Spheres

woodchip · Post by **woodchip** » Sun Oct 17, 2004 1:08 pm

Anyone ever wonder why planets are marble shaped and not columnar? Or disk shaped? If spin is only one dimensional wouldn't centrifugal (or is it centripidal) forces create a shape different than spherical?

fliptw · Post by **fliptw** » Sun Oct 17, 2004 1:19 pm

if and only if there are no attractive forced between atoms..., at which point nothing would form.

but, there are attractive forced between atoms, and as per the law of conservation, spheres would be formed.

WarAdvocat · Post by **WarAdvocat** » Sun Oct 17, 2004 1:30 pm

gravity is why they're marble shaped, but you may note equatorial bulges due to centripedal forces

But planets just don't spin very fast, relatively speaking...or you'd weigh noticably more in the polar regions.

DCrazy · Post by **DCrazy** » Sun Oct 17, 2004 1:49 pm

Woody, when you're thinking of centripetal forces, are you thinking orbits, or are you thinking axial rotation?

Think of a rock spinning in orbit around a sun. This rock's mass will cause it to generate a gravitational force, which will attract loose particles surrounding the rock. Each of these particles clinging onto the rock is going to try its hardest to get to the center of the rock, since that's where the gravity originates (assuming that the rock isn't totally weird-shaped, like a cone or something; if it is then the center of gravity is displaced from the center). Even so, we can basically assume that particles are scattered randomly throughout the universe, so it's going to be picking up particles from every direction. These particles build on top of each other to form a planet over billions of years.

woodchip · Post by **woodchip** » Sun Oct 17, 2004 1:56 pm

O.K. then,so why doesn't Saturn have a cloud of debris around it instead of its characteristic ring?

DCrazy · Post by **DCrazy** » Sun Oct 17, 2004 2:11 pm

Because the rings are centered around the equator of Saturn. There, the centripetal acceleration is greatest (because the radius of the circle is greatest), so by Newton's laws (Force = mass * acceleration) the force exerted on the particles that make up the ring is greatest.

This is the same reason that the planets' orbits are more or less centered around the equator of the sun. Even if they're pitched, the oribt is usually in the same plane as a radius of the sun, not, say, any arbitrary line of latitude.

Duper · Post by **Duper** » Sun Oct 17, 2004 2:28 pm

the same reason balloons are round, or drops of fluid in a lighter medium become spheres.

Equal pressure. I don't know the exact physics that cause it, so I won't bother throwing out guesses. I do understand the principle.

The earth is not an exact sphere as you might know. For the reason stated by DCrazy.

WarAdvocat · Post by **WarAdvocat** » Sun Oct 17, 2004 2:51 pm

not to mention the reason stated by me, plate tectonics, gravitational interaction with other bodies, including Sol & Luna.

Duper · Post by **Duper** » Sun Oct 17, 2004 3:17 pm

WarAdvocat wrote:not to mention the reason stated by me, plate tectonics, gravitational interaction with other bodies, including Sol & Luna.

DOH!

My bad er.. yea.. what WA said ..er.. too!

Lobber · Post by **Lobber** » Sun Oct 17, 2004 3:52 pm

And one point of order: The Earth isn't spherical. It's actually Pear Shaped.

Bulges out in the Southern Hemisphere and the North Pole, sinks inward in the Northern Hemisphere and the South Pole. But the deviations from spherical are only within the parameters of 15-40 feet above or below the mean diameter of the planet.

Satelites detected and measured this on the planet, and it is due to the slight mass difference where there are continents and oceans.

Bet51987 · Post by **Bet51987** » Sun Oct 17, 2004 4:21 pm

This is Kool. I love science. I am taught that rocks in space won't be round unless they have a magnetic core. Without it, they will stay in the shape observed.
The Earth is round because of it's magnetic core. Everything is going to the center. Wind, erosion, earthquakes, rain, etc, help it achieve this.
For all "practical purposes"....the Earth is round.
This is my favorite class, and the teacher is cute too.

Bettina

WarAdvocat · Post by **WarAdvocat** » Sun Oct 17, 2004 5:20 pm

bet51987 wrote:I am taught that rocks in space won't be round unless they have a magnetic core. Without it, they will stay in the shape observed.
The Earth is round because of it's magnetic core. Everything is going to the center.

That is over-simplified to the point of innacuracy. The correct answer that that bodies with a MOLTEN (or tectonically active) core will tend to be spherical or will tend to become quite spherical, while bodies without a molten (tectonically inactive) core will tend to remain the same shape unless they interact with another body in some way.

In general bodies with a molten core will also have magnetic cores, however, the converse is not true. It is quite possible to have a magnetic core that is not molten.

woodchip · Post by **woodchip** » Sun Oct 17, 2004 5:47 pm

So to get a molten core enough space thingys have to accrete by gravitational attraction so that pressure builds enough to melt rock. So why isn't the material in Saturns rings accreting into larger bodies?

Mobius · Post by **Mobius** » Sun Oct 17, 2004 6:42 pm

bet51987 wrote:This is Kool. I love science. I am taught that rocks in space won't be round unless they have a magnetic core. Without it, they will stay in the shape observed.
The Earth is round because of it's magnetic core. Everything is going to the center. Wind, erosion, earthquakes, rain, etc, help it achieve this.
For all "practical purposes"....the Earth is round.
This is my favorite class, and the teacher is cute too.
Bettina

Sorry Bettina - you are getting some Bad Science there. ALL sufficiently large objects will become spherical (or with 1% tolerance thereof) due to nothing except gravity. It has nothing to do with magnetism, liquid cores, or any other force or object WHATSOEVER.

Everything is going to the center. Wind, erosion, earthquakes, rain, etc, help it achieve this.

You are partially correct here. Yes, erosion will make objects MORE spherical (i.e. smooth out mountains etc) but it won't CREATE a spherical object. For the actions you speak of, you need an atmosphere, and nothing non-spherical is large enough to capture an atmosphere - so your argument is a bit backwards... The spherical body MAY become large enough to hold an atmosphere capable of eventually smoothing the surface of the (almost) spherical object/planet.

For an indication of what size (Mass) an object has to be before it'll become spherical (Remember, "round" is a 2-dimensional object) due to gravity, we need to look at the smallest spherical objects in the solar system. Pluto's moon "Charon" is likely to be spherical, but there are no spherical asteroids - they are not large enough. So, objects must be larger than asteroids (FYI: Pluto is about the size of Eath's Moon) before they become spherical.

Currently, there is some debate about what constitutues a "Planet", and personally, I'm inclined to side with the people who say 'Any spherical object in an orbit about our Sun (as opposed to "orbit around another body") is a planet.'

FYI: Both Deimos and Phobos (which are Martian Moons) are captured asteroids and hence not spherical.

EDIT: any liquid object in space will defy the ideas above. Shattered planetary fragments ejected into orbit as a liquid (created by kinetic impact) will form spheres due to the law of surface tension (Seeks the smallest surface area). This includes magma, water, and even ice. (Ice flows like water, just slower). If a liquid object "appears" in space, it will assume a spherical shape and then freeze in that shape. Later additions or impacts may change it's shape if it is below the mass limit described above.

WarAdvocat · Post by **WarAdvocat** » Sun Oct 17, 2004 6:42 pm

what is up w/ double posts anyhow

WarAdvocat · Post by **WarAdvocat** » Sun Oct 17, 2004 6:48 pm

Mobius wrote: Sorry Bettina - you are getting some Bad Science there. ALL sufficiently large objects will become spherical (or with 1% tolerance therefor) due to nothing except gravity. It has nothing to do with magnetism, liquid cores, or any other force or object WHATSOEVER.

For an indication of what size (Mass) an object has to be before it'll become spherical (Remember, "round" is a 2-dimensional object) due to gravity, we need to look at the smallest spehircal objects in the solar system. Pluto's moon "Charon" is likely to be spherical, but there are no spherical asteroids - they are not large enough. So, objects must be larger than asteroids and smaller than Pluto's Moon (Pluto is about the size of Eath's Moon) before they become spherical.

You might want to qualify your remarks...Some disclaimers about rocky bodies and surface tension perhaps

Mobius · Post by **Mobius** » Sun Oct 17, 2004 6:56 pm

woodchip wrote:So to get a molten core enough space thingys have to accrete by gravitational attraction so that pressure builds enough to melt rock. So why isn't the material in Saturns rings accreting into larger bodies?

Because the "chaotic" nature, constant impacts, and high speeds prevent this. Remember, the rings were made by gravitic tidal effects ripping a small moon apart.

Duper · Post by **Duper** » Sun Oct 17, 2004 7:01 pm

Mobius wrote:Currently, there is some debate about what constitutues a "Planet", and personally, I'm inclined to side with the people who say 'Any spherical object in an orbit about our Sun (as opposed to "orbit around another body") is a planet.'

I certainly hope that the Criteria being put foreth to define a planet is a bit more involved than that. Those are some weak parameters to define a planet. This would include ANY piece of "dirt" that is orbiting the sun that is close to spherical to hold the sacred title of PLANET.

Bet51987 · Post by **Bet51987** » Sun Oct 17, 2004 7:35 pm

WarAdvocat wrote:
bet51987 wrote:I am taught that rocks in space won't be round unless they have a magnetic core. Without it, they will stay in the shape observed.
The Earth is round because of it's magnetic core. Everything is going to the center.
That is over-simplified to the point of innacuracy. The correct answer that that bodies with a MOLTEN (or tectonically active) core will tend to be spherical or will tend to become quite spherical, while bodies without a molten (tectonically inactive) core will tend to remain the same shape unless they interact with another body in some way.

In general bodies with a molten core will also have magnetic cores, however, the converse is not true. It is quite possible to have a magnetic core that is not molten.

We make it simple so we can move on to other things.....IF we need to go deeper for some reason, we can.
Bettina

DCrazy · Post by **DCrazy** » Sun Oct 17, 2004 7:40 pm

There's nothing wrong with that if what they teach you is correct...

Bet51987 · Post by **Bet51987** » Sun Oct 17, 2004 7:42 pm

WarAdvocat wrote:
Mobius wrote: Sorry Bettina - you are getting some Bad Science there. ALL sufficiently large objects will become spherical (or with 1% tolerance therefor) due to nothing except gravity.

I may have misunderstood. I'm thinking of a plain rock without a gravity core.

Bet51987 · Post by **Bet51987** » Sun Oct 17, 2004 7:45 pm

And.....the Earth has a "magnetic center". Whether it's molton is secondary. The gravitational pull from the core is what pulls everything to the center...over millions of years......rounding everything out.

http://www.gsfc.nasa.gov/scienceques2002/20021101.htm

Spaceboy · Post by **Spaceboy** » Sun Oct 17, 2004 8:01 pm

gravity= sphere. cirlce=stickman.

water droplets in space tend to try to hold spheres, not stickmen.

Duper · Post by **Duper** » Sun Oct 17, 2004 11:16 pm

bet51987 wrote:And.....the Earth has a "magnetic center". Whether it's molton is secondary. The gravitational pull from the core is what pulls everything to the center...over millions of years......rounding everything out.

http://www.gsfc.nasa.gov/scienceques2002/20021101.htm

And I guess that settles that. Way to google girl!

Lobber · Post by **Lobber** » Sun Oct 17, 2004 11:53 pm

I want to point out that liquids do not exist in space for any length of time, since it is air pressure that keeps the liquid in its shape, and lack of air pressure raises the evaporation point.

woodchip · Post by **woodchip** » Mon Oct 18, 2004 4:20 am

Mobius wrote:
woodchip wrote:So to get a molten core enough space thingys have to accrete by gravitational attraction so that pressure builds enough to melt rock. So why isn't the material in Saturns rings accreting into larger bodies?
Because the "chaotic" nature, constant impacts, and high speeds prevent this. Remember, the rings were made by gravitic tidal effects ripping a small moon apart.

Don't know as I buy into that. You have the same three "chaotic" criteria in the asteroid belt but you still have accretion asteroids that form togeather.

Sage · Post by **Sage** » Mon Oct 18, 2004 5:49 am

The thread title reminded me of the Sonic & Knuckles special stages. Get Blue Spheres.

Asrale · Post by **Asrale** » Mon Oct 18, 2004 11:32 am

PLANETESIMAL!!! ACCRETION!!! BZZZZZFIZZZZZFITTT!

Sorry I just had to come in here and say that when I saw what the thread was about.

Took an astronomy class in college, wow that feels like ages ago now!

Robo · Post by **Robo** » Mon Oct 18, 2004 12:35 pm

The large mass of moons, satelites, planets and stars causes a warpage of 4d space-time (simply known as gravity). This warpage is equal from every direction, so every particle in that object is equally trying to move to the centre of gravity with an equal force in every direction - thus resulting in a sphere.

In the case of fluids, this is to do with pressure, which works along the same lines (only from the outside).

DCrazy · Post by **DCrazy** » Mon Oct 18, 2004 3:05 pm

And the fact that pressure is a 3D force, whereas gravity transcends 3-space.

roid · Post by **roid** » Tue Oct 19, 2004 5:56 am

woodchip if you have a bunch of balls on the ends of individual ropes, you can swing them all around you, on a single plane, a 2 dimensional ring of balls like saturn. (like a hulahoop, but say 10 or so individually spaced balls instead of 1 hoop).

but when those balls start banging into eachother, and you, watch the emerging chaos. it'l be a mess, it definitely won't be a neat ring anymore. and when the chaos finally settles the balls will all be touching eachother, all trying to get to the centre point.

i think it's prettymuch when individual objects start interacting with one another, causing more and more chaos: when it settles you prettymuch get a sphere.

i seem to have fergotten why DISCS of debris form around planets, and not just a haze (discs also form around blackholes). if the magnetism of the major body is at fault then that would be easy to explain.

but i'm not sure if you can really blame it on centrifical force. anything out there in space orbiting earth won't care the slightest how EARTH is moving. earth's spin will exhert no force on any orbiting body, since they don't touch (not even earth's equally spinning atmosphere will touch it).

damnit, i can't answer satisfactionary.

WarAdvocat · Post by **WarAdvocat** » Tue Oct 19, 2004 6:13 am

there IS no one satisfactory answer.

Given that the current theory is that planetary formation is the same as star system formation, except in microcosm, it is thought that rings around planets are created as the planet accretes and are composed of "spun-off" stuff that never managed to become part of the planet (real LOSER stuff since it didn't even have the gumption to form a MOON), or they're a result of impact incidents (and/or complex tidal interactions) between either the planet and/or it's satellites and/or orbital debris and/or rogue objects....

Or all of the above....

According to http://www.astronomycafe.net/ one of the moons of Saturn disappeared about 15 years ago. Observations made with the Hubble telescope lead scientists to believe that the moon was involved in an impact event. The only remains of the disappearing moon are two smaller moonlets which now exist, each in different orbits.

And we can't forget the function of "shepherd moons"

All in all a fascinating topic to research! Thanks.

woodchip · Post by **woodchip** » Tue Oct 19, 2004 6:42 am

So maybe there is a case for "size matters". Smaller bodies such as moons don't seem to have orbiting bodies. Asteroids may accrete but I don't recall any case where a larger asteroid has smaller ones in orbit.
The curious thing about Saturns rings are the well defined bands that are not only well defined visually but also temperature wise:

http://tinyurl.com/6oo77

So if we ascribe to the chaos theory, wouldn't we see a more homogenus blending of the rings?

roid · Post by **roid** » Tue Oct 19, 2004 7:05 am

woodchip wrote:So if we ascribe to the chaos theory, wouldn't we see a more homogenus blending of the rings?

at one time, yes.

perhaps from a collision of now extinct moons, or perhaps just dust from the birth of the solar system: originally they was probabaly a haze around saturn, of randomly orbiting particles. then after some time it turned into a neater and homogenus orbiting disc (for reasons that this thread is trying to explain i guess). and then after some time the disc seperates into various bands. given enough time these bands would probabaly get thinner and thinner and aggregate into small chunks and then moons, which would eventually swallow other moons, and further effect the rings with it's (the new moons') gravity.

given enough time, you would have only a few major bodies orbiting saturn, if anything at all (saturn may eat em up itself). it'd all depend on so many factors, and would take so long for any significant changes.

it's like predicting tea leaf patterns on the bottom of a mug, only it would probably take billions of years for the tea to stop moving so the leaves can settle.

Tricord · Post by **Tricord** » Tue Oct 19, 2004 8:53 am

Let me roll a ball here (no pun intended

)

A huge chunk of mass has a centripedal gravitational field, in which you have a certain potential energy towards the middle point. The system tries to achieve a state of lowest energy (warm things cool down, etc.) and so everything will be sucked to the middle point. However, since all that matter has a certain volume, it has to arrange itself in some way around the middle, in which each particle is as close as it can get. The most compact volume (meaning each particle is as close as it can get to the middle) around a middle point is a sphere.

WarAdvocat · Post by **WarAdvocat** » Wed Oct 20, 2004 6:13 am

Thank you captain obvious

And there's no such thing as a "centripedal" gravitational field as far as I know...but it sure sounds cool!

roid · Post by **roid** » Wed Oct 20, 2004 8:44 pm

but there is a centripeTal gravitational force.

i centri-pedal on my bike.

(heh actually i only discovered the correct spelling coz i had to look the word up in a dictionary)