THE COMPOSITION OF IRON METEORITES: A STUDY BY FACTOR ANALYSIS

A factor analysis has been performed on nickel and trace element data for iron meteorites. The technique shows that the present distribution of these elements is the result of three processes. These can be identified from the elements involved:

• 1 Ga, Ge, Sb and Zn (condensation and accretion).
• 2 Ni, Pd, Co and Cu (oxidation and sulphuration).
• 3 Ir, Au, As, Re, Pt, Os, Ru and Cr (an igneous event).

The distribution of Mo, however, is not readily explicable in terms of these processes. Within the groups IAB and IIAB only one process is required for all elements, but in groups IIIAB and IVA the situation for Ga, Ge and Sb is more complex.     

Space weathering and the low sulfur abundance of Eros

The surprisingly low S/Si ratio of Asteroid 433 Eros measured by the NEAR Shoemaker spacecraft probably reflects a surface depletion rather than a bulk property of the asteroid. The sulfur X-ray signal originates at a depth <10 μm in the regolith. The most efficient process for vaporizing minerals at the heliocentric distance of Eros are sputtering by solar wind ions and hypervelocity impacts. These are the same processes that account for the changes in optical properties of asteroids attributed to “space weathering” of lunar surface materials, although the relative importance of sputtering and impacts need not be the same for the Moon and asteroids. Troilite, FeS, which is the most important sulfide mineral in meteorites, and presumably on S-type asteroids like Eros, can be vaporized by much less energy than other major minerals, and will therefore be preferentially lost. Within 10⁶ years either process can remove sulfide from the top 10-100 μm of regolith. Sulfur will be lost into space and some sulfur will migrate to deeper regolith layers. We also consider other possible mechanisms of surficial sulfur depletion, such as mineral segregation in the regolith and perhaps even incipient melting. Although we consider solar wind sputtering the most likely cause of the sulfur depletion on Eros, we cannot entirely rule out other processes as causes of the sulfur deficiency. Laboratory simulations of the relevant processes can address some of the open questions. Simulations will have to be carried out in such a way that potential sulfur loss processes as well as resurfacing can be studied simultaneously, requiring a large and complex environmental chamber.     

The natural thermoluminescence of meteorites VI: Carbon-14, thermoluminescence and the terrestrial ages of meteorites

Abstract Research on meteorite finds, especially those from the Antarctic and from desert regions in Australia, Africa, and America, has become increasingly important, notably in studies of possible changes in the nature of the meteorite flux in the past. One important piece of information needed in the study of such meteorites is their terrestrial age which can be determined using a variety of methods, including ¹⁴C, ³⁶Cl, and ⁸¹Kr. Natural thermoluminescence (TL) levels in meteorites can also be used as an indicator of terrestrial age. In this paper, we compare ¹⁴C-determined terrestrial ages with natural TL levels in finds from the Prairie States (central United States), a group of finds from Roosevelt County (New Mexico, USA), and a group from the Sahara Desert. We find that, in general, the natural TL data are compatible with the ¹⁴C-derived terrestrial ages using a 20 °C TL decay curve for the Prairie States and Roosevelt County and a 30 °C decay curve for the Saharan meteorites. We also present TL data for a group of meteorites from the Sahara desert which has not been studied using cosmogenic radionuclides. Within these data there are distinct terrestrial age clusters which probably reflect changes in meteorite preservation efficiency over ～ 15, 000 years in the region.     

The slow rotation of 253 Mathilde

CCD photometry of the NEAR mission fly-by target 253 Mathilde is presented. Measurements taken during 52 nights of observations, from February to June 1995, allow a rotation period of 17.406±0.010 days and a lightcurve amplitude of 0.45±0.02 mag to be determined. A B-V color index of 0.67±0.02 and a V-R of 0.35±0.02 are measured, which are compatible with C-type membership. The determination of the phase relation results in H = 10.28±0.03 and G = 0.12±0.06. Indications that the lightcurve is not strictly singly-periodic are found. A power-spectrum analysis detects a secondary frequency f₂ = 0.0322±0.0010 d⁻¹, which is interpreted as evidence for a complex rotation state.     

Properties of chondrules in EL3 chondrites,comparison with EH3 chondrites,and the implications for the formation of enstatite chondrites

Abstract— The study of chondrules provides information about processes occurring in the early solar system. In order to ascertain to what extent these processes played a role in determining the properties of the enstatite chondrites, the physical and chemical properties of chondrules from three EL3 chondrites and three EH3 chondrites have been examined by optical, cathodoluminescence (CL), and electron microprobe techniques. Properties examined include size, texture, CL, and composition of both individual phases and bulk chondrules. The textures, distribution of textures, and composition of silicates of the EL3 chondrules resemble those of EH3 chondrules. However, the chondrules from the two classes differ in that (1) the size distribution of the EL chondrules is skewed to larger values than EH chondrules, (2) the enstatite in EL chondrules displays varying shades of red CL due to the presence of fine‐grained sulfides and metal in the silicates, and (3) the mesostasis of EH chondrules is enriched in Na relative to that of EL chondrules. The similarities between the chondrules of the two classes suggest similar precursor materials, while the differences suggest that there was not a single reservoir of meteoritic chondrules, but that their origin was fairly local. The differences in the size distribution of chondrules in EH and EL chondrites may be explained by aerodynamic and gravitational sorting during accumulation of the meteoric material, while differences in CL and mesostasis properties may reflect differences in formation conditions and cooling rate following chondrule formation. We argue that our observations are consistent with the formation of enstatite chondrites in a thick dynamic regolith on their parent body.     

Investigation of biological, chemical and physical processes on and in planetary surfaces by laboratory simulation

The recently established Arkansas-Oklahoma Center for Space and Planetary Science has been given a large planetary simulation chamber by the Jet Propulsion Laboratory, Pasadena, California. When completely refurbished, the chamber will be dubbed Andromeda and it will enable conditions in space, on asteroids, on comet nuclei, and on Mars, to be reproduced on the meter-scale and surface and subsurface processes monitored using a range of analytical instruments. The following projects are currently planned for the facility. (1) Examination of the role of surface and subsurface processes on small bodies in the formation of meteorites. (2) Development of in situ sediment dating instrumentation for Mars. (3) Studies of the survivability of methanogenic microorganisms under conditions resembling the subsurface of Mars to test the feasibility of such species surviving on Mars and identify the characteristics of the species most likely to be present on Mars. (4) The nature of the biochemical “fingerprints” likely to have been left by live organisms on Mars from a study of degradation products of biologically related molecules. (5) Testing local resource utilization in spacecraft design. (6) Characterization of surface effects on reflectivity spectra for comparison with the data from spacecraft-borne instruments on Mars orbiters.