The observational evidence for mass distribution in the meteoritic complex

The integrated mass indexS for solid particles in the solar system is defined by the equationN = (const)m ^–S, whereN is the number of particles counted down to a lower limit of massm. Independent values ofS found from various observational programs are reviewed. These lie generally in the range from 0.4 to 1.4 for the average background of particles, and include data from lunar craters, satellite impacts, meteors, meteorites and asteroids. The trend ofS with mass is reasonably well established for masses less than one gram, but there are many gaps in our knowledge concerning the objects of greater mass.Paper dedicated to Prof. Harold C. Urey on the occasion of his 80th birthday on 29 April 1973.     

Collisional evolution of the asteroid belt

A new synthesis of asteroid collisional evolution is motivated by the question of whether most asteroids larger than ∼1 km size are strengthless gravitational aggregates (rubble piles). NEAR found Eros not to be a rubble pile, but a shattered collisional fragment, with a through-going fracture system, and an average of about 20 m regolith cover. Of four asteroids visited by spacecraft, none appears likely to be a rubble pile, except perhaps Mathilde. Nevertheless, current understanding of asteroid collisions and size-dependent strength, and the observed distribution of rotation rates versus size, have led to a theoretical consensus that many or most asteroids larger than 1 km should be rubble piles. Is Eros, the best-observed asteroid, highly unusual because it is not a rubble pile? Is Mathilde, if it is a rubble pile, like most asteroids? What would be expected for the small asteroid Itokawa, the MUSES-C sample return target? An asteroid size distribution is synthesized from the Minor Planet Center listing and results of the Sloan Digital Sky Survey, an Infrared Space Observatory survey, the Small Main-belt Asteroid Spectroscopic Survey and the Infrared Astronomical Satellite survey. A new picture emerges of asteroid collisional evolution, in which the well-known Dohnanyi result, that the size distribution tends toward a self-similar form with a 2.5-index power law, is overturned because of scale-dependent collision physics. Survival of a basaltic crust on Vesta can be accommodated, together with formation of many exposed metal cores. The lifetimes against destruction are estimated as 3 Gyr at the size of Eros, 10 Gyr at ten times that size, and 40 Gyr at the size of Vesta. Eros as a shattered collisional fragment is not highly unusual. The new picture reveals the new possibility of a transition size in the collisional state, where asteroids below 5 km size would be primarily collisional breakup fragments whereas much larger asteroids are mostly eroded or shattered survivors of collisions. In this case, well-defined families would be found in asteroids larger than about 5 km size, but for smaller asteroids, families may no longer be readily separated from a background population. Moreover, the measured boulder size distribution on Eros is re-interpreted as a sample of impactor size distributions in the asteroid belt. The regolith on Eros may result largely from the last giant impact, and the same may be true of Itokawa, in which case about a meter of regolith would be expected there. Even a small asteroid like Itokawa may be a shattered object with regolith cover.     

Collisional growth of planetesimals

Safronov's (1972) demonstration that relative velocities of planetesimals would be comparable to the dominant size bodies' escape velocities, combined with a plausible size distribution that has most mass in the largest bodies, yielded his evolution model with limited growth of the largest planetesimal with respect to its next largest neighbors. A numerical simulation of planetesimal accretion (Greenberget al., 1978) suggests that at least over one stage of collisional accretion, velocities were much lower than the escape velocity of the largest bodies, because the bulk of the mass still resided in km-scale bodies. The low velocities at this early stage may conceivably have permitted early runaway growth, which, in turn, would have kept the velocities low and permitted continued runaway growth of the largest bodies.Paper presented at the European Workshop on Planetary Sciences, organised by the Laboratorio di Astrofisica Spaziale di Frascati, and held between April 23–27, 1979, at the Accademia Nazionale del Lincei in Rome, Italy.     

Collisional evolution of the early asteroid belt

We present numerical results obtained by a simulation of the collisional process between asteroids and scattered comets from the Uranus–Neptune zone. This mechanism allows the use of single exponent incremental size distributions for the initial belt reaching a final distribution that matches the observed population very well. Since the cometary bombardment was extremely efficient removing mass from the primordial asteroid belt in a very short time, we always obtained belts with total masses less than 0.001 M_⊕ after ≈ 2×10⁷ yrs. This result allows processes with an important initial mass preserving Vestas basaltic crust.     

Surface electronic states of meteoritic nanodiamonds

Abstract— The C K edge of Orgueil nanodiamonds (Cδ diamonds) was acquired by electron energy‐loss spectroscopy (EELS), with an energy resolution of 300 meV. The spectra show peaks at 282.5, 284.7, and 286.4 eV, which occur in the band gap below the main diamond edge and are absent from the bulk diamond spectrum. These peaks are attributed to transitions from C 1s surface core levels to unoccupied surface states, and arise from single and π‐bonded dangling bonds and C‐H bonds. A shoulder to the main absorption edge at 287.8 eV may correspond to hydrocarbon adsorbates. These results can be used to further our understanding of Cδ diamond structure and may reveal the presence of a fullerene‐like surface. The unique surface electronic states of the Cδ diamond surfaces are expected to affect their optical properties, which are dependent on features such as extent of H coverage, particle size, and surface structure.     

The formation of plessite in meteoritic metal

Abstract— Plessite is a mixture of body‐centered cubic (bcc) kamacite (α), face‐centered cubic (fcc) taenite (γ), and/or ordered FeNi‐tetrataenite (γ“) phases and is observed in the metal of iron, stony‐iron, and chondritic meteorites. The formation of plessite was studied by measuring the orientation of the bcc and fcc phases over large regions of plessite using electron backscatter diffraction (EBSD) analysis in five ataxites, the Carlton IAB‐IIICD iron, and zoneless plessite metal in the Kernouve H6 chondrite. The EBSD results show that there are a number of different orientations of the bcc kamacite phase in the plessite microstructure. These orientations reflect the reaction path γ (fcc)→α₂ (bcc) in which the α₂ phase forms during cooling below the martensite start temperature, M_s, on the close‐packed planes of the parent fcc phase according to one or more of the established orientation relationships (Kurdjumov‐Sachs, Nishiyama‐Wasserman, and Greninger‐Troiano) for the fcc to bcc transformation. The EBSD results also show that the orientation of the taenite and/or tetrataenite regions at the interfaces of prior α₂ (martensite) laths, is the same as that of the single crystal parent taenite γ phase of the meteorite. Therefore, the parent taenite γ was retained at the interfaces of martensite laths during cooling after the formation of martensite. The formation of plessite is described by the reaction γ→α₂ + γ→α + γ. This reaction is inconsistent with the decomposition of martensite laths to form γ phase as described by the reaction γ→α₂→α + γ, which is the classical mechanism proposed by previous investigators. The varying orientations of the fine exsolved taenite and/or tetrataenite within decomposed martensite laths, however, are a response to the decomposition of α₂ (martensite) laths at low temperature and are formed by the reaction α₂→α + γ.     

Effects of sonochemical treatment on meteoritic nanodiamonds

A nanodiamond‐rich fraction (NDF) separated from the Orgueil meteorite was subjected to a high‐intensity ultrasonic treatment in a weakly acidic aqueous solution. After sedimentation by centrifugation, two fractions of grains (suspension, designated as OD7C and sediment, designated as OD7D) with different properties have been obtained. The following effects of the sonication were revealed from comparison of the contents and isotope compositions of C, N, and Xe released during stepped pyrolysis and combustion of the fractions OD7C and OD7D, the initial NDF and two grain‐size fractions (OD10 and OD15) produced without sonication (a) surface layer of the sonicated diamond grains is modified to different extent in comparison with nontreated ones, (b) in some grains concentrations of the bulk N and Xe a reduced significantly, and (c) nondiamond nitrogen containing phases (e.g., Si₃N₄) have been destroyed. It is suggested that combined effects of the sonication and centrifugation observed for the fractions OD7C and OD7D are due to differences in surface chemistry of the nanodiamond grains, which statistically influences behavior of nanoparticles during the sonication resulting in their preferential modification in the different reaction zones of the cavitating fluid.     

Collisional records in LL-chondrites

Abstract One-third of all the LL-chondrites have exposure ages of ～15 Ma and were exposed to cosmic rays following a collisional break-up. The probability that the 15-Ma peak represents a random signal is calculated to be less than 2%. Considerably lower probabilities are obtained if only LL5s or subgroups of high ⁴⁰Ar retention are used. Furthermore, we show that the peak shape agrees with statistical constraints obtained from multiple analyses of samples from the St. Severin LL6-chondrite. The frequency in and out of the 15-Ma peak varies significantly for different petrographic LL-types. The radiogenic ⁴⁰Ar retention systematics (most LL-chondrites retained ⁴⁰Ar_rad) shows that no substantial heat pulse resulted in the 15-Ma collisional event. Interestingly, smaller exposure age clusters at ～28 Ma and ～40 Ma match up well with clusters in the histogram of L-chondrites. The distribution of LL-exposure ages is not consistent with that expected for a quasi-continuous injection of LL material into a resonance zone of the asteroid belt. The near absence of exposure ages shorter than 8 Ma either indicates a lack of recent collisional events or considerably longer transfer times than inferred from dynamical considerations.     

Collisional excitation of interstellar molecules

An effective straight-line trajectory (EST) approach has been proposed for computation of collision-induced rotational line widths (half widths at half maximum) and excitation rates in case of atom-molecule system under the frame work of Smithet. al. (1976) formalism and molecule-molecule systems under the framework of a normalized semi-classical perturbative approach. An arbitrary multiplication factor RX with its optimum value can be used as a measure of significance of the curved trajectories of the colliding particles. The EST approach has been tested for HCL-Ar system and further applied to CO-He, CO-H₂, and OCS-H₂ systems of interstellar interest. The line width computations in case of HCl–Ar system are in accordance with the trend of the experimental values. The results obtained by the present approach are in good agreement with the theoretical computations using semi-perturbative approach. The present approach provides significant information about the lower and upper bound of the collision-induced rates even for the perturbers in rotationally excited states. The EST approach is not expensive in nature. It can be generalized for vibrational-electronic excitations.     

Collisional excitation of interstellar molecules

A normalised perturbative theory is used to compute the rates for alll-doublet transitions of hydrogen cyanide in the (01¹0) state having 4J16 perturbed by H₂ and He. Wherever the experimental data are available, they are in good agreement with the theoretical values.     

The contribution of impulsive meteoritic impact vapourization to the Hermean exosphere

The exosphere of Mercury has been the object of many investigations and speculations regarding its composition, formation, depletion and dynamics. While vapourization of Mercurian surface materials by meteorite impacts has been often considered to be a less important contributor to the exosphere than other potential processes, larger objects coming from the Main Asteroid Belt could cause high local and transient enhancements in the density of the exosphere. Vapourization by such impacts is an almost stoichiometric process, and thus would contain valuable information about the surface composition. We investigate some exospheric effects of impact vapourization for meteorites with radii of 1, 10 cm, and 1 m, with particular reference to the missions that will explore Mercury during the next decade (MESSENGER and BepiColombo). Because of their higher probabilities, impacts of objects in the two smaller size ranges will surely occur during the lifetimes of the two missions. The enhancement of the exospheric density on the dayside of Mercury would be appreciable for the 10-cm and 1-m meteorites (some orders of magnitude, especially for Al, Mg, Si, and Ca). Such events could allow detection, for the first time, of refractory species like Al, Mg, and Si, which are expected to exist on the surface but have not yet been detected in the exosphere. Ca could be detectable in all cases, even if produced by impacting objects as small as 1 cm in radius. The lower exospheric background on the night side should allow easier identification of Na and K produced by impulsive events, even if their generally high background values make this eventuality less likely.     

A Numerical Check of the Collisional Resurfacing Scenario

Earth, Moon, and Planets - We present a numerical investigation aimed at checkingthe so-called Collisional Resurfacing scenario (CR) throughone important prediction it makes:do the observed...     

Collisional excitation of interstellar cyclopropenylidene

Theoretical rotational excitation rates were computed for C3H2 in collisions with He atoms at temperatures from 30 to 120 K. The intermolecular forces were obtained from accurate self-consistent field and perturbation theory calculations, and collision dynamics were treated within the infinite-order sudden approximation. The accuracy of the latter was examined by comparing with the more exact coupled states approximation.     

Galileiite: A new meteoritic phosphate minera

Abstract— A new mineral named galileiite, NaFe₄(PO₄)₃, has been found within troilite nodules in iron meteorites of the IIIA and IIIB groups. the mineral is optically positive (ω = 1.72, ω = 1.75), colorless in transmitted light and pale amber in reflected light. Grains of galileiite are very small, generally 10 μm or less; rarely, grains are up to 30 μm. It is associated with Ca-free graftonite (or Ca-free sarcopside), chromite and, occasionally, schreibersite. Johnsomervilleite may occur within troilite nodules in the same meteorite as galileiite, but they have never been observed together in the same troilite nodule. Because of the small sample size, single crystal x-ray work was not successful; however, Gandolfi diffraction measurements were made. The three strongest diffraction peaks are 2.71 Å, 3.01 Å and 4.13 Å. On the basis of its composition and similar diffraction pattern, it is considered to be related to johnsomervilleite, fillowite and chladniite, all of which are rhombohedral and isostructural. Galileiite may also be rhombohedral, but that is yet to be demonstrated.     

Trace ferric ion in lunar and meteoritic titanaugites

Evidence for presence of Fe³⁺ in lunar rocks is furnished by heating them in air to 200-225°C for two hours. This causes a large decrease in the same charge transfer bands attributed to Fe³⁺ that can be enhanced by heating the same rocks in air at 500°C. This data is interpreted as evidence that the Fe³⁺ was not in equilibrium in the melt but was produced by cosmic radiation subsequent to the rock formation. The decrease of the Fe³⁺ charge-transfer bands is accompanied by decrease in intensity of spin-allowed Fe²⁺ bands attributed toM ₁ sites in the pyroxene in rock 12018. This decrease in the Fe²⁺ bands is attributed to decrease in the Fe²⁺ Fe³⁺ charge-transfer intensification of these Fe²⁺ spin-allowed transitions when radiation-produced Fe³⁺ is partially-reduced by the low-temperature heating.The reaction of Fe³⁺ on heating to 200-225°C is probably Fe³⁺ + Ti³⁺ Fe²⁺ + Ti⁴⁺.This is the reverse of the reaction caused by cosmic ray bombardment of the rock on the lunar surface. Possible tetrahedrally coordinated Fe³⁺ is present in the meteoritic and lunar augites as suggested by comparison of their spectra to that of terrestrial augite high in Fe³⁺. This would have been present in the original melt and is distinct from radiation produced Fe³⁺ in theM-sites.The polarized absorption spectra of single crystal pigeonite and augite from rock 12021 before heating, and augite from 12018 after heating are compared to that of meteoritic titanaugite in the Angra dos Reis meteorite and terrestrial titanaugite from Maui, Hawaii. The absorption spectrum of meteoritic hypersthene (Tatahouine) is also included for comparison to the pigeonite.     

Collisional baryonic dark matter haloes

If dark haloes are composed of dense gas clouds, as has recently been inferred, then collisions between clouds lead to galaxy evolution. Collisions introduce a core in an initially singular dark matter distribution, and can thus help to reconcile scale-free initial conditions – such as are found in simulations – with observed haloes, which have cores. A pseudo-Tully–Fisher relation, between halo circular speed and visible mass (not luminosity), emerges naturally from the model: M _vis∝ V ^7/2.
Published data conform astonishingly well to this theoretical prediction. For our sample of galaxies, the mass–velocity relationship has much less scatter than the Tully–Fisher relation, and holds as well for dwarf galaxies (where diffuse gas makes a sizeable contribution to the total visible mass) as it does for giants. It seems very likely that this visible-mass/velocity relationship is the underlying physical basis for the Tully–Fisher relation, and this discovery in turn suggests that the dark matter is both baryonic and collisional.     

Collisional broadening and shift of the alkali resonance lines

We use the 6-8-12 interatomic potential, with a suitable scaling for the relevant atomic radii, to reproduce the measured broadening and shift of the alkali resonance lines perturbed by noble gases, at temperatures of 500 K.Then, using the fit to the data with helium and neon as perturbers, we compute new values for the broadening and shift of the alkali resonance lines due to atomic hydrogen, at the temperature of 5000 K, which are of interest in the analysis of solar and stellar spectra, and discuss the reliability of these results.     

Hypervelocity capture of meteoritic particles in nonsilica aerogels

Abstract– The Stardust mission captured particles from the comet 81P/Wild 2 in gradient density silica aerogel and returned the collected samples to earth in 2006. The analyses of these particles have revealed several new insights into the formation of our solar system. However, since the aerogel used as the capture material was silica, the elemental analyses of the silica‐rich particles were made more complicated in certain ways due to the mixing of the silicon of the particles and that of the aerogel. By using a nonsilica aerogel, future elemental analyses of silica–rich particles captured in aerogel could be made more straightforward. Resorcinol/formaldehyde (RF), alumina, and zirconia aerogels were impact tested with meteoritic fragments and the captured fragments were mapped with synchrotron‐based X‐ray microprobe (XRM) and the particles were analyzed with X‐ray fluorescence (XRF). The resorcinol/formaldehyde aerogel proved to be the best capture material, in that it could be keystoned and XRF could be used to locate and analyze particles that were less than 10 μm.     

The meteoritic origin of Tutankhamun's iron dagger blade

Scholars have long discussed the introduction and spread of iron metallurgy in different civilizations. The sporadic use of iron has been reported in the Eastern Mediterranean area from the late Neolithic period to the Bronze Age. Despite the rare existence of smelted iron, it is generally assumed that early iron objects were produced from meteoritic iron. Nevertheless, the methods of working the metal, its use, and diffusion are contentious issues compromised by lack of detailed analysis. Since its discovery in 1925, the meteoritic origin of the iron dagger blade from the sarcophagus of the ancient Egyptian King Tutankhamun (14th C. BCE) has been the subject of debate and previous analyses yielded controversial results. We show that the composition of the blade (Fe plus 10.8 wt% Ni and 0.58 wt% Co), accurately determined through portable x‐ray fluorescence spectrometry, strongly supports its meteoritic origin. In agreement with recent results of metallographic analysis of ancient iron artifacts from Gerzeh, our study confirms that ancient Egyptians attributed great value to meteoritic iron for the production of precious objects. Moreover, the high manufacturing quality of Tutankhamun's dagger blade, in comparison with other simple‐shaped meteoritic iron artifacts, suggests a significant mastery of ironworking in Tutankhamun's time.