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Re: asteroid capture AND ring material escape?


>But still, at least Phobos and Deimos must have been knocked towards
>Mars by some collision, 

That is not so unusual.  Jupiter is close to the asteroid belt and
perturbing the orbits of the asteroids.  Mars, being on the other side
of the asteroid belt, is the lucky recipient of the asteroids sent
its way courtesy of Jupiter (of course, we're talking on a time scale of 
millions of years).

>and decelerated (relative to Mars) sufficiently
>in that same (or some other) collision.

This is the tricky part.  Mars has two moons.  The second moon may have
collided with the first moon to get captured.  How did the first moon
get captured?  Passing through the atmosphere for an aerobraking effect?
Not very likely.  Perhaps being in close proximity of the asteoroid belt
helps increase the odds for Mars.  No one has a good answer for this one.  

> All that because the
>escape/capture velocity of the planet is so much smaller than the
>heliospheric orbit velocity. In the case of Jupiter, the speed of the
>heliospheric orbit is much lower, and the escape velocity is much
>higher, so there an asteroid-capture would be much more "common", maybe
>even a decelerating collision would not be needed.. the nearer the Sun,
>the harder it is to capture a small body into a planet-circling orbit,
>right? Is my understanding of celestial mechanics right?

You're correct.  For Jupiter, collisions are not required for orbit capture,
mainly because Jupiter has the four large moons:  Io, Europa, Ganymede and 
Callisto.  In fact, these are planet-sized moons (ie: Ganymede is larger than
Mercury).  An asteroid would only need to fly by close to one of these
moons to get slowed down enough (via gravity assist) to get captured.
Also, Jupiter has such a deep gravity well, that it may well capture objects
temporarily in orbit.  Comet Shoemaker-Levy 9 was one such instance.
Comet Shoemaker-Levy 9 made a close flyby in Jupiter, estimated around the
1920's, and was captured in a highly elliptical orbit, where is continued
to fly close to Jupiter each time around.  One such encounter was too close 
(1992) and the comet broke up into pieces  The following orbit (1994) was 
even closer and the comet fragments ran into Jupiter.   If the comet had not 
impacted Jupiter in 1994, it would of evenutally been ejected out of the 
Jovian system or impacted one of the moons.  There was another comet in a 
similar situation which did get captured into a temporary orbit, and then 
was ejected out of the Jovian system.

>And btw, I must add: Doesn't Jupiter have 8 (not four) outer satellites
>which seem to be remnants of two separate asteroids (4 each)? At 11 and
>22 million km.

Correct. The outer most four are also in retrograde orbit, a sure sign that
they were captured.  Phoebe, the outermost moon of Saturn, is also in
a retrograde orbit.  Triton is in retrograde orbit around Neptune, so
a lot of people highly suspect Triton to be a captured Kuiper Belt object.

>As far as I remember, the gas giants' rings would "vanish" unless there
>was some supply of new material (moons inside/near rings). Therefore, a
>ring caused by impact ejecta should have vanished *quite* quickly...

Correct again, the rings need to be replenished in order to survive.
Jupiter and Saturn have small satellites near their rings, apparently
keeping the rings supplied with material via meteor impacts.  If
the Earth had a ring, it wouldn't have lasted very long. 

>Not counting the Van Allen or other belts caused by the magnetic field,
>or the human made debris, is there *any* dust/rocks circling Earth? Say
>at Moon's distance? I was just thinking of "regular" meteorite bombing
>on the Moon, and since it doesn't take a *big* explosion to eject debris
>off the gravity well there...

None have been detected.

Ron Baalke

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