Noble gas space exposure ages of individual interplanetary dust particles

Abstract— The He, Ne, and Ar compositions of 32 individual interplanetary dust particles (IDPs) were measured using low‐blank laser probe gas extraction. These measurements reveal definitive evidence of space exposure. The Ne and Ar isotopic compositions in the IDPs are primarily a mixture between solar wind (SW) and an isotopically heavier component dubbed “fractionated solar” (FS), which could be implantation‐fractionated solar wind or a distinct component of the solar corpuscular radiation previously identified as solar energetic particles (SEP). Space exposure ages based on the Ar content of individual IDPs are estimated for a subset of the grains that appear to have escaped significant volatile losses during atmosphere entry. Although model‐dependent, most of the particles in this subset have ages that are roughly consistent with origin in the asteroid belt. A short (<1000 years) space exposure age is inferred for one particle, which is suggestive of cometary origin. Among the subset of grains that show some evidence for relatively high atmospheric entry heating, two possess elevated ²¹Ne/²²Ne ratios generated by extended exposure to solar and galactic cosmic rays. The inferred cosmic ray exposure ages of these particles exceeds 10⁷ years, which tends to rule out origin in the asteroid belt. A favorable possibility is that these ²¹Ne‐rich IDPs previously resided on a relatively stable regolith of an Edgeworth‐Kuiper belt or Oort cloud body and were introduced into the inner solar system by cometary activity. These results demonstrate the utility of noble gas measurements in constraining models for the origins of interplanetary dust particles.     

Aerogel keystones: Extraction of complete hypervelocity impact events from aerogel collectors

Abstract— In January 2006, the Stardust mission will return the first samples from a solid solar system body beyond the Moon and the first samples of contemporary interstellar dust ever collected. Although sophisticated laboratory instruments exist for the analysis of Stardust samples, techniques for the recovery of particles and particle residues from aerogel collectors remain primitive. Here, we describe our recent progress in developing techniques for extracting small volumes of aerogel, which we have called “keystones,” which completely contain particle impacts but minimize the damage to the surrounding aerogel collector. These keystones can be fixed to custom‐designed micromachined silicon fixtures (so called “microforklifts”). In this configuration, the samples are self‐supporting, which can be advantageous in situations where interference from a supporting substrate is undesirable. The keystones may also be extracted and placed onto a substrate without a fixture. We have also demonstrated the capability of homologously crushing these unmounted keystones for analysis techniques that demand flat samples.     

TOF‐SIMS analysis of Allende projectiles shot into silica aerogel

Abstract— Powdered Allende projectiles were fired into silica aerogel at 6.1 km/sec in order to evaluate particle retrieval and analysis techniques for samples from the Stardust mission. Since particles may disintegrate and ablate along the penetration paths in a high‐porosity aerogel, TOF‐SIMS analysis may be a suitable method to determine the distribution of such materials along the tracks as well as potential compositional modifications. Therefore, two ?350 μm‐sized tracks, residing at the surface of a keystone specimen that was flattened between two silicon chips, were analyzed. TOF‐SIMS allows for a detailed study of the chemical composition of particles that survived the impact mostly intact and of fine‐grained material from disintegrated projectiles. In the investigated keystone, material from light gas gun debris dominated. Besides the two tracks, a continuous, 40‐μm‐thick surface layer of implanted material—probably gun residue—was found. One of the two analyzed tracks is compositionally distinct from this surface layer and is likely to contain residual material of an Allende projectile. The analyses clearly demonstrate that tracks, resulting from impactors in the 5–10 μm size range, can be successfully analyzed with TOF‐SIMS.     

Ring Galaxies: The Dynamical Effect of the Halo and the Role of Shear Viscosity and Pressure Gradient Forces

The peculiar ring galaxies are formed as a result of a cosmic interaction. An intruder galaxy plunges through the center of a rotating disk galaxy triggering radially expanding density waves inside the disk. In this paper we exploited SPH simulations with the aim to examine the role of a “live” halo and/or bulge in driving the morphology of the interacting galaxy. Moreover we explore the effects of different implementations of the shear viscosity and of the pressure gradient in the code.     

Semi-empirical solar line blanketing

The line blanketing procedure described in an earlier paper has been tested by a series of blanketed solar model atmosphere calculations incorporating methodological and data variations. The results of those tests show the method to be reasonably insensitive to data uncertainties and to certain assumptions employed. The method was also checked by calculating several blanketed stellar atmospheres for comparison with blanketed atmospheres computed by other investigators. Except at the highest atmospheric levels the comparisons of the model structures were good and the blanketing method appears to be validated.Central solar intensities were computed for two semi-empirical solar models and for a theoretical model using the present blanketing method. The results are compared with the observations of Labs and Neckel (1968) in the region 3300 Å to 6500 Å. It is found the blanketing improves agreement of the model predictions with observations in a substantial way, particularly at short wavelengths. Limb-darkening predictions with blanketing were also made for these models at four wavelengths and compared with observations. The blanketing generally produces an important improvement in the comparisons; however, the amount of blanketing to be included for limb-darkening is uncertain.The temperature distribution of the blanketed theoretical model was compared with semi-empirical solar models and a blanketed model by Athay (1970) including non-LTE effects. Over a large range in optical depth the agreement is reasonably good; at small optical depths (₀ 0.01) large divergences are seen which may be due to the effects of non-LTE and to the neglect of strong lines in the present blanketing method. In addition to the good structural comparisons, the quality of the blanketed theoretical model in terms of comparison with central intensity and limb darkening is nearly as good as the semi-empirical models. We conclude that theoretical model atmospheres of solar type or nearly solar type computed with current constant-flux programs and with blanketing included are of high quality.Possible improvements in the blanketing method of importance in some instances are suggested; these include the use of a picket-fence procedure, the inclusion of strong lines, and the consideration of non-LTE effects. Further applications to the solar UV region and to stellar atmospheres are suggested.Publications of the Goethe Link Observatory, Indiana University, No. 131.     

The significance of the magnetism observed in lunar rocks

Partial thermal remanence experiments on lunar igneous rocks indicate that the magnetization of lunar rocks is not a normal single component thermoremanent magnetization. The magnetization therefore may not have been acquired at the time of initial cooling of the rock and thus should be used cautiously in making estimates of the intensity of the ancient lunar magnetic field.Contribution No. 201, Geosciences Division, The University of Texas at Dallas.