Stardust Interstellar Preliminary Examination II: Curating the interstellar dust collector, picokeystones, and sources of impact tracks

Abstract

Abstract-We discuss the inherent difficulties that arise during "ground truth" characterization of the Stardust interstellar dust collector. The challenge of identifying contemporary interstellar dust impact tracks in aerogel is described within the context of background spacecraft secondaries and possible interplanetary dust particles and b-meteoroids. In addition, the extraction of microscopic dust embedded in aerogel is technically challenging. Specifically, we provide a detailed description of the sample preparation techniques developed to address the unique goals and restrictions of the Interstellar Preliminary Exam. These sample preparation requirements and the scarcity of candidate interstellar impact tracks exacerbate the difficulties. We also illustrate the role of initial optical imaging with critically important examples, and summarize the overall processing of the collection to date.

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. With the conservative assumption that all of the interplanetary dust is < 2 µm, this equates to 100 small particles (80 interstellar and 20 interplanetary), of which 20% should be interplanetary.…”

“…All but two were identified through the Stardust@home project (9,10), in which volunteers searched online for tracks in digital micrographs of the aerogel collector. We extracted a subset of these tracks in volumes of aerogel, called "picokeystones" (10,23), and mounted them between 70 nm-thick Si 3 N 4 membranes to protect from loss and contamination.…”

“…All but two were identified through the Stardust@home project (9,10), in which volunteers searched online for tracks in digital micrographs of the aerogel collector. We extracted a subset of these tracks in volumes of aerogel, called "picokeystones" (10,23), and mounted them between 70 nm-thick Si 3 N 4 membranes to protect from loss and contamination. Picokeystones were subsequently analyzed at one or more of six synchrotrons with techniques including Scanning Transmission X-ray Microscopy (STXM) (12), Fourier Transform Infrared Spectroscopy (FTIR) (11), X-Ray Fluorescence spectroscopy (XRF) (13)(14)(15) and X-Ray Diffraction (XRD) (16).…”

“…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).…”

Stardust Interstellar Preliminary Examination III: Infrared spectroscopic analysis of interstellar dust candidates

“…The picokeystones are extracted on barbed polysilicon forks, then carefully sandwiched between two 50–70 nm‐thick Si 3 N 4 windows (Frank et al. ). This mounting technique simultaneously protects the samples from contamination, reduces risk of loss by physically trapping the samples, and allows for all synchrotron‐based analyses, except tomography, which has a reduced resolution due to limitations in the angular scan range.…”