Rubble pile

In astronomy, a rubble pile is a celestial body that is not a monolith, consisting instead of numerous pieces of rock that have coalesced under the influence of gravity. Rubble piles have low density because there are large cavities between the various chunks that make them up.

The small near-Earth asteroid 25143 Itokawa is a prime example of a rubble pile, with numerous boulders covering its surface

The asteroids Bennu and Ryugu have a measured bulk density which suggests that their internal structure is a rubble pile.[1][2] Many comets and most smaller minor planets (<10 km in diameter) are thought to be composed of coalesced rubble.[3][4]

Minor planets
Rotation periods of a large number of minor planets.[lower-alpha 1] Most smaller bodies have a period between 2.2 and 20 hours, and are thought to be rubble piles. Bodies rotating faster than 2.2 hours, however, must be monolithic, as they would fly apart otherwise. This explains why there are so few fast-spinning minor planets.[3]

Most smaller asteroids are thought to be rubble piles.[3]

Rubble piles form when an asteroid or moon (which may originally be monolithic) is smashed to pieces by an impact, and the shattered pieces subsequently fall back together, primarily due to self-gravitation. This coalescing usually takes from several hours to weeks.[5]

When a rubble-pile asteroid passes a much more massive object, tidal forces change its shape.[6]

Scientists first suspected that asteroids are often rubble piles when asteroid densities were first determined. Many of the calculated densities were significantly less than those of meteorites, which in some cases had been determined to be pieces of asteroids.

Many asteroids with low densities are thought to be rubble piles, for example 253 Mathilde. The mass of Mathilde, as determined by the NEAR Shoemaker mission, is far too low for the volume observed, considering the surface is rock. Even ice with a thin crust of rock would not provide a suitable density. Also, the large impact craters on Mathilde would have shattered a rigid body. However, the first unambiguous rubble pile to be photographed is 25143 Itokawa, which has no obvious impact craters and is thus almost certainly a coalescence of shattered fragments.

The asteroid 433 Eros, the primary destination of NEAR Shoemaker, was determined to be riven with cracks but otherwise solid. Other asteroids, possibly including Itokawa, have been found to be contact binaries, two major bodies touching, with or without rubble filling the boundary.

Large interior voids are possible because of the very low gravity of most asteroids. Despite a fine regolith on the outside (at least to the resolution that has been seen with spacecraft), the asteroid's gravity is so weak that friction between fragments dominates and prevents small pieces from falling inwards and filling the voids.

All the largest asteroids (1 Ceres, 2 Pallas, 4 Vesta, 10 Hygiea, 704 Interamnia) are solid objects without any macroscopic internal porosity. This may be because they have been large enough to withstand all impacts, and have never been shattered. Alternatively, Ceres and some few other of the largest asteroids may be massive enough that, even if they were shattered but not dispersed, their gravity would collapse most voids upon recoalescing. Vesta, at least, has withstood intact one major impact since its formation and shows signs of internal structure from differentiation in the resultant crater that assures that it is not a rubble pile. This serves as evidence for size as a protection from shattering into rubble.

Comets
Nucleus of comet 67P/Churyumov–Gerasimenko imaged by Rosetta

Observational evidence suggest that the cometary nucleus may not be a well-consolidated single body, but may instead be a loosely bound agglomeration of smaller fragments, weakly bonded and subject to occasional or even frequent disruptive events, although the larger cometary fragments are expected to be primordial condensations rather than collisionally derived debris as in the asteroid case.[7][8][9][10][11] However, in situ observations by the Rosetta mission indicate that it may be more complex than that.[12]

Moons
Phobos imaged by the Mars Reconnaissance Orbiter

The moon Phobos, the larger of the two natural satellites of the planet Mars, is also thought to be a rubble pile bound together by a thin regolith crust about 100 m (330 ft) thick.[13][14] Spectroscopy of Phobos' composition suggests that Phobos may be a captured main-belt asteroid.[15][16]

See also

References
  1. Data source, reference: Warner, B.D., Harris, A.W., Pravec, P. (2009). Icarus 202, 134-146.[lower-alpha 2] Updated 2016 September 6. See: www.MinorPlanet.info
  1. Chesley, Steven R.; Farnocchia, Davide; Nolan, Michael C.; Vokrouhlický, David; Chodas, Paul W.; Milani, Andrea; Spoto, Federica; Rozitis, Benjamin; Benner, Lance A.M.; Bottke, William F.; Busch, Michael W.; Emery, Joshua P.; Howell, Ellen S.; Lauretta, Dante S.; Margot, Jean-Luc; Taylor, Patrick A. (2014). "Orbit and bulk density of the OSIRIS-REx target Asteroid (101955) Bennu". Icarus. 235: 5–22. arXiv:1402.5573. Bibcode:2014Icar..235....5C. doi:10.1016/j.icarus.2014.02.020. ISSN 0019-1035. S2CID 30979660.
  2. Hayabusa-2: Asteroid mission exploring a 'rubble pile'. Paul Rincon, BBC News. 19 March 2019.
  3. "About Light Curves". Minor Planet Center. Retrieved 24 April 2020.
  4. Walsh, Kevin J. (14 September 2018). "Rubble Pile Asteroids". Annual Review of Astronomy and Astrophysics. 56 (1): 593–624. arXiv:1810.01815. Bibcode:2018ARA&A..56..593W. doi:10.1146/annurev-astro-081817-052013. ISSN 0066-4146. S2CID 119530506.
  5. Michel, Patrick; Benz, Willy; Tanga, Paolo; Richardson, Derek C. (November 2001). "Collisions and Gravitational Reaccumulation: Forming Asteroid Families and Satellites". Science. 294 (5547): 1696–1700. Bibcode:2001Sci...294.1696M. doi:10.1126/science.1065189. PMID 11721050. S2CID 6470148.
  6. Solem, Johndale C.; Hills, Jack G. (March 1996). "Shaping of Earth-Crossing Asteroids by Tidal Forces". Astronomical Journal. 111: 1382. Bibcode:1996AJ....111.1382S. doi:10.1086/117884.
  7. Weissman, P. R. (March 1986). "Are cometary nuclei primordial rubble piles?". Nature. 320 (6059): 242–244. Bibcode:1986Natur.320..242W. doi:10.1038/320242a0. ISSN 0028-0836. S2CID 4365705.
  8. Tidal Disruption of Asteroids and Comets. William Bottke. Southwest Research Institute in Boulder, Colorado. 1998.
  9. Stardust at Comet Wild 2. (PDF) Harold A. Weaver, Science 18 JUNE 2004, Vol 304.
  10. Interior of the Cometary Nucleus. University of California, Los Angeles.
  11. Asphaug, E.; Benz, W. (1994). "Density of comet Shoemaker–Levy 9 deduced by modelling breakup of the parent 'rubble pile'". Nature. 370 (6485): 120–124. Bibcode:1994Natur.370..120A. doi:10.1038/370120a0. S2CID 4336930.
  12. Khan, Amina (31 July 2015). "After a bounce, Rosetta's Philae lander serves up cometary surprises". Los Angeles Times. Retrieved 11 November 2015.
  13. "Phobos is Slowly Falling Apart". NASA. SpaceRef. 10 November 2015. Retrieved 11 November 2015.
  14. "NASA – Phobos". Solarsystem.nasa.gov. Archived from the original on 24 June 2014. Retrieved 4 August 2014.
  15. "Close Inspection for Phobos". One idea is that Phobos and Deimos, Mars's other moon, are captured asteroids.
  16. Landis, G. A. "Origin of Martian Moons from Binary Asteroid Dissociation," American Association for the Advancement of Science Annual Meeting; Boston, MA, 2001; abstract.
  17. Warner, Brian D.; Harris, Alan W.; Pravec, Petr (July 2009). "The asteroid lightcurve database". Icarus. 202 (1): 134–146. Bibcode:2009Icar..202..134W. doi:10.1016/j.icarus.2009.02.003.

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