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Fraser
2006-Jan-18, 04:57 PM
SUMMARY: The composition of many planets, asteroids and meteorites could be explained by the theory of "hit-and-run" collisions. Scientists originally believed that the four terrestrial planets (Mercury, Venus, Earth and Mars) were formed slowly over time as material built up by accretion. But it's entirely possible that objects sideswiped each other, and continued on; although, with both parties pretty banged up. Large objects don't even have to touch to do massive damage to each other through their gravitational influence.

View full article (http://www.universetoday.com/am/publish/hit_run_planets.html)
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GBendt
2006-Jan-22, 12:23 AM
Hi,

This article is really reasonable. It is well known that the conversion of pressurized hot rock into liquid lava takes place as soon as the pressure is reduced by a crack of the crust that overlies the hot rock. This mechanism feeds the volcanism on Hawaii.

As planetesimals collide, this will cause a local rise of pressure which will produce a sudden rise in heat. This heat melts and vapourizes matter of the both the planetesimals where they touch each other. Matter will fly into space. Further, a shock wave will propagate through the bodies of both planetesimals. This is a longitudinal wave where a conditon of high pressure is followed by one of low pressure. If the shock wave is strong enough, this low pressure may cause the hot rock to melt up instantaneously, which may release hot gases that were dissolved in the pressurized rock, similar to the process of cavitation. Thus, formerly solid matter may become instable and may be disrupted.
The result may be a dense cloud of debris, of which the faster particles may escape into space, creating a variety of smaller bodies, meteorids etc. , while the slower parts of the cloud will form one or more new planetesimals, due to the gravity that acts within the debris cloud.

A body of the size of mars may have a mass of 5e23 kg. If another body of this size collides with the first one, they will collide slowly, as they are moving in about the same direction as they collide: planets of a solar system follow the same rotational direction. Further, the speed of such a planetesimal may be some 30 km/s, which means that it will take a mars-sized body (6000 km diametre) more than three minutes to "fully enter" the scene. The process of collision will take even longer, as the difference in speeds of both bodies will be less. Thus, everything takes place as in very very slow motion. There is no bang.
Large pieces of the planetesimal matter may stay solid (otherwise we would not be able to find crystal structures in most iron meteorites on earth).

Even if the debris cloud particles move at speeds differing by several km/s, the gravity caused by the combined mass of both bodies will be strong enough to pull most of these particles back to form a new body.

Regards,

GŁnther

GOURDHEAD
2006-Jan-22, 12:14 PM
...A body of the size of mars may have a mass of 5e23 kg. If another body of this size collides with the first one, they will collide slowly, as they are moving in about the same direction as they collide: planets of a solar system follow the same rotational direction. Further, the speed of such a planetesimal may be some 30 km/s, which means that it will take a mars-sized body (6000 km diametre) more than three minutes to "fully enter" the scene. The process of collision will take even longer, as the difference in speeds of both bodies will be less. Thus, everything takes place as in very very slow motion. There is no bang.......GŁntherIs it safe to rule out chaotic interaction arising within a system of several planets such that a giant can reverse, or initiate the reversal of, the path of a smaller rocky planet to cause its path to be retrograde with respect to the majority of the planets and cause it to assume a trajectory that leads to a head-on collision with some other object in the sysem? Then there is the posibility of collision with an intruder from another system which could have a wide range of directions and magnitudes for its velocity.

ToSeek
2006-Jan-23, 04:01 PM
This appears to make the collision that created the Moon less of a rarity.