Small Planetary Bodies Research Articles

The Stickney Impact of Phobos: A Dynamical Model

Phobos, the largest moon of Mars, has dimensions typical of minor asteroids, approximately 19 × 22 × 27 km. It is probably the best studied small planetary body in the Solar System, imaged in detail by both the Viking orbiters and the partially successful Phobos 2 probe, and the subject of ongoing Earth-based spectral observation. One large crater, Stickney, dominates an entire hemisphere and places fundamental constraints on cratering and disruption models for small planetary bodies. A complex fracture pattern intersects the surface as a series of ∼100-m wide regolith-filled grooves; these probably originated with the cratering event and therefore tell us a great deal about the fragmentation process at a size scale far beyond the reaches of the laboratory. The tidal effects of Mars (Phobos is within the Roche limit) and the complexity of Phobos' geometry (it is often described as a potato) make detailed model comparisons difficult; what we seek to accomplish here is an order-of-magnitude understanding of impact and fracture physics at the size scale of planetesimals, together with a fundamental explanation for the observed current state of Phobos.We model the Stickney impact in axial symmetry using the hydrocode SALE 2D with the Tillotson equation of state and the Grady-Kipp fragmentation rheology. Given the crater diameter (11.3 km, slightly larger than the mean radius of Phobos), scaling relationships predict the impactor size corresponding to a given impact velocity. The corresponding impactor is then established as the initial condition for the hydrocode run, so that at t = 0 the impactor is just touching the target at a velocity vi. The flow field established in the impacted hemisphere excavates Stickney, although only ∼20% of the crater ejecta escapes. The nonescaping fraction, if blanketed uniformly over Phobos, adds ∼60 m or more of new regolith. Surface particle velocities away from the crater average ∼0.7 m/sec. Unconsolidated surface materials (crater rims, for instance) are shaken by the impact with typical ballistic transport distances of ∼50 to 100 m, further degrading the surface. The interior of Phobos fractures on a size scale of several kilometers—consistent with the spacing of the surface grooves. The Stickney event did not, however, result in a porosity sufficient to explain the unusually low body density. The fracture grooves correlate with the impact geometry, suggesting homogeneity (as opposed to a rubble pile) prior to the impact. If so, the low density of Phobos is compositional in nature.

Phosphates in pallasite meteorites as probes of mantle processes in small planetary bodies

THE stony-iron pallasite meteorites, which are generally believed to come from the core–mantle boundaries of asteroids, contain small quantities of phosphate minerals. Here we use trace element analyses of these phosphates to investigate the magmatic history of the silicate portions of pallasites. In Eagle Station and seven other pallasites, the phosphates have relatively low concentrations of rare earth elements (REE) and are strongly enriched in heavy relative to light REE. These patterns are consistent with formation of phosphate by subsolidus reactions between metal and silicate, in which phosphate inherits the REE pattern of olivine. In Spring-water and Santa Rosalia, calcium-rich phosphates have higher concentrations of REE, are enriched in light relative to heavy REE and have negative europium anomalies. These patterns are consistent with crystallization of phosphate from a europium-depleted chondritic liquid. Tihis is unlikely to have happened near the base of the differentiating parent-body mantle, because phosphates are late-crystallizing phases; this suggests that some pallasites may come from regions of their parent bodies much nearer the surface than the core–mantle boundary.

The effect of large Larmor radius on nonlinear Alfv�n waves

The effect of large Larmor radius on the nonlinear behaviour of Alfven waves propagating parallel to a uniform magnetic field in a compressible fluid is investigated with the aid of LLR-MHD equations. It is shown that asymptotic evolution of these waves is governed by the modified nonlinear Schrodinger equation. The dispersion is provided by the large Larmor radius effect in the magnetic field equation. It is suggested that these calculations can have a bearing on the investigation of the structure of MHD waves in both laboratory and space plasmas, e.g., imploding θ pinches, laser-blow-off plasma experiment, recent barium releases in the magnetosphere, plasma flow near small planetary bodies such as comets and plasma dynamic near collisionless shock fronts.

Evidence from the Semarkona ordinary chondrite for 26A1 heating of small planets

The radioactive decay of 26Al, to form 26Mg, may have constituted an important heat source for melting of small planetary bodies in the early Solar System. Although large excesses of radiogenic 26Mg, arising from in situ decay of 26Al (half life ~7.2 x 105 yr), have been found in ten carbonaceous and ordinary chondritic meteorites1,2, the distribution of 26Al in the solar nebula and its importance as a heat source remain to be determined. All previous observations of excess 26Mg are confined to Al-rich minerals in refractory, Ca,Al-rich inclusions (CAIs). An assessment of the distribution of 26Al in the Solar System from these data is difficult because CAIs are only a minor constituent of chondritic meteorites associated with high-temperature nebular processes, and because the initial 26Al/27Al ratio inferred from the CAI data is not constant3 but varies from the commonly measured value of ∼ 5 x 10−5 to values <1 x 10−7. Here we report the first observation of radiogenic 26Mg in non-refractory meteoritic material, a plagio-clase-bearing, olivine-pyroxene clast chondrule in the Semarkona ordinary chondrite. The inferred initial abundance of 26Al (26Al/27Al = (7.7±2.1)xl0−6) is sufficient to produce incipient melting in well insulated bodies of chondritic composition. We conclude that planetary accretion and differentiation must have begun on a timescale comparable to the half life of 26Al and that, even if widespread melting did not occur, 26Al heating played a significant role in thermal metamorphism on small planets.

Manganese—chromium isotope systematics and the development of the early Solar System

High-precision measurements of the chromium isotope compositions of samples from meteorites reveal anomalies in the 53Cr/52Gr ratio which are believed to arise from in situ decay of the extinct short-lived nuclide 53Mn. The decay of 53Mn to 53Cr in the early Solar System provides an additional chronometer with which to constrain the formation times of the small planetary bodies from which the meteorites originated. A comparison of chromium and titanium isotope anomalies shows them to be imperfectly correlated, bearing witness to the complexity of early Solar System processes.

Replacement textures in CAI and implications regarding planetary metamorphism

Textural and chemical features of five coarse-grained, calcium-aluminum-rich inclusions from the Allende meteorite indicate that some of the melilite in these inclusions was formed by a secondary metamorphic event and not by primary crystallization from a melt or by a sequential nebular condensation process. These inclusions contain embayed pyroxene surrounded by melilite. Physically separated pyroxene crystals are often in optical continuity indicating that they were once part of larger single crystals that have been partly replaced by melilite. Other evidences of metamorphism include reaction textures between melilite and spinel, and metamorphic textures such as kink-band-like features, lobate sutured grain boundaries, and 120° triple-points. This type of metamorphic process requires the addition of Ca which we propose came from calcite or by introduction of a fluid phase. We believe that the most likely environment for this metamorphic process is on a small planetary body, and not in the solar nebula. The results of this study are compatible with oxygen isotopic heterogeneities within CAI, and provide a mechanism for producing lower temperature alteration phases and the rim phases found in these inclusions. We conclude that planetary processes must thus be considered in the formation history of CAI, and that it is necessary to reconsider the classification system of these objects in light of the replacement process proposed here.

The escape of molecular hydrogen and the synthesis of organic nitriles in planetary atmospheres

What is the influence of hydrogen escape from the atmosphere of small planetary bodies on the synthesis of organic molecules in that atmosphere? To answer this question, laboratory experiments have been performed to study the evolution of different reducing model atmospheres submitted to electrical discharges, with and without the simulation of H 2 escape. A study of mixtures of nitrogen and methane shows a very strong effect of H 2 escape on the formation of organic nitriles, the only nitrogen containing organics detected in the gas phase. These are HCN, CH  CCN, (CN) 2, CH 2CHCN, CH 3 CN and CH 3CH 2CN. The yield of synthesis of most of these compounds is noticeably increased, up to several orders of magnitude, when hydrogen escape is simulated. The escape of H 2 from the atmosphere of the primitive Earth may have played a crucial role in the formation of reactive organic molecules such as CHCCN or (CN) 2, which can be considered as important prebiotic precursors. These experimental results may also explain extant data concerning the nature and relative abundance of organics present in the atmosphere of Titan, a planetary satellite which may be an ideal model within our solar system for the study of organic cosmochemistry and exobiology.

The isotopic composition and concentration of Ag in iron meteorites and the origin of exotic silver

The isotopic composition of Ag and the concentration of Ag and Pd have been determined in Canyon Diablo (IA), Grant (IIIB), Hoba, Santa Clara, Tlacotepec and Warburton Range (IVB), Piñon and Deep Springs (anom.). Troilite from Grant and Santa Clara have also been analyzed. All of these meteorites, with the exception of Canyon Diablo, give 107Ag 109Ag in the metal phase that is greater than the terrestrial value with the enrichments of 107Ag ranging from ~2% to 212%. These data show that Ag of anomalous isotopic composition is common to all IVB and anomalous meteorites. The results on Grant suggest that the anomalies may be widespread including more common meteorite groups. There is a general correlation of 107Ag 109Ag with Pd Ag except for the data from FeS of Santa Clara. It is concluded that the excess 107Ag is the result of decay of 107Pd, a nuclide that is extinct at present with an abundance of 107Pd 108Pd of about 3 × 10 −5. The troilite in Grant exhibits normal 107Ag 109Ag to within errors, a high Ag concentration and a low ratio of 108Pd 109Ag ~0.17. Grant metal has 107Ag 109Ag that is ~2% greater than normal and a high ratio of 108Pd 109Ag ~ 10 3. The data from Grant appear to represent a 107Pd- 107Ag isochron and indicate that the cooling rate at elevated temperatures was sufficiently rapid to preserve substantial isotopic differences between metal and troilite. Troilite in Santa Clara was found to contain Ag with a very high 107Ag 109Ag ratio (108% above normal), an Ag concentration only a factor of three above the metal and a high value of 108Pd 109Ag ~1.3 × 10 4. The troilite has a higher 107Ag 109Ag than the metal. These data are not compatible with a simple model of in situ decay and subsequent local Ag redistribution between metal and troilite during cooling. These data suggest that Ag in Santa Clara and possibly other IVB meteorites is made up of almost pure 107Ag produced from 107Pd decay and 109Ag produced by nuclear reactions with only a small amount of “normal” Ag. This indicates an intense energetic particle bombardment history in the early solar system (~10 20 p/m 2) which occurred after the formation of small planetary bodies. We infer that a T-Tauri activity by the early sun contributed to some late stage “nucleosynthesis” and the heating of a dust cloud. In addition, implications on the early thermal evolution of iron meteorites are presented based on 107Pd decay and models of the cooling history.

The Blue Angel: I. The mineralogy and petrogenesis of a hibonite inclusion from the Murchison meteorite

A detailed mineralogie, chemical, and petrologic study of the Blue Angel, a relatively large (~1.5 mm) hibonite-containing inclusion from the Murchison meteorite, was performed in an attempt to understand the mechanisms of formation and modification of hibonite-rich inclusions. The Blue Angel inclusion is composed of roughly equal amounts of hibonite and calcite, with minor amounts of spinel, perovskite, diopside, and an Fe-rich silicate. The inclusion can be divided into three roughly concentric zones—a hibonite-rich core, a calcite-rich mantle, and a spinel-rich layered rim. The mineral chemistry and petrography of the Blue Angel are consistent with a three-stage formation history: 1. (1) an early stage of nebular condensation which produced the hibonite, perovskite, and spinel; 2. (2) a moderate temperature stage of aqueous alteration and metamorphism occurring on a small planetary body containing CO 2 and H 2O during which calcite was formed in the inclusion; and 3. (3) the final emplacement of the Blue Angel into its present position in Murchison. The study of the Blue Angel indicates that extensive alteration of Ca-Al-rich inclusions (CAI) may have occurred by aqueous alteration and thermal metamorphism followed by explosive mixing processes on a parent body. Such metamorphic reactions may involve formation and destruction of phases, such as melilite and diopside, which have been previously thought to be primary condensates. The mechanisms proposed for the formation and modification of the Blue Angel help to explain the secondary phases and oxygen isotope anomalies found in many CAI and eliminate the need for invoking kinetically-complicated back-reactions at very low pressures with a cool part of the solar nebula. The contribution of planetary metamorphism in the formation and alteration of CAI must be considered along with nebular processes in order to understand the formation of carbonaceous chondrites.

Asteroidal agglutinate formation and implications for asteroidal surfaces

A large number of shock recovery experiments that address the ease of impact melt formation as a function of peak shock pressure lead to the conclusion that impacts at 5 km/sec into fragmental, porous surfaces will produce agglutinate-type glasses; no shock melts are produced at these velocities in dense silicate target rocks. While agglutinitic glasses dominate lunar surface soils, they are virtually absent in gas-rich, brecciated meteorites. This apparent paucity—if not complete lack—of agglutinate-type glasses is also inferred from remote IR-reflectance spectroscopy. The need to identify mechanisms that inhibit agglutinate formation on asteroidal sufaces was recognized previously and was predominantly attributed to lower projectile velocities and different gravitational environments. We will argue in this paper that additional mechanisms may be required. Specifically we propose that spall processes at a target's free surface play a major role in asteroidal surface evolution. At 5 km/sec collision velocity, a target ( R T) to projectile ( R P radius ratio of R T R P ≈ 100 delineates the boundary between an “infinite half-space” and a “finite”-sized target. In the first case, collisional energy is expended in a pure cratering regime; in the latter, additional displacement of target material in the form of spallation products occurs. The spall volume may exceed the crater volume by an order of magnitude. Therefore fragmental impact deposits on small planetary bodies may be entirely controlled by spall products, rather than crater ejecta. Because tensile failure occurs at <0.2 GPa stress, spall velocities are measured in meters per second (contrary to crater ejecta) and therefore spallation products are efficiently retained even in low gravitational environments. Spall products are also more coarse grained than crater ejecta; they are also highly biased toward petrographically “unshocked” (<0.2 GPa) rocks. Thus asteroidal surface deposits should be more coarse grained and less shocked than lunar ones—consistent with meteorite evidence and remote-sensing observations. Because spall volume exceeds crater ejecta volume, the total growth rate of asteroidal surface deposits is accelerated, leading to relatively short surface residence times of individual meteorite components, another significant difference between lunar and asteroidal surface materials.

Trace elements as quantitative probes of differentiation processes in planetary interiors

Abundances of trace elements in extrusive igneous rocks may be used as petrological and geochemical probes of the source regions of the rocks if differentiation processes, partition coefficients, phase equilibria, and initial concentrations in the source region are known. The characteristic trace element signature that each mineral in the source region imparts on the magma forms the conceptual basis for trace element modeling. The task of the trace element geochemist is to solve mathematically the inverse problem. Given trace element abundances in a magma, what is the mode of its source region? The most successful modeling has been performed for small planetary bodies which underwent relatively simple igneous differentiation events. An example is the eucrite parent body, a planet which produced basalts at ⋍4.6 Gy. and has been quiescent ever since. This simple differentiation history permits the calculation of its bulk composition (a feldspathic peridotite) and has led to the tentative identification of asteroid 4 Vesta as the eucrite parent body. The differentiation of iron meteorite groups in parent body cores is amenable to similar treatment. Quantitative calculations are currently hampered by the paucity of experimentally determined solid metal/liquid metal partition coefficients. The ‘anomalous’ behavior of Cr, however, suggests that IIIA, B irons and main group pallasites equilibrated with troilite, spinel, ferromagnesian silicates, or some combination thereof. The moon has undergone more complex differentiation, and quantitative geochemical modeling is correspondingly more difficult. Nevertheless, modeling the two‐stage evolution of mare basalts raises the possibility that the primordial moon did not have chondritic relative abundances of such refractory elements as Ca, Al, U, and the rare‐earth elements. The nonchondritic element ratios are characteristic of planetary, not nebular, fractionation processes and are consistent with the derivation of the moon from a precursor planet, possibly the earth. Alternatively, they may be artifacts of our inadequate understanding of differentiation in a deep, rapidly convecting magma ocean. The enormous complexity of terrestrial evolution over geologic time generally precludes quantitative geochemical modeling of the earth. For example, calculations investigating the petrogenesis of calc‐alkaline andesites are restricted to limiting the range of plausible hypotheses. In cases where Nd isotopic systematics indicate that the source region of a basalt suite had an approximately chondritic Sm/Nd ratio (e.g., Columbia River Plateau), semiquantitative calculations of the nature of the mantle may be possible. Geochemical modeling may have predictive value for an unsampled planet such as Mercury which may have had a relatively simple thermal history.

On the effects of higher convection modes on the thermal evolution of small planetary bodies

The effects of higher modes of convection on the thermal evolution of a small planetary body is investigated. Three sets of models are designed to specify an initially cold and differentiated, an initially hot and differentiated, and an initially cold and undifferentiated Moon-type body. The strong temperature dependence of viscosity enhances the thickening of lithosphere so that a lithosphere of about 400 km thickness is developed within the first billion years of the evolution of a Moon-type body. The thermally isolating effect of such a lithosphere hampers the heat flux out of the body and increases the temperature of the interior, causing the solid-state convection to occur with high velocity so that even the lower modes of convection can maintain an adiabatic temperature gradient there. It is demonstrated that the effect of solid-state convection on the thermal evolution of the models may be adequately determined by a combination of convection modes up to the third or the fourth order harmonic. The inclusion of higher modes does not affect the results significantly.

Possible Magnetic Interaction of Asteroids with the Solar Wind

Investigation of extraterrestrial objects is habitually accompanied by the observation of extraterrestrial magnetic fields, which often have significant effects on the solar wind in the vicinity of the objects. Should we be surprised to find this experience repeated at an asteroid?A recent study considered the conditions that would have to be satisfied by the surface magnetic field of a small planetary body at asteroidal distances so that the field would be an obstacle capable of stopping the solar wind or deflecting it sufficiently to generate a detectable magnetic interaction (Greenstadt, 1971).