Primitive Meteorite Contains Cometary Surprise

doi: 10.2138/gselements.16.3.220

Rhonda Stroud

Comets, and some primitive asteroids, preserve the microscopic remnants of the basic building blocks of rocky bodies in our solar system. Analyzing these building blocks can provide important clues both to the source of the water for Earths’ oceans and to the inventory of organic matter delivered to the early Earth. In general, samples from comets preserve a greater abundance of organic matter and primordial presolar dust grains from the protosolar molecular cloud, and generations of stars older than the sun, than do samples from asteroids. Meteorites, which come from asteroids, generally have less organic matter and preserve fewer presolar grains. This is because asteroids and comets formed at different places in the solar nebula, and, thus sampled different distributions of organic and mineral components. Furthermore, the warmer conditions on asteroids allowed for liquid water and greater subsequent physical and chemical alteration of the accreted material.

The recent publication “A Cometary Building Block in a Primitive Asteroidal Meteorite” in Nature Astronomy (Nittler et al. 2019) revealed that despite the tens of astronomical units of physical separation between where Kuiper Belt comets formed and where carbonaceous (C-type) asteroids formed, the parent asteroid of the primitive meteorite LaPaz Icefield 02342 captured and preserved a fragment of material that was typically accreted to the more distant comets (Fig. 1).

Figure 1. Schematic to show the migration of a cometary building block from the Kuiper Belt inward to the accretion zone for the carbonaceous (C-type) asteroids outside the orbit of Jupiter, and, thus, the eventual location of primitive meteorite LaPaz Icefield 02342’s parent body in the main Asteroid Main Belt.

LaPaz Icefield 02342 was found in Antarctica in 2002 and belongs to a class of primitive meteorites known as Renazzo-type carbonaceous (CR) chondrites. The first indication to the research team that there was something very unusual in their polished thin section of this meteorite came from scanning electron microscope images that showed a poppy-seed–sized (~100 μm) inclusion (Fig. 2). This inclusion was extremely carbon rich, ~70% C by area, but full of submicrometer mineral grains and readily distinguished from the epoxy used to stabilize the thin section. Team members Carlos Moyano-Cambero and Jemma Davidson identified this inclusion, or carbon-rich clast (CRC), as an important region for isotopic characterization, in addition to nearby regions with features more typical of CR chondrite meteorite matrix. Using the NanoSIMS 50L ion microprobe at the Carnegie Institution of Washington (USA), Moyano-Cambero, Davidson, and Larry Nittler were able to find large differences in the O, H and N isotopic compositions of material in the carbon-rich clast compared to the surrounding matrix. The clast contained a much greater concentration of O-rich grains with isotopic compositions indicative of a presolar origin, unusual Na-rich sulfate grains with unusual O isotopes, and organic matter with H and N isotopic compositions similar to that found in ultracarbonaceous Antarctic micrometeorites that are thought to derive from comets.

Figure 2. False color (red) backscatter secondary scanning electron microscope image of the cometary building block incorporated into the matrix of LaPaz Icefield 02342.

Additional evidence for the origin of the inclusion as a bit of cometary building-block material came from scanning transmission electron microscope (STEM) and X-ray absorption spectroscope (XAS) data. Scientists Bradley De Gregorio and Rhonda Stroud (United States Naval Research Laboratory, Washington DC, USA) extracted cross-sections from the carbon-rich clast and the matrix outside the clast to better understand the fine-grained mineralogy and the organic functional chemistry distributions. The STEM images of the clast cross-sections strongly resembled those from ultracarbonaceous Antarctic micrometeorites, and were reminiscent of images from chondritic porous interplanetary dust particles (IDPs) collected in the stratosphere, our main source of presumed cometary material (Fig. 3). Organic carbon in the clast-enveloped assemblages of amorphous Mg-rich silicates with embedded metal and sulfide grains, similar to the GEMS (glass with embedded metal and sulfides) found in IDPs (which have yet to be definitively identified as individual matrix components in any meteorite). This carbon likely served to protect the presolar silicates in the clast against alteration by liquid water on the host asteroid. In contrast, the STEM images from the LaPaz matrix were consistent with prior observations of matrix from other primitive CR chondrites: namely, abundant silicates and sulfides, isolated nanoscale-blebs of organic matter, and low degrees of porosity. The XAS results supported the conclusion that infiltration of asteroidal water was more pervasive in the matrix than in the clast, in that the organic matter in the interior of the clast was less oxidized than the organic matter in the nearby matrix.

Figure 3. Annular dark field scanning transmission electron microscope image of a cross-section that spans the boundary between the matrix and the carbon-rich clast (CRC) cometary building block. The matrix is dominated by fine-grained silicates and Fe–Ni sulfides, whereas the clast is porous and is dominated by organic carbon with discrete silicate–metal and sulfide assemblages, indistinguishable from the GEMS (glass with embedded metal and sulfides) that are found in cometary interplanetary dust particles.

This first-of-its-kind find of cometary materials embedded in a bit of asteroid tells us that the ingredients of C-type asteroids include microscopic assemblages that formed in the Kuiper Belt and then migrated inward to the vicinity of Jupiter. Based on oxygen and hydrogen isotope measurements, such materials may also have contributed to the surface carbon and ocean water of the early Earth. Because the incorporation of the cometary material into the meteorite provides a thermal barrier to ensure a safe journey through the Earth’s atmosphere, additional discoveries of such clasts in other primitive meteorites in the future may provide some of the best-preserved cometary components available for laboratory study until sample return from a comet surface is achieved. In the near term, sample return from the C-type asteroids Ryugu, by the Hayabusa2 mission, and Bennu, by the OSIRIS-REx mission, could provide an abundance of similar materials through which to track the early history of the solar system.

Reference

Nittler LR and 7 coauthors (2019) A cometary building block in a primitive asteroidal meteorite. Nature Astronomy 3: 659-666