H. Wänke

Chemistry of Rocks and Soils at Meridiani Planum from the Alpha Particle X-ray Spectrometer

The Alpha Particle X-ray Spectrometer on the Opportunity rover determined major and minor elements of soils and rocks in Meridiani Planum. Chemical compositions differentiate between basaltic rocks, evaporite-rich rocks, basaltic soils, and hematite-rich soils. Although soils are compositionally similar to those at previous landing sites, differences in iron and some minor element concentrations signify the addition of local components. Rocky outcrops are rich in sulfur and variably enriched in bromine relative to chlorine. The interaction with water in the past is indicated by the chemical features in rocks and soils at this site.

Chemical composition and accretion history of terrestrial planets

The high concentrations of moderately siderophile elements (Ni, Co, etc.) in the Earth’s mantle and the similarity of their Cl normalized abundances to those of moderately volatile elements (F, Na, K, Rb) and some elements such as In, which under solar nebula conditions are highly volatile, are striking. To account for the observed abundances, inhomogeneous accretion of the Earth from two components has been proposed. In this model accretion started with the highly reduced component A devoid of all elements more volatile than Na, followed by accretion of more and more oxidized material (component B), containing all elements in Cl abundances. Recent observations have brought almost conclusive evidence that SNC meteorites are martian surface rocks ejected by huge impacts. By assuming that Mars is indeed the parent body of SNC meteorites, the bulk composition of Mars is estimated. The data on the composition of Mars obtained in this way clearly show that the two-component model is also valid for Mars. The striking depletion of all elements with chalcophile character in the martian mantle indicates that, contrary to the Earth, Mars accreted almost homogeneously (H. Wanke, Phil. Trans. R. Soc. Lond. A 303, 287 (1981)).

Constitution of terrestrial planets

Reliable estimates of the bulk composition are so far restricted to the three planetary objects from which we have samples for laboratory investigation, i.e. the Earth, the Moon and the eucrite parent asteroid. The last, the parent body of the eucrite— diogenite family of meteorites, an object that like Earth and Moon underwent magmatic differentiations, seems to have an almost chondritic composition except for a considerable depletion of all moderately volatile (Na, K, Rb, F, etc.) and highly volatile (Cl, Br, Cd, Pb, etc.) elements. The Moon is also depleted in moderately volatile and volatile elements compared to carbonaceous chondrites of type 1 (Cl) and also compared to the Earth. Again normalized to Cl and Si the Earth’s mantle and the Moon are slightly enriched in refractory lithophile elements and in magnesium. It might be that this enrichment is fictitious and only due to the normalization to Si and that both Earth’s mantle and Moon are depleted in Si, which partly entered the Earth’s core in metallic form. The striking depletion of the Earth’s mantle for the elements V, Cr and Mn can also be explained by their partial removal into the core. The similar abundances of V, Cr and Mn in the Moon and in the Earth’s mantle indicate the strong genetic relationship of Earth and Moon. Apart from their contents of metallic iron, all siderophile elements, moderately volatile and volatile elements, Earth and Moon are chemically very similar. It might well be that, with these exceptions and that of a varying degree of oxidation, all the inner planets have a similar chemistry. The chemical composition of the Earth’s mantle, for which reliable and accurate data have recently been obtained from the study of ultramafic nodules, yields important information about the accretion history of the Earth and that of the inner planets. It seems that accretion started with highly reduced material, with all Fe as metal and even Si and Cr, V and Mn partly in reduced state, followed by the accretion of more and more oxidized matter.

The Mars Odyssey Gamma-Ray Spectrometer Instrument Suite

The Mars Odyssey Gamma-Ray Spectrometer is a suite of three different instruments, a gamma subsystem (GS), a neutron spectrometer, and a high-energy neutron detector, working together to collect data that will permit the mapping of elemental concentrations on the surface of Mars. The instruments are complimentary in that the neutron instruments have greater sensitivity to low amounts of hydrogen, but their signals saturate as the hydrogen content gets high. The hydrogen signal in the GS, on the other hand, does not saturate at high hydrogen contents and is sensitive to small differences in hydrogen content even when hydrogen is very abundant. The hydrogen signal in the neutron instruments and the GS have a different dependence on depth, and thus by combining both data sets we can infer not only the amount of hydrogen, but constrain its distribution with depth. In addition to hydrogen, the GS determines the abundances of several other elements. The instruments, the basis of the technique, and the data processing requirements are described as are some expected applications of the data to scientific problems.

Chemistry and accretion history of Mars

Using element correlations observed in SNC meteorites and general cosmochemical constraints, Wänke & Dreibus (1988) have estimated the bulk composition of Mars. The mean abundance value for moderately volatile elements Na, P, K, F, and Rb and most of the volatile elements like Cl, Br, and I in the Martian mantle exceed the terrestrial values by about a factor of two. The striking depletion of all elements with chalcophile character (Cu, Co, Ni, etc.) indicates that Mars, contrary to the Earth, accreted homogeneously, which also explains the obvious low abundance of water and carbon. SNC meteorites and especially the shergottites are very dry rocks, they also contain very little carbon, while the concentrations of chlorine and especially sulphur are higher than those in terrestrial rocks. As a consequence we should expect SO2 and HC1 to be the most abundant compounds in Martian volcanic gases. This might explain the dominance of sulphur and chlorine in the Viking soils. In turn SO2, being an excellent greenhouse gas, may have been of major importance for the warm and wet period in the ancient Martian history. Episodic release of larger quantities of SO2 stored in liquid or solid SO2 tables in the Martian regolith triggered by volcanic intrusions as suggested here could lead to a large number of warm and wet climate periods of the order of a hundred years, interrupted by much longer cold periods characterized by water ice and liquid of solid SO2. Sulphur (FeS) probably also governs the oxygen fugacity of the Martian surface rocks.

Chemical Composition of Rocks and Soils at the Pathfinder Site

As Viking Landers did not measure rock compositions, Pathfinder (PF) data are the first in this respect. This review gives no proof yet whether the PF rocks are igneous or sedimentary, but for petrogenetic reasons they could be igneous. We suggest a model in which Mars is covered by about 50% basaltic and 50% andesitic igneous rocks. The soils are a mixture of the two with addition of Mg-sulfate and -chloride plus iron compounds possibly derived from the hematite deposits.

Experimental determination of metal/silicate partition coefficients for P, Co, Ni, Cu, Ga, Ge, Mo, and W and some implications for the early evolution of the Earth

Metal/silicate partition coefficients were determined at 1600°C for P, Ga, Ge and W and at 1300°C for P, Fe, Co, Ni, Cu, Ga, Ge, Mo and W. Experiments span a range of two orders-of-magnitude in oxygen fugacity. Good correlations between logƒO2 and the log of the partition coefficients were observed for all elements. The slopes of these correlations reflect the number of oxygen atoms associated with the metal oxides in the silicate phase. The composition of the silicates was basaltic, but variable with respect to FeO; from about 4% FeO at the most reducing experiments to 45% FeO at the most oxidizing experiments. Within these variations no dependence of the partition coefficients on silicate composition was observed, except for Ni and Co. The unusual slopes in the log DNi and log DCo vs. logƒO2 correlations can be explained by a strong dependence of the NiO and CoO activities on the liquid silicate composition. Reasonable agreement is found with literature data, except for a major discrepancy in the Mo-partition coefficient. Since many of the literature data were obtained from experiments with concentration levels in the percent range and since the experiments reported here contained the metals only in the ppm range, the present data are in agreement with Henrys law. The distribution of siderophile elements in the upper mantle can be explained by accretion of increasingly oxidizing material. After accretion of some 80 to 90% of the Earth the oxygen fugacity is sufficiently high that Fe, Ni, Co and some other siderophile elements will be quantitatively retained in the upper mantle. The small amount of metal still segregating to the core will suffice to extract highly siderophile elements. The final 1% of accreting material is so oxidizing that metal segregation is completely inhibited. The pattern of siderophile elements observed in the upper mantle is broadly consistent with results from calculations based on this model and utilizing metalsilicate partition coefficients reported here.

On the chemistry of the Allende inclusions and their origin as high temperature condensates

Abstract The abundances of 45 elements have been determined in a Ca,Al-rich chondrule from the Allende meteorite. The analytical techniques used were instrumental neutron and gamma activation analyses together with radiochemical neutron activation analysis. The refractory lithophile elements Al, Ca, Ti, Sc, V, Sr, Y, Zr, Nb, REE, Hf, Ta, (W) and U and the refractory siderophile elements Re, Os, Ir, Ru and Pt show a mean enrichment factor of 20 relative to C1-chondrites. Most of the other elements, which have been determined, are depleted relative to C1-chondrites. These measurements confirm the suggestion that non-magmatic processes are responsible for these enrichments. Especially the agreement of the measured abundances with those from the predictions of condensation calculations is excellent. Furthermore, by comparing the concentrations of some elements in a surface piece of the chondrule to a central part, a steep concentration gradient was found for the elements Fe, Mn, Na, K, Cl and Br. This suggests that these elements are of secondary origin, having entered the chondrule through the surface.

Chemical systematics of the shergotty meteorite and the composition of its parent body (Mars)

Abstract We report chemical data for 60 elements by INAA and RNAA in two bulk samples, for 30 elements in various mineral separates of Shergotty, and results of leaching experiments with 1M HCl on powdered aliquots of Shergotty and BETA 79001, lithologies A and B. Shergotty is homogeneous in major element composition but heterogeneous with respect to LIL trace elements (~20%). The heterogeneity is even greater for volatile and siderophile trace elements. The mineral data, including three clinopyroxene fractions with variable FeO contents, maskelynite and minor phases (Ti-magnetite, ilmenite, quartz, K-rich phase), show that major minerals do not account for the rare earth elements (REE) in the bulk meteorite. Instead, the REE are to a large extent concentrated in accessory whitlockite and apatite (shown by leaching with 1M HCl): together with the majority of REE (La, 96%, Yb 70%), Cl and Br are quantitatively dissolved by leaching. The REE patterns of the leachate of Shergotty and EETA 79001 are different. The Shergotty leachate may consist of two components. Component l is similar to that of EETA 79001 leachate (whitlockite), component 2 is enriched in light REE and may be responsible for the higher LREE contents of Shergotty in comparison to the other shergottites. There is some evidence that Shergotty was an open system and component 2 was introduced after crystallization. The REE patterns of the residues of Shergotty and EETA 79001 are identical indicating that the parent magmas of both meteorites are compositionally similar. Based on cpx separates with the lowest REE content, the REE pattern in the Shergotty parent magma was calculated. It is enriched in LREE and has a subchondritic Nd Sm ratio. The negative Eu anomaly in the phosphates indicates that at least some plagioclase crystallized before phosphate. Based on several element correlations in SNC meteorites, it was suggested (Dreibus and Wanke, 1984) that both the Shergotty parent body (SPB, very probably Mars), and the Earth accreted from the same two chemically different components: component A, highly reduced and devoid of volatile elements and an oxidized component B containing also volatile elements. The SPB (Mars) mantle is 2–4 times richer in volatile and moderately siderophile elements than the Earth, indicating a higher portion of component B in the SPB. The concentrations of chalcophile elements in the SPB mantle are low, reflecting equilibration with a sulfide phase and subsequent segregation of sulfide into the core. Unlike the Earth (Wanke, 1981), the SPB (Mars) may therefore have accreted almost homogeneously.

Determination of the chemical composition of Martian soil and rocks: The alpha proton X ray spectrometer

The alpha proton X ray spectrometer (APXS) for the Mars Pathfinder mission is designed to provide a complete and detailed analysis of chemical elements in Martian soil and rocks near the landing site. The APXS instrument is carried on the Pathfinder Microrover, which will provide transportation to places of interest on the Martian surface. It consists of a complex sensor head, mounted on a simple but sophisticated APXS deployment mechanism (ADM) outside the warm electronics box (WEB) of the Microrover, and the instrument electronics, mounted inside the WEB. The ADM permits the instrument sensor head to be placed against soil and rock samples in arbitrary positions, ranging from horizontal to vertical, in order to perform in situ analysis. The possibility to transport the APXS to an arbitrary location, preselected on Earth, and to perform in situ analysis there, constitutes one of the most exciting aspects of the Pathfinder mission. The principle of the APXS technique is based on three interactions of alpha particles from a radioisotope source with matter: simple Rutherford backscattering, production of protons from (α, p) reactions on light elements, and generation of characteristic X rays upon recombination of atomic shell vacancies created by a bombardment. Measurement of the intensities and energy distributions of these three components yields information on the abundance of chemical elements in the sample. In terms of sensitivity and selectivity, data are partly redundant and partly complementary: alpha backscattering is superior for light elements (C, O), while proton emission is mainly sensitive to Na, Mg, Al, Si, S, and X ray emission is more sensitive to heavier elements (Na to Fe and beyond). A combination of all three measurements enables determination of all elements (with the exception of H and He) present at concentration levels above typically a fraction of 1%.