Surface evolution and disruption of near-Sun asteroids ($q\le 0.15$ au): is the population bluing physical, which processes dominate resurfacing and destruction, and what is the incidence of perihelion mass loss?

Asteroids evolving to perihelion distances $q\le 0.15$ au are heated to subsolar temperatures above 1000 K and are destroyed faster than dynamical lifetimes predict, yet the physical processes that modify and ultimately disrupt them remain unidentified at the population level. Holt et al. (2022) surveyed the optical colors, rotation periods, and activity of the 53 then-known near-Sun asteroids and found a color distribution bluer than the general near-Earth asteroid population, but with large overlap, no trends with orbital or rotational properties, and activity limits 1–3 orders of magnitude shallower than Phaethon's perihelion mass loss. The open problem is threefold and joint: establish whether the tentative population bluing is a robust physical consequence of near-Sun resurfacing rather than selection or composition; allocate causally among the competing near-Sun processes (thermal fatigue, YORP spin-up and mass shedding, tidal resurfacing, meteoroid bombardment, devolatilization/thermal erosion) for the population and individual objects; and measure or bound the incidence and typical rates of perihelion mass loss at Phaethon-like levels. Follow-up work through 2026 has advanced pieces — laboratory evidence for instantaneous thermal erosion of dark primitive material, population-level evidence for tidal disruption, sodium-driven (not dusty) activity on Phaethon, and colors of two new extreme objects that are notably not blue — but none of the three population-level questions is settled.

Background

Near-Earth asteroids (NEAs) whose orbits carry them to small perihelion distances ($q$, the closest approach to the Sun) experience a uniquely extreme environment: at $q\le 0.15$ au the subsolar surface temperature exceeds 1000 K, hot enough to weaken, degas, and reshape surface material. Granvik et al. (2016) discovered from debiased NEA population models that asteroids disappear faster than dynamical removal allows once $q$ drops well inside Mercury's orbit, and inferred that they are destroyed by some rapid, still-unidentified process they termed 'super-catastrophic disruption' — a disruption so complete that no large fragments survive. Candidate near-Sun surface-modifying and destroying processes include: thermal fatigue (repeated day–night thermal cycling cracking surface rock), the YORP effect (Yarkovsky–O'Keefe–Radzievskii–Paddock: the torque from asymmetric absorption and re-radiation of sunlight, which can spin an asteroid up until it sheds mass or disrupts), tidal resurfacing or disruption during close planetary encounters, meteoroid bombardment, and devolatilization — the loss of trace volatiles or thermally labile minerals that can both eject material and alter the surface's optical color. Several of these processes can drive ordinary dark, red, carbonaceous surface material toward neutrally or bluer colored, dehydration-altered surfaces, as observed on (3200) Phaethon ($q\approx 0.14$ au), the large blue B-type parent of the Geminid meteor shower and the only near-Sun asteroid with well-documented recurrent perihelion activity (imaged by the SOHO and STEREO solar observatories, whose coronagraphs routinely observe objects very close to the Sun). Holt et al. (2022) attempted observation of 35 of the 53 then-known asteroids with $q\le 0.15$ au, reporting colors and spectral slopes for 22 objects and rotation periods for three, plus a compilation of all known physical properties, probabilistic source regions, and dwell times at low $q$. They found the near-Sun color distribution bluer than the NEA distribution overall but strongly overlapping it, with no correlation of color against perihelion distance, Tisserand parameter (a dynamical discriminator between asteroidal and cometary orbits), or rotation period, and no detected activity — attributing the scatter to unknown per-object dynamical histories, compositions, and competing surface-alteration processes. Their coronagraph-archive search could only exclude perihelion mass loss 1–3 orders of magnitude above Phaethon's rate for the rest of the population, and only nine objects had known rotation periods, too few to test the prediction that enhanced YORP torques near the Sun produce an excess of fast rotators. They therefore posed, as the study's open problems, the population-level questions that the general dynamical models, the per-object data, and the activity searches were individually too weak to answer. To settle those questions, Holt et al. also identified a need for a larger homogeneous color sample, per-object dynamical integrations yielding dwell times, rotation-period statistics sufficient to test the enhanced-YORP fast-rotator prediction, albedo and spectroscopic characterization (including near-infrared diagnostics), and deeper targeted perihelion activity searches.

Problem Statement

For the population of near-Sun asteroids with perihelion distances $q\le 0.15\ \mathrm{au}$, determine the physical state of their surfaces and the processes and timescales governing their surface evolution and disruption, by answering three linked questions at the population level. (i) Robustness of bluing: is the optical color distribution of the $q\le 0.15\ \mathrm{au}$ population significantly bluer than that of the general near-Earth asteroid population as a robust consequence of near-Sun resurfacing physics, rather than an artifact of sample selection, compositional diversity, survivorship bias (preferential destruction of dark, red, primitive material), or measurement uncertainties? (ii) Causal allocation: which of the competing near-Sun processes — thermal fatigue/cracking, YORP-driven spin-up with mass shedding, tidal resurfacing during planetary close encounters, meteoroid bombardment, and devolatilization or instantaneous thermal erosion — dominates surface alteration and destruction for the population and for individual objects, given their dynamical histories (dwell times at low $q$), surface physical properties (albedo, thermal inertia, roughness), and compositional classes? (iii) Activity incidence: what is the incidence and what are the typical mass-loss rates of perihelion activity among the $q\le 0.15\ \mathrm{au}$ population at sensitivity levels comparable to Phaethon's observed perihelion activity?

References

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