by neufer » Wed Apr 09, 2014 6:50 pm
Anthony Barreiro wrote:
This illustration shows Chariklo as roughly spherical. Would an asteroid of 250 km diameter be able to pull itself into a spherical shape? Would that make Chariklo a dwarf planet?
http://en.wikipedia.org/wiki/Dwarf_planet#Size_and_mass wrote:
<<The lower size and mass limits of dwarf planets is determined by the requirements of achieving a hydrostatic equilibrium shape, but the size or mass at which an object attains this shape depends on its composition and thermal history. Empirical observations suggest that the lower limit will vary according to the composition and thermal history of the object.
For a body made of rigid silicates, such as the stony asteroids, the transition to hydrostatic equilibrium should occur at a diameter of approximately 600 km and a mass of some 3.4×1020 kg. For a body made of less rigid water ice, the limit should be about 320 km and 1019 kg. Assuming that Methone's elongated shape reflects the balance between the tidal force exerted by Saturn and its own gravity, its diameter only 3 km suggests that it is composed of icy fluff. In the asteroid belt, Ceres is the only body that clearly surpasses the silicaceous limit (though it is actually a rocky–icy body), and its shape is an equilibrium spheroid. 2 Pallas and 4 Vesta, however, are rocky and are just below the limit. Pallas, at 525–560 km and 1.85–2.4×10
20 kg, is "nearly round" but still somewhat irregular. Vesta, at 530 km and 2.6×10
20 kg, deviates from an ellipsoid shape primarily due to a large impact basin at its pole.
Among icy bodies, the smallest thought to be in hydrostatic equilibrium when the concept of dwarf planet was being debated was Mimas, at 396 km and 3.75×1019 kg. The largest irregular body known in the outer Solar System is Proteus, nearly-but-not-quite round at 405–435 km and an assumed mass of ≈4.4×1019 kg. Bodies like Mimas may have had a warmer thermal history than Proteus, or their shape may have resolved after a collision. Neither body is pure ice as used to calculate the lowest limit, however, and Mike Brown suggested that the practical lower limit for an icy dwarf planet is likely to be somewhere under 400 km. There are about 100 TNOs currently estimated to be above this size. However, it has since been discovered that Mimas is not in hydrostatic equilibrium, and that its ellipsoidal shape is due to its past history, rather like the more extreme case of tiny Phoebe. The smallest Saturnian moon confirmed to be in hydrostatic equilibrium is Rhea, at 1,530 km, whereas the largest not in equilibrium is Iapetus, at 1,470 km. These findings have not been discussed in the context of dwarf planets, but Iapetus and Rhea are in the size range of Makemake (1,415–1,445 km) and larger than Haumea (1,180–1,310 km).>>
[quote="Anthony Barreiro"]
This illustration shows Chariklo as roughly spherical. Would an asteroid of 250 km diameter be able to pull itself into a spherical shape? Would that make Chariklo a dwarf planet?[/quote]
[list]Probably not.[/list]
[quote=" http://en.wikipedia.org/wiki/Dwarf_planet#Size_and_mass"]
<<The lower size and mass limits of dwarf planets is determined by the requirements of achieving a hydrostatic equilibrium shape, but the size or mass at which an object attains this shape depends on its composition and thermal history. Empirical observations suggest that the lower limit will vary according to the composition and thermal history of the object. [b][color=#FF0000]For a body made of rigid silicates, such as the stony asteroids, the transition to hydrostatic equilibrium should occur at a diameter of approximately 600 km and a mass of some 3.4×10[sup]20[/sup] kg.[/color] [color=#0000FF]For a body made of less rigid water ice, the limit should be about 320 km and 10[sup]19[/sup] kg.[/color][/b] Assuming that Methone's elongated shape reflects the balance between the tidal force exerted by Saturn and its own gravity, its diameter only 3 km suggests that it is composed of icy fluff. In the asteroid belt, Ceres is the only body that clearly surpasses the silicaceous limit (though it is actually a rocky–icy body), and its shape is an equilibrium spheroid. 2 Pallas and 4 Vesta, however, are rocky and are just below the limit. Pallas, at 525–560 km and 1.85–2.4×10[sup]20[/sup] kg, is "nearly round" but still somewhat irregular. Vesta, at 530 km and 2.6×10[sup]20[/sup] kg, deviates from an ellipsoid shape primarily due to a large impact basin at its pole.
[b][color=#0000FF]Among icy bodies, the smallest thought to be in hydrostatic equilibrium when the concept of dwarf planet was being debated was Mimas, at 396 km and 3.75×10[sup]19[/sup] kg. The largest irregular body known in the outer Solar System is Proteus, nearly-but-not-quite round at 405–435 km and an assumed mass of ≈4.4×10[sup]19[/sup] kg. Bodies like Mimas may have had a warmer thermal history than Proteus, or their shape may have resolved after a collision. Neither body is pure ice as used to calculate the lowest limit, however, and Mike Brown suggested that the practical lower limit for an icy dwarf planet is likely to be somewhere under 400 km.[/color][/b] There are about 100 TNOs currently estimated to be above this size. However, it has since been discovered that Mimas is not in hydrostatic equilibrium, and that its ellipsoidal shape is due to its past history, rather like the more extreme case of tiny Phoebe. The smallest Saturnian moon confirmed to be in hydrostatic equilibrium is Rhea, at 1,530 km, whereas the largest not in equilibrium is Iapetus, at 1,470 km. These findings have not been discussed in the context of dwarf planets, but Iapetus and Rhea are in the size range of Makemake (1,415–1,445 km) and larger than Haumea (1,180–1,310 km).>>[/quote]