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.