Energy dissipation at the silica glass/compressed aerogel interface: The fate of Wild 2 mineral grains and fragments smaller than ~100 nm

Abstract

Allocation FC6,0,10,0,26 from Stardust track 10 shows a slightly wavy silica glass/compressed silica aerogel interface exposing a patchwork of compressed silica aerogel domains and domains of silica glass with embedded Wild 2 materials in ultra‐thin TEM sections. This interface is where molten silica encountered compressed silica aerogel at temperatures <100 °C, and probably near room temperature, causing steep thermal gradients. An Mg, Fe‐olivine grain, and a plagioclase‐leucite intergrowth survived without melting in silica glass. A Mg‐, Al‐, Ca‐, K‐bearing silica globule moved independently as a single object. Two clusters of pure iron, low‐Ni iron, and low‐Ni, low‐sulfur Fe‐Ni‐S grains also survived intact and came to rest right at the interface between silica glass/compressed silica aerogel. There are numerous Fe‐Ni‐S nanograins scattered throughout MgO‐rich magnesiosilica glass, but compositionally similar Fe‐Ni‐S are also found in the compressed silica aerogel, where they are not supposed to be. This work could not establish how deep they had penetrated the aerogel. Iron nanograins in this allocation form core‐ring grains with a gap between the iron core and a surrounding ring of thermally modified aerogel. This structure was caused when rapid, thermal expansion of the core heated the surrounding compressed aerogel that upon rapid cooling remained fixed in place while the iron core shrank back to its original size. The well‐known volume expansion of pure iron allowed reconstruction of the quench temperature for individual core‐ring grains. These temperatures showed the small scale of thermal energy loss at the silica glass/compressed silica aerogel interface. The data support fragmentation of olivine, plagioclase, and iron and Fe ± low‐Ni grains from comet 81P/Wild 2 during hypervelocity capture.

Synthesis of the morphological description of cometary dust at comet 67P/Churyumov-Gerasimenko

“…All particles suffered alteration due to the capture, dominantly through heating to temperatures above the melting point of silica. Larger particles over a micrometre in size are often reasonably well preserved due to their higher thermal inertia, whilst sub-micrometre dust was only able to survive when shielded by a larger particle (Brownlee et al 2006;Rietmeijer 2016).…”

GEMS, hydrated chondritic IDPs, CI‐matrix material: Sources of water in 81P/comet Wild 2

“…Brownlee et al. () submitted that “if abundant hydrated silicates >200 nm existed in Wild 2, there should be clear evidence of them in the analyzed samples.” There is none, but ~200 nm anhydrous Ca,K‐aluminosilica and Mg‐rich olivine grains survived (Rietmeijer ).…”

The Moon: An Archive of Small Body Migration in the Solar System

“…Recognition of cometary debris in the lunar regolith would require evidence of mineralogical, composition and isotopic similarity to previously analysed cometary particles, such as those returned by the NASA Stardust mission to comet 81P/Wild-2 and the terrestrial interplanetary dust particle (IDP) collection (e.g., Rietmeijer 1998;McKeegan et al 2006;Zolensky et al 2006;Busemann et al 2009;Frank et al 2014). To date, given these constraints, no cometary silicate debris has been directly identified in lunar regolith samples.…”