The smallest comet 81P/Wild 2 dust dances around the CI composition

Abstract

The bulbous Stardust track #80 (C2092,3,80,0,0) is a huge cavity. Allocations C2092,2,80,46,1 nearest the entry hole and C2092,2,80,47,6 about 0.8 mm beneath the entry hole provide evidence of highly chaotic conditions during capture. They are dominated by nonvesicular low‐Mg silica glass instead of highly vesicular glass found deeper into this track which is consistent with the escape of magnesiosilica vapors generated from the smallest comet grains. The survival of delicate (Mg,Al,Ca)‐bearing silica glass structures is unique to the entry hole. Both allocations show a dearth of surviving comet dust except for a small enstatite, a low‐Ca hypersthene grain, and a Ti‐oxide fragment. Finding scattered TiO2 fragments in the silica glass could support, but not prove, TiO2 grain fragmentation during hypervelocity capture. The here reported dearth in mineral species is in marked contrast to the wealth of surviving silicate and oxide minerals deeper into the bulb. Both allocations show Fe‐Ni‐S nanograins dispersed throughout the low‐Mg silica glass matrix. It is noted that neither comet Halley nor Wild 2 had a CI bulk composition for the smallest grains. Using the analogs of interplanetary dust particles (IDPs) and cluster IDPs it is argued that a CI chondritic composition requires the mixing of nonchondritic components in the appropriate proportions. So far, the fine‐grained Wild 2 dust is biased toward nonchondritic ferromagnesiosilica materials and lacking contributions of nonchondritic components with Mg‐Fe‐Ni‐S[Si‐O] compositions. To be specific, “Where are the GEMS”? The GEMS look‐alike found in this study suggests that evidence of GEMS in comet Wild 2 may still be found in the Stardust glass.

Comet 67P/Churyumov–Gerasimenko preserved the pebbles that formed planetesimals

“…In the Stardust samples of comet 81P/Wild 2, the glass bulk compositions match a GEMS composition but there is little evidence for spherical grains. Only two single spherical amorphous grains with embedded metal and sulfide grains have been reported (Leroux & Jacob 2013), (Rietmeijer 2015), which could have been made during impact collection. To test the sensitivity of our model to the actual composition of 67P silicates in the dust aggregates sampled by GIADA, in Fig.…”

At the interface of silica glass and compressed silica aerogel in Stardust track 10: Comet Wild 2 is not a goldmine

“…Silica glass in types B and C tracks show low‐MgO contents (Leroux et al. ; Rietmeijer , ; Stodolna et al. , ) that match the low‐MgO composition in the SiO 2 ‐MgO phase diagram (Roskosz et al.…”

“…The idea behind the BKG correction is the removal from further consideration all contributions due to chemical impurities in preflight silica aerogel. The procedure works quite well for silica glass with embedded Fe‐Ni‐S nanograins and very small amorphous and/or crystalline inclusions (Rietmeijer ). The measured and BKG‐corrected abundances for the compressed silica aerogel show very little change in the O, Si, Ni, and Zn abundances after BKG correction (Table ), which is as expected.…”

“…). In detail, at least for the smallest comet particles, the average composition actually dances around the CI composition (Rietmeijer ). Early Stardust sample analyses showed that (1) the postflight aerogel was not always 100% pure silica (Lanzirotti et al.…”

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

“…The uncertainty about GEMS origins became urgent when interpreting the Stardust minerals (Rietmeijer , ). The GEMS‐like objects in Stardust are probably the results of mixing [Mg‐Fe‐Si‐O] and [Fe‐Ni‐S] melts (Rietmeijer ) but this process cannot explain the presence of GEMS in the matrix of CP IDPs. The STARDUST mission to Jupiter‐Family (J‐F) comet 81P/Wild 2 (hereafter Wild 2) (Brownlee et al.…”