Exploring Dynamical and Evolutionary Processes via Debiased Main Belt Asteroid Size-Frequency Distributions

Collisions, gravitational forces, and non-gravitational forces like YORP and Yarkovsky drift are three major processes that shape planetary systems.  Due to their small sizes and lack of atmospheric weathering, asteroids are the most well-preserved tracers of these forces.  The Main belt between Mars and Jupiter is the most numerous asteroid population known and houses dozens of identified collisional families of varying ages and compositions, fossilizing evidence of these evolutionary processes at diverse time stamps and under different compositional contexts.  Fundamental Main belt asteroid (MBA) properties like shape and size-frequency distribution (SFDs) are key constraints on solar system evolutionary models, yet these properties for the Main belt’s constituent subpopulations remain either unknown or inadequately constrained due to biases in the surveys upon which they are based.    

Our project aims to constrain shape and spin pole distributions, model survey efficiencies, and construct debiased SFDs for prominent collisional families in the Main belt and for dynamically distinct regions of non-family (i.e., “background”) MBAs.  By comparing debiased SFDs of old versus young families of similar composition, we can ascertain how quickly and in what ways continuous evolutionary forces are reshaping small body populations.  We can also help understand how impact physics differ between different types of material by comparing debiased SFDs and shape distributions of carbonaceous families to more siliceous families.  The shape and spin pole distributions determined here are used to assess how quickly ongoing collisional grinding softens asteroid dimensions and how efficiently YORP realigns rotation axes.  At the completion of this study, we will also improve the mass budget estimate of the Main belt using our debiased SFDs to better constrain dynamical and collisional depletion and accumulation models. 

We implement the LEADER software (Nortunen et al. 2017) to constrain pole latitude and a:b axis ratio distributions for our dynamical subpopulations using archived 12-micron asteroid photometry taken during the NEOWISE space mission’s cryogenic phase.  The NEOWISE dataset is uniquely well suited for debiasing due to far fewer observational biases, a regular survey cadence, and well understood and stable survey sensitivities.  We will use this information to measure detection efficiency functions over multiple trials and across several parameters down to diameters of a few kilometers, then apply them to the observed subpopulations to determine debiased SFDs that (uniquely, in our understanding) take into account empirical asteroid shape properties.  In this presentation, we report the preliminary findings of our debiased MBA SFD project.