Refractory materials in comet samples

Abstract

Transmission electron microscope examination of more than 250 fragments, >1 μm from comet Wild 2 and a giant cluster interplanetary dust particle (GCP) of probable cometary origin has revealed four new calcium‐aluminum‐rich inclusions (CAIs), an amoeboid olivine aggregate (AOA), and an additional AOA or Al‐rich chondrule (ARC) object. All of the CAIs have concentric mineral structures and are composed of spinel + anorthite cores surrounded by Al,Ti clinopyroxenes and are similar to two previous CAIs discovered in Wild 2. All of the cometary refractory objects are of moderate refractory character. The mineral assemblages, textures, and bulk compositions of the comet CAIs are similar to nodules in fine‐grained, spinel‐rich inclusions (FGIs) found in primitive chondrites and like the nodules may be nebular condensates that were altered via solid–gas reactions in the solar nebula. Oxygen isotopes collected on one Wild 2 CAI also match FGIs. The lack of the most refractory inclusions in the comet samples may reflect the higher abundances of small moderately refractory CAI nodules that were produced in the nebula and the small sample sizes collected. In the comet samples, approximately 2–3% of all fragments larger than 1 μm, by number, are CAIs and nearly 50% of all bulbous Stardust tracks contain at least one CAI. We estimate that ~0.5 volume % of Wild 2 material and ~1 volume % of GCP is in the form of CAIs. ARCs and AOAs account for <1% of the Wild 2 and GCP grains by number.

The Effect of Jupiter's Formation on the Distribution of Refractory Elements and Inclusions in Meteorites

“…One of the most surprising findings of the Stardust comet sample return mission was the presence of fragments of chondrules, AOAs, and CAIs in the comet Wild 2, that must have formed in the inner solar system (Simon et al 2008;Bridges et al 2012;Joswiak et al 2017). It is estimated that 0.5 vol% of the sample return material is CAIs (Joswiak et al 2017). This is comparable to the abundance in CM and CR chondrites formed in our model at about 4 AU, despite the fact that comets are presumed to form much further out in the disk, at tens of AU.…”

Dust-to-Gas and Refractory-to-Ice Mass Ratios of Comet 67P/Churyumov-Gerasimenko from Rosetta Observations

“…Comets consist of a mixture of refractory materials in the classical sense (minerals), an organic "soup" that ranges from simple molecules more volatile than water ice, such as methane and methanol, to heavy and refractory molecules such as kerogen-like insoluble organic matter (IOM), and a number of frozen volatiles (ices) like H 2 O, CO 2 , CO, H 2 S, NH 3 , and multiple other minor species (Bockelée-Morvan et al 2004;Mumma and Charnley 2011;. Comet nuclei have a very low density/high porosity (Weissman et al 2004;Weissman and Lowry 2008;, and the few samples available on Earth that likely originate from comets (chondritic-porous interplanetary dust particles, ultra-carbonaceous Antarctica micro-meteorites), as well as grains collected in the coma of 81P/Wild by the Stardust mission, all exhibit a very fine-grained texture (Joswiak et al 2017). These compositional and physical characteristics point to a rather slow and gentle accretion under cold conditions 100 K, i.e.…”

Fine‐grained material associated with a large sulfide returned from Comet 81P/Wild 2

“…Recently, Joswiak et al. () found that calcium‐aluminum‐rich inclusion (CAI)‐like fragments account for ≈1 vol% in cometary samples including Wild 2 and a giant cluster IDP named U2‐20GCP. Joswiak et al.…”

“…Joswiak et al. () make a strong case that U2‐20 has a cometary origin, and have noted (personal communication) that it has coarse‐grained rocks (CAI‐like) in direct contact with FGM including GEMS and EAs. Wooden et al.…”

“…), and while there is substantial evidence that some specific CP‐IDPs are cometary (Joswiak et al. ), there is no definitive link between specific CP‐IDPs and specific comets. CP‐IDPs commonly contain 100–500 nm amorphous silicates known as GEMS (glass with embedded metals and sulfides), and crystalline silicate aggregates of similar and larger size called equilibrated aggregates (EAs).…”