Survival of refractory presolar grain analogs during Stardust‐like impact into Al foils: Implications for Wild 2 presolar grain abundances and study of the cometary fine fraction

Abstract

We present results of FIB–TEM studies of 12 Stardust analog Al foil craters which were created by firing refractory Si and Ti carbide and nitride grains into Al foils at 6.05 km s−1 with a light‐gas gun to simulate capture of cometary grains by the Stardust mission. These foils were prepared primarily to understand the low presolar grain abundances (both SiC and silicates) measured by SIMS in Stardust Al foil samples. Our results demonstrate the intact survival of submicron SiC, TiC, TiN, and less‐refractory Si3N4 grains. In small (<2 μm) craters that are formed by single grain impacts, the entire impacting crystalline grain is often preserved intact with minimal modification. While they also survive in crystalline form, grains at the bottom of larger craters (>5 μm) are typically fragmented and are somewhat flattened in the direction of impact due to partial melting and/or plastic deformation. The low presolar grain abundance estimates derived from SIMS measurements of large craters (mostly >50 μm) likely result from greater modification of these impactors (i.e., melting and isotopic dilution), due to higher peak temperatures/pressures in these crater impacts. The better survivability of grains in smaller craters suggests that more accurate presolar grain estimates may be achievable through measurement of such craters. It also suggests small craters can provide a complementary method of study of the Wild 2 fine fraction, especially for refractory CAI‐like minerals.

A study of the observed shift in the peak position of olivine Raman spectra as a result of shock induced by hypervelocity impacts

“…There is a separate TEM study of residue in impact craters in Al at 6 km s −1 in Croat et al. (). All the authors seem to agree that significant amounts of projectile material remain in the craters even at high impact speeds.…”

“…A more detailed TEM study of projectile residues is reported in Wozniakiewicz et al (2012aWozniakiewicz et al ( , 2012b, where the presence of unmelted material after impacts of minerals into Al at around 6 km s À1 is extensively reported. There is a separate TEM study of residue in impact craters in Al at 6 km s À1 in Croat et al (2015). All the authors seem to agree that significant amounts of projectile material remain in the craters even at high impact speeds.…”

“…; Croat et al. ). However, the capture process may have left residual strain in the crystalline lattice.…”

“…Subsequently, it was shown by Burchell et al (2008b) that not only were distinct fragments of several mineralic impactors on Al foil detectable lining the crater after impact at 6.1 km s À1 but that they had also preserved sufficient crystallinity to be identifiable by Raman spectroscopy. Later work has studied such craters in detail and shown that the fragments can comprise a relatively large fraction of the original grain, particularly for small impactors (e.g., see Wozniakiewicz et al 2012b;Croat et al 2015). However, the capture process may have left residual strain in the crystalline lattice.…”

The survivability of phyllosilicates and carbonates impacting Stardust Al foils: Facilitating the search for cometary water

“…1A). This facility has been used extensively to simulate the Stardust capture conditions by accelerating and impacting chosen projectiles to~6 km s À1 into Stardust analogue targets (e.g., Kearsley et al 2006Kearsley et al , 2007Kearsley et al , 2008Kearsley et al , 2009Burchell et al 2008aBurchell et al , 2008bWozniakiewicz et al 2009Wozniakiewicz et al , 2011Wozniakiewicz et al , 2012aWozniakiewicz et al , 2012bPrice et al 2010;Floss et al 2013;Croat et al 2015).…”