Recovery and Classification of Spherules from the Pacific Ocean Site of the CNEOS 2014 January 8 (IM1) Bolide

On 2014 January 8, US government satellite sensors detected three atmospheric detonations in rapid succession about 84 km north of Manus Island (Loeb et al. 2023). Analysis of the trajectory suggested an interstellar origin of the causative object CNEOS 2014 January 8: an arrival velocity relative to Earth more than ∼45 km s⁻¹, and a vector tracked back to outside the plane of the ecliptic (Siraj & Loeb 2022a). The bolide broke apart at an unusually low altitude of ∼17 km. The object was substantially stronger than any of the other 272 objects in the CNEOS catalog, including the ∼5%-fraction of iron meteorites (Siraj & Loeb 2022b). The fireball light energy suggest that about 500 kg of material was ablated by the fireball and converted into ablation spherules with a small efficiency (Siraj & Loeb 2023).

The expedition searched for remnants of the bolide, labeled hereafter IM1. It utilized a 40 m catamaran workboat, the M/V Silver Star. A 200 kg sled was used with 300 neodymium magnets mounted on both of its sides and video cameras mounted on the tow-bridle. Approximately 0.06 km² were sampled in the target area. The fine material collected on the neodymium magnets was extracted and brought in a wet slurry up to a laboratory. Subsequently, both magnetic and non-magnetic separations were processed through sieves and dried. Spherules were handpicked with tweezers using a binocular zoom microscope. They ranged in size from 100 μm to 2 mm. We obtained a total of 850 spherules by this method.

Most samples (780) were first analyzed by microXRF with a Bruker Tornado M4 for their bulk major element composition, followed by imaging (SEM and chemical mapping) and spot chemical analyses of about 100 samples with a JEOL Model JXA 8230 Electron Probe Microanalyzer (EPMA). Measurements of elemental abundances for about 60 major and trace elements were performed for 70 samples with an iCAP TQ triple quadrupole ICP-MS (ThermoFisher Scientific).

Cosmic spherules are sub-divided into three compositional types (Blanchard et al. 1980). These are the silicate-rich spherules or S-type, the Fe-rich spherules or I-type and glassy spherules or G-types. Relatively rare spherules have been labeled differentiated as they have similarities to achondrite meteorites and have been treated as a subgroup of S-type spherules (Folco & Cordier 2015).

The major element compositions of 745 spherules from the IM1 site, measured by microXRF, are plotted in a Mg–Si–Fe ternary diagram (Figure 1(a)). About 22% of the spherules have low Mg and plot close to the Si–Fe side of the diagram. The high-Si part of this group plots within the range of terrestrial igneous rocks that are shown for comparison. These spherules are thus called differentiated, meaning they are likely derived from crustal rocks of a differentiated planet; we label them as D-type spherules, characterized by Mg/Si < 1/3.

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For the primitive spherules we use 100Fe/(Fe+Si+Mg) > 90 to distinguish I-types from S- and G-types. The S- and G-type dividing line is 100Si/(Fe+Si+Mg) = 50, relatively consistent with previous literature (Brownlee et al. 1997; Taylor et al. 2000; Folco & Cordier 2015). The primitive spherule groups are compared to reference values of Bulk Earth, bulk silicate Earth (BSE), and CI meteorites that are and should be on the primitive trend.

The differentiated spherules are divided into high Sr (>450 ppm) and low Sr groups (<450 ppm). They are further subdivided on the Si-content by using 100Si/(Fe+Si+Mg) = 60 as the dividing line for high Sr spherules. The D-type spherules have been divided into four distinct groups. This results in 8 distinct spherule groups that are all shown in Figure 1(a).

We use a different ternary plot to identify spherules with particularly high contents of refractory lithophile elements, based on the enrichments of Be, La and U relative to Mg and Fe. First the concentrations of these elements are divided by the concentrations of the same elements in CI chondrites. The CI-normalized values (Be_CI, La_CI, U_CI, Mg_CI and Fe_CI) are used to calculate the plotting parameters for the ternary diagram. The BeLaU parameter (Be_CI+La_CI+U_CI)/(Mg_CI+Fe_CI+Be_CI+La_CI+U_CI) was divided by 100, while the M-parameter was multiplied by 10 to make use of the entire area inside the ternary diagram. We define a BeLaU spherule as having BeLaU > 80, and we define high and low Si varieties based on the Al/Fe ratio. This procedure identifies 10 of D-type spherules as BeLaU low Si spherules and 2 as BeLaU high Si spherules. Based on 60 elements from the periodic table, their unique chemical composition is shown in Figure 1(b). The BeLaU composition shows an excess of Be, La and U, and other elements by up to three orders of magnitude relative to the solar system standard of CI chondrites. Many of these enhanced elements are 1–2 orders of magnitude more abundant than shown in Figure 2 of Rudraswami et al. (2016) for S-type spherules.

It has been claimed that the compositions of BelaU spherules are consistent with coal ash (Gallardo 2023). The National Institute of Standards and Technology (NIST) has provided standards of coal fly ash. The best documented standard for many elements is SRM 1633a (Jochum et al. 2005). We compare the average composition of BeLaU spherules (Loeb et al. 2023) for 55 elements with the SRM1633a coal fly ash standard in Figure 1(c). Many volatile elements (Zn, As, Se, Cd, Tl, Pb and Bi) are enriched in the coal fly ash by factors of about 10–100 compared to the BeLaU spherules. Some refractory elements (Be, Ca, Cr, Fe, Y, Tm, Yb, Lu W) are depleted by factors of 3 to 10 in coal fly ash when compared BeLaU spherules. Thus, BeLaU spherules do not have the composition of coal fly ash making the claim of Gallardo (2023) invalid.