DIX Planetary Science Seminar
Asteroids are fragments of planetesimals, and as such constitute one of the most pristine samples we have for studying the early solar system and subsequent evolution. The present-day surfaces of asteroids have been shaped by their initial formation conditions along with collisions since then that have formed craters, revealed subsurface materials, and contributed to their unique shapes. Detecting such features requires high spatial resolution, and one of the few facilities capable of achieving the necessary resolution is the Atacama Large Millimeter/submillimeter Array (ALMA), an interferometer in Chile. We imaged 15 main-belt asteroids with ALMA at wavelengths of 0.8mm or 1.2mm, where thermal emission from the surface and immediate subsurface dominates. Thermal emission depends on the thermal inertia and dielectric constant of the emitting material, and these properties depend on composition (particularly metal content), roughness, and scattering. We run a thermophysical model to fit our data for thermal inertia and dielectric constant. We focus on the asteroid (4) Vesta, which has been visited by the Dawn spacecraft and is the source of the Howardite-Eucrite-Diogenite (HED) meteorites, making it the best target for comparisons between wavelength regimes and techniques. We find a region of low dielectric constant overlapping with a region of eucritic mineralogy, suggesting that the heterogeneity in mineralogy on Vesta's surface also corresponds to heterogeneity in thermophysical properties.
------------------------------------------------------------------------------------------------------
Most organic carbon in primitive chondritic meteorites occurs as a solid macromolecular material known as insoluble organic matter (IOM). Its origin, accretion, and evolution within chondritic parent bodies carries major significance since it likely represents the main form of organic material delivered to terrestrial planets. It remains unclear whether IOM, or its precursors, were inherited from the interstellar medium or formed locally in the protoplanetary disk, and how strongly its present composition reflects primary synthesis versus parent-body alteration. We first show that carbon isotope variations in IOM from the main carbonaceous chondrite groups strongly correlate with mass-independent oxygen isotope anomalies of their host meteorites. Since the latter are generally interpreted to record incorporation of, or exchange with, nebular H2O generated by photochemical processing of CO gas, the carbon isotope variations may reflect the same process. We propose that two distinct IOM precursors, associated with volatile-rich and -poor dust reservoirs, were accreted into chondrite parent bodies. Their carbon isotope compositions suggest formation over a relatively warm range of temperatures in the disk (~200-400 K), which may relate to hydrocarbon chemistry occurring near the soot line and in the warm molecular layer. We then examine IOM structural ordering using Raman spectroscopy. This has traditionally been used to trace thermal metamorphism in chondrite parent bodies; however, intra-meteorite structural variations in primitive samples may instead record distinct precursor populations.