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Author: baalkebaalke Date: Nov 20, 2007 23:10
http://www.jpl.nasa.gov/news/news.cfm?release=2007-132
Astronomers Say Moons Like Ours Are Uncommon
Jet Propulsion Laboraboty
November 20, 2007
The next time you take a moonlit stroll, or admire a full, bright-
white
moon looming in the night sky, you might count yourself lucky. New
observations from NASA's Spitzer Space Telescope suggest that moons
like
Earth's - that formed out of tremendous collisions - are uncommon in
the
universe, arising at most in only 5 to 10 percent of planetary
systems.
"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."
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Author: N:dlzc D:aol T:com (dlzc)N:dlzc D:aol T:com (dlzc) Date: Nov 21, 2007 03:52
...
> "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."
Not only does that not follow (dust is blasted by two
mostly-liquid bodies interacting), 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.
All they can say for sure with this type of survey is that if
dust is a product, it gets collected really fast. It seems to
me.
David A. Smith
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Author: Steve WillnerSteve Willner Date: Nov 21, 2007 23:09
> ...
>> "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 \(dlzc\)" cox.net>
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? How could a planetary-scale collision fail to form
dust?
> 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.
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Author: robert caseyrobert casey Date: Nov 22, 2007 02:54
>
> 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...
>
>
Surface tension on the liquids becomes a non-issue for large "drops".
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Author: N:dlzc D:aol T:com (dlzc)N:dlzc D:aol T:com (dlzc) Date: Nov 22, 2007 03:05
Dear robert casey:
"robert casey" ix.netcom.com> wrote in message
news:13k9oan8ko720b9@corp.supernews.com...>
>> 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...
>
> Surface tension on the liquids becomes a
> non-issue for large "drops".
Well we are certainly talking large drops here.
Did they strike at high speed (so that viscosity won't matter)?
Did they hit head on (so that material must fountain out)?
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Author: dlzcdlzc Date: Nov 28, 2007 15:21
On Nov 27, 6:00 pm, Yousuf Khan yahoo.com> wrote:> N:dlzcD:aol T:com (dlzc) wrote:
>
>> Did they strike at high speed (so that viscosity won't matter)?
>> Did they hit head on (so that material must fountain out)?
>
>> We ended up with a differential velocity that allowed us to "spin
>> off" the lighter elements, which indicates to me that the answer
>> to both questions was "no". We were able to sort out the lighter
>> stuff, and preferentially lobe that out, possibly because it
>> cooled at a lower temperature. Even had time to cool down a bit,
>> before pinching off.
>
>> But we can say matter of factly that lack of a dust cloud rules
>> out moons like ours? It seems like a big stretch to me.
>
> I have my own doubts about the young Earth collision with a
> Mars-sized planet to form the Moon. They've already named
> the hypothetical Mars-sized planet, Theia or Orpheus! Anyways
> to answer your questions about that theory, from the animations ...
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Author: Steve WillnerSteve Willner Date: Nov 28, 2007 18:09
On Nov 21, 8:36 pm, "N:dlzc D:aol T:com \(dlzc\)" cox.net>
wrote:> With no solid surface, two interacting "liquid bodies" or even
> one solid, one liquid, simply meld together. Look at water
> droplets merging.
You are applying everyday experience to size and energy scales where
it isn't valid. A moon-forming collision involves energies per atom
vastly greater than van der Waals energies. The prior state of the
planet's surface or interior is irrelevant. Lots of material gets
blasted into space. Most settles back on the "earth" or "moon," but
some goes into solar orbit. This latter material, initially gas, is
what condenses into dust.
SW> How could a planetary-scale collision fail to form dust?
> No dusty surface.
As noted above, the main mechanism is not that pre-existing dust is
released. Rather, individual atoms or small groups are ejected and
later condense into larger dust particles. (Collisions in the
asteroid belt today involve much smaller energies. Existing rock is
fragmented and ejected.)
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Author: dlzcdlzc Date: Nov 28, 2007 20:32
Dear Steve Willner:
On Nov 28, 10:09 am, Steve Willner cfa.harvard.edu> wrote:> On Nov 21, 8:36 pm, "N:dlzcD:aol T:com \(dlzc\)" cox.net>
> wrote:
>>> With no solid surface, two interacting "liquid bodies" or even
>> one solid, one liquid, simply meld together. Look at water
>> droplets merging.
>
> You are applying everyday experience to size and energy
> scales where it isn't valid. A moon-forming collision involves
> energies per atom vastly greater than van der Waals energies.
I keep looking for water-ball / water-ball interactions on Skylab or
ISS, to see if such has been documented. I hear what you are saying,
but ...
> The prior state of the planet's surface or interior is irrelevant.
> Lots of material gets blasted into space. Most settles back
> on the "earth" or "moon,"
... "eventually" ...
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Author: Peter MunnPeter Munn Date: Dec 2, 2007 18:33
Leafing through alt.sci.planetary, I read Steve Willner's message of
Fri, 30 Nov 2007:
>On Nov 28, 2:32 pm, dlzc cox.net> wrote:>
>As far as I know. The oxygen isotope ratios in particular are
>otherwise very hard to explain. Of course it may be that no one has
>thought of the right model yet, but giant impact certainly explains a
>lot.
I get the impression that only rather unlikely giant impact scenarios
achieve the tough constraints required: namely, a large moon formed with
a high enough orbit that it doesn't get pulled down by tidal drag.
Plus, unless the formation is very late in the process of clearing out
remnant planetoids, the moon's orbit also has to withstand the net drag
of subsequent lunar impacts. On the other hand, could a very late giant
impact with few really large subsequent lunar impacts be consistent with
the unevenness of the lunar gravity field?
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