Asteroid missions: Goals for elemental abundance analysis

Since it is known that there is a diversity of surface types among asteroids and assumed that asteroids represent several different bulk compositions and stages in planetary evolution, a first mission to the asteroid belt must study and compare several asteroids of differing types. Both very primitive and highly evolved asteroids should be studied. Identifications of any asteroid with a known type of meteorite will permit the attachment of a large body of accurate data to a known location, and thereby secure many commonly made assumptions as facts. Thus, it is essential that remote analysis of asteroids be able to distinguish among the compositions of known meteorites. Determination of the absolute abundances of Mg, Al, Si, Ca, Fe, Ni, and S will permit meteorite types to be distinguished. Analysis of additional elements such as C and H and other trace elements will permit more certainty in identification. Remote sensing of primitive asteroids should permit the detection of water on or outgassing from asteroid surfaces. An important goal will be to determine the degree to which remote observations of surfaces reflect real differences in interior compositions; hence, accurate determinations of densities will be essential. High-resolution photography of asteroidal surfaces may yield information on the heterogeneity of the surfaces.     

Ice clathrate as a possible source of the atmospheres of the terrestrial planets

The presence and compositions of atmospheres on the terrestrial planets do not follow directly from condensation models which would have Earth accreting near 500°K. No single mechanism yet proposed adequately accounts for the abundances of noble gases and carbon and nitrogen in the atmospheres. We show that the composition of clathrates forming at low temperatures in cold regions of the nebula can be predicted. Addition of about 1 ppm clathrate material to the Earth can explain observed abundances of Ar, Kr, and Xe. Condensation and adsorption processes occuring at 400–500°K are necessary to explain the observed abundances of Ne, H₂O, C, and N. Possible sources of clathrates could be cometary bodies formed in the outer solar system.     

Estuarine macrobenthos in Calcasieu Lake,Louisiana: Community and trophic structure

Macrobenthic assemblages in Calcasieu Lake estuary (Louisiana) were sampled at 11 sites from October 1983 through November 1985. The sites were numerically dominated by subsurface-deposit feeders, consisting mostly of polychaetes. Greater densities of macrofauna were collected at the northern (upper) stations of the lake than were collected in West Cove or the southern stations. Abundances of polychaetes, oligochaetes, and amphipods at the upper lake stations accounted for most of the differences among stations. The numerical dominance by detritivores (97% of the fauna) and lack of strong sediment or salinity gradients across the estuary, resulted in an absence of temporal pattern in trophic structure of the macrofauna.     

Trace elements in rims and interiors of Chainpur chondrules

Trace elements were measured in the rims and interiors of nine chondrules separated from the Chainpur LL-3 chondrite. Whole rock samples of Chainpur and samples of separated rims were also measured. Chondrule rims are moderately enriched in siderophile and volatile elements relative to the chondrule interiors. The enriched volatile elements include the lithophilic volatile element Zn. The moderate enrichment of volatiles in chondrule rims and the lack of severe depletion in chondrules can account for the complete volatile inventory in Chainpur. These results support a three-component model of chondrite formation in which metal plus sulfide, chondrules plus rims and matrix silicates are mixed to form chondrites.     

Aluminum-26 in meteorites—VII. Ureilites,their unique radiation history

Cosmogenic ²⁶A1 activities have been measured by γ-γ coincidence counting in the three ureilites which had not previously been studied. The values in dpm/kg are: Dingo Pup Donga, 38.4 ± 2.4; North Haig, 39.3 ± 4.8; Dyalpur, 55.8 ± 4.8. Five of the six known ureilites thus have lower ²⁶A1 contents, 63 per cent to 77 per cent, than the calculated saturation values, in marked contrast to most other stony meteorites. This cannot be attributed to short cosmic ray exposure ages. Nor do size and depth effects account for the narrow range of ²⁶A1 activities, because a nuclear particle track study indicates that preatmospheric radii were highly variable, from ≥ 40 cm for Goalpara to only a few cm for Dingo Pup Donga. By default, the most likely explanation is that the ureilites had much smaller or much larger orbits than all other stony meteorites.     

Some studies of an unusual eucrite: Ibitira

The Ibitira eucrite is remarkable both for its vesicles and its unbrecciated nature. It consists of ~63 vol. % pyroxene (Wo₁₄En₃₈Fs₄₈), 31% plagioclase (An_95–96), ~2% of nickel-iron, troilite, ilmenite, titanian chromite, and 4% of a silica polymorph. It has a mean track density of 1.8 ± 0.3 × 10⁶ cm^?2, mainly due to cosmic rays. Its pre-atmospheric radius must have been at least 10 cm.The absence of complex radiation effects and presence of vesicles place constraints on the thickness of the Ibitira basalt flow. From the freezing time calculations of Provost and Bottinga, it appears that Ibitira came from a flow no less than 2.5 m and probably no more than 10 or 20 m thick. However, this estimate depends strongly on the viscosity of the melt, which is not well known.     

Rare gases and fossil particle tracks in the Kenna ureilite

Fossil particle tracks and spallation-produced He and Ne in the Kenna ureilite indicate that it existed in space as a small object for 23 m.y. In our study of Kenna, we found no evidence of trapped He or Ne. Large amounts of heavy rare gases occur in Kenna in concentrations typical of ureilites. In a step-wise release of gases, the isotopic compositions of Kr and Xe were found to be constant above 600°C, revealing the presence of a single retentively sited component. The Xe isotopic abundances are characterized by 124:126:128:129:130:131:132:134:136 = 0.471:0.414:8.280:103.61: 16.296:81.92:100:37.70:31.23. This isotopic composition is distinct from AVCC (average carbonaceous chondritic), but similar to compositions known for some time in certain temperature fractions of Renazzo, Murray and Murchison. Kenna-type Xe appears to be one of the several components found in carbonaceous chondrites.

Binz et al. (Geochim. Cosmochim. Acta 39, 1576–1579, 1975) have recently found that many volatile trace elements are strongly depleted in ureilites. Thus, the relatively large amounts of heavy rare gases present in ureilites did not result from a mixture of a volatile-rich component with the ureilite host. It appears that some material rich in carbon and heavy rare gases was incorporated into a differentiated ureilite host. All current hypotheses which purport to explain the origin of trapped gases in meteorites encounter difficulty in accounting for trapped gases in ureilites in a straightforward manner.     

Modeling the morphological diversity of impact craters on icy satellites

Impact craters on icy satellites display a wide range of morphologies, some of which have no counterpart on rocky bodies. Numerical simulation studies have struggled to reproduce the diversity of features, such as central pits and transitions in crater depth with increasing diameter, observed on the icy Galilean satellites. The transitions in crater depth (at diameters of about 26 and 150 km on Ganymede and Callisto) have been interpreted as reflecting subsurface structure. Using the CTH shock physics code, we model the formation of craters with diameters between 400 m and about 200 km on Ganymede using different subsurface temperature profiles. Our calculations include recent improvements in the model equation of state for H₂O and quasi-static strength parameters for ice. We find that the shock-induced formation of dense high-pressure polymorphs (ices VI and VII) creates a gap in the crater excavation flow, which we call discontinuous excavation. For craters larger than about 20 km, discontinuous excavation concentrates a hot plug of material (>270 K and mostly on the melting curve) in the center of the crater floor. The size and occurrence of the hot plug are in good agreement with the observed characteristics of central pit craters, and we propose that a genetic link exists between them. We also derive depth versus diameter curves for different internal temperature profiles. In a 120 K isothermal crust, calculated craters larger than about 30 km diameter are deeper than observed and do not reproduce the transition at about 26 km diameter. Calculated crater depths are shallower and in good agreement with observations between about 30 and 150 km diameter using a warm thermal gradient representing a convective interior. Hence, the depth-to-diameter transition at about 26 km reflects thermal weakening of ice. Finally, simulation results generally support the hypothesis that the anomalous interior morphologies for craters larger than 100 km are related to the presence of a subsurface ocean.     

Geochemical and mineralogical evidence for Sahara and Sahel dust additions to Quaternary soils on Lanzarote,eastern Canary Islands,Spain

Africa is the most important source of dust in the world today, and dust storms are frequent on the nearby Canary Islands. Previous workers have inferred that the Sahara is the most important source of dust to Canary Islands soils, with little contribution from the Sahel region. Soils overlying a late Quaternary basalt flow on Lanzarote, Canary Islands, contain, in addition to volcanic minerals, quartz and mica, exotic to the island’s bedrock. Kaolinite in the soils also likely has an exotic origin. Trace‐element geochemistry shows that the soils are derived from varying proportions of locally derived basalt and African dust. Major‐element geochemistry, clay mineralogy and interpretation of satellite imagery suggest that dust additions to the Canary Islands come not only from the Sahara Desert, but also from the Sahel region.     

Composition and sources of extractable organic matter from a sediment core in Lake Kivu,East African rift valley

Lake Kivu is a gas-charged East African rift lake with currently anoxic bottom water. The extractable compounds and residual organic matter of a short sediment core have δ¹³C values typical of lacustrine microbial detritus. The total extracts consist primarily of polar compounds such as n-alkanoic acids, hydroxyalkanoic acids, triterpenoids, steroids and monosaccharides, with minor amounts of n-alkanes and n-alkanols. These tracer compounds and δ¹³C values indicate that the organic matter in the surficial and deeper sedimentary record was dominated by bacterial sources. The sapropelic sediment between these horizons contains organic matter from primarily algal with lesser bacterial input. Terrestrial organic markers are minor in all samples. The major fractions of the compounds in the total extracts were oxidized in the upper water column prior to transit through the anoxic bottom water to sedimentary deposition. The sapropelic horizon may reflect lake water turnover with ventilation or hydrothermal activity and consequently increased algal blooms.