Darwin glass formed by impact melting, probably during excavation of the 1.2 km diameter Darwin crater, Tasmania, Australia. The glass was ejected up to 20 km from the source crater and forms a strewn field of 400 km^2. There is at least 11,250 m^3 of glass in the strewn field and relative to the size of the crater this is the most abundant ejected impact glass on Earth. The glass population can be subdivided on the basis of shape (74% irregular, 20% ropy, 0.5% spheroid, 6% droplet, and 0.7% elongate) and color (53% dark green, 31% light green, 11% black, and 5% white). The white glasses contain up to 92 wt% SiO2 and are formed from melting of quartzite. Black glasses contain a minimum of 76 wt% SiO2 and formed from melting of shale. Systematic variations in the proportion of glasses falling into each of the color and shape classes relative to distance from the crater show: 1) a decrease in glass abundance away from the crater; 2) the largest fragments of glass are found closest to the crater; 3) small fragments (2 g) dominate finds close to the crater; 4) the proportion of white glass is greatest closest to the crater; 5) the proportion of black glass increases with distance from the crater and 6) the proportion of splashform glasses increases with distance from the crater. These distribution trends can only be explained by the molten glass having been ballistically ejected from Darwin crater during impact and are related to 1) the depth of excavation from the target rock stratigraphy and/or 2) viscosity contrasts between the high and low SiO2 melt. The high abundance and wide distribution of ejected melt is attributed to a volatile charged target stratigraphy produced by surface swamps that are indicated by the paleoclimate record.

The Brunflo fossil meteorite was found in the 1950s in mid-Ordovician marine limestone in the Grde quarry in Jmtland. It originates from strata that are about 5 million years younger than similar limestone that more recently has yielded >50 fossil meteorites in the Thorsberg quarry at Kinnekulle, 600 km to the south. Based primarily on the low TiO2 content (about 1.8 wt%) of its relict chromite the Brunflo meteorite had been tentatively classified as an H chondrite. The meteorite hence appears to be an anomaly in relation to the Kinnekulle meteorites, in which chromite composition, chondrule mean diameter and oxygen isotopic composition all indicate an L-chondritic origin, reflecting an enhanced flux of meteorites to Earth following the disruption of the L chondrite parent body 470 Ma. New chondrule-size measurements for the Brunflo meteorite indicate that it too is an L chondrite, related to the same parent-body breakup. Chromite maximum diameters and well-defined chondrule structures further show that Brunflo belongs to the L4 or L5 type. Chromites in recently fallen L4 chondrites commonly have low TiO2 contents similar to the Brunflo chromites, adding support for Brunflo being an L4 chondrite. The limestone in the Grde quarry is relatively rich (about 0.45 grain kg^(-1)) in sediment-dispersed extraterrestrial chromite grains (63 m) with chemical composition similar to those in L chondrites and the limestone (1-3 grains kg^(-1)) at Kinnekulle, suggesting that the enhanced flux of L chondrites prevailed, although somewhat diminished, at the time when the Brunflo meteorite fell.

Found during the 2004 U.S. Antarctic Search for Meteorites season, LaPaz Icefield (LAP) 04841 represents an addition to the LaPaz lunar basalts suite and brings the total mass collected to 1.93 kg. The presence of FeNi grains, troilite, and the anorthositic composition of plagioclase are evidence for the lunar origin of this meteorite. Pyroxene and olivine Mn/Fe values plot along the trend set for lunar basalts. Analyses of chromite grains provide a V/(Al + Cr) ratio of 1.33 +/- 13, translating to an fO2 one log unit below the IW buffer, in accordance with previous fO2 estimates for lunar basalts. Application of the Zr-cooling speedometer, for ilmenite and ulvspinel pairs, gives a cooling rate of 5.2 degrees C/day, matching previous estimates of cooling rates for the LaPaz lunar meteorites and Apollo mare basalts. Mineral modes and chemistries, as well as trace-element patterns, provide compelling evidence for pairing of this meteorite to others in the LaPaz lunar basalt suite.

Meteor Crater is one of the first impact structures systematically studied on Earth. Its location in arid northern Arizona has been ideal for the preservation of the structure and the surviving meteoric material. The recovery of a large amount of meteoritic material in and around the crater has allowed a rough reconstruction of the impact event: an iron object 50 m in diameter impacted the Earths surface after breaking up in the atmosphere. The details of the disruption, however, are still debated. The final crater morphology (deep, bowl-shaped crater) rules out the formation of the crater by an open or dispersed swarm of fragments, in which the ratio of swarm radius to initial projectile radius Cd is larger than 3 (the final crater results from the sum of the craters formed by individual fragments). On the other hand, the lack of significant impact melt in the crater has been used to suggest that the impactor was slowed down to 12 km/s by the atmosphere, implying significant fragmentation and fragments separation up to 4 initial radii. This paper focuses on the problem of entry and motion through the atmosphere for a possible Canyon Diablo impactor as a first but necessary step for constraining the initial conditions of the impact event which created Meteor Crater. After evaluating typical models used to investigate meteoroid disruption, such as the pancake and separated fragment models, we have carried out a series of hydrodynamic simulations using the 3D code SOVA to model the impactor flight through the atmosphere, both as a continuum object and a disrupted swarm. Our results indicate that the most probable pre-atmospheric mass of the Meteor Crater projectile was in the range of 4x10^8 to 1.2x10^9 kg (equivalent to a sphere 4666 m in diameter). During the entry process the projectile lost probably 30% to 70% of its mass, mainly because of mechanical ablation and gross fragmentation. Even in the case of a tight swarm of particles (Cd <3), small fragments can separate from the crater-forming swarm and land on the plains (tens of km away from the crater) as individual meteorites. Starting from an impactor pre-atmospheric velocity of ~18 km/s, which represents an average value for Earth-crossing asteroids, we find that after disruption, the most probable impact velocity at the Earths surface for a tight swarm is around 15 km/s or higher. A highly dispersed swarm would result in a much stronger deceleration of the fragments but would produce a final crater much shallower than observed at Meteor Crater.

We present a textural comparison of localized shock melt pockets in Martian meteorites and glass pockets in terrestrial, mantle-derived peridotites. Specific textures such as the development of sieve texture on spinel and pyroxene, and melt migration and reaction with the host rock are identical between these two apparently disparate sample sets. Based on petrographic and compositional observations it is concluded that void collapse/variable shock impedance is able to account for the occurrence of pre-terrestrial sulfate-bearing secondary minerals in the melts, high gas emplacement efficiencies, and S, Al, Ca, and Na enrichments and Fe and Mg depletion of shock melt compositions compared to the host rock; previously used as arguments against such a formation mechanism. Recent experimental studies of xenoliths are also reviewed to show how these data further our understanding of texture development and can be used to shed light on the petrogenesis of shock melts in Martian meteorites.

Radiometric age dating of Martian rocks and surfaces at known locations for which crater densities can be determined is highly desirable in order to fully understand Martian history. Performing K-Ar age dating of igneous rocks on Mars by robots, however, presents technical challenges. Some of these challenges can be defined by examining Ar-Ar data acquired on Martian meteorites, and others can be evaluated through numerical modeling of simulated K-Ar isochrons like those that would be acquired robotically on Martian rocks. Excess 40Ar is present in all shergottites. Thus for Martian rocks, the slopes of K-Ar isochrons must be determined to reasonable precision in order to calculate reliable ages. Model simulations of possible isochrons give an indication of some requirements in order to define a precise rock age: Issues addressed here are: how many K-Ar analyses should be made of rocks thought to have the same age; what range of K concentrations should these analyzed samples have; and what analytical uncertainty in K-Ar measurements is desirable. Meteorite data also are used to determine the D/a^2 diffusion parameters for Ar in plagioclase and pyroxene separates of several shergottites and nakhlites. These data indicate the required temperatures and times for heating similar Martian rocks in order to extract Ar. Quantitatively extracting radiogenic 40Ar could be difficult, and degassing cosmogenic Ar from mafic phases even more so. Considering all these factors, robotic K-Ar dating of Martian rocks may be achievable, but will be challenging.