arXiv subjects

Zack Gainsforth

Publications and source records attributed to Zack Gainsforth.

6 recordsLinked to original sources

arXiv2021

Automatic detection of impact craters on Al foils from the Stardust interstellar dust collector using convolutional neural networks

NASA's Stardust mission utilized a sample collector composed of aerogel and aluminum foil to return cometary and interstellar particles to Earth. Analysis of the aluminum foil begins with locating craters produced by hypervelocity impacts of cometary and interstellar dust. Interstellar dust craters are typically less than one micrometer in size and are sparsely distributed, making them difficult to find. In this paper, we describe a convolutional neural network based on the VGG16 architecture that achieves high specificity and sensitivity in locating impact craters in the Stardust interstellar collector foils. We evaluate its implications for current and future analyses of Stardust samples.

astro-ph.IM↗

arXiv2020

Chondrules from high-velocity collisions: thermal histories and the agglomeration problem

We assess whether chondrules, once-molten mm-sized spheres filling the oldest meteorites, could have formed from super-km/s collisions between planetesimals in the solar nebula. High-velocity collisions release hot and dense clouds of silicate vapor which entrain and heat chondrule precursors. Thermal histories of CB chondrules are reproduced for colliding bodies $\sim$10--100 km in radius. The slower cooling rates of non-CB, porphyritic chondrules point to colliders with radii $\gtrsim$ 500 km. How chondrules, collisionally dispersed into the nebula, agglomerated into meteorite parent bodies remains a mystery. The same orbital eccentricities and inclinations that enable energetic collisions prevent planetesimals from re-accreting chondrules efficiently and without damage; thus the sedimentary laminations of the CB/CH chondrite Isheyevo are hard to explain by direct fallback of collisional ejecta. At the same time, planetesimal surfaces may be littered with the shattered remains of chondrules. The micron-sized igneous particles recovered from comet 81P/Wild-2 may have originated from in-situ collisions and subsequent accretion in the proto-Kuiper belt, obviating the need to transport igneous solids across the nebula. Asteroid sample returns from Hayabusa2 and OSIRIS-REx may similarly contain chondrule fragments.

astro-ph.EP↗

arXiv2018

Fine-grained Material Associated with a Large Sulfide returned from Comet 81P/Wild 2

In a consortium analysis of a large particle captured from the coma of comet 81P/Wild 2 by the Stardust spacecraft, we report the discovery of a field of fine-grained material (FGM) in contact with a large sulfide particle. The FGM was partially located in an embayment in the sulfide, so appears to have been largely protected from damage during hypervelocity capture in aerogel. Some of the FGM particles are indistinguishable in their characteristics from common components of chondritic-porous interplanetary dust particles (CP-IDPs), including glass with embedded metals and sulfides (GEMS) and equilibrated aggregates (EAs). The sulfide exhibits surprising Ni-rich lamellae, which may indicate that this particle experienced a long-duration heating event after its formation but before incorporation into Wild 2. We discuss the relationship of the FGM to the sulfide, to other Wild 2 particles and to the history of the Solar nebula.

astro-ph.EP↗

arXiv2018

Measurement of the Oxidation state of Fe in the ISM using X-ray Absorption Spectroscopy

The oxidation state of iron in the interstellar medium (ISM) can provide constraints on the processes that operated on material in the protosolar disk. We used synchrotron-based X-ray absorption spectra of several mineral standards and two kinds of primitive extraterrestrial materials to constrain the oxidation state and mineralogy of the host-phase of ISM Fe as measured by X-ray observations of Fe-L ISM absorption from the {\em Chandra X-ray Observatory}. Given the initial oxidation state of the building blocks of the Solar System, we explore nebular and parent-body processes that eventually led to the remarkable diversity of oxidation states of primitive Solar System materials. Oxidation of cometary material appears to have taken place in the nebula, before incorporation into cometary nuclei, although the mechanism is unknown.

astro-ph.EP↗

arXiv2015

Oxygen Isotopic Composition of Coarse- and Fine-grained Material from Comet 81P/Wild 2

Individual particles from comet 81P/Wild 2 collected by NASA's Stardust mission vary in size from small sub-$\mu$m fragments found in the walls of the aerogel tracks, to large fragments up to tens of $\mu$m in size found towards the termini of tracks. The comet, in an orbit beyond Neptune since its formation, retains an intact a record of early-Solar-System processes that was compromised in asteroidal samples by heating and aqueous alteration. We measured the O isotopic composition of seven Stardust fragments larger than $\sim$2 $\mu$m extracted from five different Stardust aerogel tracks, and 63 particles smaller than $\sim$2 $\mu$m from the wall of a Stardust track. The larger particles show a relatively narrow range of O isotopic compositions that is consistent with $^{16}$O-poor phases commonly seen in meteorites. Many of the larger Stardust fragments studied so far have chondrule-like mineralogy which is consistent with formation in the inner Solar System. The fine-grained material shows a very broad range of O isotopic compositions ($-70<\Delta^{17}$O$<+60$) suggesting that Wild 2 fines are either primitive outer-nebula dust or a very diverse sampling of inner Solar System compositional reservoirs that accreted along with a large number of inner-Solar-System rocks to form comet Wild 2.

astro-ph.EP↗

arXiv2013

Model for quantitative tip-enhanced spectroscopy and the extraction of nanoscale-resolved optical constants

Near-field infrared spectroscopy by elastic scattering of light from a probe tip resolves optical contrasts in materials at dramatically sub-wavelength scales across a broad energy range, with the demonstrated capacity for chemical identification at the nanoscale. However, current models of probe-sample near-field interactions still cannot provide a sufficiently quantitatively interpretation of measured near-field contrasts, especially in the case of materials supporting strong surface phonons. We present a model of near-field spectroscopy derived from basic principles and verified by finite-element simulations, demonstrating superb predictive agreement both with tunable quantum cascade laser near-field spectroscopy of SiO$_2$ thin films and with newly presented nanoscale Fourier transform infrared (nanoFTIR) spectroscopy of crystalline SiC. We discuss the role of probe geometry, field retardation, and surface mode dispersion in shaping the measured near-field response. This treatment enables a route to quantitatively determine nano-resolved optical constants, as we demonstrate by inverting newly presented nanoFTIR spectra of an SiO$_2$ thin film into the frequency dependent dielectric function of its mid-infrared optical phonon. Our formalism further enables tip-enhanced spectroscopy as a potent diagnostic tool for quantitative nano-scale spectroscopy.

cond-mat.mtrl-sci↗