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Re: asteroid capture AND ring material escape?


-----Original Message-----
From: Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
To: meteorite-list@meteoritecentral.com
<meteorite-list@meteoritecentral.com>
Date: Monday, March 22, 1999 5:52 PM
Subject: Re: asteroid capture AND ring material escape?


OK, Jupiter & Saturn have small satellites to replenish their rings via
meteor impact.

But that's only half the story.  Io, for example, loses material into space
during some of the more explosive volcanic eruptions there.  So, explosive
volcanism on moons can also>replenish the rings around planets.


Regarding the question about rocks or dust being  detected circling the
earth this side of the moon,
I would like to mention that as many as 50 mini-asteroids, with unusual
spectral color properties, pass between the earth and the moon daily.
(Science 258, pg 403, Oct. 16, 1992)  "Mini" in this case is considered as
less than 500 meters in diameter.  Instruments cannot yet detect those less
than 5 meters in diameter.  Most have perihelia near 1.0 AU, and many have
low eccentricity.  One possibility suggested is that they could be lunar
ejecta.  Here are just a few of the many references:
Rabinowitz et al., 1993, Nature 363,704-706.
Rabinowitz and Wetherill, 1995, Meteoritics 30, 563-564, (abstract).
Chyba, 1993, Nature 363, 701-703.
    I'm not saying they qualify as earth ring material, but they do
demonstrate that there are plenty of rocks out there on a daily basis
between the earth and the moon.                                    Darryl
Futrell





>>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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