Angular Momentum and Planetary Dynamos

StarDreamer · 2007-04-30 13:28:32

I have seen many proposals on here for giving either Mars or Venus a moon and also for tweaking the nagular momentum of the planet or else reigniting the internal planetary dynamo. I am uncertain of the physics of this and suspect there may be rather severe constraints on what we should even consider hoping to achieve.

Let's explore this. Here are some options:
1) Parking NEA, NVA or NMA (asteroids) into orbit.
2) Lifting mass off the planet surface to make a moon. Mostly proposed for Venus only.
3) Lobbing iron-nickel impactors at our neighbours and also incidentally cleaning up the terrestrial neighbourhood.

And now here are the concerns for each:
1) Can we move a mass the size of an asteroid both with the speed and the precision to bring it into orbit around another body? This means both overcoming its initial inertia and orbital mechanics and then braking imparted inertia to bring about orbit?
a) As a subset of this method, would it be feasible to accrete matter onto either Phobos or Deimos? They are, after all, already in orbits and that is the most difficult part of the whole enterprise.
b) What is the mass of these sets of asteroids available for moon-building? Are there enough NVAs to create a satellite with enough gravity to form itself into a sphere? Likewise with the NMAs? Perhaps more with Mars than with Venus. I'd like to pair up Eros with Venus ....

2) Can we build a moon from scratch, a la Death Star fashion? I would not want to attempt this with Mars, but I have seen several proposals on here for Venus, as a depository for excess carbon. I'd rather see it take excess sulphur personally, but let's play with this. You'd want to begin with a hollow iron sphere positioned in orbit. Perhaps magnetise the whole sphere to encourage passive accretion by space dust? Or would solar energy rob it of its field? Could we lift enough carbon and sulphur out of the Venus gravity well to make this work? At what point would there be enough matter on the surface of this moon to shield the hollow core from solar radiation and other interference? If it could be shielded, then a metallic magnetised core could be built inside.

3) Lobbing impactors at planets is the cheapest way to add energy into a planetary system, but the process is inherently chaotic and we cannot know all the consequences of our actions in advance. After all, it was impactors on Mars which brought pieces of Mars to our Antarctica. And impactors on Luna send bits of moonrock down here to us. I'm guessing impactors on Venus might have consequences only for Mercury, but can we be sure? Impactors affect angular momentum, but do not create moons, unless we are talking about Velikovskian collisions.

Which brings us to the related topic of angular momentum. Does giving a planet a moon help angular momentum? I expect not, but perhaps there are better informed people on here.

Earth got its Moon out of a cataclysmic collision in the early solar system. The theory is that we had a Lagrange companion which accreted enough matter to lose its position and come towards us. Theorists name this object Theia, the mother of the Moon (Selena). Theia struck us a glancing blow which vaporised our crust and sent most of it into orbit. Theia was nearly the size of Mars. Theia is not the Moon -- Theia was engulfed into the Earth itself. But Earth had evolved sufficiently to have stratified into layers, with iron and nickel and uranium sinking into the core, while silicates floated on top. Theia blasted off the silicate layer and sent it into orbit. That is why the composition of the Moon nearly mirrors that of our crust, but it lacks the materials found in our deeper layers. Theia created the living Earth of today -- the surviving fragments of original crust became the cratons around which the continents congealed. It made plate tectonics possible. Pangea. Some of the oldest plates only fit together neatly if you increase the curvature of the surface (ie: if the whole planet is about 20% smaller). The Atlantic has opened and closed many times, but the Pacific has always existed -- this is where the crust was lost. Theia collided with Earth WITH the direction of rotation, thus giving our planet an extra spin. When the Moon coalesced above our Roche Limit, it became a DRAG on the angular momentum of Earth. It was the collision which imparted this angular momentum; it has been the Moon which has dissipated it ever since, through tidal friction. After Theia, the Earth had a day of 5-6 hours and a year of 1500+ days. When cyanobacteria mats were growing in our oceans, the rotation had slowed to 21hrs per day and 450 days per year. We know this because the fossil record preserves their growth rates of one layer per day. When Luna was born, it induced oceanic tides in excess of 300ft which washed over the whole planet while the bulk of the water stayed mounded up under the Moon. :shock: The Moon is still receding 10cm per year. The L1 point between the Earth-Moon system and the Sun is thus also gradually moving away, but I suspect not at the same rate. My suspicion is that the distance between Luna and L1 is slowly being compressed. I wonder if there will come a time in the far future when the radius of Luna will exceed the distance between its orbital path and L1? Then it could break free.

Late collisions of massive objects were a feature of the solar system in the first 100 Ma or so. Each orbit had more than one contender for dominance because accretion was happening everywhere at once. Ever wonder how Uranus got knocked over on its side? Or why its magnetic axis is off center from its geometric axis (ie: not tilted, but displaced -- not passing through the core)? It got knocked over.

Venus also experienced a late collision of cataclysmic proportions. Its orbital plane is slightly tilted to the solar ecliptic. It is not merely tidally locked to the Sun, but slightly retrograde. Venus got punched head-on by its Langrange companion. Consequently its energy was not added into the planet but subtracted. It got stopped nearly dead in its tracks. This depleted its full store of angular momentum. It engulfed its impactor. And the full thick silicate crust remained behind, which is another reason for the runaway greenhouse.

The bodies with the most angular momentum in the solar system are Jupiter first and Earth second. This gives us great orbital stability, like a gyroscope. If the matter on the Moon were to be returned to Earth in the direction of our rotation, then, indeed, like a skater bringing in her arms, we would spin like a top -- a day in five hours. As the inner planets accreted matter, their orbits walked outwards. Jupiter never budged. Consequently the matter in the asteroid belt got squeezed up against this immoveable object and could not pull it together.

Having said all this, I do not know how we can impart more angular momentum to Mars or Venus through use of a moon. The only way I can see is to use iron-nickel impactors aimed off-center with the direction of rotation. With Venus, even this may not work -- because its motion is retrograde, the planet would have to be slowed, stopped, reversed, to get it spinning in the right direction. And it would be *very* unstable. And it would add heat to Venus. :shock:

I'd like to ship all Earth's plutonium to Mars and drop it into the core. That would put starch in its sails.....

Bryan

RickSmith · 2007-05-08 23:46:59

And now here are the concerns for each:
1) Can we move a mass the size of an asteroid both with the speed and the precision to bring it into orbit around another body? This means both overcoming its initial inertia and orbital mechanics and then braking imparted inertia to bring about orbit?

Bryan

Sure, if you can move an asteriod it is not that much more difficult to put it into an orbit. The trick is to start with a distant orbit around Mars and then thrust for a few minutes at the furthest point of the elipse to circularize it and make it shrink.

a) As a subset of this method, would it be feasible to accrete matter onto either Phobos or Deimos? They are, after all, already in orbits and that is the most difficult part of the whole enterprise.
b) What is the mass of these sets of asteroids available for moon-building? Are there enough NVAs to create a satellite with enough gravity to form itself into a sphere? Likewise with the NMAs? Perhaps more with Mars than with Venus. I'd like to pair up Eros with Venus ....

Bryan

Sure, but it is not that hard to get an orbit.

I don't have the numbers before me but I doubt that there are enough NMA to make a circular moon. The moon has to get 800 to 900 km in diameter to squish into a sphere and that requires a LOT of asteriods. However by heating them we can make it a lot easier for them to slump together.

2) Can we build a moon from scratch, a la Death Star fashion? I would not want to attempt this with Mars, but I have seen several proposals on here for Venus, as a depository for excess carbon. I'd rather see it take excess sulphur personally, but let's play with this. You'd want to begin with a hollow iron sphere positioned in orbit. Perhaps magnetise the whole sphere to encourage passive accretion by space dust?
Or would solar energy rob it of its field?

Bryan

Most space dust is not magnetic.

The solar wind would not rob a perminant magnet of its field. An electro-magnet would do work on the solar wind and thus would require more energy to be constantly be fed into it.

Could we lift enough carbon and sulphur out of the Venus gravity well to make this work? At what point would there be enough matter on the surface of this moon to shield the hollow core from solar radiation and other interference? If it could be shielded, then a metallic magnetised core could be built inside.

Bryan

No reason we couldn't lift all of Venus out of Venus' gravity field with enough energy. Speaking practically tho, I can not see this happening even with a He3 economy.

As for shielding the magnetic field generators inside; you can't have it both ways. If you want to have a magnetic field to do work on the solar wind, than the solar wind will do work on you. If you shield your inner magnets with mu metal or superconductors, then the field won't reach outside your shield.

3) Lobbing impactors at planets is the cheapest way to add energy into a planetary system, but the process is inherently chaotic and we cannot know all the consequences of our actions in advance. After all, it was impactors on Mars which brought pieces of Mars to our Antarctica. And impactors on Luna send bits of moonrock down here to us. I'm guessing impactors on Venus might have consequences only for Mercury, but can we be sure? Impactors affect angular momentum, but do not create moons, unless we are talking about Velikovskian collisions.

Bryan

Huge impacts on Venus would almost certainly put crud into space that would find its way to Earth. However most will take millions of years to get here, be small and be spread out widely over time. So it wouldn't worry me too much.

Which brings us to the related topic of angular momentum. Does giving a planet a moon help angular momentum? I expect not, but perhaps there are better informed people on here.

Bryan

I'm not sure what you mean by help angular momentum. But a tiny amount of mass in orbit holds a giant amount of angular momentum. For example, all the planets masses added together are less than 1/1000 the mass of the sun, but the planets hold more than 99% of the solar systems angular momentum.

Mars has no moon so its obliquity precess far more than the Earth's. Even a relatively small amount of mass in an orbit will significantly stabilize the system. So I think the answer to your question is a yes.

Earth got its Moon out of a cataclysmic collision in the early solar system. ...
I wonder if there will come a time in the far future when the radius of Luna will exceed the distance between its orbital path and L1? Then it could break free.

Bryan

Luna is actually composed of mantle level rocks with high melting points. The L1 point will always stay between the Earth and Luna as is a mathematical construct based on the mass of Luna and Earth itself. Yes, people say that before the solar system ends, Luna will escape Earth orbit and enter orbit around the sun. From there it could either hit the Earth, end up in orbit again or get kicked further away.

Late collisions of massive objects were a feature of the solar system in the first 100 Ma or so. Each orbit had more than one contender for dominance because accretion was happening everywhere at once. Ever wonder how Uranus got knocked over on its side? Or why its magnetic axis is off center from its geometric axis (ie: not tilted, but displaced -- not passing through the core)? It got knocked over.

Bryan

The angle of the magnetic field has to do with the flow of charged particles and interference with the solar magnisphere. Also a planetary magnetic field is temporary. Uranus' magnetic field has built up the way it is thru recent chance and physics not because of a late impact. However late impacts are the accepted reason why it is tilted on its side.

Venus also experienced a late collision of cataclysmic proportions. Its orbital plane is slightly tilted to the solar ecliptic. It is not merely tidally locked to the Sun, but slightly retrograde. Venus got punched head-on by its Langrange companion. Consequently its energy was not added into the planet but subtracted. It got stopped nearly dead in its tracks. This depleted its full store of angular momentum. It engulfed its impactor. And the full thick silicate crust remained behind, which is another reason for the runaway greenhouse.

Bryan

If you are talking about a large body in the Venus - Sun L4 or L5 point it can't be that large. L4 & 5 are only stable if the small body in those points are 5% or less of the mass of the planet.

Having said all this, I do not know how we can impart more angular momentum to Mars or Venus through use of a moon. The only way I can see is to use iron-nickel impactors aimed off-center with the direction of rotation. With Venus, even this may not work -- because its motion is retrograde, the planet would have to be slowed, stopped, reversed, to get it spinning in the right direction. And it would be *very* unstable. And it would add heat to Venus. :shock:

Bryan

Sure this would work. You would need a huge amount of mass and Venus (for terraforming) would actually like to lose mass. But speading up the Martian day (to even out more the temperature swings) while adding mass and heat to Mars would be very welcome.

I'd like to ship all Earth's plutonium to Mars and drop it into the core. That would put starch in its sails.....

Bryan

99.9999% of the Plutonium on Earth is man made (with the exceptions of that natural breeder reactor in the Congo). There is not enough made to put a dent on Mars' core.

Thanks for the interesting post StarDreamer.

Warm regards, Rick.

StarDreamer · 2007-05-10 23:33:39

Sure, if you can move an asteriod it is not that much more
difficult to put it into an orbit. The trick is to start with a distant orbit
around Mars and then thrust for a few minutes at the furthest point of the
elipse to circularize it and make it shrink.
I don't have the numbers before me but I doubt that there are enough NMA to make
a circular moon. The moon has to get 800 to 900 km in diameter to squish into a
sphere and that requires a LOT of asteriods. However by heating them we can
make it a lot easier for them to slump together.

I know Ceres is spherical (at about these dimensions), but aren't Pallas and
Vesta nearly so as well, at about half the size. Even so, yes, a lot of matter.
Maybe if we include some FeNi and some plutonium it will heat up.

2) Can we build a moon from scratch, a la Death Star fashion? I would not want
to attempt this with Mars, but I have seen several proposals on here for Venus,
as a depository for excess carbon. I'd rather see it take excess sulphur
personally, but let's play with this. You'd want to begin with a hollow iron
sphere positioned in orbit. Perhaps magnetise the whole sphere to encourage
passive accretion by space dust? Or would solar energy rob it of its field?
Most space dust is not magnetic. The solar wind would not rob a perminant magnet
of its field. An electro-magnet would do work on the solar wind and thus would
require more energy to be constantly be fed into it.
As for shielding the magnetic field generators inside; you can't have it both
ways. If you want to have a magnetic field to do work on the solar wind, than
the solar wind will do work on you. If you shield your inner magnets with mu
metal or superconductors, then the field won't reach outside your shield.

What do you mean by "do work on the solar wind"? Are you making the distinction
between an induced magnetic field versus a permanent magnet? What is the
functional difference? I understand that some (all?) of the Jovian moons have
induced magnetic fields because of their orbits within Jupiter's magnetosphere.
Does this mean that if you took a moon out of Jupiter's gravity well that it
would lose its magnetic field? How does one make a permanent magnet??

Would a magnetised moon be "highly desirable" for the longterm viability
of terraformation? Or merely "window dressing"?

Venus has only the faintest of magnetic fields, but I'm not sure why this has
come about. Surely it has a molten core?? It has a molten surface! But liquid
metal should generate some magnetic dynamics, yes? Or is it so weak because of
its retrograde rotation? I wonder if Venus got a retrograde rotation by being
knocked upside down, in effect? I gather that Uranus has its north pole below
the ecliptic and its south pole above the ecliptic, meaning that it got knocked
over beyond just 90*. Venus may be exhibiting its original rotation, but if its
physical poles have fully reversed, then it would end up rotating in reverse.

Which brings us to the related topic of angular momentum. Does giving a planet a
moon help angular momentum? I expect not, but perhaps there are better informed
people on here.
I'm not sure what you mean by help angular momentum. But a tiny amount of mass
in orbit holds a giant amount of angular momentum. For example, all the planets
masses added together are less than 1/1000 the mass of the sun, but the planets
hold more than 99% of the solar systems angular momentum.
Mars has no moon so its obliquity precess far more than the Earth's. Even a
relatively small amount of mass in an orbit will significantly stabilize the
system. So I think the answer to your question is a yes.

Ahaaa! Yes, and this was my biggest question. Does the "help" change depending
upon the process used to acquire the moon? I see a lot of people on here talking
about lifting mass (carbon from Venus for ex) and putting it into orbit. But lifting
mass changes the angular momentum of the planet it is being lifted from. If energy
can be neither created nor destroyed in a closed system, then how does lifting matter
into orbit actually help?? I'm trying to work out whether taking matter off
one side of the ledger and adding it to the other side of the ledger actually improves
the conditions on the first side of the ledger or if it is all a zero-sum game and
ultimately pointless.

Do Phobos and Deimos contribute more than marginally to the stability of Mars? Does
the swarm of satellites buzzing around Earth affect our own orbital dynamics? (We have
lifted mass and put it into orbit!)

If we did create a moon for Venus, should it revolve in the "normal" manner or in
keeping with the retrograde rotation of the planet itself? Which manner will lead it
to increase its distance over time and which manner will lead it to fall into the Roche
Limit and be destroyed, a la Triton at Neptune or Deimos at Mars?

Earth got its Moon out of a cataclysmic collision in the early solar system. ...
I wonder if there will come a time in the far future when the radius of Luna
will exceed the distance between its orbital path and L1? Then it could break
free.
Luna is actually composed of mantle level rocks with high melting points. The
L1 point will always stay between the Earth and Luna as is a mathematical
construct based on the mass of Luna and Earth itself. Yes, people say that
before the solar system ends, Luna will escape Earth orbit and enter orbit
around the sun. From there it could either hit the Earth, end up in orbit again
or get kicked further away.

Not the L1 point within the Earth-Moon system -- I mean the L1 between
the binary Earth-Moon system and the Sun. Because the barycenter is within the Earth's
crust or mantle, but the Moon is receding, L1 to the Sun must also be receding, because
the total diameter of Earth-Moon (the Moon's orbit) is expanding. But because the centre
of mass (barycenter) is so remote from the system perimeter (outward lunar surface),
I think the distance from the Moon to the Solar L1 must be getting compressed over time.
Could L1 approach or intersect the Moon at perihelion?

Late collisions of massive objects were a feature of the solar system in the
first 100 Ma or so. Each orbit had more than one contender for dominance because
accretion was happening everywhere at once. Ever wonder how Uranus got knocked
over on its side? Or why its magnetic axis is off center from its geometric axis
(ie: not tilted, but displaced -- not passing through the core)? It got knocked
over.
The angle of the magnetic field has to do with the flow of charged particles and
interference with the solar magnisphere. Also a planetary magnetic field is
temporary. Uranus' magnetic field has built up the way it is thru recent chance
and physics not because of a late impact. However late impacts are the accepted
reason why it is tilted on its side.

Magnetic axis, not magnetic angle. On Earth, yes, the magnetic axis and the rotational
axis are not at identical angles, but both intersect at the geometric centre of Earth.
What I am saying about Uranus is that its magnetic axis does not bisect the planet into
two equal halves -- its centre is off-centre, geometrically. It's a chord.

Venus also experienced a late collision of cataclysmic proportions. Its orbital
plane is slightly tilted to the solar ecliptic. It is not merely tidally locked
to the Sun, but slightly retrograde. Venus got punched head-on by its Langrange
companion. Consequently its energy was not added into the planet but subtracted.
It got stopped nearly dead in its tracks. This depleted its full store of
angular momentum. It engulfed its impactor. And the full thick silicate crust
remained behind, which is another reason for the runaway greenhouse.
If you are talking about a large body in the Venus - Sun L4 or L5 point it can't
be that large. L4 & 5 are only stable if the small body in those points are 5%
or less of the mass of the planet.

Standard theory today regarding the creation of the Moon is that an impactor
the size of Mars collided with Earth after stratification of the core had already
taken place. This impactor is named Theia and it occupied L5 in Earth's own orbital
path. It was a competitor in the race to "clear the orbit", as the IAU would have it.

99.9999% of the Plutonium on Earth is man made (with the exceptions of that
natural breeder reactor in the Congo). There is not enough made to put a dent
on Mars' core.
Thanks for the interesting post StarDreamer.
Warm regards, Rick.

A natural breeder reactor in the Congo ?? Okay -- spill! :-)

Bryan

RickSmith · 2007-05-11 00:22:06

I know Ceres is spherical (at about these dimensions), but aren't Pallas and
Vesta nearly so as well, at about half the size. Even so, yes, a lot of matter.
Maybe if we include some FeNi and some plutonium it will heat up.

At this URL is 24 pictures of Vesta.

http://www.solarviews.com/raw/ast/vesta24.gif

I guess the question is how round is round? To my eye, it is visibly not a sphere.

What do you mean by "do work on the solar wind"? Are you making the distinction between an induced magnetic field versus a permanent magnet? What is the functional difference? I understand that some (all?) of the Jovian moons have induced magnetic fields because of their orbits within Jupiter's magnetosphere.
Does this mean that if you took a moon out of Jupiter's gravity well that it
would lose its magnetic field? How does one make a permanent magnet??

I am not saying anything at all as to what causes the magnetic field. The solar wind is made up of charged particles. They will be accelerated by the magnetic field. (Which does work on them.) The sun's own magnetic field is coupled to the solar wind. It is far from clear to me what would happen to a small moon that is given an artificial magnetic field.
Will the accelerations on it cancel out as it orbits Venus? To answer that we need to know where the poles of the magnet are, if the moon is tide locked with the planet, etc.
However, since the magnetic field is accelerating particles even a super conductor will need more power to pump up the energy lost accelerating the solar wind.

You can not make a regular moon a permanent magnet. They are not made of ferromagnetic materials.

Venus has only the faintest of magnetic fields, but I'm not sure why this has
come about. Surely it has a molten core??

It likely does. I think that the very weak magnetic field is caused by a very slow rotation relative to the sun's magnetosphere which is passes thru and (thru induction) is coupled with.

Ahaaa! Yes, and this was my biggest question. Does the "help" change depending upon the process used to acquire the moon? ...
Do Phobos and Deimos contribute more than marginally to the stability of Mars? Does the swarm of satellites buzzing around Earth affect our own orbital dynamics? (We have lifted mass and put it into orbit!)

I am confused by your question. I think we may be talking about to different things.

I'm talking about stabilizing Mars' climate by making a large moon (much more massive than Phobos and Demos). Are you talking about changing the rotation rate?

The masses of the satellites are so tiny they almost certainly have no noticeable effect on Earth.

If we did create a moon for Venus, should it revolve in the "normal" manner or in keeping with the retrograde rotation of the planet itself? Which manner will lead it to increase its distance over time and which manner will lead it to fall into the Roche Limit and be destroyed, a la Triton at Neptune or Deimos at Mars?
[/color]

We could theoretically build a moon in any orbit that we wanted with any rotation that we wish. We could put a moon into such an orbit that it would either spiral out or in. The moon could either speed up or slow down Venus' rotation rate. (Tho the kick would have to be pretty big to get it out of its tidal lock with the Earth.)

Magnetic axis, not magnetic angle. On Earth, yes, the magnetic axis and the rotational axis are not at identical angles, but both intersect at the geometric centre of Earth. What I am saying about Uranus is that its magnetic axis does not bisect the planet into two equal halves -- its centre is off-centre, geometrically. It's a chord.
[/color]

My understanding is that the Earth's magnetic axis does not go thru the center of the Earth either.

RickSmith · 2007-05-11 01:03:07

Bryan asked about the breeder reactors they found in the Congo basin.

I've read two articles in Scientific American about these.

The basic story goes like this. Uranium is not water soluble
However uranium oxide is a polar molecule so it DOES
dissolve in water. The Congo basin has plenty of uranium
ores and as these weathered, the uranium oxidized and
was carried into the water.

When they reached the mouth of the river in the delta the
rotting plant material pulled all the oxygen out of the water.
The uranium atoms lost their oxygen and being no longer
soluble, percipitated out of solution. Thus long ropy strings
of uranium atoms were concentrated in the mud.

When the mud was further compressed the uranium was
packed into an even smaller volume. A mass of uranium
in a small enough volume will turn into a reactor if it has
something to act as a moderator. Geological processes
later lifted the rocks into a series of hills.

These became the rich uranium mines in Oklo in the
Belgium Congo.

Now water acts as a neutron moderator. (Fast neutrons
are hard for other uranium atoms to catch. A moderator
slows them down.) The sediments were under the water
table so when things were wet, the water moderating the
reactor caused the reactors to go critical. This caused the
water to boil away so the reactor shut down. After several
hours the rock would cool enough for water to percolate
back into the soil and the whole thing would go critical
again boiling off the water and causing the cycle to repeat.

These reactors bred up a lot of plutonium while burning up
the U235.

They have found 15 reactors in the area. The largest in its
several million year life cooked up 8 tonnes of plutonium if
I remember correctly.

I found a reference to it at this URL:

http://adamant.typepad.com/seitz/2006/0 … of_af.html

From that URL:
"Oklo was soon staked out by Legionnaires as a cadre of
Ecole Polytechnique alumni excavated the 15 natural
reactors within the mine that had gone critical and
fissioned away the U-235 1.8 billion years before. Once
they had been dug up, analyzed and entered into the
annals of science, the last of the usual suspects were put
back on the street, and the miners of Oklo returned to
excavating the still rich ore."

Look up breeder reactors in the Congo in the Scientific
American indexes and you can get the details.

While trying to find a URL talking about this I found:

Natural Breeder Reactor Found in Australia

Apparently they have found natural reactors in Australia
as well.

The natural world is so INTERESTING. Far stranger and
wonderful than the things that people can think up in
Science Fiction novels.

Warm regards, Rick.

StarDreamer · 2007-05-11 14:54:37

Very interesting about the breeder reactors.

In answer to your question:

I am confused by your question. I think we may be talking about two different things. I'm talking about stabilizing Mars' climate by making a large moon (much more massive than Phobos and Demos). Are you talking about changing the rotation rate?

The question would be both. There are many benefits to having a moon, some attainable, some not, some delusional. Would a substantial Moon positively affect the rotation rate of Venus? Or Mars?

At a certain size, the moon would begin to give benefits in stability of the system and its seasons and climates. At a larger size, it might affect the rotation of the host planet (but my understanding of Earth's Luna was that it slowed us down by tidal friction ...??), or the magnetic field, or the planetary core.

How true is this?
Bryan

RickSmith · 2007-05-11 15:54:20

Theoretically, you could change the rotation rate with a moon almost the size of Venus itself. However, realisitcally, so much mass is not rotating very much that we are stuck with what we have got.

Rick

StarDreamer · 2007-05-11 19:09:34

Theoretically, you could change the rotation rate with a moon almost the size of Venus itself. However, realisitcally, so much mass is not rotating very much that we are stuck with what we have got.
Rick

Would it truly need to be so big? Luna changed the rotation of Earth without being the size of the Earth. We know that, at the time cyanobacteria were laying down mats in our young oceans over 500 Mya, because these mats grow one layer per day, thus making rings like tree rings, that the year had more than 450 days in it. But our orbit around the Sun was approximately the same. Therefore there were the same number of hours in a year, but fewer hours per day because Earth rotated faster. Luna has receded thousands of miles since then and the tidal friction has slowed down Earth's rotation.

I am asking these questions about angular momentum because I have read so many posts on here by people proposing to create moons for Mars or Venus with the goal of affecting angular momentum of the planet. My (limited) understanding of physics suggests to me that this will not work. But I am intrigued by the possibility, so I am trying to goad people into debating this thoughtfully to see if there is any substance to the dream at all.

What effects would a massive moon have on a planet? Would a moon just outside the Roche Limit have greater umphh than a more distant one? The tidal friction would generate heat, in both objects. What else?

Bryan

RickSmith · 2007-05-11 19:21:39

To some extent it depends on the amount of time you want to wait. Luna is 1/81st of Earth's mass but it took millions of years to add a couple hours on to our day. In billions of years it added 14 hours onto the Earth's rotation rate.

People are presumably doing this because they want a new world in the lifetime of themselves or their great grand children. Would anyone seriously plan to move a small planet's worth of mass around the solar system so that in a billion years (when the sun is even hotter) Venus will have a rotation rate of 230 Earth days (down from 243 Earth days)?

I am saying it CAN be done using the laws of physics. I don't think it will ever be done because for a fraction of that energy expenditure you can move to other solar systems and likely find easier to terraform planets.

Warm regards, Rick.

StarDreamer · 2007-05-11 20:44:22

okay -- so the physics passes the smell test but the project fails on the cost-benefit analysis. I can live with that!

So then here is the next problem for Venus -- since Venus has such a weak magnetic field, the solar wind hits the atmosphere directly, giving the planet a tail like a comet. If we reduce the weight of atmosphere there from 92 bars down to 3 or 4 bars (by whatever method -- not important), do we lose the rest to solar wind? Does Venus afford any protections to cosmic radiation, solar wind, etc, in its current state?

RickSmith · 2007-05-11 22:55:03

... since Venus has such a weak magnetic field, the solar wind hits the atmosphere directly, ... do we lose the rest to solar wind? Does Venus afford any protections to cosmic radiation, solar wind, etc, in its current state?

Mars likely started off with an atmosphere of around 3 to 5 bar and it has taken billions of years to lose it. (With much of the nitrogen being reacted by lightning into Nitrates). It will likewise take millions of years to make a noticiable dent in Venus' new 3 bar atmosphere. Very likely, the gasses from volcanoes will more than make up for any losses

Rick

Spatula · 2007-05-21 12:32:12

... since Venus has such a weak magnetic field, the solar wind hits the atmosphere directly, ... do we lose the rest to solar wind? Does Venus afford any protections to cosmic radiation, solar wind, etc, in its current state?
Mars likely started off with an atmosphere of around 3 to 5 bar and it has taken billions of years to lose it. (With much of the nitrogen being reacted by lightning into Nitrates). It will likewise take millions of years to make a noticiable dent in Venus' new 3 bar atmosphere. Very likely, the gasses from volcanoes will more than make up for any losses
Rick

Billions of years. The only thing it can lose is hydrogen, from water in the upper atmosphere, which is a slow complicated process.

Venus has only the faintest of magnetic fields, but I'm not sure why this has
come about. Surely it has a molten core?? It has a molten surface! But liquid
metal should generate some magnetic dynamics, yes? Or is it so weak because of
its retrograde rotation? I wonder if Venus got a retrograde rotation by being
knocked upside down, in effect?

The retrograde rotation was likely caused by a collision.

The weak magnetic field is not caused by the slow spin. Astronomers calculated only a marginally weaker field than Earth for a planet with Venus's size and motion. It's not the core cooling off. Considering the abundance of active volcanoes and a surface that recycles over 500 million years, that's not the problem.

You'll find that the answer is actually very intricate. A magnetic field doesn't require a hot core, but one with a significant temperature disparity between the core and inner mantle. The core has to be losing heat, and at the moment it isn't. The mantle doesn't have currents that can bring cold outer-mantle material in contact with the core. The planet's thick crust causes it to lose heat through different methods. There are geological formations all over the surface that illustrate this.

Mars, similarly, doesn't have a 'cold core'. The core is actually quite hot, probably liquid, but so much of the mantle has cooled and hardened that the planet's geologic activity is now down to one active volcano, on the entire planet, at any given time, if any. Again no currents, so again no magnetic dynamo.

StarDreamer · 2007-06-05 09:35:27

Venus has only the faintest of magnetic fields, but I'm not sure why this has come about. Surely it has a molten core?? It has a molten surface! But liquid metal should generate some magnetic dynamics, yes? Or is it so weak because of its retrograde rotation? I wonder if Venus got a retrograde rotation by being knocked upside down, in effect?
The retrograde rotation was likely caused by a collision.

Yes, exactly. But the distinction I am trying to make is between two different ways of getting retrograde motion. One would be a collision which absorbs the prograde spin, slows it, stops it, reverses its direction to Rx, all while leaving the poles aligned properly. The other way is to knock the planet upside down (Uranus got knocked more than halfway there!) while leaving its prograde spin intact. A planet with prograde spin but its north pole pointing down will appear to be spinning Rx in comparison to the other planets.

You'll find that the answer is actually very intricate. A magnetic field doesn't require a hot core, but one with a significant temperature disparity between the core and inner mantle. The core has to be losing heat, and at the moment it isn't. The mantle doesn't have currents that can bring cold outer-mantle material in contact with the core. The planet's thick crust causes it to lose heat through different methods. There are geological formations all over the surface that illustrate this.

So Venus has yet another vicious cycle working against us -- if the atmosphere cools enough to let heat escape, then the magnetosphere gets stronger and it retains its atmosphere more tightly against the solar wind, leading to reheating. It has reached equilibrium in this direction.

~~ Bryan

Spatula · 2007-06-05 11:25:12

So Venus has yet another vicious cycle working against us -- if the atmosphere cools enough to let heat escape, then the magnetosphere gets stronger and it retains its atmosphere more tightly against the solar wind, leading to reheating. It has reached equilibrium in this direction.

I'm not going to speculate about the magnetosphere. It does seem possible that removing the thick atmosphere will cause the planet's internal heat loss to speed up. Really, we'll know more about this stuff when the barrage of spacecraft studying Venus map its temperatures and internal structure better.

This shouldn't affect temperatures, so far as I know.

StarDreamer · 2007-06-05 15:48:00

Maybe Venus is an example of planetary "arrested development" -- it looks how Earth might have looked soon after its birth, with all its heat trapped under heavy gas. It was carbon-lifeforms which got us out of that. Mars, on the other hand, is an example of "accelerated ageing", where it had a promising start but flew through that phase and was left with nothing to show for it.

~~Bryan

StarDreamer · 2007-06-12 06:08:54

If the majority of the mass in the Asteroid belt (by which I mean Ceres, Vesta, Juno, Pallas) could somehow be merged into the planet Mars, probably by collision, how would the orbits of Mars, Earth and Venus be affected?

This is a big "what if", and, yes, pure science fiction. Please don't question the methodology -- this is just a thought experiment. Assuming we had the technology and the resources to clear the Belt and add its mass to Mars, then what else happens?

I am guessing that Mars migrates into a slightly higher orbit around the Sun and the Martian year picks up a few days, unless Mars itself also gains speed and inertia with the mass and the year remains nearly constant? What would be the net gain in Mars gravity?

If Mars moves to a higher orbit, then does Earth also? And Venus? My guess is that all inner planets expand their orbits somewhat and get longer years, but that Jupiter does not budge. Is it enough to change our climates? Does this completely destabilise the inner solar system or do the planets eventually sort themselves out?

Conversely, if we could give Venus a moon, somehow, does it nudge Earth and Mars outwards in their orbits?

X · 2007-06-14 22:37:23

I was going to answer your question about the change to Mars' gravity, but some punk apparently removed the formula for grvaity from wikipedia. Now the only formula I can find requires a radius which isn't really something I want to be playing with with non-spherical bodies is it instead of volume? If anyone has the formula for gravity using volume could you post it here.

StarDreamer · 2007-06-15 00:47:23

What I was trying to express has something to do with the conservation of mass and energy. When protoEarth got its big impactor, which birthed Luna like Eve from Adam's rib, did that also prompt Earth to seek a higher orbit about the Sun? And if Earth did take a higher orbit, did that also nudge Mars out (into the deep freeze) and give Venus some breathing space as well to walk out, by means of orbital resonance (Bode's Law)? If so, then that event (because Earth picked up significant mass and angular momentum from that impact) would also have squeezed the asteroid belt, nudged Mars out of the biozone and perhaps allowed Venus to cool a bit. This might explain the dearth of water at Venus too -- because the planet formed closer to the Sun originally.

Antius · 2007-06-15 01:40:06

On Mars, the Tharsis region provides the balancing effect that is provided by the moon on Earth. The Tharsis bulge always aligns itself on the equator, effectively stabilising the planet.

Antius · 2007-06-15 01:40:31

On Mars, the Tharsis region provides the balancing effect that is provided by the moon on Earth. The Tharsis bulge always aligns itself on the equator, effectively stabilising the planet.

StarDreamer · 2007-06-19 05:47:17

On Mars, the Tharsis region provides the balancing effect that is provided by the moon on Earth. The Tharsis bulge always aligns itself on the equator, effectively stabilising the planet.

How great a deformation of the sphere does the Tharsis bulge create? I know this is where the volcanoes are, but is there further deformation additional to the volcano mounts?

Is there a large impact crater at the antipodes?

On Earth, I always suspected things like the Siberian Traps or the Deccan Traps might be triggered by impacts on the opposing side of the planet. The waves of energy will refocus there. It is known, for example, that atomic bomb tests by France in the south Pacific would provoke a small earthquake in France itself because it is at their antipodes.

RickSmith · 2007-06-21 13:43:37

Is there a large impact crater at the antipodes [of Tharsis]?

Hellas Impact Basin is opposite Tharsis.

Warm Regards, Rick

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