On the rotational properties of M-type asteroids: recent evidence for higher rotation rates

TL;DR

The study investigates whether metallic (M-type) asteroids rotate faster than the general asteroid population. It combines Mahlke's probabilistic taxonomy with SBDB rotation and diameter data and applies two independent analyses: a diameter-dependent spline regression of rotation frequency and a diameter-agnostic Monte Carlo bootstrapping test, both incorporating robust statistical controls. The results indicate that M-types rotate faster on average, with a positive log-frequency offset ($eta_1>0$) and a consistent empirical signal across methods (e.g., $p=1.071\times10^{-6}$ for the regression and $p=0.0053$ for the bootstrap), suggesting higher internal cohesion in metal-rich bodies. While recognizing sample size and classification limitations, the convergence across methodologies supports a physical interpretation that metal-rich asteroids sustain higher spin rates, informing models of differentiation, collisional evolution, and the interpretation of the Psyche mission data; future work with larger, more complete datasets and in situ measurements will further clarify these rotational properties.

Abstract

Rotational dynamics of asteroids carry important information about their internal structure, collisional history and material composition. This work investigates whether metallic (M-type) asteroids exhibit systematically higher rotation rates than the broader asteroid population. Using recent Mahlke's taxonomy and a multi-method statistical approach that combines classical hypothesis testing, regression modeling, and resampling techniques, we consistently find evidence that metallic asteroids rotate, on average, faster. The results remain robust across different ways of controlling for asteroid size and sampling biases. While limitations remain due to sample size, heterogeneous data sources, and possible selection effects, the convergence of independent methods strengthens the conclusion that metallic composition is associated with higher rotation rates. This finding supports the hypothesis that metallic bodies possess greater internal cohesion, with implications for the formation and collisional evolution of small bodies in the Solar System.

Figures (3)

• Figure 1: Rotation frequency versus diameter for the asteroid sample. The background in light gray shows the general asteroid population (19 940 objects from the SBDB with measured diameters and rotation periods). Overplotted are the candidate M-type asteroids according to Mahlke et al. (2022): gray crosses correspond to objects with intermediate probabilities ($0.5 < P(M) < 0.8$), while black points mark high-confidence M-types with $P(M) > 0.8$.
• Figure 2: Box plot of the residual distributions between the data and the spline model. On the left, the residuals of the general population. On the right, the residuals of the M-type asteroids, which show less dispersion and higher centrality. The Mann-Whitney U test yields a $p$-value of 0.003, with M-type asteroids showing a median residual higher by 0.326 in log-frequency than the general population.
• Figure 3: Result of the Monte Carlo bootstrapping simulation. The histogram shows the result of 10,000 repetitions: in each repetition, 175 random asteroids are selected and their geometric mean is calculated. The distribution is largely normal, with $\mu=2.117\mathrm{\,\,d^{-1}}$, $\sigma=0.200\mathrm{\,\,d^{-1}}$. However, when specifically the 175 Mahlke M-types are selected, the obtained geometric mean is $\mu=2.692\mathrm{\,\,d^{-1}}$, yielding an empirical $p$-value of $0.0053$