Stardust Interstellar Preliminary Examination XI: Identification and elemental analysis of impact craters on Al foils from the Stardust Interstellar Dust Collector

Abstract

Abstract-The Stardust Interstellar Preliminary Examination team analyzed thirteen Al foils from the NASA Stardust interstellar collector tray in order to locate candidate interstellar dust (ISD) grain impacts. Scanning electron microscope (SEM) images reveal that the foils possess abundant impact crater and crater-like features. Elemental analyses of the crater features, with Auger electron spectroscopy, SEM-based energy dispersive X-ray (EDX) spectroscopy, and scanning transmission electron microscope-based EDX spectroscopy, demonstrate that the majority are either the result of impacting debris fragments from the spacecraft solar panels, or intrinsic defects in the foil. The elemental analyses also reveal that four craters contain residues of a definite extraterrestrial origin, either as interplanetary dust particles or ISD particles. These four craters are designated level 2 interstellar candidates, based on the crater shapes indicative of hypervelocity impacts and the residue compositions inconsistent with spacecraft debris.

Evidence for interstellar origin of seven dust particles collected by the Stardust spacecraft

“…Modeling indicates that very few IDP impacts on the SIDC are expected to coincide with the "midnight" direction where interstellar impacts occur (10,19) and we observed no tracks in the angular range where IDPs should have their maximum flux, indicating that the IDP background is small. Based on the observed angular distribution of captured particles, and model trajectories, the statistical likelihood of an interplanetary origin for all three interstellar dust candidates in aerogel is <0.03% (10,20) The ecliptic longitude of the interstellar dust radiant that best fits the observed trajectories of the three candidates in aerogel is somewhat larger than anticipated (9,18,20) based on observations from Ulysses and Galileo, but this may indicate a real long-term radiant shift which is consistent with the long-term increase in radiant longitude in neutral helium (30 (32), who predicted a total collected particle count of 120 (80 < 2µm and 40 > 2µm diameter) interstellar particles and 20 IDPs.…”

“…We identified 25 crater-like features after an automated scanning electron microscope-based search of 13 individual Al foils (19). Elemental analysis, by either Auger electron spectroscopy or energy dispersive X-ray spectroscopy (EDS), indicates that most of these features are impacts from fragments of the spacecraft solar panels.…”

“…Thus, the diameters of the particles that produced the craters range from ~0.2 to 0.3 µm. We extracted cross-sections of these craters with focused ion beam milling, and then analyzed the cross-sections with scanning transmission electron microscopy (STEM) (19).…”

“…The identification of these seven impacts is the result of a massively-distributed, volunteerbased, search of optical micrographs of the aerogel collectors, manual and automated searches of scanning electron micrographs of aluminum foils, extensive coordinated sample analyses, laboratory hypervelocity impact experiments, and numerical modeling of ISD propagation in the heliosphere. These are described in detail in a series of papers (9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) published contemporaneously with this article; further supplementary details can be found on Science online (21).…”

Population characteristics of submicrometer-sized craters on regolith particles from asteroid Itokawa

“…Sub-micrometer-sized elongate and shallow craters were also identified on Al foils of the Stardust Intersteller Dust Collector. They are thought to have been formed by secondary ejecta through oblique impacts ( Burchell et al, 2012;Stroud et al, 2014 ). The locally concentrated craters tend to exhibit oblique impact features ( Fig.…”