Detailed calculations are made to test the predictions of Duley, Millar & Williams (1978) concerning the chemical reactivity of interstellar oxide grains. A method is established for calculating interaction energies between atoms and the perfect crystal with or without surface vacancy sites. The possibility of reactions between incident atoms and absorbed atoms is investigated. We conclude that H2 formation can occur on the perfect crystal surfaces, and that for other diatomic molecules the important formation sites are the |$\text F_{\text s ^-}$| and |$\text V_2^2$|-centres. The outline by Duley, Millar & Williams (1979) of interstellar oxide grain growth and destruction is justified by these calculations.
Tar sands (also referred to as asphaltic sandstones, heavy-oil deposits, or bitumen-impregnated sandstones) in W. Kentucky have been recognized as a potential mineral resource for over 100 yr. These deposits have become the subject of increasing interest as a potential petroleum resource. Previous studies have provided estimates of the potential resources of shallow mineable tar-sand deposits. These investigations have concentrated on the outcrop areas where tar sands are present at or near the surface. However, data on subsurface deposits have been lacking and currently no reports are available which evaluate the potential oil resources associated with the occurrences of deeper tar-sand deposits in W. Kentucky. The Kentucky Geological Survey has initiated a project to inventory and evaluate the oil-resource potential of asphaltic sandstones in the subsurface of W. Kentucky. The purpose of this study is to evaluate the subsurface occurrences of asphaltic sandstones in the Big Clifty Sandstone Member of the Golconda Formation in a portion of the tar-sand area of W. Kentucky.
INTRODUCTION Classic volcanism prevalent on terrestrial planets and volatile-poor protoplanets, such as asteroid Vesta, is based on silicate chemistry and is often expressed by volcanic edifices (unless erased by impact bombardment). In ice-rich bodies with sufficiently warm interiors, cryovolcanism involving liquid brines can occur. Smooth plains on some icy satellites of the outer solar system have been suggested as possibly cryovolcanic in origin. However, evidence for cryovolcanic edifices has proven elusive. Ceres is a volatile-rich dwarf planet with an average equatorial surface temperature of ~160 K. Whether this small (~940 km diameter) body without tidal dissipation could sustain cryovolcanism has been an open question because the surface landforms and relation to internal activity were unknown. RATIONALE The Framing Camera onboard the Dawn spacecraft has observed >99% of Ceres’ surface at a resolution of 35 m/pixel at visible wavelengths. This wide coverage and resolution were exploited for geologic mapping and age determination. Observations with a resolution of 135 m/pixel were obtained under several different viewing geometries. The stereo-photogrammetric method applied to this data set allowed the calculation of a digital terrain model, from which morphometry was investigated. The observations revealed a 4-km-high topographic relief, named Ahuna Mons, that is consistent with a cryovolcanic dome emplacement. RESULTS The ~17-km-wide and 4-km-high Ahuna Mons has a distinct size, shape, and morphology. Its summit topography is concave downward, and its flanks are at the angle of repose. The morphology is characterized by (i) troughs, ridges, and hummocky areas at the summit, indicating multiple phases of activity, such as extensional fracturing, and (ii) downslope lineations on the flanks, indicating rockfalls and accumulation of slope debris. These morphometric and morphologic observations are explained by the formation of a cryovolcanic dome, which is analogous to a high-viscosity silicic dome on terrestrial planets. Models indicate that extrusions of a highly viscous melt-bearing material can lead to the buildup of a brittle carapace at the summit, enclosing a ductile core. Partial fracturing and disintegration of the carapace generates slope debris, and relaxation of the dome’s ductile core due to gravity shapes the topographic profile of the summit. Modeling of this final phase of dome relaxation and reproduction of the topographic profile requires an extruded material of high viscosity, which is consistent with the mountain’s morphology. We constrained the age of the most recent activity on Ahuna Mons to be within the past 210 ± 30 million years. CONCLUSION Cryovolcanic activity during the geologically recent past of Ceres constrains its thermal and chemical history. We propose that hydrated salts with low eutectic temperatures and low thermal conductivities enabled the presence of cryomagmatic liquids within Ceres. These salts are the product of global aqueous alteration, a key process for Ceres’ evolution as recorded by the aqueously altered, secondary minerals observed on the surface. Perspective view of Ahuna Mons on Ceres from Dawn Framing Camera data (no vertical exaggeration). The mountain is 4 km high and 17 km wide in this south-looking view. Fracturing is observed on the mountain’s top, whereas streaks from rockfalls dominate the flanks.
We investigate a model in which HAC-coated silicate grains subjected to low-velocity shocks release carbon to the gas in the form of atoms and PAHs. Both HAC and PAH are assumed to be subject to chemistry, including reactions with oxygen atoms, carbon atoms and carbon ions. The released species contribute to a conventional post-shock chemistry, during which the HAC and PAHs may be further eroded or restored. We find that the parameter of crucial importance is the reaction efficiency for atomic oxygen with both HAC and PAH. If this efficiency is less than 100 per cent, then HAC mantles are (at least) restored in a period of about 106 yr. PAHs are destroyed on time-scales of less than about 105 yr unless the oxidation efficiency is very small (∼ 10−3). A definitive conclusion must await laboratory studies of oxidation; however, expected oxidation efficiencies of 0.1 would support the view that HAC mantles on grains in diffuse clouds can be restored in the intervals ( ∼ 106 yr) between periodic low-velocity shocks, that PAHs are post-shock transients, and that chemistry in diffuse clouds is significantly affected by the gas–grain interaction.
D. A. Williams ( UMIST, Manchester, U. K. ) . I wish to highlight some of the interesting and possibly controversial points that were raised. Professor Tayler gave us a very good introduction to the subject and I expect we shall discuss the questions that he raised on abundance anomalies and in particular the survival of grains. There are two particular aspects that interest me. First there is deuterium fractionation in the interstellar medium and it is, of course, known that deuterium fractionation occurs in meteorite material. That seems to indicate that material was fractionated in cold conditions and that the conditions have remained cold ever since because, if the temperature gets above ca . 200 K, that fractionation will disappear. The other point that I found particularly interesting in recent literature is the detection of diamond in the carbonaceous component of certain meteorites and again this seems to indicate a low-temperature regime for that particular material; diamond not being the most stable form of carbon. In Professor Kroto’s stimulating talk there were raised a number of questions, but not so much about chemistry of the interstellar medium as on chemistry in the laboratory or possibly chemistry in circumstellar regions; the main question that I would expect to hear discussed today is that of the applicability of what he has done. Very exciting though it is, there is some uncertainty about the applicability of his work to the problems that we are considering in this particular meeting. The conditions that you might find in the circumstellar regions are obviously not going to be quite like the conditions produced in the laboratory. The second important question that I would expect to be addressed in discussion now is the following. As material moves out of the circumstellar regions into the interstellar regions are the structures that Professor Kroto was describing expected to persist or not? He mentioned that C 60 may be formed in Bunsen burners and he also said that C 60 is very stable. If that is so why is all carbon on the earth not in the form of C 60 ? There must be some destruction mechanisms applying to these structures. Actually, if one makes amorphous carbon by having a surface in a carbon rich medium then, in fact, one does not get the sort of structures that he talked about. A mixture of diamond-like and graphitic-like regions of fairly small extent, perhaps a few tens of angstroms, is found.
Characterized by large paterae and late Noachian wrinkle-ridged plains, the ~1.2 million km2 Malea Planum region (MPR) has been grouped into a circum-Hellas volcanic province and likely represents the oldest of the large volcanic areas on Mars. Being key to Mars' early volcanic, tectonic, and climate evolution, we conducted a comprehensive photogeological investigation of the MPR using multiple datasets including THEMIS-IR as a basemap. We identified 26 geomorphologic units and derived apparent model ages based on crater size-frequency distribution measurements for six of them. Along with stratigraphic, morphologic, hyperspectral, and gravimetric analyses, as well as findings by previous works in the surrounding regions, our chronostratigraphy resulted in a complete landscape formation model of the mapping area. At 3.9–3.8 Ga, Malea and Pityusa Paterae form, probably as volcanic collapse calderas geographically controlled by Hellas-concentric faults. Pityusa Patera hosts folded deposits, possibly pyroclastics emplaced and shortened during patera formation as a piston-type caldera. Around 3.8–3.7 Ga, i.e., during the same time the ridged plains of the Hellas basin are formed, up to ~3.9 million km3 of volcanic and clastic/ballistic deposits partially sourced by Pityusa/Malea activity and/or by now-obscured vents are emplaced and superpose Pityusa and Malea Paterae, thus covering any potential features associated with them. Assuming the wrinkle-ridged plains to entirely consist of basaltic deposits with ~2 wt% H2O, outgassing might have produced ~0.8 m Global Equivalence Layer of water and/or 3.9 hPa of H2, which could have temporarily increased ambient temperatures, potentially enabling fluvial and lacustrine processes across the Malea-Hellas regions. After plains emplacement, doming above a shallow magma chamber and its subsequent partial evacuation forms Amphitrites Patera as a caldera on a ~1.5 km high, broad rise collocated with a positive ~2.6 x 10-3 m/s2 free-air, but no significant Bouguer gravity anomaly. Smooth crater fills throughout the area that often show high thermal inertias as well as enrichments of plagioclase and clay minerals might represent partially leached pyroclastic deposits resulting from this patera formation. Between 3.7 and 3.6 Ga, the northern slope of Amphitrites Patera is heavily dissected by low-viscosity flow processes that drain towards the Hellas basin floor and leave behind the Axius Valles amongst others, forming one of the densest martian valley networks (~0.08 km-1). 1,777 km long Mad Vallis and other smaller channels traversing the entire MPR and connecting the South Pole area with the Hellas basin are also formed around this time. Based on the geologic context and feasibility studies, we favor glacial meltwater/mud or low-viscosity lavas sourced from Amphitrites' summit over a catastrophic sapping event as cause for the Axius Valles. Following this, the Barnard impact event deposits ejecta on the surrounding flow features southeast of Amphitrites Patera; sinuous valleys and ridges are formed inside Barnard crater, likely by meltwater from ice sheets that might also have occupied Amphitrites Patera. Around 3.5 Ga, ~80-140 m (i.e., up to ~140,000 km3) of layered, friable materials are emplaced across large parts of the MPR as far north as 60°S. These materials are an extension of the circum-south polar Dorsa Argentea Formation (DAF), possible lag deposits from wet-based glaciation. Entrained within these deposits are dark, fine-grained materials, likely pyroclastics potentially sourced from volcanic activity at Peneus Patera, which might have formed around the same time, with bounding faults penetrating the wrinkle-ridged plains but without completely resurfacing its interior floor. Combining structural analyses of radial wrinkle ridges within Peneus Patera with a piston-type caldera model similar to Pityusa Patera (Bernhardt and Williams, 2021) would imply the collapse of a magma chamber at 19.5 to 26 km depth, i.e., potentially in the mid-crust. Up to ~210,000 km3 of friable airfall deposits, possibly sourced by ongoing/recurring Peneus activity, then cover the entire MPR but are eroded except where they are armored by superposing impact ejecta, thus forming numerous pedestal craters. In the Amazonian, these pedestals are themselves covered by up to few 10s of 1000s of km3 of atmosphere-derived volatiles and fines, which are then also sculpted into a second, distinctly younger pedestal crater population. Ongoing erosion of pyroclastic materials entrained in DAF deposits across the MPR and elsewhere continue to provide mafic fines that potentially supply the formation of transversal and barchanoid dune fields in local depressions, e.g., within Pityusa Patera and on the floors of larger impact craters throughout the MPR and Noachis Terra to the northwest, which is contrary to previous theories of more local supplies. In conclusion, our investigation of the MPR, which included a comprehensive map and chronostratigraphic as well as morphometric analyses, shows that the area experienced a complex volcanic, tectonic, eolian as well as most likely (glacio-)fluvial history and acted as corridor between the south polar area and the Hellas basin. In total, ~294,000 km3 of material were eroded from the MPR in multiple episodes, i.e., not just in one catastrophic event. This might have contributed close to a third of the originally one million km3 of hummocky materials on the Hellas basin floor (Bernhardt et al., 2016a). Similar to Pityusa Patera, Peneus Patera formed as relatively deep-seated caldera. Activity related to Amphitrites and Peneus Paterae likely contributed to ridged plains formation and the associated volatile release as well as mobilization had significant environmental effects.
