Discovery of moganite in a lunar meteorite as a trace of H2O ice in the Moon's regolith.

Moganite, a monoclinic SiO2 phase, has been discovered in a lunar meteorite. Silica micrograins occur as nanocrystalline aggregates of mostly moganite and occasionally coesite and stishovite in the KREEP (high potassium, rare-earth element, and phosphorus)-like gabbroic-basaltic breccia NWA 2727, although these grains are seemingly absent in other lunar meteorites. We interpret the origin of these grains as follows: alkaline water delivery to the Moon via carbonaceous chondrite collisions, fluid capture during impact-induced brecciation, moganite precipitation from the captured H2O at pH 9.5 to 10.5 and 363 to 399 K on the sunlit surface, and meteorite launch from the Moon caused by an impact at 8 to 22 GPa and >673 K. On the subsurface, this captured H2O may still remain as ice at estimated bulk content of >0.6 weight %. This indicates the possibility of the presence of abundant available water resources underneath local sites of the host bodies within the Procellarum KREEP and South Pole Aitken terranes.

Mineralogy and petrology of comet 81P/Wild 2 nucleus samples.

The bulk of the comet 81P/Wild 2 (hereafter Wild 2) samples returned to Earth by the Stardust spacecraft appear to be weakly constructed mixtures of nanometer-scale grains, with occasional much larger (over 1 micrometer) ferromagnesian silicates, Fe-Ni sulfides, Fe-Ni metal, and accessory phases. The very wide range of olivine and low-Ca pyroxene compositions in comet Wild 2 requires a wide range of formation conditions, probably reflecting very different formation locations in the protoplanetary disk. The restricted compositional ranges of Fe-Ni sulfides, the wide range for silicates, and the absence of hydrous phases indicate that comet Wild 2 experienced little or no aqueous alteration. Less abundant Wild 2 materials include a refractory particle, whose presence appears to require radial transport in the early protoplanetary disk.

Interplanetary dust from the explosive dispersal of hydrated asteroids by impacts.

The Earth accretes about 30,000 tons of dust particles per year, with sizes in the range of 20-400 microm (refs 1, 2). Those particles collected at the Earth's surface--termed micrometeorites--are similar in chemistry and mineralogy to hydrated, porous meteorites, but such meteorites comprise only 2.8% of recovered falls. This large difference in relative abundances has been attributed to 'filtering' by the Earth's atmosphere, that is, the porous meteorites are considered to be so friable that they do not survive the impact with the atmosphere. Here we report shock-recovery experiments on two porous meteorites, one of which is hydrated and the other is anhydrous. The application of shock to the hydrated meteorite reduces it to minute particles and explosive expansion results upon release of the pressure, through a much broader range of pressures than for the anhydrous meteorite. Our results indicate that hydrated asteroids will produce dust particles during collisions at a much higher rate than anhydrous asteroids, which explains the different relative abundances of the hydrated material in micrometeorites and meteorites: the abundances are established before contact with the Earth's atmosphere.