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Earth like worlds orbiting gas giants

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James Nicoll

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Aug 26, 2001, 11:40:36 AM8/26/01
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In article <b3030854.01082...@posting.google.com>,
Johnny1A <sherm...@hotmail.com> wrote:
>I thought I'd put this up for discussion. A theme I've run across
>more than once is the concept of an Earth-like world (large body,
>oceans and oxygen-nitrogen atmosphere, continents, etc) in orbit
>around a large gas giant rather than directly orbiting a system's
>primary.
>
>I ran across it again just recently in an RPG setting, and it got me
>to thinking again. Just how plausible is this concept?
>
>Obviously, it wouldn't work in our Solar System, the gas giants are
>too far out, and Jupiter, the most likely candidate to have a big
>enough satellite, is shrouded in intense radiation belts.
>
>But that doesn't mean other Systems _have_ to be like Sol. What would
>be required for such a world to exist and survive? What are the
>biggest stumbling blocks?

You might want to ask this in rec.arts.sf.science. I have taken
the liberty of cross-posting and setting follow-ups to that group.

The good news is, we now know of a lot of gas giants much,
much closer to their stars than Jupiter is to Sol. Apparently gas
giants can form in the outer systems and then migrate inward. This
would be seriously bad news for any terrestrial planets in the
inner system except I seem to recall gas giants form much faster
than terrestrial style planets, so what might be happening is the
gas giant preventing the formation of terrestrial planets by hoovering
up the inner system material rather than swatting each world like a
bug on the way in. More material for the moons to form out of, anyway.

Most of the close orbiting gas giants we know are very close
indeed, much closer than Mercury is to Sol. This is not useful for
your puroses but this may be due to a bias in the method we use to
detect extra-solar worlds and there certainly is at least one gas
giant orbiting in its star's liquid water zone. If you google on
extra-solar planets, you should get a useful list of gas giants which
might have potentially bearing moons.


John Savard

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Aug 27, 2001, 1:29:35 AM8/27/01
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On 26 Aug 2001 11:40:36 -0400, jdni...@panix.com (James Nicoll)
wrote, in part:

>>A theme I've run across
>>more than once is the concept of an Earth-like world (large body,
>>oceans and oxygen-nitrogen atmosphere, continents, etc) in orbit
>>around a large gas giant rather than directly orbiting a system's
>>primary.

>>Just how plausible is this concept?

>>Obviously, it wouldn't work in our Solar System, the gas giants are
>>too far out, and Jupiter, the most likely candidate to have a big
>>enough satellite, is shrouded in intense radiation belts.

> The good news is, we now know of a lot of gas giants much,


>much closer to their stars than Jupiter is to Sol.

>so what might be happening is the


>gas giant preventing the formation of terrestrial planets by hoovering
>up the inner system material rather than swatting each world like a
>bug on the way in. More material for the moons to form out of, anyway.

>there certainly is at least one gas


>giant orbiting in its star's liquid water zone.

As it is somewhat cumbersome to subscribe to a new newsgroup, I am
posting this where the original poster is likely to see it.

As you note, and as mentioned in recent news stories, gas giants have
been discovered in other solar systems closer to their primaries than
Jupiter is to our Sun.

This takes care of one problem.

But two problems remain:

1) Won't such a gas giant have a dangerous radiation belt, like that
of Jupter, and

2) Won't such a gas giant leave so little material in its orbit that
all its moons will be too small to hold an atmosphere?

I don't think we know enough to completely eliminate such a scenario,
and it is attractive for SF, since it allows multiple habitable
planets to be mutually reachable by conventional rocketry instead of
by FTL transport. But it at least appears to me that this will be an
uncommon situation.

And a gas giant looming in the night sky is certainly *impressive* -
even just for a cover painting, and far more so in a movie.

Thus, I see this scenario as being used in SF, despite it being a bit
on the improbable side.

John Savard
http://home.ecn.ab.ca/~jsavard/index.html
http://plaza.powersurfr.com/jsavard/other/slrint.htm

Conrad Hodson

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Aug 27, 2001, 6:16:27 AM8/27/01
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On Mon, 27 Aug 2001, John Savard wrote:


> >>Obviously, it wouldn't work in our Solar System, the gas giants are
> >>too far out, and Jupiter, the most likely candidate to have a big
> >>enough satellite, is shrouded in intense radiation belts.
>

> But two problems remain:
>
> 1) Won't such a gas giant have a dangerous radiation belt, like that
> of Jupter, and
>
> 2) Won't such a gas giant leave so little material in its orbit that
> all its moons will be too small to hold an atmosphere?

Why would you assume that a gas giant must have airless and irradiated
moons on the Jovian model, when our own system has a major gas giant with
a far less hostile radiation environment, and a moon with a massive
atmosphere?

Until very recently, planetary science has been limited to one solar
system and nine planets. Go back and read the very authoritative
pronouncements that gas giants "of course" couldn't form close to stars,
that the model of our own system with rocky worlds neatly inside icy/gas
giant worlds was so logical that it had to prevail everywhere.

Now look at the evidence that's come in about the actual positions of
Jovian-scale worlds. It's just one more example of how close to the
beginning point we are in learning about our neighborhood. IMHO, we need
to theorize of course, but pretending we know for sure about the
generalities of planetary formation at this early time is utter folly and
hubris.

Conrad Hodson

Mike Combs

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Aug 27, 2001, 2:31:22 PM8/27/01
to
Conrad Hodson wrote:
>
> Why would you assume that a gas giant must have airless and irradiated
> moons on the Jovian model, when our own system has a major gas giant with
> a far less hostile radiation environment, and a moon with a massive
> atmosphere?

That's an interesting point to ponder. Since we now know that there's
no reason to rule out gas giants close to the sun, what if Saturn had
been in our position in the solar system? Would Titan have been a world
upon which life could have evolved?

What a lovely sky such a world would have had...

--


Regards,
Mike Combs
----------------------------------------------------------------------
Arthur: You know, it's at times like this... that I really wish I'd
listened to what my mother told me when I was young.
Ford: Why, what did she tell you?
Arthur: I don't know, I didn't listen!

Frank Henriquez

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Aug 27, 2001, 6:38:05 PM8/27/01
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In article <3B8A91FA...@nospam.comchgnospam2ti>,
Mike Combs <mike...@nospam.comchgnospam2ti> wrote:

>That's an interesting point to ponder. Since we now know that there's
>no reason to rule out gas giants close to the sun, what if Saturn had
>been in our position in the solar system? Would Titan have been a world
>upon which life could have evolved?

The main reason that Titan has a thick atmosphere is BECAUSE it is so
far away from the Sun. If you move Titan to 1AU from the Sun, it
wouldn't be able to hold on to its atmosphere as it warms up (Titan's
surface gravity is a little less than our Moon's).

If an icy object like Titan had formed near the Earth's orbit, the
atmosphere would have escaped, and most of the ices melted and
sublimated away. The result would have been a small, rocky, airless moon.

Frank

--
Frank Henriquez Programmer/Analyst Jules Stein Eye Institute, UCLA
fr...@ucla.edu http://www.bol.ucla.edu/~frank/index.htm

John Savard

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Aug 27, 2001, 7:52:46 PM8/27/01
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On Mon, 27 Aug 2001 03:16:27 -0700, Conrad Hodson <con...@efn.org>
wrote, in part:

>IMHO, we need
>to theorize of course, but pretending we know for sure about the
>generalities of planetary formation at this early time is utter folly and
>hubris.

All I said was that such planets seemed to be unlikely - not that I
knew they were impossible. Not even all Jupiter's moons are within its
radiation belts, and, as you note, there are smaller gas giants.

Miguel Farah F.

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Aug 27, 2001, 7:47:33 PM8/27/01
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In rec.arts.sf.science John Savard <jsa...@ecn.ab.sblok.ca.nowhere> wrote:
>[...]

>1) Won't such a gas giant have a dangerous radiation belt, like that
>of Jupter, and

>2) Won't such a gas giant leave so little material in its orbit that
>all its moons will be too small to hold an atmosphere?

The rocky planet wouldn't necessarily have to form near the gas giant.
It could for at a farther orbit and then -still in the early age of
the solar system- be moved downward (by a big meteorite hit, for
example) until the gas giant traps it.

Yes, implausible, but still possible. Me thinks.

--
MIGUEL FARAH // mig...@nn.cl
#include <disclaimer.h> // http://www.nn.cl/~miguel
<*>
"- What do you think of the world situation?
- I couldn't think of a better place for it!"
- B.C.

Johnny1A

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Aug 27, 2001, 11:09:24 PM8/27/01
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Conrad Hodson <con...@efn.org> wrote in message news:<Pine.GSU.4.21.010827...@garcia.efn.org>...

Good point! I've been thinking about this since I first posted it.
The closest thing to 'worlds' in our Solar System orbiting a gas giant
are the Galilean Satellites. Their composition is interesting: Io is
rocky/sulfur/etc, Europa seems to be mostly rock with a deep ice
layer/oecan, and Ganymede and Callisto are mixtures (apparently) of
rock and ice with a high percentage of ices.

I assume that if Jupiter orbited Sol at 1 A.U., the ices would be gone
or melted from these bodies, the we'd have rockballs comparable to
Luna. I might be wrong, though, I'm not up on the latest results from
the probes and telescopes.

For an Earth-like world, we need iron, nickel, and oxides of various
sorts, with a very low but vitally important percentage of volatiles
such as water. We might also need a bigger gas giant than Jupiter.
What effect would it have on Jupiter, if any, if a body the size of
Earth orbited in place of one of the Galilean satellites?

We have to assume that our hypothetical world orbits farther out than
at least Io or Europa, I _assume_, because of the effect of tides.
Tidal stess is usually credited with being the energy source for the
volcanoes on Io.

One more factor: wouldn't our Earth-like world tend to end up
rotation-locked, with one face always pointing toward the gas giant?
I'm not sure about that.

Shermanlee

Mike Williams

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Aug 28, 2001, 2:04:08 AM8/28/01
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Wasn't it John Savard who wrote:

>All I said was that such planets seemed to be unlikely - not that I
>knew they were impossible. Not even all Jupiter's moons are within its
>radiation belts, and, as you note, there are smaller gas giants.

Not only that, but it's quite possible that life could adapt to living
in considerably higher levels of radiation than we can. Our DNA repair
mechanisms have evolved to be only as good as they need to be to handle
the levels of radiation that naturally occur on Earth. Even so, there
are some microbes that thrive inside nuclear reactors at high radiation
levels. Life on a high radiation world would tend to quickly evolve
better repair mechanisms for its DNA (or alien equivalent).

--
Mike Williams
Gentleman of Leisure

Mike Williams

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Aug 28, 2001, 2:35:40 AM8/28/01
to
Wasn't it Johnny1A who wrote:

>Good point! I've been thinking about this since I first posted it.
>The closest thing to 'worlds' in our Solar System orbiting a gas giant
>are the Galilean Satellites. Their composition is interesting: Io is
>rocky/sulfur/etc, Europa seems to be mostly rock with a deep ice
>layer/oecan, and Ganymede and Callisto are mixtures (apparently) of
>rock and ice with a high percentage of ices.
>
>I assume that if Jupiter orbited Sol at 1 A.U., the ices would be gone
>or melted from these bodies, the we'd have rockballs comparable to
>Luna. I might be wrong, though, I'm not up on the latest results from
>the probes and telescopes.
>
>For an Earth-like world, we need iron, nickel, and oxides of various
>sorts, with a very low but vitally important percentage of volatiles
>such as water. We might also need a bigger gas giant than Jupiter.

Say, for example, the planet that orbits HD28185, which is 5.6 times the
mass of Jupiter and has a circular orbit right in the middle of the
"habitable zone" of a star that's very similar to Sol.

>What effect would it have on Jupiter, if any, if a body the size of
>Earth orbited in place of one of the Galilean satellites?

Slightly different tides, but nothing to worry about.

>We have to assume that our hypothetical world orbits farther out than
>at least Io or Europa, I _assume_, because of the effect of tides.
>Tidal stess is usually credited with being the energy source for the
>volcanoes on Io.

Io is tidally locked to Jupiter and in a circular orbit, so Jupiter
alone doesn't cause any tidal stress on Io. The stresses are due to the
other moons in the system. If Io were the only moon of Jupiter there
would be no volcanism.

Note that Mimas is much closer to Saturn than Io is to Jupiter, and
experiences a tidal force from Saturn that's 3.55 times the Jupiter
tidal force on Io. No volcanism has been detected on Mimas.

>One more factor: wouldn't our Earth-like world tend to end up
>rotation-locked, with one face always pointing toward the gas giant?
>I'm not sure about that.

It certainly would be tidally locked, all the moons of Jupiter, Saturn
and Uranus are. So the day length is governed by the orbital period. We
wouldn't want the moon to be very far from the gas giant, or the days
would be so long that the temperature would stray outside the habitable
zone. We can orbit faster, if the gas giant is more massive, and fairly
reasonable day lengths could be obtained at the distances of Titan or
Ganymede.

Conrad Hodson

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Aug 28, 2001, 5:56:14 AM8/28/01
to
On 27 Aug 2001, Johnny1A wrote:
>
> I assume that if Jupiter orbited Sol at 1 A.U., the ices would be gone
> or melted from these bodies, the we'd have rockballs comparable to
> Luna. I might be wrong, though, I'm not up on the latest results from
> the probes and telescopes.

One would assume so. Io would probably look much the same as now, Europa
might look like a smaller version of Luna with a bit of
volcanism. Callisto and Ganymede might be comparable to Luna (depending
on their actual rocky content.)

>
> For an Earth-like world, we need iron, nickel, and oxides of various
> sorts, with a very low but vitally important percentage of volatiles
> such as water. We might also need a bigger gas giant than Jupiter.
> What effect would it have on Jupiter, if any, if a body the size of
> Earth orbited in place of one of the Galilean satellites?
>

Next to none. Especially if we orbited well out from the gas giant, which
would be desirable for other reasons.

> We have to assume that our hypothetical world orbits farther out than
> at least Io or Europa, I _assume_, because of the effect of tides.
> Tidal stess is usually credited with being the energy source for the
> volcanoes on Io.

IIRC, this isn't just a matter of distance from Jupiter but of
perterbations by the other satellites as they pass. If this is true, Io
wouldn't be tidally heated if it were Jupiter's only large satellite, no
matter how close to Jupiter it was.

>
> One more factor: wouldn't our Earth-like world tend to end up
> rotation-locked, with one face always pointing toward the gas giant?
> I'm not sure about that.

If it were anywhere close to the gas giant, you'd expect it to
tide-lock. (However, my hubris alarm just went off. All through my
youth, every astronomy book claimed Mercury had been tide-locked by the
Sun, too. Turns out it actually has fallen into a resonant situation, and
sunlight does sweep the whole surface.) Until we can say why Luna and the
Galilean satellites tidelock, and why Mercury did not, I'd hesitate to get
dogmatic about what would happen to an Earth-sized satellite of a gas
giant.

OTOH, that might not be a disaster in itself. Such a world might end up
with a long day-night cycle (equal to the orbital period, in the case of
tide-lock) but that wouldn't have to be a deal-breaker for a biosphere.
The _sun_ would still rise and set--just the gas giant would be fixed in
the sky.


Conrad Hodson

Mike Combs

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Aug 28, 2001, 1:58:59 PM8/28/01
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Frank Henriquez wrote:
>
> The main reason that Titan has a thick atmosphere is BECAUSE it is so
> far away from the Sun.

Thanks for reminding me of something that I should have remembered
before posting.

Mike Williams

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Aug 28, 2001, 3:03:08 PM8/28/01
to
Wasn't it Conrad Hodson who wrote:
>On 27 Aug 2001, Johnny1A wrote:
>> One more factor: wouldn't our Earth-like world tend to end up
>> rotation-locked, with one face always pointing toward the gas giant?
>> I'm not sure about that.
>
>If it were anywhere close to the gas giant, you'd expect it to
>tide-lock. (However, my hubris alarm just went off. All through my
>youth, every astronomy book claimed Mercury had been tide-locked by the
>Sun, too. Turns out it actually has fallen into a resonant situation, and
>sunlight does sweep the whole surface.) Until we can say why Luna and the
>Galilean satellites tidelock, and why Mercury did not, I'd hesitate to get
>dogmatic about what would happen to an Earth-sized satellite of a gas
>giant.

The "why" is pretty well known. The tidal force on the Moon and the
Galilean satellites is considerably stronger than that on Mercury. I
guess that we just didn't know where the tidal strength cut off point
was likely to be for tidal locking.

The tidal force is proportional to the mass of the primary and inversely
proportional to the cube of the distance.

Let's work in units of Earth mass and Lunar distance, and call the tidal
force acting on the Moon from Earth 1.0, then:-

Jupiter's mass is about 317.9
Sun's mass is about 332946
Io distance from Jupiter is about 1.10
Europa distance is about 1.75
Ganymede distance is about 2.79
Calisto distance is about 4.98
Mercury distance from Sun is about 150.8

Using: Tide ~= Mass/distance^3

Io tidal force is 317.9/1.10^3 = 239 times that at the Moon
Europa tidal force = 59.6
Ganymede tidal force = 14.7
Calisto tidal force = 2.71
Moon tidal force = 1.0 by definition
Mercury tidal force = 0.097

So the tidal force at Mercury from the Sun is considerably weaker than
that at the Moon or at the Galilean satellites.

The idea that Mercury was tide locked was an observational error rather
than a failure of theory.

Michael Ash

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Aug 28, 2001, 4:19:48 PM8/28/01
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In article <Pine.GSU.4.21.01082...@garcia.efn.org>,
Conrad Hodson <con...@efn.org> wrote:

>OTOH, that might not be a disaster in itself. Such a world might end up
>with a long day-night cycle (equal to the orbital period, in the case of
>tide-lock) but that wouldn't have to be a deal-breaker for a biosphere.
>The _sun_ would still rise and set--just the gas giant would be fixed in
>the sky.

What effect would an huge, very visible, stationary landmark in the sky
have on developing civilization on such a world?

Navigation would be a lot easier in the hemisphere that can see the gas
giant, if nobody had compasses. Would a world like this have a magnetic
field strong enough to use compasses? If not, then the giant will be
even more useful that way. If compasses are practical, then I could see
the far side civilizations discovering them and using them, with
seafarers who kept exclusively to the near side scorning them. This
could make technology develop faster on the far side. Without compasses,
the ease of seagoing trade on the near side could make technology
develop faster there.

There are other effects possible as well. I could imagine huge holy wars
between people who worshipped the giant and people who thought it was
just imaginary, since they had never seen it. Would astronomy be helped
by the presence of a huge reminder of things existing in the sky, or
would the correct idea of the mechanics of the solar system be impeded
by it?

--
"From now on, we live in a world where man has walked on the moon.
And it's not a miracle, we just decided to go." -- Jim Lovell

Mike Ash - <http://www.mikeash.com/>, <mailto:ma...@mikeash.com>

Thomas Palm

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Aug 28, 2001, 4:36:42 PM8/28/01
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Michael Ash wrote:
> There are other effects possible as well. I could imagine huge holy wars
> between people who worshipped the giant and people who thought it was
> just imaginary, since they had never seen it.

Wouldn't it be sort of difficult for the non-believers to make
an invasion of the land of the worshippers without seeing the
very thing they didn't believe in? Wouldn't it be easier for the
worshippers to invite leaders of the far side to come witness
the giant in the sky rather than invade them?

Holy wars more or less require beliefs that can't easily be
verifed.

> Would astronomy be helped
> by the presence of a huge reminder of things existing in the sky, or
> would the correct idea of the mechanics of the solar system be impeded
> by it?

It might block out much of the sky making it hard for observers
facing it to see other stellar objects which would certainly
slow development of astronomy. On the other hand there is still
the far side where that kind of discoveries could be made.
Still it would probably take a long time before the realization
that the small moving lights in the sky were equivalent to the
huge, stationary giant in their sky.

Mike Williams

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Aug 28, 2001, 4:33:12 PM8/28/01
to
Wasn't it Michael Ash who wrote:

>Would astronomy be helped
>by the presence of a huge reminder of things existing in the sky, or
>would the correct idea of the mechanics of the solar system be impeded
>by it?

They may be able to observe that the shadow transits of their own moon
looks just like those of the other moons of the gas giant. In which case
they'd be pretty quick to get the idea that they live on something
that's just like the other moons.

ROU I'll Zapp the First Six, You get the Rest

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Aug 28, 2001, 6:26:49 PM8/28/01
to
In message <Pine.GSU.4.21.01082...@garcia.efn.org>,
Conrad Hodson <con...@efn.org> writes
<snip>

>> One more factor: wouldn't our Earth-like world tend to end up
>> rotation-locked, with one face always pointing toward the gas giant?
>> I'm not sure about that.
>
>If it were anywhere close to the gas giant, you'd expect it to
>tide-lock. (However, my hubris alarm just went off. All through my
>youth, every astronomy book claimed Mercury had been tide-locked by the
>Sun, too. Turns out it actually has fallen into a resonant situation, and
>sunlight does sweep the whole surface.) Until we can say why Luna and the
>Galilean satellites tidelock, and why Mercury did not, I'd hesitate to get
>dogmatic about what would happen to an Earth-sized satellite of a gas
>giant.
>
>OTOH, that might not be a disaster in itself. Such a world might end up
>with a long day-night cycle (equal to the orbital period, in the case of
>tide-lock) but that wouldn't have to be a deal-breaker for a biosphere.
>The _sun_ would still rise and set--just the gas giant would be fixed in
>the sky.

What about the eclipses that you'd get every day around midday (on one
side of the planet only), assuming the plane of the moon's orbit was
roughly in the same plane as the sun/planet? Siesta time?

--
ROU I'll Zapp the First Six, You get the Rest

Johnny1A

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Aug 29, 2001, 12:15:10 AM8/29/01
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Michael Ash <ma...@mikeash.com> wrote in message news:<mail-20D001.1...@news-server.wi.rr.com>...

> In article <Pine.GSU.4.21.01082...@garcia.efn.org>,
> Conrad Hodson <con...@efn.org> wrote:
>
> >OTOH, that might not be a disaster in itself. Such a world might end up
> >with a long day-night cycle (equal to the orbital period, in the case of
> >tide-lock) but that wouldn't have to be a deal-breaker for a biosphere.
> >The _sun_ would still rise and set--just the gas giant would be fixed in
> >the sky.
>
> What effect would an huge, very visible, stationary landmark in the sky
> have on developing civilization on such a world?
>
> Navigation would be a lot easier in the hemisphere that can see the gas
> giant, if nobody had compasses. Would a world like this have a magnetic
> field strong enough to use compasses? If not, then the giant will be
> even more useful that way. If compasses are practical, then I could see
> the far side civilizations discovering them and using them, with
> seafarers who kept exclusively to the near side scorning them. This
> could make technology develop faster on the far side. Without compasses,
> the ease of seagoing trade on the near side could make technology
> develop faster there.

That raises another question: electrical and magnetic effects. Even
if the gas giant doesn't have a huge radiation belt, (I think it
would, but as Conrad has pointed out we lack a lot of data), there are
probably going to be some interesting magnetic interactions between
our hypothetical world and the gas giant. At the very least, I
suspect that you would get spectacular auroras.
I'm not sure what effect, if any, all this would have on magnetic
compasses.

Further, if the gas giant orbits closer to its star than Jupiter does
to Sol, we'd have the potential for stronger stellar-planetary
interaction. IIRC, I think a theory was floated a year or two ago
that a big gas giant orbiting near a star could lead to a situation
with periodic large flares, for example.

If our civilization is ocean-travelling, I suspect their tides and
currents are going to be rather more complex than Earth's, especially
if there are other large satellary bodies.

> There are other effects possible as well. I could imagine huge holy wars
> between people who worshipped the giant and people who thought it was
> just imaginary, since they had never seen it. Would astronomy be helped
> by the presence of a huge reminder of things existing in the sky, or
> would the correct idea of the mechanics of the solar system be impeded
> by it?

Could go either way, and if there are multiple civiliations it would
probably do both. By the time the telescope is invented, there will
be a lot more interesting things to see than from Earth! Even a
small, primitive telescope could probably reveal a _lot_ of detail
about the other big satellites and the gas giant itself.

As a side-effect, when technology reached space-flight levels, there'd
be a lot of potential targets, unless our world is the only satellite.
(That strikes me as unlikely, but like Conrad, I have to note that we
lack data again.) On the other hand, that huge gravity field from the
gas giant might make fuel requirements somewhat of a barrier. Is it
cheaper in fuel to go from one moon of Jupiter to another than it
would be to go from Earth to the Moon?

Shermanlee

Michael Ash

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Aug 29, 2001, 2:33:04 AM8/29/01
to
In article <b3030854.01082...@posting.google.com>,
sherm...@hotmail.com (Johnny1A) wrote:

[other interesting stuff snipped]

>As a side-effect, when technology reached space-flight levels, there'd
>be a lot of potential targets, unless our world is the only satellite.
> (That strikes me as unlikely, but like Conrad, I have to note that we
>lack data again.) On the other hand, that huge gravity field from the
>gas giant might make fuel requirements somewhat of a barrier. Is it
>cheaper in fuel to go from one moon of Jupiter to another than it
>would be to go from Earth to the Moon?

We do have quite a bit of data, actually. Four gas giants, albeit none
very close to the sun. Each giant in our system has a large number of
bodies of various sizes orbiting it. Unless being close to the sun would
somehow clear out everything except our one planet, I would definitely
expect a bunch of other moons.

For maneuver, look at Galileo. It's been zipping between all the major
moons of Jupiter for several years, and it can't have carried that much
fuel up there. As long as the civilization understands how to do weird
gravitational assists, which seems likely in their situation, then they
ought to be able to get around pretty easily.

Conrad Hodson

unread,
Aug 29, 2001, 9:33:02 AM8/29/01
to

Very true--though it only works if there _are_ other large moons. Right
now we don't know whether gas giants can have satellites as big as
Earth; certainly none of the ones in this system do. Or perhaps an
Earth-sized satellite snaffles up so many planetesimals during its
accretion that the only other satellites are small and distant--too small
to make transit shadows.

The answers to this sort of thing may become clear if we get telescopes
good enough to really observe small bodies in other systems. Until then,
we just don't know.

Conrad Hodson

Brian McGuinness

unread,
Aug 29, 2001, 12:27:43 PM8/29/01
to
"Miguel Farah F." <mig...@nn.cl> wrote in message news:<9mem6l$a8d$9...@heriberto.webhost.cl>...

> In rec.arts.sf.science John Savard <jsa...@ecn.ab.sblok.ca.nowhere> wrote:
> >[...]
> >1) Won't such a gas giant have a dangerous radiation belt, like that
> >of Jupter, and
>
> >2) Won't such a gas giant leave so little material in its orbit that
> >all its moons will be too small to hold an atmosphere?
>
> The rocky planet wouldn't necessarily have to form near the gas giant.
> It could for at a farther orbit and then -still in the early age of
> the solar system- be moved downward (by a big meteorite hit, for
> example) until the gas giant traps it.
>
> Yes, implausible, but still possible. Me thinks.

Early in the history of the planetary system, while the gas giant
still had an extended envelope of gas surrounding it, a passing
terrestrial planet might be captured into orbit with the gas envelope
slowing it enough to be captured. If it was captured into a large
enough orbit the radiation might not be a problem (I don't know what
the radiation levels are in Jupiter's magnetotail). A possible
problem is the planet breaking up during capture. Jupiter's outermost
eight moons are thought to be two bodies that were captured and broke
up.

By the way, a good source for information on extrasolar planets is
http://exoplanets.org/

--- Brian

Frank Henriquez

unread,
Aug 29, 2001, 3:53:31 PM8/29/01
to

...

>As a side-effect, when technology reached space-flight levels, there'd
>be a lot of potential targets, unless our world is the only satellite.
> (That strikes me as unlikely, but like Conrad, I have to note that we
>lack data again.) On the other hand, that huge gravity field from the
>gas giant might make fuel requirements somewhat of a barrier. Is it
>cheaper in fuel to go from one moon of Jupiter to another than it
>would be to go from Earth to the Moon?

The delta-v required to travel between Jupiter's inner moons is similar
to the delta-v required to travel between the Earth and Mars or Venus.
Not easy!

Paul F. Dietz

unread,
Aug 29, 2001, 7:24:32 PM8/29/01
to
Frank Henriquez wrote:

> The delta-v required to travel between Jupiter's inner moons is similar
> to the delta-v required to travel between the Earth and Mars or Venus.
> Not easy!

Unless you use electrodynamic tethers.

I imagine a civilization on one of these moons generating their
power from the motion of their moon through the planet's magnetic
field. It's a shame our sun's magnetic field isn't that strong
(unless that would cause superflares, in which case Never Mind.)

Paul

Joseph Michael Bay

unread,
Aug 29, 2001, 10:33:12 PM8/29/01
to
Conrad Hodson <con...@efn.org> writes:

>> sorts, with a very low but vitally important percentage of volatiles
>> such as water. We might also need a bigger gas giant than Jupiter.
>> What effect would it have on Jupiter, if any, if a body the size of
>> Earth orbited in place of one of the Galilean satellites?

>Next to none. Especially if we orbited well out from the gas giant, which
>would be desirable for other reasons.

You've met my dad?

--
Joe Bay
Stanford University
Department of Something or Other

Johnny1A

unread,
Aug 29, 2001, 10:52:17 PM8/29/01
to
brian.b.m...@lmco.com (Brian McGuinness) wrote in message news:<3fd57b8c.01082...@posting.google.com>...

It the planet was captured, it's rotation rate, orbital inclination,
and axial tilt might all be odd, compared to the 'natural' satellites
of the gas giant. I wonder how long it would take the gas giant to
slow a 24-hour day into whatever final state it would settle in?

Shermanlee

John Savard

unread,
Aug 30, 2001, 2:07:10 AM8/30/01
to
On 28 Aug 2001 21:15:10 -0700, sherm...@hotmail.com (Johnny1A)
wrote, in part:

>Is it
>cheaper in fuel to go from one moon of Jupiter to another than it
>would be to go from Earth to the Moon?

The reason this idea is so attractive is because it is *definitely*
cheaper to go from one moon to another than to travel to another solar
system. This scenario allows multiple planets in the habitable zone
(in addition to satellites, there could be one at each Trojan point of
the gas giant - and that, at least, has fewer obstacles, as well).

Erik Max Francis

unread,
Aug 30, 2001, 2:33:35 AM8/30/01
to
John Savard wrote:

> The reason this idea is so attractive is because it is *definitely*
> cheaper to go from one moon to another than to travel to another solar
> system. This scenario allows multiple planets in the habitable zone
> (in addition to satellites, there could be one at each Trojan point of
> the gas giant - and that, at least, has fewer obstacles, as well).

I doubt a terrestrial planet in a star-jovian Trojan point would be
stable. The arrangement is only stable if the secondary (the jovian) is
about 25 times less massive than the primary (the star), but in any case
the tertiary (the terrestrial planet) is taken to have negligible mass.

I don't know offhand precisely how a tertiary object with nonnegligible
mass affects the situation, but I suspect it's destabilizing.

--
Erik Max Francis / m...@alcyone.com / http://www.alcyone.com/max/
__ San Jose, CA, US / 37 20 N 121 53 W / ICQ16063900 / &tSftDotIotE
/ \ The woman's movement is no longer a cause but a symptom.
\__/ Joan Didion
Alcyone Systems' Daily Planet / http://www.alcyone.com/planet.html
A new, virtual planet, every day.

Jaak Suurpere

unread,
Aug 30, 2001, 4:01:15 AM8/30/01
to
Michael Ash <ma...@mikeash.com> wrote in message news:<mail-D0D11E.0...@news-server.wi.rr.com>...

> In article <b3030854.01082...@posting.google.com>,
> sherm...@hotmail.com (Johnny1A) wrote:
>
> [other interesting stuff snipped]
>
> >As a side-effect, when technology reached space-flight levels, there'd
> >be a lot of potential targets, unless our world is the only satellite.
> > (That strikes me as unlikely, but like Conrad, I have to note that we
> >lack data again.) On the other hand, that huge gravity field from the
> >gas giant might make fuel requirements somewhat of a barrier. Is it
> >cheaper in fuel to go from one moon of Jupiter to another than it
> >would be to go from Earth to the Moon?
>
> We do have quite a bit of data, actually. Four gas giants, albeit none
> very close to the sun. Each giant in our system has a large number of
> bodies of various sizes orbiting it. Unless being close to the sun would
> somehow clear out everything except our one planet, I would definitely
> expect a bunch of other moons.
>
Or except if the clearing were to be due to 1 of the satellites being
Earth-sized rather than Mercury-sized.
Jupiter has 4 major satellites - 2 Mercury-sized ones, and 2
Moon-sized ones. Then none before the size of few hundred kilometres.
Saturn has Titan which is around the size of Mercury, then none to
below 2000 km - and then a bunch over 1000 km and below.
Uranus has a bunch in the size of hundreds to above thousand km, but
nothing bigger.
Neptune has 1 big satellite and the rest are far smaller.

Conrad Hodson

unread,
Aug 30, 2001, 6:47:26 AM8/30/01
to
On Wed, 29 Aug 2001, Michael Ash wrote:
> We do have quite a bit of data, actually. Four gas giants, albeit none
> very close to the sun. Each giant in our system has a large number of
> bodies of various sizes orbiting it. Unless being close to the sun would
> somehow clear out everything except our one planet, I would definitely
> expect a bunch of other moons.

I would guess it has a lot to do with angular momentum in the proto-gas
giant clump in the accretion disk. Close to the sun might well be less of
an issue than the fact that Earth far exceeds the mass of all the gas
giant satellites in our whole solar system. In other words, getting an
Earthlike satellite of that gas giant may already be a stretch; it might
take so many planetisimals that nothing in the Io-to-Callisto size range
formed.

Or maybe not. We thought we knew that gas giants _had_ to form well away
from their stars; that's the way it happened here, and it seemed to have
happened for a very plausible reason. Well, almost all of those
predictions seem to have failed dismally, and it seems sensible in the
face of that to back off on all the _a priori_ theorizing and wait for
some more actual data. Since our best theories completely failed to
predict the existence of superjovian planets to _sunward_ of a liquid
water zone, we probably shouldn't be too glib about predicting the details
of what those worlds are like and especially how they formed.

Once again, the universe turns out to be bigger (and far more
interesting!) than the one we "knew". It also gives writers a lot of
freedom; right now it looks as if there are _many_ planetary options
besides the ones in our solar system, and it could easily take decades to
get instruments good enough to yank the bases out from under a generation
of good stories.

Conrad Hodson

Miguel Farah F.

unread,
Aug 30, 2001, 8:00:55 PM8/30/01
to
In rec.arts.sf.science John Savard <jsa...@ecn.ab.sblok.ca.nowhere> wrote:
>On 28 Aug 2001 21:15:10 -0700, sherm...@hotmail.com (Johnny1A)
>wrote, in part:

>>Is it
>>cheaper in fuel to go from one moon of Jupiter to another than it
>>would be to go from Earth to the Moon?

>The reason this idea is so attractive is because it is *definitely*
>cheaper to go from one moon to another than to travel to another solar
>system. This scenario allows multiple planets in the habitable zone
>(in addition to satellites, there could be one at each Trojan point of
>the gas giant - and that, at least, has fewer obstacles, as well).

I'll give in - I can't hide it anymore: I don't know what a Trojan
point is. Can anybody enlighten me?

Thanks.

--
MIGUEL FARAH // mig...@nn.cl
#include <disclaimer.h> // http://www.nn.cl/~miguel
<*>

"Simple, change the gravitational constant of the Universe."
- Q

Erik Max Francis

unread,
Aug 30, 2001, 9:57:31 PM8/30/01
to
"Miguel Farah F." wrote:

> I'll give in - I can't hide it anymore: I don't know what a Trojan
> point is. Can anybody enlighten me?

One of the (two) stable Lagrange points. For an object in orbit around
a primary (and at least ~25 times less massive), points 60 degrees ahead
and behind (so they're two points) of the second object are points where
negligibly small objects can move in synchrony with the secondary object
and be stable (small perturbations not cause them to depart from the
points).

In the real Solar System, junk tends to collect at the Trojan points of
major bodies. There are collections of asteroids at these points in
Jupiter's orbit; they're called, not surprisingly, the Trojans.

--
Erik Max Francis / m...@alcyone.com / http://www.alcyone.com/max/
__ San Jose, CA, US / 37 20 N 121 53 W / ICQ16063900 / &tSftDotIotE

/ \ The quickest way of ending a war is to lose it.
\__/ George Orwell
Polly Wanna Cracka? / http://www.pollywannacracka.com/
The Internet resource for interracial relationships.

BC

unread,
Aug 30, 2001, 10:10:14 PM8/30/01
to
A Trojan orbit is the other term for the L4 and L5 Lagrangian points. They are
located at 60 degrees leading and following the secondary mass orbit in a
binary system where the primary body's mass is at least 25 times the mass of
the secondary body. This is a special solution to the classic Three-Body
Problem of stable gravitational orbits. I recently stumbled over a site with
multimedia java recreation of this and several other solutions to stable
orbits, check out http://www.burtleburtle.net/bob/physics/kempler.html.

BC

Johnny1A

unread,
Aug 30, 2001, 11:19:30 PM8/30/01
to
Frank Henriquez <fr...@ucla.edu> wrote in message news:<frank-9129D7....@news.ucla.edu>...

> In article <b3030854.01082...@posting.google.com>,
> sherm...@hotmail.com (Johnny1A) wrote:
>
> ...
>
> >As a side-effect, when technology reached space-flight levels, there'd
> >be a lot of potential targets, unless our world is the only satellite.
> > (That strikes me as unlikely, but like Conrad, I have to note that we
> >lack data again.) On the other hand, that huge gravity field from the
> >gas giant might make fuel requirements somewhat of a barrier. Is it
> >cheaper in fuel to go from one moon of Jupiter to another than it
> >would be to go from Earth to the Moon?
>
> The delta-v required to travel between Jupiter's inner moons is similar
> to the delta-v required to travel between the Earth and Mars or Venus.
> Not easy!
>
> Frank

Granted, but another factor would play in: shorter travel times.
Since our starting and ending points are closer together in absolute
terms, we don't need to carry as much food, oxygen, water, etc. Other
consumables are probably comparably reduced.

It's really amazing how much difference modest weight savings can make
in space flight, since every gram of mass requires a bigger engine for
the same acceleration, which means more fuel, requiring a bigger tank,
meaning more mass, meaning a bigger engine, etc, etc (we all know
_that_ progression!).

(Unless, of course, we need thick radiation shielding or the like that
would make up for the mass saved in consumables.)

Shermanlee

Johnny1A

unread,
Aug 30, 2001, 11:23:34 PM8/30/01
to
sherm...@hotmail.com (Johnny1A) wrote in message news:<b3030854.01082...@posting.google.com>...


Another question to ponder: could our Earth-like world have its own
natural satellites, even as it orbits the gas giant? I recall reading
a while back that someone calculated that moons of moons inevitably
crash or break up. On the other hand, why wouldn't the same thing
apply to Luna, since it's a moon of Earth which is a moon of the Sun?
Is it viable for a moon to have moons?

I ask because some scientists suspect that the presence of Luna was
critical for the formation of a viable biosphere on Earth.

Absent a moon, could the gas giant fulfill the same role, if
necessary?

Shermanlee

Jordan S. Bassior

unread,
Aug 31, 2001, 12:01:51 AM8/31/01
to
Shermanlee said:

> On the other hand, why wouldn't the same thing
>apply to Luna, since it's a moon of Earth which is a moon of the Sun?
>Is it viable for a moon to have moons?

Actually, the Earth and Luna are technically a twin planet. Isn't that ironic?
The body we get the word "moon" from, technically isn't. :)
--
Sincerely Yours,
Jordan
--
"To urge the preparation of defence is not to assert the imminence of war. On
the contrary, if war were imminent, preparations for defense would be too
late." (Churchill, 1934)
--

Matt Ruff

unread,
Aug 31, 2001, 3:12:16 AM8/31/01
to
"Miguel Farah F." wrote:
>
> I'll give in - I can't hide it anymore: I don't know what a
> Trojan point is.

First guy out of the horse is the point.

-- M. Ruff

Erik Max Francis

unread,
Aug 31, 2001, 3:25:53 AM8/31/01
to
Matt Ruff wrote:

GO GO GO (too much Counter-Strike)

--
Erik Max Francis / m...@alcyone.com / http://www.alcyone.com/max/
__ San Jose, CA, US / 37 20 N 121 53 W / ICQ16063900 / &tSftDotIotE

/ \ Walk a mile in my shoes / And you'd be crazy too
\__/ Tupac Shakur
Rules for Buh / http://www.alcyone.com/max/projects/cards/buh.html
The official rules to the betting card game, Buh.

Karl M. Syring

unread,
Aug 31, 2001, 4:13:06 AM8/31/01
to
"Erik Max Francis" <m...@alcyone.com> schrieb

> "Miguel Farah F." wrote:
>
> > I'll give in - I can't hide it anymore: I don't know what a Trojan
> > point is. Can anybody enlighten me?
>
> One of the (two) stable Lagrange points. For an object in orbit around
> a primary (and at least ~25 times less massive), points 60 degrees ahead
> and behind (so they're two points) of the second object are points where
> negligibly small objects can move in synchrony with the secondary object
> and be stable (small perturbations not cause them to depart from the
> points).
>
> In the real Solar System, junk tends to collect at the Trojan points of
> major bodies. There are collections of asteroids at these points in
> Jupiter's orbit; they're called, not surprisingly, the Trojans.

To give a link to may favorite map of inner solar system:
http://cfa-www.harvard.edu/iau/lists/InnerPlot.html
Spot the Trojan points.

Karl M. Syring

Ross Presser

unread,
Aug 31, 2001, 1:39:24 PM8/31/01
to
Erik Max Francis <m...@alcyone.com> wrote:

> In the real Solar System, junk tends to collect at the Trojan
> points of major bodies. There are collections of asteroids at
> these points in Jupiter's orbit; they're called, not surprisingly,
> the Trojans.

If I recall correctly, the asteroid grouping was first discovered, then
named the Trojans (after something to do with pursuing Jove, I guess),
then when it was realized why these points were stable the name Trojan
got transferred to the points.

--
Ross Presser * ross_p...@imtek.com
"I am -NOT- Cthulhu! Go away! I have no statement! The fish went
insane all on their own!" -- Jonathan Day, "things an octopus might
say"

BC

unread,
Aug 31, 2001, 11:35:11 PM8/31/01
to

Johnny1A wrote:

>
> Another question to ponder: could our Earth-like world have its own
> natural satellites, even as it orbits the gas giant? I recall reading
> a while back that someone calculated that moons of moons inevitably
> crash or break up. On the other hand, why wouldn't the same thing
> apply to Luna, since it's a moon of Earth which is a moon of the Sun?
> Is it viable for a moon to have moons?

This depends on the mass ratio of the three bodies; and whether there are other bodies of nonnegligable mass, i.e. large moons,
in orbit around the jovian. As to a moon being the size of Luna in ratio to a world the size of Earth; this would be impossible
as neither the Earth or Luna orbit the other, instead they orbit a barycenter which is outside Earth's center of mass making
this highly unstable around a jovian world.

>
>
> I ask because some scientists suspect that the presence of Luna was
> critical for the formation of a viable biosphere on Earth.

Not only the presence but the formation of Luna were critical to the development of a 'living planet'. 70% of Earth's crust is
missing, formed into Luna and making land possible by allowing/causing tectonics and large basins to contain all the water;
this might occur in a jovian's neighborhood. Without Luna in orbit, the terrestrial world depends on perturbations from another
source to keep tectonics and possibly a magnetic field similar to Earths, from the liquid core being 'stirred'; one or more
large moons co-orbiting the jovian would probably suffice.

>
>
> Absent a moon, could the gas giant fulfill the same role, if
> necessary?

By itself, I doubt it. The terrestrial world would probably be tide locked, although based on Mercury it might slowly rotate.
Either way, my previous statement answers this; you probably need other moons co-orbiting the Jovian.

>
>
> Shermanlee

John Savard

unread,
Sep 1, 2001, 12:51:30 PM9/1/01
to
On Wed, 29 Aug 2001 23:33:35 -0700, Erik Max Francis <m...@alcyone.com>
wrote, in part:
>John Savard wrote:

>> The reason this idea is so attractive is because it is *definitely*
>> cheaper to go from one moon to another than to travel to another solar
>> system. This scenario allows multiple planets in the habitable zone
>> (in addition to satellites, there could be one at each Trojan point of
>> the gas giant - and that, at least, has fewer obstacles, as well).

>I doubt a terrestrial planet in a star-jovian Trojan point would be
>stable. The arrangement is only stable if the secondary (the jovian) is
>about 25 times less massive than the primary (the star), but in any case
>the tertiary (the terrestrial planet) is taken to have negligible mass.

>I don't know offhand precisely how a tertiary object with nonnegligible
>mass affects the situation, but I suspect it's destabilizing.

Well, there's lots of junk in Jupiter's Trojan points, so they appear
to work - and the Earth's mass definitely is negligible compared to
that of Jupiter.

However, for life to evolve, one needs stability over _billions_ of
years, so you may well have a valid objection. Or, conversely, the
Trojan points may be so stable that they subject the planet to
excessive amounts of meteor and asteroid bombardment, by sweeping up
passing objects from too large an area.

I pointed that out as an additional possibility, though, because it
_might_ be less "iffy" than an orbit around the gas giant, and so it
creates another possibility.

Erik Max Francis

unread,
Sep 1, 2001, 2:19:37 PM9/1/01
to
John Savard wrote:

> Well, there's lots of junk in Jupiter's Trojan points, so they appear
> to work - and the Earth's mass definitely is negligible compared to
> that of Jupiter.

For differing values of "negligible." The mass of the objects already
in Jupiter's Trojans are definitely negligible with respect to Jupiter.
But I wouldn't say at the drop of a hat that Earth's mass is negligible
with respect to Jupiter's in this matter.

--
Erik Max Francis / m...@alcyone.com / http://www.alcyone.com/max/
__ San Jose, CA, US / 37 20 N 121 53 W / ICQ16063900 / &tSftDotIotE

/ \ Love is the triumph of imagination over intelligence.
\__/ H.L. Mencken

Johnny1A

unread,
Sep 1, 2001, 11:58:14 PM9/1/01
to
Erik Max Francis <m...@alcyone.com> wrote in message news:<3B9126B9...@alcyone.com>...

> John Savard wrote:
>
> > Well, there's lots of junk in Jupiter's Trojan points, so they appear
> > to work - and the Earth's mass definitely is negligible compared to
> > that of Jupiter.
>
> For differing values of "negligible." The mass of the objects already
> in Jupiter's Trojans are definitely negligible with respect to Jupiter.
> But I wouldn't say at the drop of a hat that Earth's mass is negligible
> with respect to Jupiter's in this matter.

Jupiter masses ~300 times the mass of Earth. Actually, I think it's a
bit mroe than that. I suspect that would qualify as negligible, but I
don't have detailed calculations to back it up.

Shermanlee

Matt McIrvin

unread,
Sep 2, 2001, 1:55:42 AM9/2/01
to
In article <gh8T1KA8...@econym.demon.co.uk>,
Mike Williams <mi...@nospam.please> wrote:

> Io is tidally locked to Jupiter and in a circular orbit, so Jupiter
> alone doesn't cause any tidal stress on Io.

Your point is correct, but pedantically speaking, the word "stress"
refers to the arrangement of the tidal forces themselves; so there *is*
tidal stress from Jupiter, it's just constant in time with respect
to the body of Io, and so cannot cause the volcanism on its own.

--
Matt McIrvin

Matt McIrvin

unread,
Sep 2, 2001, 2:05:50 AM9/2/01
to
In article <msYC2HAs...@econym.demon.co.uk>,
Mike Williams <mi...@nospam.please> wrote:

> Using: Tide ~= Mass/distance^3
>
> Io tidal force is 317.9/1.10^3 = 239 times that at the Moon
> Europa tidal force = 59.6
> Ganymede tidal force = 14.7
> Calisto tidal force = 2.71
> Moon tidal force = 1.0 by definition
> Mercury tidal force = 0.097
>
> So the tidal force at Mercury from the Sun is considerably weaker than
> that at the Moon or at the Galilean satellites.
>
> The idea that Mercury was tide locked was an observational error rather
> than a failure of theory.

On the other hand, the Sun's tide has affected Mercury's rotation;
it's locked resonantly to exactly 2/3 of an orbital revolution.

The rotation of Venus may have a subtle tidal resonance with Earth: at
closest approach, the same side of Venus is always facing us. Some
pretty good, if large-scale, radar maps of the surface were made from
the Arecibo radio telescope in Puerto Rico, but they could only get one
side that way.

--
Matt McIrvin

Matt McIrvin

unread,
Sep 2, 2001, 2:08:48 AM9/2/01
to
In article <mail-20D001.1...@news-server.wi.rr.com>,
Michael Ash <ma...@mikeash.com> wrote:

> What effect would an huge, very visible, stationary landmark in the sky
> have on developing civilization on such a world?
>
> Navigation would be a lot easier in the hemisphere that can see the gas
> giant, if nobody had compasses. Would a world like this have a magnetic
> field strong enough to use compasses? If not, then the giant will be
> even more useful that way. If compasses are practical, then I could see
> the far side civilizations discovering them and using them, with
> seafarers who kept exclusively to the near side scorning them. This
> could make technology develop faster on the far side. Without compasses,
> the ease of seagoing trade on the near side could make technology
> develop faster there.
>
> There are other effects possible as well. I could imagine huge holy wars
> between people who worshipped the giant and people who thought it was
> just imaginary, since they had never seen it. Would astronomy be helped
> by the presence of a huge reminder of things existing in the sky, or
> would the correct idea of the mechanics of the solar system be impeded
> by it?

There was a situation something like this in Buzz Aldrin and John
Barnes' _Encounter with Tiber_.

--
Matt McIrvin

Matt McIrvin

unread,
Sep 2, 2001, 2:19:22 AM9/2/01
to
In article <mail-D0D11E.0...@news-server.wi.rr.com>,
Michael Ash <ma...@mikeash.com> wrote:

> In article <b3030854.01082...@posting.google.com>,
> sherm...@hotmail.com (Johnny1A) wrote:
>

> [other interesting stuff snipped]


>
> >As a side-effect, when technology reached space-flight levels, there'd
> >be a lot of potential targets, unless our world is the only satellite.
> > (That strikes me as unlikely, but like Conrad, I have to note that we
> >lack data again.) On the other hand, that huge gravity field from the
> >gas giant might make fuel requirements somewhat of a barrier. Is it
> >cheaper in fuel to go from one moon of Jupiter to another than it
> >would be to go from Earth to the Moon?

[...]


> For maneuver, look at Galileo. It's been zipping between all the major
> moons of Jupiter for several years, and it can't have carried that much
> fuel up there. As long as the civilization understands how to do weird
> gravitational assists, which seems likely in their situation, then they
> ought to be able to get around pretty easily.

Galileo's in a rather different situation. It isn't exactly zipping
between the major moons. It fell in from interplanetary space, and
it's far from having the fuel to soft-land anywhere, or even to get
into a tight circular orbit around Jupiter. All this time it's been in
a quite eccentric orbit, shaped sort of like the orbit of Halley's
Comet around the Sun, that periodically takes it far out from the
inner Jovian system. When it falls through the inner system, it's
possible to target it to fly close to one or another of the Galilean
moons.

--
Matt McIrvin

Matt McIrvin

unread,
Sep 2, 2001, 2:27:25 AM9/2/01
to
In article <3B8EEF0B...@alcyone.com>,

Erik Max Francis <m...@alcyone.com> wrote:

> In the real Solar System, junk tends to collect at the Trojan points of
> major bodies. There are collections of asteroids at these points in
> Jupiter's orbit; they're called, not surprisingly, the Trojans.

There are also some little moons of Saturn at Trojan points with
respect to larger moons (these could possibly provide more empirical
data about mass differences). The Saturn system has all kinds of
wonderfully bizarre classical mechanics in it.

--
Matt McIrvin

Mike Williams

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Sep 2, 2001, 3:17:06 AM9/2/01
to

However, the presence of such objects doesn't tell us whether those
orbits have sufficient long term stability. Some of the bizarre
mechanics going on around Saturn are unlikely to last for more than a
few million years. During the short period that we've been observing
them they look quite stable, and it's only by performing theoretical
calculations that we can determine whether they can last.

--
Mike Williams
Gentleman of Leisure

Kai Henningsen

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Sep 2, 2001, 10:59:00 AM9/2/01
to
sherm...@hotmail.com (Johnny1A) wrote on 29.08.01 in <b3030854.01082...@posting.google.com>:

> I wonder how long it would take the gas giant to
> slow a 24-hour day into whatever final state it would settle in?

What makes you think it would start at 24h (instead of end there after a
long time of tidal brakes by Luna)?

It seems rotation periods somewhere in the vicinity of 10 hours are rather
more common in our system.


Kai
--
http://www.westfalen.de/private/khms/
"... by God I *KNOW* what this network is for, and you can't have it."
- Russ Allbery (r...@stanford.edu)

Kai Henningsen

unread,
Sep 2, 2001, 10:44:00 AM9/2/01
to
mmci...@world.std.com (Matt McIrvin) wrote on 02.09.01 in <mmcirvin-DC8C91.02055002092001@[192.168.123.1]>:

> The rotation of Venus may have a subtle tidal resonance with Earth:

Holy batman. How do you do a tidal resonance for Venus from Earth, but not
the other way around, given the rather similar mass and density of the
two?

Robert Shaw

unread,
Sep 2, 2001, 5:41:38 PM9/2/01
to

"Kai Henningsen" <kaih=884T2...@khms.westfalen.de> wrote
> mmci...@world.std.com (Matt McIrvin) wrote

>
> > The rotation of Venus may have a subtle tidal resonance with Earth:
>
> Holy batman. How do you do a tidal resonance for Venus from Earth, but not
> the other way around, given the rather similar mass and density of the
> two?
>
That needn't be a problem.

The most obvious reason is the Moon. The largest torque exterted on the
Earth is applied by the Moon, with solar tides coming second. That's
enough to make the system asymmetric and keep the Earth from falling
into resonance with Venus.

Even with the Moon, mutual tidal locking wouldn't be automatic. Venus
could have started much closer to the resonance than the Earth did, in
which case there would have been time enough for the tidal forces to slow
Venus's rotation until it reached resonance, but not for the same to have
happened to Earth.


--
Matter is fundamentally lazy:- It always takes the path of least effort
Matter is fundamentally stupid:- It tries every other path first.
That is the heart of physics - The rest is details.- Robert Shaw


Johnny1A

unread,
Sep 2, 2001, 9:52:21 PM9/2/01
to
kaih=884T2...@khms.westfalen.de (Kai Henningsen) wrote in message news:<884T2...@khms.westfalen.de>...

> sherm...@hotmail.com (Johnny1A) wrote on 29.08.01 in <b3030854.01082...@posting.google.com>:
>
> > I wonder how long it would take the gas giant to
> > slow a 24-hour day into whatever final state it would settle in?
>
> What makes you think it would start at 24h (instead of end there after a
> long time of tidal brakes by Luna)?
>
> It seems rotation periods somewhere in the vicinity of 10 hours are rather
> more common in our system.
>
>
> Kai

I was simply thinking of Earth as an example.

Shermanlee

Frank Henriquez

unread,
Sep 4, 2001, 7:23:00 PM9/4/01
to
In article <b3030854.0108...@posting.google.com>,
sherm...@hotmail.com (Johnny1A) wrote:


>Another question to ponder: could our Earth-like world have its own
>natural satellites, even as it orbits the gas giant?

Unlikely to have a stable orbit. If you take a look at the projected
in-orbit lifespan of the Europa Orbiter, you'll be shocked (90 days or
so).

>I ask because some scientists suspect that the presence of Luna was
>critical for the formation of a viable biosphere on Earth.

Remember, this is based on a sample of 1. If Europa has a biosphere, or
if Mars has/had a biosphere, that theory would need some reworking.

Frank

--
Frank Henriquez Programmer/Analyst Jules Stein Eye Institute, UCLA
fr...@ucla.edu http://www.bol.ucla.edu/~frank/index.htm

Johnny1A

unread,
Sep 4, 2001, 10:53:49 PM9/4/01
to
sherm...@hotmail.com (Johnny1A) wrote in message news:<b3030854.01090...@posting.google.com>...

By the way, here's another factor involving the rotation that I've
thought of: the planetary weather.

On Earth, the fact that the planet rotates is responsible for a lot of
the facets of our weather system. What influence would the tide-lock
or resonance lock of the habitable satellite have on the local
weather?

Shermanlee

Mike Williams

unread,
Sep 5, 2001, 3:49:23 AM9/5/01
to
Wasn't it Johnny1A who wrote:

>By the way, here's another factor involving the rotation that I've
>thought of: the planetary weather.
>
>On Earth, the fact that the planet rotates is responsible for a lot of
>the facets of our weather system. What influence would the tide-lock
>or resonance lock of the habitable satellite have on the local
>weather?

Good point.

One theory:-

Because the rotation rate would be considerably slower[1] the Coriolis
forces would be much weaker, and therefore the storm systems would have
less of a tendency to rotate. The weather systems would tend to be more
like large simple convection cells. I think that there'd also be less of
a tendency for large structures like the trade winds and jet streams to
form. If the storm cells don't move around as much, they'd have more
time to gain energy, which would be exacerbated by the longer day
length, allowing more time for the sun's energy to pump up the storm.
Perhaps there would tend to be very powerful storms in terms of rain
intensity, lightning and hail but without the high wind speeds that we
get in hurricanes.

Another theory:-

Because the rotation rate would be considerably slower the Coriolis
forces would be much weaker, and therefore the storm systems would have
less of a tendency to rotate. The weather systems would tend to be more
like large simple convection cells. Cold air would be able to flow
directly in to the heart of the base of the storm cell, instead of being
deflected into circular motion by Coriolis forces. This inflow would
cool the heart of the storm system and prevent the storm from becoming
powerful.

In either case:-

The overall climatic impact of these differences would probably mean
that there would be less mixing of air masses (and similarly with ocean
currents) so the dry areas would tend to stay drier and the hot areas
would stay hotter.


[1]Assuming that the moon is far enough from the gas giant that its
orbit is considerably longer than one day, because otherwise we may have
rather high radiation levels.

Conrad Hodson

unread,
Sep 6, 2001, 4:16:46 AM9/6/01
to
On Wed, 5 Sep 2001, Mike Williams wrote:
>
> Because the rotation rate would be considerably slower[1] the Coriolis
> forces would be much weaker, and therefore the storm systems would have
> less of a tendency to rotate. The weather systems would tend to be more
> like large simple convection cells. I think that there'd also be less of
> a tendency for large structures like the trade winds and jet streams to
> form. If the storm cells don't move around as much, they'd have more
> time to gain energy, which would be exacerbated by the longer day
> length, allowing more time for the sun's energy to pump up the storm.
> Perhaps there would tend to be very powerful storms in terms of rain
> intensity, lightning and hail but without the high wind speeds that we
> get in hurricanes.

Minor point; hurricanes are formidable because of their size and duration,
not because they have the highest windspeeds. Those are found in
tornadoes, and the system you describe above would still have those, as
well as violent squalls and downburst winds. Though, with a weaker
Coriolis, tornadoes themselves might be wider and less intense.

Conrad Hodson

Hop David

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Sep 6, 2001, 10:20:19 AM9/6/01
to

Mike Williams wrote:

> In either case:-
>
> The overall climatic impact of these differences would probably mean
> that there would be less mixing of air masses (and similarly with ocean
> currents) so the dry areas would tend to stay drier and the hot areas
> would stay hotter.

And the longer days and nights would also make for greater temperature
extremes. For this reason I believe the outer moons would be less
hospitable and earth like.

>
>
> [1]Assuming that the moon is far enough from the gas giant that its
> orbit is considerably longer than one day, because otherwise we may have
> rather high radiation levels.

Not every gas giant has Jupiter's powerful magnetic field. So I don't think
it's a given that every inner moon will suffer the high levels of radiation
that Io endures.


-- Hop
http://clowder.net/hop/index.html
http://www.tabletoptelephone.com/~hopspage/HopsTiles.html

Conrad Hodson

unread,
Sep 7, 2001, 5:21:49 AM9/7/01
to
On Thu, 6 Sep 2001, Hop David wrote:
>
> Not every gas giant has Jupiter's powerful magnetic field. So I don't think
> it's a given that every inner moon will suffer the high levels of radiation
> that Io endures.

Very good point. Here's another: we know from "fossil" magnetism of
Earth's igneous rocks that our own magnetic field has reversed itself at
irregular intervals for at least the last hundred million years. We also
have a _very_ poor record of predicting the magnetic fields of planets
before space probes arrived to take real-life measurements. This suggests
that our theories of planetary magnetism, er, need work.

Given all this, it seems very foolish to say that "Jupiter has powerful
radiation belts that affect the moons out to Ganymede"--and assume that
it's always been so. All we have are tiny snapshots of those planet's
magnetic/radiation histories. For all we know, a hundred thousand years
ago, Saturn could have had extremely powerful fields and radiation belts,
and Jupiter none. Maybe if we can someday date and study rocks or ice
samples from various moons, we can get some historical perspective. But
right now we have none whatsoever, and ought to keep that in mind as we
write and speculate.

Conrad Hodson

Hop David

unread,
Sep 7, 2001, 2:12:20 PM9/7/01
to

Conrad Hodson wrote:

ISTR speculation that Jupiter's magnetic field comes from a spinning ball of
metallic hydrogen.

Don't Saturn, Uranus and Neptune have much weaker fields? I would guess these
planets don't have sufficent gravity to pressurize the hydrogen into a metallic
state. If this is the case, I believe the gas giant's mass be a major factor in
the strength of it's magnetic field.

But I suspect you're correct - we don't really know.

Frank Henriquez

unread,
Sep 7, 2001, 6:29:50 PM9/7/01
to
In article <3B990E03...@tabletoptelephone.com>,
Hop David <hops...@tabletoptelephone.com> wrote:

>ISTR speculation that Jupiter's magnetic field comes from a spinning ball of
>metallic hydrogen.
>
>Don't Saturn, Uranus and Neptune have much weaker fields?

ISTR that one possible reason for the "benign" radiation environment
around Saturn (about as good - or bad - as in low Earth orbit) is its
ring system.

Turning Io into rubble to see if it has an effect on the radiation
environment around Jupiter might be an interesting experiment...

Conrad Hodson

unread,
Sep 7, 2001, 7:09:13 PM9/7/01
to

On Fri, 7 Sep 2001, Hop David wrote:
>
> ISTR speculation that Jupiter's magnetic field comes from a spinning ball of
> metallic hydrogen.
>
> Don't Saturn, Uranus and Neptune have much weaker fields? I would guess these
> planets don't have sufficent gravity to pressurize the hydrogen into a metallic
> state. If this is the case, I believe the gas giant's mass be a major factor in
> the strength of it's magnetic field.

> But I suspect you're correct - we don't really know.
>

Yep. For one instance, Saturn is assumed to have a big layer of liquid
metallic hydrogen just like Jupiter--which should be conductive and
convecting too, it's hard to imagine any reason why such would _not_
happen. The convecting/conducting liquid dynamo model has been around for
fifty years or so, and it's probably right, but it's very obvious that
there are details that are far from simple, and the processes just might
be near-chaotic. Much the way that the overall circulation of our
atmosphere and ocean currents are known and predictable, but the details
are much less so. It's possible that trying to predict (in advance of
actual probe or remote observation) what sort of magnetic environment
surrounds a given gas giant may be equivalent to asking a meteorologist
whether it will be raining in Philadelphia this time next year--or whether
our own field will be normal, reversed or absent a million years from next
Tuesday.

Conrad Hodson

Nancy Lebovitz

unread,
Sep 7, 2001, 9:06:44 PM9/7/01
to
>Michael Ash <ma...@mikeash.com> wrote in message news:<mail-20D001.1...@news-server.wi.rr.com>...
>> In article <Pine.GSU.4.21.01082...@garcia.efn.org>,
>> Conrad Hodson <con...@efn.org> wrote:
>>
>> >OTOH, that might not be a disaster in itself. Such a world might end up
>> >with a long day-night cycle (equal to the orbital period, in the case of
>> >tide-lock) but that wouldn't have to be a deal-breaker for a biosphere.
>> >The _sun_ would still rise and set--just the gas giant would be fixed in
>> >the sky.

>>
>> What effect would an huge, very visible, stationary landmark in the sky
>> have on developing civilization on such a world?
>>
>> Navigation would be a lot easier in the hemisphere that can see the gas
>> giant, if nobody had compasses. Would a world like this have a magnetic
>> field strong enough to use compasses? If not, then the giant will be
>> even more useful that way. If compasses are practical, then I could see
>> the far side civilizations discovering them and using them, with
>> seafarers who kept exclusively to the near side scorning them. This
>> could make technology develop faster on the far side. Without compasses,
>> the ease of seagoing trade on the near side could make technology
>> develop faster there.
>
The weather on the gas giant would be visible--the folks on the planet
would be less inclined to think that everything in the sky is stable
and perfect. They might also get some idea of the mechanics behind
their own weather relatively early--at least the idea that weather
patterns move over large distances.
--
Nancy Lebovitz na...@netaxs.com www.nancybuttons.com

TechnoCowGeek

unread,
Oct 4, 2001, 7:00:39 AM10/4/01
to
> Or jump, even. Tyrannosaurs were giants, and they carried their tails
high when
> moving fast, (obvious from the hip and balance), so if Rex could spot the
fence
> _and knew what it was_ he could easily bound over it with one leap, skip,
or
> even running stride.

Depends on height of the T-rex. Fences are generally 4 feet high, although
simple electric ones can be as short as 3 feet high.

> A cow fleeing something the size of a combine harvester chasing it at 30
mph
> while roaring and showing 6" long teeth in a mouth the size of a small
car
> would probably try to run _through_ the fence. Heck, a _human_ might try
to,
> with a nightmare like THAT barrelling down on him.

...Roaring while running? Wonderfully dramatic, or should I say cinematic,
but ever tried to yell while running? Predators are generally silent, even
when wounded.

...Cows that are used to a pasture know where the fence is, and tend to
avoid it by bolting to either side. They can also make reasonably sharp
turns as long as they have enough room to maneuver. Cows can also jump.
That's why barbed wire or electric is run along the top of fencing - the
sweet little dears easily jump a 4 foot fence from a standstill. When
spooked and running, they can jump even higher.

Most likely the cows would stampede, heading towards the nearest cover -
preferably dense woods. When cornered, a few would bolt and lead the
stampeding herd. Predators are drawn to movement, so oddly enough Rex might
chase the ones that bolted instead of the ones just standing there.

...As for Farmer Brown, the sound of a running T-rex would be hard to miss.
The difficult part would be distinguishing between Rex chasing cattle, and
passing cars with the Bass turned up too loud.


TechnoCowGeek
------------------
"Let's not confuse the issue with facts."


TechnoCowGeek

unread,
Oct 4, 2001, 7:01:01 AM10/4/01
to
>Until they came to the barbed wire fence, anyway.
> That would be a surprise for _both_ of them. For Rex, possibly a fatal
one, if
> he tangled his feet in it at full run and came down hard on his head
(lacking
> effective arms, that would be the outcome if his legs were tangled so he
> couldn't save himself with a skip).

Coyotes easily snap an electric fence just by running into it. Depending on
speed, massive Rex should be able to snap the much sturdier barbed wire
fence, or at least tear it loose from the posts. Fencing with the 4x4
openings is a different story - kind of like trying to run through a 4 ft
tall chain link fence.

TechnoCowGeek

unread,
Oct 4, 2001, 7:02:17 AM10/4/01
to

> Mind you, it might find eating a cow wrapped in barbed wire
> unpleasant--perhaps if this happened often you'd see them avoiding
> cattle. Sort of the way my cat learned not to catch hornets.
>
> Conrad Hodson
>

My cats are more determined predators. If something stings them, they bite
it. If it stings them in the mouth, they bite harder until the stubborn prey
is dead.

They also have the annoying habit of bringing home live prey in their jaws
to share with the big cats (read: us). Imagine adorable young Rex carrying
back an alive and screaming Farmer Brown in his jaws, to show off to mommy
Rex - or the human that raised Rex.

Luke Campbell

unread,
Oct 4, 2001, 10:20:49 PM10/4/01
to
TechnoCowGeek wrote:

> > Or jump, even. Tyrannosaurs were giants, and they carried their tails
> high when
> > moving fast, (obvious from the hip and balance), so if Rex could spot the
> fence
> > _and knew what it was_ he could easily bound over it with one leap, skip,
> or
> > even running stride.
>
> Depends on height of the T-rex. Fences are generally 4 feet high, although
> simple electric ones can be as short as 3 feet high.

Adult T. rex was aprox. 4 meters high at the hips. That's a good 14 feet, with
a clearance of roughly 7 feet at the belly and lower projections of the pubic
bone. It should be able to step over most fences. Whether it could jump is an
open question, but if it could, it could probably clear a fence.

> > A cow fleeing something the size of a combine harvester chasing it at 30
> mph
> > while roaring and showing 6" long teeth in a mouth the size of a small
> car
> > would probably try to run _through_ the fence. Heck, a _human_ might try
> to,
> > with a nightmare like THAT barrelling down on him.
>
> ...Roaring while running? Wonderfully dramatic, or should I say cinematic,
> but ever tried to yell while running? Predators are generally silent, even
> when wounded.

I tend to agree.

> ...As for Farmer Brown, the sound of a running T-rex would be hard to miss.
> The difficult part would be distinguishing between Rex chasing cattle, and
> passing cars with the Bass turned up too loud.

Not necessarily. Note that elephants of the same mass are quite silent when
moving. T. rex may or may not have been similarly quiet. Don't confuse the
special effects of Jurassic Park with reality.

Luke

Vegard Valberg

unread,
Oct 5, 2001, 7:52:51 AM10/5/01
to
What people seem to be forgetting in the debate about T-Rex and the
fences is that the fence in question was taller than the T-Rex itself,
and quite a bit taller too, in fact I think that it was about six to ten
feet taller than the T-Rex. Now whether or not it could still make the
jump that I do not know, but I am a bit bothered about this constant
mention of four and six feet high fences.

--
- Vegard Valberg

My e-mail adress is <Vval...@online.no>,
that is two v's, not one W.

Karl M. Syring

unread,
Oct 5, 2001, 8:13:24 AM10/5/01
to
"Vegard Valberg" <vval...@online.no> schrieb

> What people seem to be forgetting in the debate about T-Rex and the
> fences is that the fence in question was taller than the T-Rex itself,
> and quite a bit taller too, in fact I think that it was about six to ten
> feet taller than the T-Rex. Now whether or not it could still make the
> jump that I do not know, but I am a bit bothered about this constant
> mention of four and six feet high fences.

You are much too late on the thread. This special discussion started on the
premise a T. rex time traveller chasing a cow, no special dino fences
involved. I would find the scene even more amusing if the T. rex topples
over and has a bad time while trying to get back on it's feet.

Karl M. Syring


Lee DeRaud

unread,
Oct 5, 2001, 10:54:48 AM10/5/01
to
On Fri, 5 Oct 2001 14:13:24 +0200, "Karl M. Syring" <syr...@email.com>
wrote:

ObSFmovie: "Caveman"...the scene where the dino eats the fermented
pumpkins (or whatever) and gets drunk. The facial expressions and
'hand' movements are hilarious.

Lee

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