Magmatic sulfide mineralogy has been studied in 2 olivine-phyric shergottites (DaG 476 and SaU 005) and 4 basaltic shergottites (Zagami, Shergotty, Los Angeles, and NWA 480). Modal abundances of magmatic sulfides, as estimated by image analysis on thin section, are high (0.16 to 0.53 area percent) and correlate positively with abundances of Fe-Ti oxides. Sulfides are mesostasis minerals, being mostly interstitial grains or locally enclosed in post-cumulus melt inclusions (e.g., SaU 005) in olivine. Sulfides in shergottites are composed of major pyrrhotite containing pentlandite exsolutions associated with minor amounts of Cu sulfides (chalcopyrite and/or cubanite). Hot desert finds (e.g., DaG 476) show abundant fracture-filling iron (oxy)hydroxides of probable terrestrial origin. Unaltered sulfides show metal-rich hexagonal pyrrhotite compositions with metal/sulfur (M/ S) ratio ranging between 0.936 +/- 0.005 and 0.962 +/- 0.01. This compositional range corresponds to the two-phase structural domain 2C + nC of the Fe-S system; however, the high-temperature disordered hexagonal 1C pyrrhotite structure would be in better agreement with magnetic properties of shergottites. Ni contents in pyrrhotite increase from Los Angeles (<0.05 at%) to Shergotty, Zagami, and NWA 480 (0.2-0.5 at%), and DaG 476 and SaU 005 (up to 3 at%). The higher Ni values of pyrrhotite in olivine-phyric shergottites correlate with the abundance of pentlandite exsolutions, both reflecting more primitive Ni-rich sulfide liquids where abundant olivine crystallized. This result and the strong correlation between sulfide abundances and Fe-Ti oxides argue for a primary magmatic origin of these sulfides. Although they reproduce the trend of magmatic oxygen fugacity conditions determined from Fe-Ti oxide pairs, observed pyrrhotite compositions are systematically more metaldeficient compared to those calculated from the Fe-S-O system. This suggests post-magmatic oxidation during cooling on Mars, followed by terrestrial weathering for hot desert finds.

We explore the orbital dynamics of Earth-crossing objects with the intent to understand the time scales under which an "orbital stream" of material could produce time-correlated meteorite falls. These meteoroid streams have been suggested to be associated with three well-known meteoritedropping fireballs (Innisfree, Peekskill, and Příbram). We have performed two different analyses of the statistical significance of the "orbital similarity," in particular calculating how often orbits of the same level of similarity would come from a random sample. Secondly, we have performed extremely detailed numerical integrations related to these three cases, and we find that if they were streams of objects in similar orbits, then they would become "decoherent" (in the sense that the day-of-fall of meteorites of these streams become almost random) on time scales of 10^4-10^5 yr. Thus, an extremely recent breakup would be required, much more recent that the cosmic ray exposure ages of the recovered falls in each case. We conclude that orbital destruction is too efficient to allow the existence of long-lived meteoroid streams and that the statistical evidence for such streams is insufficient; random fall patterns show comparable levels of clustering.

In the framework of international planetary exploration programs, several space missions are planned to search for organics and bio-signatures on Mars. Previous attempts have not detected any organic compounds in the Martian regolith. It is therefore critical to investigate the processes that may affect organic molecules on and below the planets surface. Laboratory simulations can provide useful data about the reaction pathways of organic material at Mars surface. We have studied the stability of amino acid thin films against ultraviolet (UV) irradiation and use those data to predict the survival time of these compounds on and in the Martian regolith. We show that thin films of glycine and D-alanine are expected to have half-lives of 22 +/- 5 hr and of 3 +/- 1 hr, respectively, when irradiated with Mars-like UV flux levels. Modelling shows that the half-lives of the amino acids are extended to the order of 10^7 years when embedded in regolith. These data suggest that subsurface sampling must be a key component of future missions to Mars dedicated to organic detection.

Northwest Africa (NWA) 1950 is a new member of the lherzolitic shergottite clan of the Martian meteorites recently found in the Atlas Mountains. The petrological, mineralogical, and geochemical data are very close to those of the other known lherzolitic shergottites. The meteorite has a cumulate gabbroic texture and its mineralogy consists of olivine (Fo66 to Fo75), low and high-Ca pyroxenes (En78Fs19Wo2- En60Fs26W14; En53Fs16Wo31-En45Fs14Wo41), and plagioclase (An57Ab41Or1 to An40Ab57Or3; entirely converted into maskelynite during intense shock metamorphism). Accessory minerals include phosphates (merrillite), chromite and spinels, sulfides, and a glass rich in potassium. The oxygen isotopic values lie on the fractional line defined by the other SNC meteorites (Delta-17O = 0.312 ppm). The composition of NWA 1950 is very similar to the other lherzolitic shergottites and suggests an origin from the same magmatic system, or at least crystallization from a close parental melt. Cosmogenic ages indicate an ejection age similar to those of the other lherzolitic shergottites. The intensity of the shock is similar to that observed in other shergottites, as shown by the occurrence of small melt pockets containing glass interwoven with stishovite.

The applicability of the Euler deconvolution method in imaging impact crater structure vis- vis delineation of source depth of the circular magnetic anomaly and/or basement depth beneath the crater is addressed in this paper. The efficacy of the method has been evaluated using the aeromagnetic data obtained over the Serra da Cangalha impact crater, northeastern Brazil. The analyses of the data have provided characteristic Euler deconvolution signatures and structural indices associated with impact craters. Also, through the interpretation of the computed Euler solutions, our understanding of the structural features present around the impact structure has been enhanced. The Euler solutions obtained indicate shallow magnetic sources that are interpreted as possibly post-impact faults and a circular structure. The depth of these magnetic sources varies between 0.8 and 2.5 km, while the Precambrian basement depth was found at ~1.5 km. This is in good agreement with the estimates of the Precambrian basement depth of about 1.1 km, calculated using aeromagnetic data. The reliability of the depth solutions obtained through the implementation of the Euler method was confirmed through the use of the existing information available in the area and the result of previous studies. We find that the Euler depth solutions obtained in this study are consistent with the results obtained using other methods.