[meteorite-list] Venus May Have Once Had A Moon

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Thu Oct 12 19:45:56 2006
Message-ID: <003401c6ee58$8a616810$0348e146_at_ATARIENGINE>

Hi,

    I don't have a copy of that graph of planetary
angular momentum versus log of planetary mass
(and no way to post it if I did), but you can find
the values for these and many more physical
parameters of the planets at this URL:
http://history.nasa.gov/SP-345/p16.htm
 The document is SP-345: Evolution of the Solar
System by Hannes Alfv?n

    TotAngMo in units of 10^46 gms^2/second:
Mer 0.906
Ven 18.5
Ter 26.7
Mars 3.52
Jup 19400.0
Sat 7840.0
Urn 1700.0
Nep 2500.0
Plu 17.9

    It also gives Specific AngMo (per unit of mass),
and the masses of the planets. I don't know whether
this graph of yours used Total or Specific AngMo,
or if this page gives the combined mass of
Earth-and-Moon, or just the Earth.

    I also stumbled across a (text) reference to a
line plot of AngMo in which all the planets fall on
the line except the Earth/Moon, which is way above
the line, but they didn't show the graph.

    The table in the URL gives other useful but hard
to find data like the moment of inertia of each planet.
About twenty years ago (pre-internet), I was writing
a crude computer program and discovered I HAD to
know the moment of inertia of the Earth (alone) and
could not find the value anywhere.
    I spent three days with my TI-66 calculator and
a stack of books calculating the moment of inertia of
an entire planet from scratch, layer by layer, inner core,
outer core, lower mantle, blah, blah, and ended up
with a value of, oh, well, about 1/3, so I used 0.33 in
the program, with which I happily spent days and weeks
(with a 2 mhz CPU) regressing the orbit of the Moon
back through geologic time by the tidal transfer of
angular momentum between Terra and Luna.
    So, I had a nostalgic warm and fuzzy moment when
I found this table which says the moment of inertia of
the Earth is 0.3335.

    Enough data there to scratch up the graph, if you
want it bad... Plot away! And it will prove...
What was that again?


Sterling K. Webb
---------------------------------------------------------------
----- Original Message -----
From: "Rob McCafferty" <rob_mccafferty_at_yahoo.com>
To: "tett" <tett_at_rogers.com>; <meteorite-list@meteoritecentral.com>
Sent: Thursday, October 12, 2006 2:36 PM
Subject: Re: [meteorite-list] Venus May Have Once Had A Moon


>
> Sadly no, I don't. They were all in my astrophysics
> notes from uni but they got thrown out in a house move
> about 18 months ago. I suppose I could hunt around for
> something. I kept my textbooks, maybe theres something
> in there.
>
> Rob McC
> --- tett <tett_at_rogers.com> wrote:
>
>> Rob,
>>
>> Do you have your graphs and calculations available
>> in an email format? I
>> would love to review this and try to understand your
>> argument.
>>
>> Cheers,
>>
>> Mike Tettenborn
>> ----- Original Message -----
>> From: "Rob McCafferty" <rob_mccafferty_at_yahoo.com>
>> To: "Ron Baalke" <baalke_at_zagami.jpl.nasa.gov>;
>> <meteorite-list_at_meteoritecentral.com>
>> Sent: Wednesday, October 11, 2006 3:57 PM
>> Subject: Re: [meteorite-list] Venus May Have Once
>> Had A Moon
>>
>>
>> > I've been saying this for years. I even tell my
>> > classes.
>> >
>> > If log angular momentum is plotted vs log Mass,
>> all
>> > planets fit nicely on a line except Venus and
>> Mercury
>> > (Earth/moon system needs to be combined).
>> > Now since angular momentum is a conserved
>> quantity, it
>> > matters not one jot how far a planet and its moon
>> > drift apart. Combine the angular momentum of Venus
>> and
>> > Mercury and they slot nicely on the line like all
>> the
>> > others.
>> > If some accuse me of favouring an idea which is
>> too
>> > neat, I'd accuse the author of this article of
>> this
>> > article of over-thinking a problem. The peculiar
>> > rotation of venus is rather nicely explained by it
>> > losing a moon, especially one as big as Mercury.
>> >
>> > Rob McC
>> > (plagariser of his Professors Ideas)
>> >
>> > --- Ron Baalke <baalke_at_zagami.jpl.nasa.gov> wrote:
>> >
>> >>
>> >> http://skytonight.com/news/4353026.html
>> >>
>> >> Why Doesn't Venus Have a Moon?
>> >> by David Tytell
>> >> Sky & Telescope
>> >> October 10, 2006
>> >>
>> >> Back when Earth was very young, our home world
>> was
>> >> steadily pummeled by
>> >> large solar system debris. While Earth withstood
>> the
>> >> barrage of hits
>> >> like a prizefighter that wouldn't fall down, one
>> >> blow nearly destroyed
>> >> the world. A Mars-size body plowed into us,
>> >> completely disrupting both
>> >> bodies and splashing massive amounts of debris
>> into
>> >> orbit which, most
>> >> astronomers agree, coalesced to form our Moon.
>> >>
>> >> But if something that large hit us, how did our
>> >> nearest-neighbor planet,
>> >> Venus, dodge the same fate? According to a new
>> >> study, it didn't.
>> >> Billions of years ago, according to work
>> announced
>> >> yesterday, Venus once
>> >> had a moon that formed the same way Earth's did.
>> >>
>> >> On Monday at the American Astronomical Society's
>> >> Division of Planetary
>> >> Sciences meeting in Pasadena, California, Caltech
>> >> undergraduate Alex
>> >> Alemi presented models created with David
>> Stevenson
>> >> of Caltech that
>> >> suggest Venus was not only slammed with a rock
>> large
>> >> enough to form the
>> >> Moon, the event happened at least twice.
>> >>
>> >> According to Alemi and Stevenson, in models of
>> the
>> >> early solar system it
>> >> is nearly impossible for Venus to avoid a big
>> hit.
>> >> Most likely, Venus
>> >> was slammed early on and gained a moon from the
>> >> resulting debris. The
>> >> satellite slowly spiraled away from the planet,
>> due
>> >> to tidal
>> >> interactions, much the way our Moon is still
>> slowly
>> >> creeping away from
>> >> Earth.
>> >>
>> >> However, after only about another million years
>> >> Venus suffered another
>> >> tremendous blow, according to the models. The
>> second
>> >> impact was opposite
>> >> from the first in that it "reversed the planet's
>> >> spin," says Alemi.
>> >> Venus's new direction of rotation caused the body
>> of
>> >> the planet to
>> >> absorb the moon's orbital energy via tides,
>> rather
>> >> than adding to the
>> >> moon's orbital energy as before. So the moon
>> >> spiraled inward until it
>> >> collided and merged with Venus in a dramatic,
>> fatal
>> >> encounter.
>> >>
>> >> "Not only have we gotten rid of the moon, but
>> we've
>> >> also done well to
>> >> explain Venus's current slow rotation rate [and
>> >> direction]," says Alemi.
>> >> If a second moon formed from the second
>> collision,
>> >> it too would have
>> >> been absorbed the way the first one was.
>> >>
>> >> The models do allow for more than two impacts,
>> but
>> >> the probability of
>> >> Venus enduring several massive collisions is low.
>> >> "You can do this with
>> >> multiple collisions, but the hypothesis is that
>> [the
>> >> net result] adds up
>> >> to a negligible contribution" to the planet's
>> final
>> >> state, says Alemi.
>> >>
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Received on Thu 12 Oct 2006 07:45:44 PM PDT