![Itokawa06 hayabusa](https://upload.wikimedia.org/wikipedia/commons/4/48/Itokawa06_hayabusa.jpg)
In
astronomy
Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Objects of interest ...
, a rubble pile is a
celestial body that is not a
monolith
A monolith is a geological feature consisting of a single massive rock (geology), stone or rock, such as some mountains. For instance, Savandurga, Savandurga mountain is a monolith mountain in India. Erosion usually exposes the geological for ...
, consisting instead of numerous pieces of rock that have coalesced under the influence of
gravity
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
. Rubble piles have low density because there are large cavities between the various chunks that make them up.
The asteroids
Bennu
Bennu is an ancient Egyptian deity linked with the Sun, creation, and rebirth. He may have been the original inspiration for the phoenix legends that developed in Greek mythology.
Roles
According to Egyptian mythology, Bennu was a self-create ...
and
Ryugu have a measured bulk density which suggests a rubble pile internal structure.
Many
comet
A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ar ...
s and most smaller
minor planet
According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the IAU officially used the term ''minor ...
s (<10 km in diameter) are thought to be composed of coalesced rubble.
Minor planets
![LCDB Period vs](https://upload.wikimedia.org/wikipedia/commons/e/e8/LCDB_Period_vs._Diameter_Plot.png)
Most smaller
asteroid
An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere.
...
s are thought to be rubble piles.
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.
When a rubble-pile asteroid passes a much more massive object, tidal forces change its shape.
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
Mathilde (minor planet designation: 253 Mathilde) is an asteroid in the intermediate asteroid belt, approximately 50 kilometers in diameter, that was discovered by Austrian astronomer Johann Palisa at Vienna Observatory on 12 November 1885. It h ...
. The mass of Mathilde, as determined by the
NEAR Shoemaker
''Near Earth Asteroid Rendezvous – Shoemaker'' (''NEAR Shoemaker''), renamed after its 1996 launch in honor of planetary scientist Eugene Shoemaker, was a robotic space probe designed by the Johns Hopkins University Applied Physics Laborator ...
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
25143 Itokawa (provisional designation ) 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 ...
, 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
''Near Earth Asteroid Rendezvous – Shoemaker'' (''NEAR Shoemaker''), renamed after its 1996 launch in honor of planetary scientist Eugene Shoemaker, was a robotic space probe designed by the Johns Hopkins University Applied Physics Laborator ...
, 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
Regolith () is a blanket of unconsolidated, loose, heterogeneous superficial deposits covering solid rock. It includes dust, broken rocks, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestria ...
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
Ceres (; minor-planet designation: 1 Ceres) is a dwarf planet in the asteroid belt between the orbits of Mars and Jupiter. It was the first asteroid discovered, on 1 January 1801, by Giuseppe Piazzi at Palermo Astronomical Observatory in Sici ...
,
2 Pallas
Pallas (minor-planet designation: 2 Pallas) is the second asteroid to have been discovered, after Ceres. It is believed to have a mineral composition similar to carbonaceous chondrite meteorites, like Ceres, though significantly less hyd ...
,
4 Vesta
Vesta (minor-planet designation: 4 Vesta) is one of the largest objects in the asteroid belt, with a mean diameter of . It was discovered by the German astronomer Heinrich Wilhelm Matthias Olbers on 29 March 1807 and is named after Vesta, the ...
,
10 Hygiea
Hygiea (minor-planet designation: 10 Hygiea) is a major asteroid and possible dwarf planet located in the main asteroid belt. With a diameter of and a mass estimated to be 3% of the total mass of the belt, it is the fourth-largest asteroid in ...
,
704 Interamnia
704 Interamnia is a large F-type asteroid. With a mean diameter of around 330 kilometres, it is the fifth-largest asteroid, after Ceres, Vesta, Pallas and Hygiea. Its mean distance from the Sun is 3.067 ( AU). It was discovered on 2 October 1 ...
) 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
![Comet 67P True color](https://upload.wikimedia.org/wikipedia/commons/f/f9/Comet_67P_True_color.jpg)
Observational evidence suggest that the
cometary nucleus
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 sublime and produce an atmosphere su ...
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.
However, ''in situ'' observations by the
''Rosetta'' mission, indicates that it may be more complex than that.
Moons
![Phobos colour 2008](https://upload.wikimedia.org/wikipedia/commons/5/5c/Phobos_colour_2008.jpg)
The moon
Phobos, the larger of the two natural satellites of the planet
Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
, is also thought to be a rubble pile bound together by a thin regolith crust about thick.
Spectroscopy of Phobos' composition suggests that Phobos may be a captured
main-belt asteroid
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 ...
.
[Landis, G. A. "Origin of Martian Moons from Binary Asteroid Dissociation," American Association for the Advancement of Science Annual Meeting; Boston, MA, 2001]
abstract
See also
*
Circumplanetary disk
A circumplanetary disk (or circumplanetary disc) is a torus, pancake or ring-shaped accumulation of matter composed of gas, dust, planetesimals, asteroids or collision fragments in orbit around a planet. Around the planets, they are the reservoi ...
*
Comet nucleus
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 (geology), rock, dust, and frozen gases. When heated by the Sun, the gases Sublimation (physics), ...
*
List of slow rotators (minor planets)
This is a list of slow rotators—minor planets that have an exceptionally long rotation period. This period, typically given in hours, and sometimes called rotation rate or spin rate, is a fundamental standard physical property for minor p ...
References
External links
Close-up images of Itokawa, a rubble pile asteroid*
Hyper-Velocity Impacts on Rubble Pile Asteroidspdf online @ kent.ac.uk
{{Small Solar System bodies
Astrophysics
Bodies of the Solar System