Coupled shape and spin evolution of Bennu due to the YORP effect
Bibliographic record
Abstract
The rotation rate of (101955) Bennu has been observed to increase, providing evidence of the YORP effect in action. Bennu is a rubble pile with little strength. At the current spin-up rate, the rotation would result in large-scale disruption in <1 My. Such an extreme scenario is predicated on the YORP torque continuing to increase the rotation. However, YORP is sensitive to the shape and can change on a short timescale as small episodes of failure can increase oblateness, reduce spin rate, and redistribute rubble on the surface. A more comprehensive model of the shape and spin evolution of Bennu is required to understand its past and future. Here, we calculate the YORP torque on a shape model of Bennu. For a random distribution of rubble, the torques on individual blocks should cancel, and the large-scale structure should control the YORP response. However, we find the calculated torque is strongly dependent on the resolution of the shape model used, suggesting that the smaller material has an influence. As the surface roughness of the model increases, the magnitude of the torque and even its sign may change. Spin rate increases that more closely match measurements are obtained with increasing small-scale roughness. Simulated models that are coarser in resolution, but possess greater roughness than the equivalent lower-resolution shape model from observations, likewise are more consistent with the observed spin-up rate. We find that surface roughness with a non-random orientation controlled by large-scale structure determines the YORP torque. Following [1], we model the evolution of a rubble pile with Bennu’s shape subject to YORP using the granular modeling tool pkdgrav and explore how the torques change as the object is deformed. The YORP torques are calculated on the present shape and applied until particles begin to move. The torques are then recomputed on the new shape, and the iteration continues. We find negligible change in the torque until the rotation period decreases to 3.6 hr from its current 4.3-hr period. At 3.53 hr, the asteroid starts to lose mass from the equator. Our results suggest that the deformation of the asteroid’s shape due to YORP does not strongly alter rotation, and that if the initial shape is known to sufficient accuracy, the future shape and spin can be predicted. [1] Cotto-Figueroa D. et al. (2015) ApJ 803, 25.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".