach is the fact that it avoids specifying a boundary condition for gas pressure at the surface, because the emitted gas flux is found at any time with the aid of the kinetic calculation. The local balance of sublimation and condensation in the interior of the porous ice can be calculated more consistently than is possible by macroscopic models only, because surface pressure and density develop in a “natural” way and no external boundary condition for the pressure must be imposed. We consider the development of temperature distribution and gas flux in ice samples in response to surface irradiation. Both pure ice and ice covered by a dust mantle are studied. The results are compared with corresponding solutions obtained on the basis of a macroscopic model, and differences are discussed in detail. Finally, experimental data obtained from previous comet simulation experiments (KOSI and related laboratory experiments) are reconsidered. In particular, temperature profiles and gas fluxes from the KOSI-9 experiment (E. Grün et al., 1993, J. Geophys. Res. 98, 15,091–15,104) are interpreted in terms of the kinetic approach.

Stability of water ice under a porous nonvolatile layer: implications to the south polar layered deposits of Mars

The analysis of the Viking IRTM data led to the conclusion that the surface layer of the Mars south polar layered deposits has very low thermal inertia. This is consistent with the layered deposits being mantled by a layer consisting of very fine dust. Analysis of the heat and mass transport through such a layer allows us to estimate the minimum thickness of a dust mantle required to guarantee water ice stability beneath it. We find that if the thermal inertia is as low as 100 J m −2 s −1/2 K −1 and the dust layer is composed of particles of about 10 μm , then a 5 mm thick layer is sufficient to keep the ground water ice stable on at least seasonal time scales.

A Multitransition HCO+Study in NGC 2264G: Anomalous Emission of theJ = 1 → 0 Line

We present multitransition observations of the HCO+ molecule toward the very young star-forming region associated with the NGC 2264G molecular outflow. Anomalous emission is observed in the lowest rotational transition; the J = 4 → 3 and J = 3 → 2 transitions clearly trace the dense core encompassing the exciting source of the molecular outflow, whereas the HCO+ J = 1 → 0 is barely detected at a much lower intensity and has a much broader line shape. Analysis of the data strongly suggests that the HCO+ J = 1 → 0 emission arising from the core is being absorbed efficiently by a cold low-density envelope around the core or a foreground cloud. This result seems exceptional, yet the J = 1 → 0 HCO+ and HCN emission from other dense cores (especially those in giant molecular clouds) may be affected. In these cases, the rare isotopes of these molecules and higher rotational transitions of the main isotopes should be used to study these regions. Two quiescent clumps, JMG 99 G1 and G2, are detected in the blue lobe of the NGC 2264G molecular outflow, close to shock-excited near-IR H2 knots. These clumps belong to the class of radiatively excited clumps, i.e., the radiation from the shock evaporates the dust mantles and initiates a photochemical process, enhancing the emission of the HCO+.