Quantitative Detection of Sub-surface Irregularities in Structure and Composition: a General Method Applied to (4) Vesta

Rationale and Objective: Understanding the composition and internal structure of asteroids and protoplanets like (4) Vesta is crucial for modeling early solar system planet formation [14, 8] and cataloging asteroids for future research, most prominently in de- veloping fields such as asteroid mining and planetary defense. The surface composition of these bodies, determined by spectral remote sensing, must be quantitatively recon-ciled with the bulk properties of the bodies to infer their interior structure [5]. This study’s objective is to predict the crustal grain density of (4) Vesta based on its global Visible and Near-Infrared (VNIR) spectrum and compare it to the known bulk density, demonstrating the viability of this method for future application to a larger dataset. Novelty and Method: The work applies a high-rigor methodology that addresses two critical challenges in spectral remote sensing: the non-linearity of light scattering in particulate regolith, which is solved by applying the Hapke (1993) radiative transfer model [6, 9] to invert reflectance to linearly-additive single-scattering albedo, and the spectral redundancy of large endmember libraries, which is resolved through heuristic filtering and Principal Component Analysis (PCA) [7]. This suite is input into the Hapke (1993) model to yield endmember mass ratios (fi). The resulting composition and literature- derived endmember densities (ρi) are used to calculate the crustal grain density (ρgrain) via the harmonic mean, subject to a full error propagation [2].Results and Contribution: The spectral unmixing yields a crustal composition of primarily low-Ca pyroxene (45%) and high-Ca pyroxene (30%). Calculating error propagation results in a crustal grain density of ρgrain = 3.26 ± 0.07 g/cm3. This value is statistically distinct from the bulk density (ρbulk = 3.46±0.01 g/cm3) measured by the Dawn mission [14, 13]. Conclusion: The positive density differential, ∆ρ = ρbulk−ρgrain = 0.20±0.07 g/cm3, is statistically significant beyond 2σ. This paper provides quantitatively robust evidence, supported by full uncertainty analysis, that the interior of (4) Vesta must possess a composition or structure (e.g., a high-density core) distinct from that of its surface. The general methodology introduced here serves as a replicable framework for inferring internal structure from remote sensing data for any solid body.