Jennifer Ann Grier

Optical maturity of ejecta from large rayed lunar craters

Lucey et al. [2000] have developed a methodology for extracting an optical maturity parameter (OMAT) from multispectral Clementine images. The OMAT parameter characterizes the overall maturity of lunar soils and crater ejecta by changes in reflectance spectra. Using these OMAT images, we surveyed large craters (≥20 km diameter) on the Moon that had previously been mapped as possessing or possibly possessing rayed ejecta. We generated average radial profiles of OMAT values for rays of these large craters. From these profiles we classified the craters into three relative age groups: (1) older than Copernicus (inferred age of ∼810 Myr), (2) intermediate, and (3) as young or younger than Tycho (inferred age of ∼109 Myr). We suspect that there is a bias to our classification scheme, such that the OMAT profiles of smaller craters look like that of larger but older craters. Nevertheless, some large craters, such as Eudoxus (67 km) and Aristillus (55 km), are now known from this study to have optically mature ejecta and therefore are suspected to be older than Copernicus (this is consistent with an age of 1.3 Gyr suggested for Aristillus by Ryder et al. [1991]). Such craters were included by McEwen et al. [1997] when estimating the density of craters younger than or contemporaneous with Copernicus. Therefore the case for a modest increase in the cratering rate (in the past 800 Myr versus the previous 2.4 Gyr) indicated from that work has been weakened [Grier and McEwen, 2001]. Given current constraints on dating large and recent lunar craters, we cannot support (or disprove) the hypothesis that there has been a significant increase in the rate of large terrestrial impact events in the past 100–400 Myr.

Noble gases in orthopyroxenite ALH84001: A different kind of martian meteorite with an atmospheric signature

Meteorite ALH84001, a recently identified martian orthopyroxenite, has xenon with higher ratios of 129Xe132Xe (up to 2.15) than any previously identified martian sample except the glass in EETA79001, suggesting that it has incorporated a relatively large amount of martian atmosphere. As such, it should be a fruitful sample to study in order to try to determine incorporation mechanisms, but it is petrogenetically very different from other martian meteorites with similar noble gas signatures. We determine a cosmic-ray exposure age of 15.8±1.6 Ma, longer than any other martian meteorites, requiring either another impact event on Mars, or another breakup of a martian fragment en route to Earth. Argon systematics suggest an age much older than the other martian meteorites and may indicate a substantial amount of martian atmospheric argon as well. Excesses of 80Kr and 82Kr in one sample are consistent with neutron capture on bromine in situ, but not with an atmosphere rich in Br-derived isotopes.

Cat Mountain: A meteoritic sample of an impact-melted asteroid regolith

Cat Mountain is a new ordinary chondrite impact melt breccia that contains several shocked chondrule-bearing clasts of L5 material. These clasts are surrounded by a total impact melt of similar composition material which appears to have cooled over a period of a few thousand years, probably within a melt breccia lens in the bottom of a large (>1 km diameter) crater on an L chondrite asteroid. Noble gas isotopes indicate that the sample was involved in at least two different impact events, approximately 880 and 20 Myr ago, following the 4.55 Ga accretion of primitive chondritic material. The 880 Ma event is responsible for the impact breccia texture of the sample, and the 20 Ma event reduced the sample to a meter-sized object. We also infer that another impact occurred between 880 and 20 Ma (possibly the ∼500 Ma event recorded in many other L chondrites) to jettison the material from the asteroid belt into an orbit that evolved into an Earth-crossing trajectory. The shock-metamorphic processes that occurred at 880 Ma redistributed the opaque phases in the meteorite and altered the crystalline characteristics of silicate phases. This reduced the reflectance of the L5 material and decreased the amplitude of its spectral absorption features. These characteristics are consistent with the spectral characteristics of some C class asteroids and suggest that some dark asteroids that appear to belong to the C class could be covered with shocked ordinary chondrite material. If one assumes that Cat Mountain came from the same asteroid as other L chondrites with the same cosmic ray exposure age, then the juxtaposition of these different materials suggests asteroids are rubble piles which are heterogeneous on a scale less than 100 m. Furthermore, the structural integrity of Cat Mountain and other L chondrites suggests the strengths of asteroid rubble piles are limited by fractures and contrasting material properties and are thus inherently weak in a ram pressure regime produced when they enter a planetary atmosphere. However, in a regime where the asteroid is the target of impact fragmentation rather than the projectile, the added porosity of a rubble pile structure will compensate for the presence of fractures and absorb a large amount of the impact energy. In this case the structural integrity of the asteroid may appear to be the same as a previously unshocked chondritic material.

Formation and relative ages of maskelynite and carbonate in ALH84001

The morphology and stoichiometry of feldspathic glass in the Martian meteorite ALH84001 indicates it is maskelynite (a diaplectic glass) rather than a flowed glass, although this glass was heterogeneously affected by a tertiary set of processes. An impact event with shock pressures in excess of 31 GPa was needed to convert the original plagioclase (An36Ab60Or4) to maskelynite. Carbonate is intimately associated with the maskelynite, and the carbonate’s radiating crystalline fabric and globular forms suggest it was produced after plagioclase was converted to maskelynite. Textures also suggest carbonate was produced at the expense of maskelynite in a dissolution-precipitation reaction that involved a carbonic fluid. This fluid system is tentatively estimated to have been active for at least a few years at temperatures <300°C, based on dissolution rates of plagioclase in mildly to strongly alkaline hydrothermal systems (which is the only analogue currently available). This reaction does not need to be mitigated by microbial life. Neither are bacteria needed to produce the radiating textures and globular forms of carbonate, which may instead reflect kinetic phenomena associated with crystal nucleation and growth. Because the carbonate was produced at the expense of maskelynite, it is younger than maskelynite, which has previously been shown to have last degassed 3.92 ± 0.04 Ga (Turner et al., 1997). However, the specific age of the carbonate and the carbonic fluid system remains unknown.

The Lunar Record of Recent Impact Cratering

The history of the accretion of extraterrestrial material onto the Earth covers the entire lifetime of the planet, from the accretion of great quantities of mass at the time the planet formed ∼4.5 Ga, to the much less dramatic infall of stones and dust to the Earth today. Impact craters are the scars left behind when some of the larger portions of this infalling material (impactors) strike the planet’s surface. Craters can therefore be used to measure the amount and frequency of mass that impacts a large body such as the Earth. Although processes rapidly eliminate craters on the Earth, the nearby Moon retains a pristine record of recent cratering events.