Obliquities of “top-shaped” asteroids may not imply reshaping by YORP spin-up

Statler, Thomas S.

doi:10.1016/j.icarus.2014.10.050

Cited by 16 publications

(12 citation statements)

References 52 publications

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“…After the formation of an oblate spheroid, it should not take much time (order of 10 4 -10 6 years for kilometer-sized asteroids 3,24 ) for post-processes such as YORP to lead to top shapes such as those currently observed. Dynamical studies of the spins of main belt asteroids show that YORP evolution is required to explain their distribution 25 .…”

Section: Resultsmentioning

confidence: 99%

“…However, observations that asteroids have the expected obliquity and are top-shaped may not be sufficient to confirm the hypothesis that they were shaped by the YORP effect. It has been showed that time scales needed to reorient an asteroid that is already symmetric in shape are much shorter than that of YORP spin-up 24 . In addition, the YORP spin-up process is not linear, as small changes in the surface topography driven by asteroids' rotation can substantially vary the YORP acceleration 32 .…”

Section: Resultsmentioning

confidence: 99%

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Collisional formation of top-shaped asteroids and implications for the origins of Ryugu and Bennu

et al. 2020

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Asteroid shapes and hydration levels can serve as tracers of their history and origin. For instance, the asteroids (162173) Ryugu and (101955) Bennu have an oblate spheroidal shape with a pronounced equator, but contain different surface hydration levels. Here we show, through numerical simulations of large asteroid disruptions, that oblate spheroids, some of which have a pronounced equator defining a spinning top shape, can form directly through gravitational reaccumulation. We further show that rubble piles formed in a single disruption can have similar porosities but variable degrees of hydration. The direct formation of top shapes from single disruption alone can explain the relatively old crater-retention ages of the equatorial features of Ryugu and Bennu. Two separate parent-body disruptions are not necessarily required to explain their different hydration levels.

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Collisional formation of top-shaped asteroids and implications for the origins of Ryugu and Bennu

et al. 2020

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“…Therefore, the presence of any water ice within Bennu depends strongly on its past dynamical history. However, previous work has shown that the obliquity component of the YORP effect rapidly moves the pole of Bennu to the stable orbit‐perpendicular configuration it is in now (within ~10 5 years; Statler, ) and that there is an ~85% probability that Bennu has not approached the Sun to within 0.4 AU (Delbó & Michel, ). Resurfacing events may also erase and create new polar cold traps over time, but from equatorial crater counts Bennu's surface is estimated to be rather old at between 100 million to 1 billion years in age (Walsh et al, ).…”

Section: Discussionmentioning

confidence: 99%

Implications for Ice Stability and Particle Ejection From High‐Resolution Temperature Modeling of Asteroid (101955) Bennu

et al. 2020

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15Key Points: 16 • Modeled temperatures indicate that water ice sublimation is not the process ejecting 17 particles from the surface of Bennu. 18• Sub-surface water ice however could be stable in small regions near the poles. 19 • The diurnal temperature curve has a large amplitude at all latitudes, which supports 20 thermal fracturing as a cause of the ejection events. 21 22 23 Abstract 24The finding by the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and 25 Security-Regolith Explorer) mission that its target (101955) Bennu is an active asteroid has 53 We find that thermal fracturing is a viable mechanism to explain the particle ejections because 54 Bennu exhibits large day-night differences in temperature. These large temperature differences 55 occur even at high latitudes on the sunward-facing sides of boulders. This widespread viability of 56 thermal fracturing is consistent with the observation of particles ejecting from various latitudes 57 on Bennu. 58

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“…Tanga et al (2009) showed in a series of simulations, that indeed the observed distribution of axial ratios and therefore the bulk shapes of asteroid can be reproduced using a N -body approach of spherical 'pebbles' with surface friction. While this interpretation of top-shaped asteroids as being shaped by YORP induced spin-up seems conclusive, Statler (2015) cautions that the evidence is not yet without doubt. As YORP strength is strongly depending on asymmetric surface features, the amplitude of the rotational part will decrease when the bulk reshaping form exactly these top-like, symmetric shapes.…”

Section: Shape Evolution By Yorp Spin-upmentioning

confidence: 83%

“…In these simulations, for low internal angles of friction no evolution of top-shaped, axisymmetric bodies could be observed. Also, the timescale of changes in obliquity, at least for nearly axisymmetric bodies, is an order of magnitude shorter than for changes in rotation rate, therefore the observed bi-modal obliquity distribution does not necessarily attribute the top-like shape to YORP spin-up (Statler 2015).…”

Section: Shape Evolution By Yorp Spin-upmentioning

confidence: 93%

Hyper-Velocity Impacts on Rubble Pile Asteroids

Deller¹

2017

Springer Theses

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SummaryMost asteroids in the size range of approximately 100 m to 100 km are rubble piles, aggregates of rocky material held together mainly by gravitational forces, and only weak cohesion. They contain high macroporosities, indicating a large amount of void space in their interiors. How these voids are distributed is not yet known, as in-situ measurements are still outstanding.In this work, a model to create rubble pile asteroid simulants for use in SPH impact simulations is presented. Rubble pile asteroids are modelled as gravitational re-aggregating remnant fragments of a catastrophically disrupted parent body, which are represented by spherical pebbles. It is shown that this approach allows to explicitly follow the internal restructuring of rubble pile asteroids during impact events, while preserving the expected properties of the bulk asteroid as known from observations and experiments. The bulk behaviour of asteroid simulants, as characterized by the stability against disruption and fragment size distribution, follows the expected behaviour and is not sensitive to the exact distribution of voids in the interior structure, but rather to the void fraction as the amount of consolidated void space in between the constituent fragment pebbles. No exact a priory knowledge of the fragment size distribution inside the body is therefore needed to use this model in impact simulations.Modelling the behaviour of the large-scale rubble pile constituents during impact events is used as a tool to infer the internal structure of asteroids by linking surface features like hills or pits to the creation of sub-catastrophic craters.In this work, the small rubble pile asteroid (2867) Šteins is analysed. The flyby of the Rosetta spacecraft at Šteins has revealed several interesting features: the large crater Diamond close to the southern pole, a hill like feature almost opposite to the crater, and a catena of crater pits extending radially from the rim of the crater.A possible link between these two structures and the cratering event is investigated in a series of impact simulations varying the interior of a plausible shape of Šteins prior to the event that formed crater Diamond. A connection between the cratering event and the hill is shown to be highly unlikely. Therefore, the hill is most likely a remnant of the formation of Šteins. Its size therefore helps to infer the initial size distribution of fragments forming the asteroid.The formation of a fracture radially from the crater can be observed for rubble pile simulants with highly collimated voids. This fracture could plausibly form the catena of pits observed on Šteins. This can therefore serve as a link between observable surface features and Šteins internal structure. The interior of Šteins is most likely an aggregate of fragments that themselves are only lightly fractured, and large void spaces might be found inside the asteroid. As Šteins seems to be a good example of a YORPoid, an asteroid that has been evolved to a top-like shape by radiative forces due to the YORP ...

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Obliquities of “top-shaped” asteroids may not imply reshaping by YORP spin-up

Cited by 16 publications

References 52 publications

Collisional formation of top-shaped asteroids and implications for the origins of Ryugu and Bennu

Collisional formation of top-shaped asteroids and implications for the origins of Ryugu and Bennu

Implications for Ice Stability and Particle Ejection From High‐Resolution Temperature Modeling of Asteroid (101955) Bennu

Hyper-Velocity Impacts on Rubble Pile Asteroids

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