DIX Planetary Science Seminar

"Early Solar System Turbulence Constrained by High Oxidation States of the Oldest Non-Carbonaceous Planetesimals"

Planetesimals formed via gravitational collapse of pebble clouds in the early Solar System (SS) constitute the parent bodies of most meteorites investigated today. Nucleosynthetic isotope anomalies of bulk meteorites have revealed a dichotomy between non-carbonaceous (NC) and carbonaceous (CC) groups, reflective of provenance in the SS protoplanetary disk. In particular, planetesimals sampling NC and CC isotopic signatures are conventionally thought to originate from the anhydrous inner disk and volatile-rich outer disk, respectively, with their segregation enforced by Jupiter's growth at the water-ice sublimation line. This framework is challenged by emerging evidence that NC iron meteorite parent bodies (IMPBs) were characterized by far higher oxidation states than previously thought, suggesting abundant liquid water in their interiors prior to core formation. Here, a model for a degassing icy planetesimal (heated by 26Al) is employed to map the conditions for liquid water production therein. Our work culminates in a threshold characteristic size for pebbles composing the said planetesimal, under which water-ice melting occurs. Assuming pebble growth by hit-and-stick collisions is limited by fragmentation, we self-consistently translate the threshold size to lower limits on early SS turbulence. This is achieved through a model for a disk evolving under both turbulence and magnetohydrodynamic disk winds. We find that for NC IMPBs to have been "wet," their constituent pebble must have been less than a few centimeters, corresponding to typical values for the Shakura-Sunyaev 𝛼 parameter in excess of ~10-3. These findings argue against a quiescent SS disk, at least within 1 Myr from the condensation of Calcium-Aluminum-rich Inclusions (CAIs). Moreover, they are concordant with astronomical constraints on disk turbulence and suggest pebble accretion played a minor role in building our rocky planets.

"Deciphering spectra of water ice on airless bodies with spectral modeling"

Lunar Trailblazer will soon be on its way to the Moon, and one of the mission goals is to study water ice in lunar permanent shadows. Understanding the physical form of the Moon's ice – fine-grained frost or pure ice chunks, for example – will help us constrain its emplacement mechanisms and estimate how much is available for future lunar missions. We have been developing a spectral modeling framework based on Hapke radiative transfer theory with applications to airless bodies to constrain the physical form, quantity, and grain size of their ice from remote sensing data. To test and refine our approach, we use Ceres as a case study. Ceres is a volatile-rich dwarf planet in the Main asteroid belt with confirmed exposures of water ice. Despite an in-situ study by NASA Dawn mission, Ceres remains a mystery. How hydrated is its crust, and what is the origin of its bright salt deposits? – these are the questions we are trying to address in our study.