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Posted by N:dlzc D:aol T:com \(dlzc\) on November 21, 2007, 8:36 pm
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Dear Steve Willner:
>> >http://www.jpl.nasa.gov/news/news.cfm?release=2007-132
>> ...
>> > "When a moon forms from a violent collision,
>> > dust should be blasted everywhere," said
>> > Nadya Gorlova of the University of Florida,
>> > Gainesville, lead author of a new study
>> > appearing Nov. 20 in the Astrophysical
>> > Journal. "If there were lots of moons forming,
>> > we would have seen dust around lots of stars -
>> > but we didn't."
>
> On Nov 20, 9:52 pm, "N:dlzc D:aol T:com \(dlzc\)"
> wrote:
>> Not only does that not follow (dust is blasted by two
>> mostly-liquid bodies interacting),
>
> What do you mean by "mostly-liquid bodies," and why
> would that affect dust formation?
With no solid surface, two interacting "liquid bodies" or even
one solid, one liquid, simply meld together. Look at water
droplets merging. At approach rates of single or a few diameters
per second...
> How could a planetary-scale collision fail to form
> dust?
No dusty surface. A solid body "sinks", and two liquid bodies
simply merge. They are assuming some sort of high energy
collision, rather than a simple merging of two bodies to yield
more angular momentum than a single fluid body could withstand.
Lobing off a body with all the lighter elements from the mantle.
>> but significant dust collections are only located in
>> our asteroid belt (or the Oort cloud), and dust
>> ejecta will have elliptical orbits making collection
>> likely.
>
> We are viewing the Solar System about 4.5 Gyr after
> the relevant collision. The relevant time for the Gorlova
> et al. study is 10 Myr or so after. The point is that
> dust wouldn't be able to go anywhere in that short a
> time.
Assuming an energetic collision. Assuming the "vapor" stayed in
orbit around the Sun, rather than "nutating" around now-merged
parents, to be scooped back up in a few "years".
>> All they can say for sure with this type of survey
>> is that if dust is a product, it gets collected really
>> fast.
>
> "Collected really fast" is, I suppose, a logical possibility.
> Do you have any suggestion for what mechanism could
> do that?
Comets seem to have their ejecta stay in orbit with them. Should
we expect differently? Did Tempel leave a little puff where the
impactor we sent hit it, or did that puff too follow the comet
around?
> (I expect the authors have considered the usual ones.)
Yes, I hate "made for public release" news blurbs too.
I am not saying they did not do a hell of an important job. All
I am saying is that they limited the type of moon formation, not
necessarily excluding how our Moon was formed.
David A. Smith
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