Calcium-aluminum-rich inclusions (CAIs) in chondritic meteorites are the oldest dated solid materials in the solar system at 4.567 Ga. What does their mineralogy (corundum, hibonite, perovskite, melilite) reveal about their formation conditions?
AThey formed by aqueous alteration on a parent body
BTheir highly refractory mineralogy (the highest-temperature condensates from a solar-composition gas) indicates they formed by condensation from the hot solar nebula, representing the first solids to form as the nebula cooled
CThey are fragments of Earth's core ejected during the Moon-forming impact
DThey formed by volcanism on a large asteroid
CAI mineralogy exactly matches thermodynamic predictions for the first minerals to condense from a cooling gas of solar composition. Corundum (Al2O3) condenses at ~1700 K, followed by hibonite (CaAl12O19), perovskite (CaTiO3), and melilite. These are the most refractory (highest condensation temperature) phases, consistent with being the first solids to form in the solar nebula. Their 4.567 Ga Pb-Pb age defines the start of the solar system.
Question 2 True / False
CI chondrites are used as the reference for solar system elemental abundances because they match the composition of the Sun's photosphere for all elements.
TTrue
FFalse
Answer: False
CI chondrites match solar photospheric abundances remarkably well for non-volatile elements (refractory lithophile, siderophile, and chalcophile elements), with agreement typically within 10-20%. However, they are depleted in the most volatile elements (H, He, C, N, O, noble gases) that were not fully incorporated into solid meteorite parent bodies. The solar photosphere better represents these volatile elements. For non-volatile elements, the CI chondrite-photosphere agreement is one of the most important observations in cosmochemistry, validating CI chondrites as proxies for bulk solar system composition.
Question 3 Short Answer
Explain what presolar grains are and what they reveal about the stellar sources of solar system material.
Think about your answer, then reveal below.
Model answer: Presolar grains are tiny mineral particles (typically < 1 um) found within primitive meteorites that formed in the outflows of ancient stars before the solar system existed. They are identified by extremely anomalous isotopic compositions that cannot be produced by any solar system process -- for example, silicon carbide (SiC) grains with 12C/13C ratios 10-100x different from solar, recording nucleosynthesis in asymptotic giant branch (AGB) stars. Nanodiamond, corundum, and graphite grains record contributions from supernovae. These grains survived the formation of the solar nebula, incorporation into parent bodies, and residence in meteorites for 4.6 Gyr, providing direct samples of pre-solar stellar material and constraining the nucleosynthetic contributions to the solar system's elemental and isotopic inventory.
Presolar grains are literally stardust -- physical samples of other stars that predate our solar system, identifiable by isotopic signatures impossible to produce locally.