The shape distribution of boulders on Asteroid 25143 Itokawa: Comparison with fragments from impact experiments

Michikami, Tatsuhiro; Nakamura, Akiko; Hirata, Naru

doi:10.1016/j.icarus.2009.10.008

Cited by 57 publications

(41 citation statements)

References 15 publications

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“…6. If we compare the 103P b/a value with those derived by Michikami et al (2010) (Michikami et al 2010). On the contrary, the 0.58 b/a value indicates that the 103P boulders ≥30 m are characterized by more elongated shapes compared with collisional laboratory fragments, therefore, suggesting a possible different origin.…”

Section: The Shape Distribution Of Boulders ≥30 M On 103pmentioning

confidence: 84%

“…The aim of this approach is to determine the b/a ratio and compare that with the boulders studied on asteroids 25 143 Itokawa and 433 Eros, as in Michikami et al (2010). This was only possible for boulders ≥30 m. As a result, in the case of smaller boulders, the ∼3 m/px scale, coupled with the use of 103P deconvolved images , makes the identification of the b/a ratio extremely difficult, and hence, only provides a confident measure of the maximum dimension of boulders.…”

Section: Dataset and Methodologymentioning

confidence: 99%

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Size-frequency distribution of boulders ≥10 m on comet 103P/Hartley 2

Pajola

Lucchetti

Bertini

et al. 2015

A&A

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Aims. We derive the size-frequency distribution of boulders on comet 103P/Hartley 2, which are computed from the images taken by the Deep Impact/HRI-V imaging system. We indicate the possible physical processes that lead to these boulder size distributions. Methods. We used images acquired by the High Resolution Imager-Visible CCD camera on 4 November 2010. Boulders ≥10 m were identified and manually extracted from the datasets with the software ArcGIS. We derived the global size-frequency distribution of the illuminated side of the comet (∼50%) and identified the power-law indexes characterizing the two lobes of 103P. The three-pixel sampling detection, together with the shadowing of the surface, enables unequivocally detection of boulders scattered all over the illuminated surface. Results. We identify 332 boulders ≥10 m on the imaged surface of the comet, with a global number density of nearly 140/km 2 and a cumulative size-frequency distribution represented by a power law with index of −2.7 ± 0.2. The two lobes of 103P show similar indexes, i.e., −2.7 ± 0.2 for the bigger lobe (called L1) and −2.6 +0.2/ − 0.5 for the smaller lobe (called L2). The similar power-law indexes and similar maximum boulder sizes derived for the two lobes both point toward a similar fracturing/disintegration phenomena of the boulders as well as similar lifting processes that may occur in L1 and L2. The difference in the number of boulders per km 2 between L1 and L2 suggests that the more diffuse H 2 O sublimation on L1 produce twice the boulders per km 2 with respect to those produced on L2 (primary activity CO 2 driven). The 103P comet has a lower global power-law index (−2.7 vs. −3.6) with respect to 67P. The global differences between the two comets' activities, coupled with a completely different surface geomorphology, make 103P hardly comparable to 67P. A shape distribution analysis of boulders ≥30 m performed on 103P suggests that the cometary boulders show more elongated shapes when compared to collisional laboratory fragments as well as to the boulders present on the surfaces of 25 143 Itokawa and 433 Eros asteroids. Consequently, this supports the interpretation that cometary boulders have different origins with respect to the impact-related asteroidal boulders.

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Section: The Shape Distribution Of Boulders ≥30 M On 103pmentioning

confidence: 84%

Section: Dataset and Methodologymentioning

confidence: 99%

Size-frequency distribution of boulders ≥10 m on comet 103P/Hartley 2

Pajola

Lucchetti

Bertini

et al. 2015

A&A

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“…In laboratory-size impact experiments, Capaccioni et al (1984) with b /a = 0.77 ± 0.11 (Capaccioni et al 1984). While boulders larger than R = 5 m on asteroid Itokawa show a slightly different value of b /a = 0.62 ± 0.19, Michikami et al (2010) argue that this might be due to the sorting by granular motion that has evidently been processing the interior of Itokawa (Miyamoto et al 2007). The effect of elongated shapes to the rubble pile simulant model is not clear.…”

Section: Treatment Of Interlocking Pebblesmentioning

confidence: 94%

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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“…Therefore, the major constituents of a rubble pile asteroid are themselves fragments of a collision, and their size distribution and shape will be a result of the disruption event that destroyed the parent body. In the case of asteroids Eros and Itokawa, Michikami, Nakamura & Hirata (2010) endorsed this scenario by showing that the shapes of boulders found on the surface match those of fragments of impact experiments.…”

Section: Introductionmentioning

confidence: 89%

A new approach to modelling impacts on rubble pile asteroid simulants

Deller

Lowry

Snodgrass

et al. 2015

Mon. Not. R. Astron. Soc.

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Many asteroids with low bulk densities must have a rubble pile structure and internal voids. Although little is known about their internal structure, numerical simulations of impact events on these asteroids rely on assumptions on how the voids are distributed. We present a new approach to model impacts on rubble pile asteroids that explicitly takes into account their internal structure. The formation of the asteroid is modelled as a rubble pile aggregate of spherical pebbles of different sizes. This aggregate is then converted into a high-resolution smoothed particle hydrodynamics (SPH) model, accounting for macroporosity inside the pebbles. We compare impact-event outcomes for a large set of internal configurations to explore the parameter space of our model-building process. The analysis of the fragment size distribution and the disruption threshold quantifies the specific influence of each input parameter. The size distribution of the pebbles used in our model is a simple power law, containing three free parameters: the slope α, the lower cut-off radius r min and the upper cutoff radius r max . The influence of all three parameters on the outcome is assessed in this paper. The existence of void space in our model increases the resistance against collisional disruption, a behaviour previously reported based on numerical simulations using a continuum description of porous material (Holsapple 2009). We show, for a set of asteroid collisions typical for small asteroids in the main belt, that no a priori knowledge of the exact size distribution of the pebbles inside the asteroid is needed, as the choice of the corresponding parameters does not directly correlate with the impact outcome.

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The shape distribution of boulders on Asteroid 25143 Itokawa: Comparison with fragments from impact experiments

Cited by 57 publications

References 15 publications

Size-frequency distribution of boulders ≥10 m on comet 103P/Hartley 2

Size-frequency distribution of boulders ≥10 m on comet 103P/Hartley 2

Hyper-Velocity Impacts on Rubble Pile Asteroids

A new approach to modelling impacts on rubble pile asteroid simulants

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