Rubble pile

Last updated
Asteroid-Bennu-OSIRIS-RExArrival-GifAnimation-20181203.gif Comet 67P on 19 September 2014 NavCam mosaic.jpg
Suspected rubble piles:

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.

Contents

Asteroids Bennu and Ryugu have a measured bulk density which suggests a rubble pile internal structure. [1] [2] Many comets and most smaller minor planets are thought to be composed of coalesced rubble. [3]

Minor planets

Rotation periods of a large number of minor planets. 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. LCDB Period vs. Diameter Plot.png
Rotation periods of a large number of minor planets. 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.

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. [4]

When a rubble-pile asteroid passes a much more massive object, tidal forces change its shape. [5]

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 up 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

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. [6] [7] [8] [9] [10] However, in situ observations by the Rosetta mission, indicates that it may be more complex than that. [11]

Moons

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. [12] [13] Spectroscopy of Phobos' composition suggests that Phobos may be a captured main-belt asteroid. [14] [15]

See also

Related Research Articles

Asteroid Minor planet that is not a comet

An asteroid is a minor planet of the inner Solar System. Historically, these terms have been applied to any astronomical object orbiting the Sun that did not resolve into a disc in a telescope and was not observed to have characteristics of an active comet such as a tail. As minor planets in the outer Solar System were discovered that were found to have volatile-rich surfaces similar to comets, these came to be distinguished from the objects found in the main asteroid belt. The term "asteroid" refers to the minor planets of the inner Solar System, including those co-orbital with Jupiter. Larger asteroids are often called planetoids.

Comet Shoemaker–Levy 9 was a comet that broke apart in July 1992 and collided with Jupiter in July 1994, providing the first direct observation of an extraterrestrial collision of Solar System objects. This generated a large amount of coverage in the popular media, and the comet was closely observed by astronomers worldwide. The collision provided new information about Jupiter and highlighted its possible role in reducing space debris in the inner Solar System.

Near-Earth object Small Solar System body whose orbit brings it close to the Earth

A near-Earth object (NEO) is any small Solar System body whose orbit brings it into proximity with Earth. By convention, a Solar System body is a NEO if its closest approach to the Sun (perihelion) is less than 1.3 astronomical units (AU). If a NEO's orbit crosses the Earth's, and the object is larger than 140 meters (460 ft) across, it is considered a potentially hazardous object (PHO). Most known PHOs and NEOs are asteroids, but a small fraction are comets.

4179 Toutatis

4179 Toutatis, provisional designation 1989 AC, is an elongated, stony asteroid and slow rotator, classified as near-Earth object and potentially hazardous asteroid of the Apollo and Alinda group, approximately 2.5 kilometers in diameter. Discovered by French astronomer Christian Pollas at Caussols in 1989, the asteroid was named after Toutatis from Celtic mythology.

Asteroid belt Circumstellar disk (accumulation of matter) in an orbit between those of Mars and Jupiter

The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes but much smaller than planets, called asteroids or minor planets. This asteroid belt is also called the main asteroid belt or main belt to distinguish it from other asteroid populations in the Solar System such as near-Earth asteroids and trojan asteroids.

Phobos (moon) The larger, inner, moon of Mars

Phobos is the innermost and larger of the two natural satellites of Mars, the other being Deimos. Both moons were discovered in 1877 by American astronomer Asaph Hall. Phobos is named after the Greek god Phobos, a son of Ares (Mars) and Aphrodite (Venus) and twin brother of Deimos. Phobos was the god and personification of fear and panic.

Asteroid impact avoidance Methods to prevent destructive asteroid hits

Asteroid impact avoidance comprises the methods by which near-Earth objects (NEO) on a potential collision course with Earth could be diverted away, preventing destructive impact events. An impact by a sufficiently large asteroid or other NEOs would cause, depending on its impact location, massive tsunamis or multiple firestorms, and an impact winter caused by the sunlight-blocking effect of large quantities of pulverized rock dust and other debris placed into the stratosphere.

25143 Itokawa Near-Earth asteroid within the Apollo Group

25143 Itokawa (provisional designation 1998 SF36) is a sub-kilometer near-Earth object of the Apollo group and a potentially hazardous asteroid. It was discovered by the LINEAR program in 1998 and later named after Japanese rocket engineer Hideo Itokawa. The peanut-shaped S-type asteroid has a rotation period of 12.1 hours and measures approximately 330 meters (1,100 feet) in diameter. Due to its low density and high porosity, Itokawa is considered to be a rubble pile, consisting of numerous boulders of different sizes rather than of a single solid body.

Accretion (astrophysics) The accumulation of particles into a massive object by gravitationally attracting more matter

In astrophysics, accretion is the accumulation of particles into a massive object by gravitationally attracting more matter, typically gaseous matter, in an accretion disk. Most astronomical objects, such as galaxies, stars, and planets, are formed by accretion processes.

Sample-return mission Spacecraft mission

A sample-return mission is a spacecraft mission to collect and return samples from an extraterrestrial location to Earth for analysis. Sample-return missions may bring back merely atoms and molecules or a deposit of complex compounds such as loose material ("soil") and rocks. These samples may be obtained in a number of ways, such as soil and rock excavation or a collector array used for capturing particles of solar wind or cometary debris.

Comet nucleus Central part of a comet

The nucleus is the solid, central part of a comet, once termed a dirty snowball or an icy dirtball. A cometary nucleus is composed of rock, dust, and frozen gases. When heated by the Sun, the gases sublimate and produce an atmosphere surrounding the nucleus known as the coma. The force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the Sun. A typical comet nucleus has an albedo of 0.04. This is blacker than coal, and may be caused by a covering of dust.

Formation and evolution of the Solar System Formation of the Solar System by gravitational collapse of a molecular cloud and subsequent geological history

The formation and evolution of the Solar System began about 4.5 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the center, forming the Sun, while the rest flattened into a protoplanetary disk out of which the planets, moons, asteroids, and other small Solar System bodies formed.

951 Gaspra

951 Gaspra is an S-type asteroid that orbits very close to the inner edge of the asteroid belt. Gaspra was discovered by Russian astronomer G. N. Neujmin in 1916. Neujmin named it after Gaspra, a Black Sea retreat that was visited by his contemporaries, such as Gorky and Tolstoy.

101955 Bennu Second most hazardous near-Earth asteroid

101955 Bennu (provisional designation 1999 RQ36) is a carbonaceous asteroid in the Apollo group discovered by the LINEAR Project on 11 September 1999. It is a potentially hazardous object that is listed on the Sentry Risk Table with the second-highest cumulative rating on the Palermo Technical Impact Hazard Scale. It has a cumulative 1-in-2,700 chance of impacting Earth between 2175 and 2199. It is named after the Bennu, the ancient Egyptian mythological bird associated with the Sun, creation, and rebirth.

Extinct comet Comet that lacks typical activity

An extinct comet is a comet that has expelled most of its volatile ice and has little left to form a tail and coma. In a dormant comet, rather than being depleted, any remaining volatile components have been sealed beneath an inactive surface layer.

Planetary surface Where the solid (or liquid) material of the outer crust on certain types of astronomical objects contacts the atmosphere or outer space

A planetary surface is where the solid material of the outer crust on certain types of astronomical objects contacts the atmosphere or outer space. Planetary surfaces are found on solid objects of planetary mass, including terrestrial planets, dwarf planets, natural satellites, planetesimals and many other small Solar System bodies (SSSBs). The study of planetary surfaces is a field of planetary geology known as surface geology, but also a focus of a number of fields including planetary cartography, topography, geomorphology, atmospheric sciences, and astronomy. Land is the term given to non-liquid planetary surfaces. The term landing is used to describe the collision of an object with a planetary surface and is usually at a velocity in which the object can remain intact and remain attached.

<span class="nowrap">(341843) 2008 EV<sub>5</sub></span>

(341843) 2008 EV5, provisional designation 2008 EV5, is a sub-kilometer asteroid, classified as a near-Earth object and potentially hazardous asteroid of the Aten group, approximately 400 metres (1,300 feet) in diameter. It was discovered on 4 March 2008, by astronomers of the Mount Lemmon Survey at Mount Lemmon Observatory near Tucson, Arizona, United States.

Asteroidal water is water or water precursor deposits such as hydroxide (OH) that exist in asteroids. The "snow line" of the Solar System lies outside of the main asteroid belt, and the majority of water is expected in minor planets (e.g., Kuiper belt objects and Centaurs. Nevertheless, a significant amount of water is also found inside the snow line, including in near-earth objects.

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. 1 2 3 "About Light Curves". Minor Planet Center. Retrieved 24 April 2020.
  4. 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.
  5. 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.
  6. 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.
  7. Tidal Disruption of Asteroids and Comets. William Bottke. Southwest Research Institute in Boulder, Colorado. 1998.
  8. Stardust at Comet Wild 2. (PDF) Harold A. Weaver, Science 18 JUNE 2004, Vol 304.
  9. Interior of the Cometary Nucleus. University of California, Los Angeles.
  10. 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. doi:10.1038/370120a0. S2CID   4336930.
  11. Khan, Amina (31 July 2015). "After a bounce, Rosetta's Philae lander serves up cometary surprises". Los Angeles Times. Retrieved 11 November 2015.
  12. "Phobos is Slowly Falling Apart". NASA. SpaceRef. 10 November 2015. Retrieved 11 November 2015.
  13. "NASA – Phobos". Solarsystem.nasa.gov. Archived from the original on 24 June 2014. Retrieved 4 August 2014.
  14. "Close Inspection for Phobos". One idea is that Phobos and Deimos, Mars's other moon, are captured asteroids.
  15. Landis, G. A. "Origin of Martian Moons from Binary Asteroid Dissociation," American Association for the Advancement of Science Annual Meeting; Boston, MA, 2001; abstract.
  16. 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.