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Hat Monster
2002-Apr-20, 12:19 AM
Exhibit 1:
The moon is thought to have formed from a massive impact in the early days of the solar system, supporting this is the absence of iron in the moon and it's low density, and also it's tidally locked rotation. The impact would have had to be shear in order to properly orbit the debris and capture the impactor.

Exhibit 2:
Venus is thought to have been tilted over by a massive impact in the early days of the solar system. The impact would have had to be shear in order to tilt the planet.

Contention:
Why is Earth not tilted over (tidal drag explains our 23.3 degrees quite well), why does Venus not have a moon?

Donnie B.
2002-Apr-20, 01:00 AM
The result of any impact, large or small, is highly contingent. There are innumerable variables; some of the most important are the exact mass and composition of the impactor, the speed and angle of the impact, the mass and composition of the "target", and the rotation rates of the two objects.

I believe that the simulation models have to be "tweaked" for very specific initial conditions to produce the Earth-Moon system as we see it today. This isn't cheating, because if such a collision occurred, it had to have some combination of these factors; and how better to determine them than to pick the ones that yield the current observations?

Of course, this doesn't "prove" that an impact created the Moon; only that it's a reasonable hypothesis. It explains the preset status quo better than other models do, so it's currently the favorite explanation.

So why would Venus be so different? Simple: different initial conditions. Perhaps the impactor was much larger or smaller, or slightly different in closing angle. Small changes could produce an impact with much less ejecta, or ejecta that quickly fell back to the surface, or largely remained below the Roche limit and so could never accrete into a single body (and whose orbits eventually decayed).

I'm not familiar with the mechanism you mention for producing Earth's polar tilt. Is it lunar tidal action you are referring to? That is, the Earth got its tilt because the Moon's orbit is tilted? But that tilt was also the result of the conditions of the impact. Whether Earth's polar tilt occurred at the time of impact, or later due to tidal drag, or some combination thereof, it still traces back to the impact.

John Kierein
2002-Apr-20, 01:16 AM
Why does tidal lock imply a collision? Any dumbbell shaped object will keep one side pointed toward the earth due to gravity gradient forces. The moon is known to be "lumpy" full of Mascons (or mass concentrations). It's no secret that it should keep one side pointed toward the earth as a result. Just because the moon seems to be "lumpy" and probably is an aglomeration of several smaller objects doesn't mean it was a created via a collision with the earth. We routinely use the gravity gradient characteristics of separated masses to gravity gradient stabilize satellites cheaply to keep one side pointed towards the earth for earth pointed payloads.

GrapesOfWrath
2002-Apr-20, 01:56 AM
On 2002-04-19 20:19, Hat Monster wrote:
(tidal drag explains our 23.3 degrees quite well)

I did a google on +"tidal drag"+"tilt"+"earth" but didn't come up with anything that explained that. What do you mean?

dapted
2002-Apr-20, 05:50 AM
Try this one http://www.jal.cc.il.us/~mikolajsawicki/tides_new2.pdf

Tidal lock is incredible fun. Some theorize that when the moon actually reaches a lock with earth it will fall apart forming a ring like that around saturn.

The moon could easily have occured while the earth was still so young that it was just a blob of magma glowing in the dark. An impact by any large object could have set up vibrations sufficient to make two blobs. Or a large magnetic or gravitaional force coming close enough could have split the blob,and a magnetic one could have made all the iron gravitate to one side.

Nothing to back this up but imagine a blob of water floating in space in the space shuttle. Let somebody hit it even slightly and it becomes many smaller blobs of water.

Something like that would explain why most of the planets are on a similar plane as they orbit the sun, maybe they were all one big blob, if you go back far enough.

Link fixed by tBA



<font size=-1>[ This Message was edited by: The Bad Astronomer on 2002-04-27 21:43 ]</font>

2002-Apr-20, 02:50 PM
<a name="20020420.8:27"> page 20020420.8:27 aka Young Earth {Boo}
My Main thingee's the Pacific Basin & its Lack
of Granits{Light Rocks} Which I think make up the Moon
and there are the warm Shallow seas..
But never mind Physics
My gripe heres Dynamics
It seams to me, that the distance to the Moon
its resession Rate, and orbital Dynamics
ARE NOT really well understood by EarthLinks
of this day {Boo, Boo, Boo}
especially its Excursions North & South
now tell me Quickly whats that Cycle called
How long does it take (in Seconds) and
at what rate does the N/S part decay ( U have A week and 1/2 left in April )

David Simmons
2002-Apr-20, 05:23 PM
On 2002-04-19 20:19, Hat Monster wrote:
Contention:
Why is Earth not tilted over ...

Well, how do we know it isn't "tilted over" since we have absolutely zero data on the inclination of the axis before the presumed collision?

Firefox
2002-Apr-20, 05:45 PM
Tidal lock is incredible fun. Some theorize that when the moon actually reaches a lock with earth it will fall apart forming a ring like that around saturn.

Wouldn't such a ring last a short time compared to Saturn's? Earth has an awfully small roche limit (it's within our atmosphere, IIRC.)


-Adam

Hat Monster
2002-Apr-21, 08:23 AM
Well, how do we know it isn't "tilted over" since we have absolutely zero data on the inclination of the axis before the presumed collision?

Rotation isn't retrograde and it's well in line with the rest of the planets for period.

Hat Monster
2002-Apr-21, 08:28 AM
Earth's Roche limit is 18,470km, well out of the atmosphere.

http://www.jpl.nasa.gov/saturn/faq.html#roche

Phobos
2002-Apr-21, 10:55 AM
I have a theory which would easily explain why some planets are tilted.

If a planet was struck by a large enough impactor, you may expect the impactor to bury itself deep below the surface of the planet. Provided that the impactor does not pass all the way through the planet, it would seem reasonable to assume that the impactor stands a good chance of eventually stopping in the magma region of the planet whereupon we would expect it to melt and become part of the planet.

Now if this happens, we would expect the impactor to have some effect on the internal rotational direction of the magma. Over time this could effect the tilt of the planet.

Phobos

Jim
2002-Apr-21, 06:28 PM
On 2002-04-21 04:23, Hat Monster wrote:


Well, how do we know it isn't "tilted over" since we have absolutely zero data on the inclination of the axis before the presumed collision?

Rotation isn't retrograde and it's well in line with the rest of the planets for period.


I think the point is, how do we know that the earth was tilted 23.5 degrees before the impact that made the moon? Why not 2.5 degrees? Or zero? Or 90 degrees t'other way?

Do we have any evidence that says the earth has always had the same tilt it does today?

Hale_Bopp
2002-Apr-22, 02:02 PM
With all due respect, please do not say "I have a theory..."

According to the National Academy of Sciences, a theory is a "well substantiated explanation of some aspect of the natural world that can incorporate facts, laws, inferences and tested hypothesis." Unless your idea can meet that definition, it cannot properly be called a theory.

The constant misuse of the word theory, even be respected scientists, damages the public perception of science. The general public can write off solid science because, "It's just a theory" because the general public does not understand the true meaning of theory.

I have an idea/hypothesis are both better terms to use. I know the potential for confusion is probably less on this board than in the general population, but it's still a good idea to practice proper usage /phpBB/images/smiles/icon_smile.gif

Rob

Donnie B.
2002-Apr-22, 10:30 PM
The only proper way to use "I have a theory..." is to follow it with "(which is mine)".

David Simmons
2002-Apr-22, 11:34 PM
On 2002-04-21 04:23, Hat Monster wrote:


Well, how do we know it isn't "tilted over" since we have absolutely zero data on the inclination of the axis before the presumed collision?

Rotation isn't retrograde and it's well in line with the rest of the planets for period.


Well, all of the planets follow Kepler's law don't they? This is a function of the mass of the primary and the distance to the primary. I guess I don't see why this proves anything about collisions one way or another. Unless it shows that since the earth's orbit is regular like all the others there was no collision with anything sizeable.

Tim Thompson
2002-Apr-26, 09:03 PM
Hat: Venus is thought to have been tilted over by a massive impact in the early days of the solar system.

Who thinks that? maybe for Uranus, but certainly not for Venus. The obliquity of Venus is only barely retrograde; tilt it a tad over 3 degrees, and it's prograde again. That's not much of a tilt.

That kind of small retrograde axis is easily explained by the fact that planetary obliquities are chaotic, and subject to large excursions over relatively short time. This is particularly the case for Venus, which has no stabilizing moon. The current obliquity of Venus is not the original one, and is the end product of chaotic excusrions over the life of the solar system (The Chaotic Obliquity of the Planets, Laskar & Robutel, Nature 361(6413): 608-612, February 18, 1993; Stabilization of the Earth's Obliquity by the Moon, Laskar, Joutel & Robutel, Nature 361(6413): 615-617, February 18, 1993; Venus' Free Obliquity, C.F. Yoder, Icarus 117(2): 250-286, October 1995).

It is also possible to pull the axis of Venus over by a torque that comes not from an impactor, but from solar & atmospheric torque (Evolution of the Spin of Venus, McCue & Dormand, Earth Moon and Planets 63(3): 209-225, December 1993).

DJ
2002-Apr-27, 01:38 PM
The moon could easily have occured while the earth was still so young that it was just a blob of magma glowing in the dark. An impact by any large object could have set up vibrations sufficient to make two blobs. Or a large magnetic or gravitaional force coming close enough could have split the blob,and a magnetic one could have made all the iron gravitate to one side.


My concern with this type of evolution would be the disfiguration of the earth due to the gravity of the moon. While the earth is technically an oblate spheroid, already suffering from gravity distortion by the moon, had this action taken place while the earth was still significantly flexible, it probably would not be a stable formation.

I think the observations more strongly suggest a moon arriving on-site after the crust of the planet, and significant interior cooling had occurred. We would definitely have distortions in the center of the planet (distribution of magma) if not.

DJ

GrapesOfWrath
2002-Apr-27, 11:42 PM
On 2002-04-27 09:38, DJ wrote:
I think the observations more strongly suggest a moon arriving on-site after the crust of the planet, and significant interior cooling had occurred. We would definitely have distortions in the center of the planet (distribution of magma) if not.

Why's that? The viscosity of the Earth's mantle is low enough that it would conform to whatever shape is necessary--or do you mean something else?

DJ
2002-Apr-28, 11:44 AM
I meant that if there was a moon onsite earlier in earth's formation, the distortions of external and internal shape might be much more apparent. The earth might have cooled with these distortions intact.

I'm considering some of the enigmas coming in from Mars, which suggests a very lopsided core.

2002-Apr-28, 11:46 AM
<a name="20020428.3:36"> page 20020428.3:36 aka Astroloplis?MarsFellow
On 2002-04-19 21:56, GrapesOfWrath wrote: To: Marsfellow
In my unwritten book, Solar System: "the Main Event"
in witch i, describe in infinite detail
the Obliteration of Astrolopolis..
{ thats the planet that did orbit between Mars & Jupiter}
[ that was in Galyear 38, this ones #39 ]
(howver thats in debait as some say this is only 24)
believe whatever number fits into your model?
and the vanishment of the main member of the Mars orbit Marsfellow himself Leaving only his moon Mars to fill the orbit {into another deminsion alltogether?

GrapesOfWrath
2002-Apr-29, 11:11 AM
On 2002-04-28 07:44, DJ wrote:
I meant that if there was a moon onsite earlier in earth's formation, the distortions of external and internal shape might be much more apparent. The earth might have cooled with these distortions intact.

I understood. My point was that the Earth is still malleable enough that it conforms fairly readily. In the early sixties, it was thought that the Earth's slight excess bulge was an artifact of its previous higher rotation rate. That indicated a viscosity much higher than is now thought--and was a strong argument against plate tectonics, at the time.

Now, we know it is not as viscous.


I'm considering some of the enigmas coming in from Mars, which suggests a very lopsided core.


That doesn't mean that Earth's core is not also lopsided. Depends upon what you mean by lopsided.

2002-Apr-29, 01:41 PM
[quote]
On 2002-04-29 07:11, GrapesOfWrath wrote:
[quote]
On 2002-04-28 07:44, DJ wrote:
5:32 A.M. HUb' ALLRIGHT WHERE: did you dissapear to in
the C U debait? hmm?


Now, we know it is not as viscous.
5:33 A.M. Viscousity has nothing to do with MUCH
Look at the video of the Alaska EarthQuake
and the bendind sidewalk. ~ Concretes no Viscous!
Matter take the form that the Gravity Field
provides at the time. Spherical field equation Spherical mass
undulating field, undulating mass. My points the Pacific Basin's missing Granit! [oh well]{never mind} 5:38 A.M.