The meteorite-asteroid connection: the infrared spectra of eucrites, shergottites, and vesta

Infrared reflectance spectra have been obtained for the meteorites Shergotty and Allan Hills (ALHA) 77005, a unique achondrite apparently related to the shergottites. Comparisons with the reflectance spectra of eucrites and asteroid 4 Vesta indicate that the surface of Vesta is covered with eucrite-like basalts and that, if shergottite-like basalts are present on the surface of Vesta, they must be a minor rock type. The paradox that both the eucrite and shergottite parent bodies should presently exist is examined. The preferred solution is that both eucrites and shergottites are derived from Vesta, and that this asteroid is compositionally and isotopically heterogeneous; however, other possible solutions cannot be ruled out.     

Mainbelt asteroids: dual-polarization radar observations

Observations of 20 asteroids in the main belt between Mars and Jupiter provide information about the nature of these objects' surfaces at centimeter-to-kilometer scales. At least one asteroid (Pallas) is extremely smooth at centimeter-to-meter scales. Each asteroid appears much rougher than the Moon at some scale between several meters and many kilometers. The range of asteroid radar albedos is very broad and implies substantial variations in porosity or metal concentration (or both). The highest albedo estimate, for the asteroid Psyche, is consistent with a surface having porosities typical of lunar soil and a composition nearly entirely metallic.     

Near-Earth asteroids: possible sources from reflectance spectroscopy

Spectra of near-Earth asteroids were compared to spectra of selected asteroids, planets, and satellites to determine possible source regions. The diversity of reflectance spectra of the near-Earth asteroids implies different mineralogical compositions and hence more than one source region. The presence of near-Earth asteroid spectral signatures similar to those of certain main-belt asteroids supports models that derive some of these asteroids from the 5:2 Kirkwood gap and the Flora family by gravitational perturbations. Planetary and satellite surfaces are different in composition than the near-Earth asteroids, which is in agreement with theoretical arguments that such bodies should not be sources. Some near-Earth asteroids supply portions of Earth's meteorite flux, but other sources must also contribute.     

Binary asteroids: evidence for their existence from lightcurves

The lightcurves of the asteroids 49 Pales and 171 Ophelia bear a striking resemblance to those of eclipsing binary stars. This evidence suggests that these asteroids are binary objects. Asteroids belonging to the Themis dynamical family have unusual lightcurves, possibly due to satellite events; these unusual lightcurves suggest that multiple objects may be formed during the disruption of asteroids in collisional events.     

The jupiter-io connection: an alfven engine in space

Much has been learned about the electromagnetic interaction between Jupiter and its satellite Io from in situ observations. Io, in its motion through the Io plasma torus at Jupiter, continuously generates an Alfvén wing that carries two billion kilowatts of power into the jovian ionosphere. Concurrently, Io is acted upon by a J x B force tending to propel it out of the jovian system. The energy source for these processes is the rotation of Jupiter. This unusual planet-satellite coupling serves as an archetype for the interaction of a large moving conductor with a magnetized plasma, a problem of general space and astrophysical interest.     

Pallasitic meteorites: implications regarding the deep structure of asteroids

Olivine compositions in pallasites exhibit a bimodal distribution and indicate a high degree of internal equilibrium. Cooling rates measured in the metal phases are uniform and consistently lower than those of most iron meteorites. These factors suggest that the pallasites were derived from few parent bodies, and that they crystallized in a highly insulated site-presumably the core of their parent body. Most iron meteorites were derived either from isolated areas closer to the surface or from other parent bodies.     

The thymus-adrenal connection: thymosin has corticotropin-releasing activity in primates

Endotoxin-free thymosin fraction 5 elevated corticotropin, beta-endorphin, and cortisol in a dose- and time-dependent fashion when administered intravenously to prepubertal cynomolgus monkeys. Two synthetic component peptides of thymosin fraction 5 had no acute effects on pituitary function, suggesting that some other peptides in thymosin fraction 5 were responsible for its corticotropin-releasing activity. In agreement with these observations, total thymectomy of juvenile macaques was associated with decreases in plasma cortisol, corticotropin, and beta-endorphin. These findings indicate that the prepubertal primate thymus contains corticotropin-releasing activity that may contribute to a physiological immunoregulatory circuit between the developing immunological and pituitary-adrenal systems.     

Ancient asteroids enriched in refractory inclusions

Calcium- and aluminum-rich inclusions (CAIs) occur in all classes of chondritic meteorites and contain refractory minerals predicted to be the first condensates from the solar nebula. Near-infrared spectra of CAIs have strong 2-micrometer absorptions, attributed to iron oxide-bearing aluminous spinel. Similar absorptions are present in the telescopic spectra of several asteroids; modeling indicates that these contain approximately 30 +/- 10% CAIs (two to three times that of any meteorite). Survival of these undifferentiated, large (50- to 100-kilometer diameter) CAI-rich bodies suggests that they may have formed before the injection of radiogenic 26Al into the solar system. They have also experienced only modest post-accretionary alteration. Thus, these asteroids have higher concentrations of CAI material, appear less altered, and are more ancient than any known sample in our meteorite collection, making them prime candidates for sample return.     

Origin of multikilometer earth- and mars-crossing asteroids: A quantitative simulation

Orbital dynamic simulations show that many asteroids in the main asteroid belt are driven toward Mars-crossing orbits by numerous weak mean motion resonances, which slowly increase the orbital ellipticity of the asteroids. In addition, half of the Mars-crossing asteroids (MCAs) transition to Earth-crossing asteroids (ECAs) in less than 20 million years. This scenario quantitatively explains the observed number of large ECAs and MCAs.