While many of you were reading Heinlien, I was reading Edgar Rice Burroughs', Thuvia Moon Maid of Mars, so I need some help. In my novel, the outpost the special forces are meeting at is a habitable moon that orbits a gas giant. The moon has red deserts, blue jungles, green oceans and really sharp, steep jagged mountains. It has an iron core thus a magnetic field but its
axial tilt is vertical. Would the weather be too crazy to be habitable and would radiation from the gas giant kill my biological creatures? The oxygen on the earth-sized moon is thin, more like one of our prehistoric atmosphere's, and the chemical composition of the soil is what causes the foliage to be blue, also as protection from the extreme winters, and summers.

Do I need to scratch the vertical axis of the moon?
 

Message #1381 left by Carolyn D. on Mar 6, 2000 at 12:09

Re: 1378-Ruthie, you haven't mentioned one of the most interesting factors-What is the color of the sky? That's bound to be affected by/affect the radiation which is screened out.

I'm not an astrophysics buff, but I vaguely remember Io being a good bet for life...or was it another of Jupiter's moons? The one that looked like a pizza?
 

Message #1383 left by Ken on Mar 6, 2000 at 14:19

Ruthie;

If by a 'vertical axial tilt', you mean a rotational axis that's perpendicular to the (sun) orbital axis, you'll have to give the gas giant a vertical axial tilt as well; the moon will be tidally locked to the gas giant.

You seem to know that life on a regular planet with such a tilt is rough, but adding a new rotation around a gas giant might help somewhat. Life on the poles is still nasty; the sun rises and sets once a year instead of once a day.  At the 'equator', life on the 'away-from-gas-giant' side is just as if there's no gas giant at all; the weather changes from equator-like to arctic-circle-like every 1/4 year. Still rough, but at least the sun rises and sets every local day. But on the equator on the gas-giant-facing-side, things get better; if the sun is ever directly-overhead it will be blocked by the gas giant itself. (The gas giant will always seem
to fixed directly overhead, and the eclipses will make the summers reasonable).

I put this basic idea in Message #1302 (in this topic). For your case, you'll still have to deal with a sun that doesn't really rise much at all during the two winters each year (one winter with strong north winds and another with strong south winds, as the sun will be shining non-stop on one of the two poles). But you can get some heat (tidal or radiative) from the gas giant, too, so that should smooth out the winters a bit.

As for the radiation, you can always dial it down by slowing down the gas giant; don't assume they'll all be just like Jupiter, which whips around in 8 hours or so. The magnetic field of the moon will also help; Ganymede's magnetic field shields its equatorial region from much of Jupiter's radiation, and your life will want to be on the equator anyway.
 

Message #1385 left by RSWebb on Mar 6, 2000 at 18:05

Jupiter might be massive enough to initiate a limited nuclear reaction, which might be the source of its radiation.

Are you saying Jupiter's rotation rate significantly increases the rate at which charged particles are expelled from its atmosphere?

At 1.43 x 10^5 kilometers diameter, and a ten hour rotation, I get a velocity of 12 km/s (pi*diamter/period), speedy to be sure, but well below Jupiter's escape velocity of 60 km/s. I suppose some particles would get a boost from Jupitier's rotation, and so would escape when  they otherwise wouldn't, but for every particle that benefited, there would be one that was
penalized. The overall effect would not neccessarily cancel out- there might be more particles fast enough to escape with a rotational boost than there are particles slow enough to esacpe but for a rotational brake- but still the rotational effect seems to me to be rather small.

So I have my doubts rotation rate plays much of a role in Jupiter's radiation. A less massive, rather than slower rotating, gas giant would seem the way to go.
 

Message #1386 left by Eric Schulman on Mar 6, 2000 at 23:03

The source of Jupiter's radiation belts isn't nuclear reactions (which Jupiter is too small to sustain). The radiation belts are caused by charged particles from the solar wind trapped in Jupiter's magnetic field (just the the Earth's Van Allen belts). The magnetic field is caused by the rotation of Jupiter's metallic hydrogen core. So as Ken pointed out, one way of decreasing the radiation in the belts is by slowing the rotation of the planet, thus decreasing the magnetic field. You could also have the moon orbiting closer to the planet than the magnetic fields, in which case it would be tidally locked and there would be gravitational
stresses (like Io). Or, you could have it far outside the magnetic fields, in which case it wouldn't be tidally locked. If the moon were very distant and the Jovian planet orbited around its star within the habitable zone (about 0.9 to 1.3 astronomical units for a star like the Sun), you'd have the the neat "life on a moon" effect without having to deal with the messy tidal locking and dangerous radiation effects.

Eric
 

Message #1388 left by RSWebb on Mar 7, 2000 at 4:53

When the fact that Jupiter radiates more energy than it receives was first discovered- this was about twenty years ago- astronomers were rather surprised, since then prevailing theories indicated that Jupiter should not be producing energy.

Touring around the web, I see the prevailing opinion now is Jupiter's excess energy comes from gravitational contraction. Few websites even acknowledge the possbility of fusion as a source.

At the time of the discovery, the astronomical community's beliefs were not quite so dogmatic. Most people hedged their bets, e.g. Bowers and Deeming Astrophysics I "a lower limit of order .05 solar masses for an object to reach the hydrogen burning main sequence. We note that Jupiter has a mass not far below this limit." (i.e. below the lower limit of the order of magnitude estimate, .005, where Jupiter has a mass of .001)  I can't say I've seen a good reason for the gravitational contraction bandwagon, which used to a a faintly disreputable theory, associated as it was with Lord Kelvin's attempt to defend
creationism.

It's true one can mathematically demonstrate that Jupiter cannot have a self-sustaining fusion reaction, but a self sustaining reaction is not neccessary; no one really knows what the effect of massive pressure is on quantum tunneling.

Anyway, it is certainly true that most scientists today say that there is no possibility of fusion reaction in Jupiter, so if you want to use conventional wisdom, that is what you should say. Personally, I'm staying with a limited fusion reaction until I see better evidence than what is out there. (I love the idea of Jupeiter as a miniature Solar System)

Can anybody explain how gravitational contraction can still be going on after 4.5 billion years?

As far as Jupiters magnetic field being caused by a rotating metallic hydrogen core, that's just a guess. Nobody really understands planetary magnetic fields, least of all Jupiter's. Currents of ionized hydrogen would provide a most satisfactory magnetic field, I am sure. Be creative in your explanations, by all means.

Ideas come and ideas go. Have you read Thomas Gold's the Deep Hot Biosphere? He literally turns upside down traditional ideas on the origin of hydrocarbons and the evolution of early life.  Here's the catch; Gold's ideas are not new. They are old ideas, being recirculated and revitalized. That's the way science sometimes goes.

Gravitational contraction, bah. Metallic hydrogen cores generating magnetic fields, bah. Long
live Jupiter the mini-Sun!
 

Message #1390 left by tnw on Mar 7, 2000 at 13:36

Re: 1381 Carolyn, I think it's Europa you're thinking of. Europa is an ocean world covered in an ice sheet. Some scientist think there might be life in the oceans.

tnw
 

Message #1392 left by Ken on Mar 7, 2000 at 18:39

On Jupiter:  There's a difference between the thermal radiation from Jupiter itself (photons), and the charged-particle radiation associated with the magnetosphere. For the first type of radiation, I think the evidence is strong for gravitational contraction;
collisions are always going to be slowly drawing the heavy elements downward as they lose angular momentum -- this will heat up the planet for billions of years to come.

For the second type... that depends on the magnetic field strength. Thanks Eric, for clarifying what I was trying to say, but I think RS has the right instinct; forget slowing down the spin, just lower the mass of the gas giant. Saturn has 20-30 times weaker magnetic field but almost the same spin rate; it's the mass that's different. The spin will help somewhat, but that's probably linear. The conductivity of the core will be a very nonlinear function of the mass, so small mass changes can make a huge difference in field strength (apparently).

Incidentally, Ruthie, if you want a gas giant with a tipped spin axis, take a look at Uranus. Interestingly, the magnetic field axis on Uranus is a whoopping 60 degrees off of the rotation  axis, which no one really understands. This is strong evidence that it doesn't have a conducting core (much less mass!), and the field is maybe due to conducting currents in the middle-layers. The small field means that Uranus might be a good model for the gas giant in your story.

But for Jupiter, I've seen the equation of state curve for high-pressure hydrogen, and I'm voting for a conducting core.
 
 

Message #1393 left by RSWebb on Mar 8, 2000 at 6:21

^There's a difference between the thermal radiation from Jupiter itself (photons), and the charged-particle radiation associated
^with the magnetosphere.

Right. Ruthie said "radiation from the gas giant" which is what threw me; to me, "radiation from the gas giant" is Jupiter's own, native born radiation. The charged particles are the *Sun's* radiation, trapped in Jupiter's magnetic field. At least, that is my understanding; I'm still not clear if anybody is claiming that charged particles emerge from Jupiter itself. I suppose they could, in fact I think they probably do, though I'm guessing at a low rate.

Ken, that is exactly right about magnetic fields, but you don't go far enough. No one really understands ANY planetary magnetic field, though there is a lot of evidence about Earth, so I'm inclined to think we're on the right track there. The explanations we see about planetary magnetic fields are scientific orthodoxy, in other words political; no one wants to offend the
heavy hitters in the field and risk defunding.

As for fusion, no one has ever studied fusion under high pressure conditions. It is easy to create stellar temperatures, by zapping deuterium with lasers, but no laboratory has come any where close to creating stellar pressures, not last I checked.

I don't think fusion shuts off like a light bulb at some particular mass. No doubt there are various thresholds, maybe even a big one around 50 Jupiter masses like theory indicates, but to say there can be no fusion processes in Jupiter is saying in effect that a negative has been proven. (some people remember to include the phrase "self-sustaining" in "no fusion processes" i.e. "no self-sustaining fusion processes", and that is a statement I agree with, but it also leaves a potentially large hole in the theory of gravitational contraction.

I took another tour around the web and everybody is saying the same thing. Gravitational contraction, gravitational contraction, gravitation contraction. So maybe there really is good evidence ruling out fusion, but I haven't seen it.
 
 

Message #1394 left by Andrew on Mar 8, 2000 at 7:26

How big is a Gauss, the unit of magnetic field strength? I specifically need to know in relation to the magnetic field of a human-habitable satellite of a gas giant.

I need three numbers as might be measured by someone standing on the surface to provoke these
responses:
1. "Well, that's about earth-normal, perhaps a little higher"
2. "Hmmm. That's fairly high."
3. "Good grief! What is going on? Much more and we'll have to evacuate!"

Andrew
 
 

Message #1395 left by Anonymous on Mar 8, 2000 at 8:05

wiping away tears of laughter:
"Incidentally, Ruthie, if you want a gas giant with a tipped spin axis, take a look at Uranus."

Ken, thank you. You just made my day!
 
 

Message #1396 left by Terry on Mar 8, 2000 at 8:05

Woops, sorry. That was me.
 
 

Message #1397 left by Rebecca on Mar 8, 2000 at 12:14

Terry, She'd have to be pretty flexible to do that, though. <g,d&rlh!>
 
 

Message #1398 left by Ken on Mar 8, 2000 at 12:33

LOL -- thanks, Terry. I'll try to throw that phrase into casual conversation more often!

RS; you may be interested to know that much of Jupiter's magnetosphere gets filled up via Io's volcanos.

Andrew, you can get all the field strength numbers you need from this fact sheet (click on the appropriate planet, then scroll down for magnetic parameters). You'll find the units in gauss*planet radius^3; this is the field strength on the equator. For a moon that's two planet radii from the planet center (one planet radius from the surface), the field drops by 2^3, or a factor of 8. (This equation only works in the ecliptic plane).

But be careful; although the field strength does determine how energetic the trapped charged particles can get, it's not the field that gets you. The standard measure of dangerous radiation is a rad (rem for a human), not a gauss.
 
 

Message #1406 left by RSWebb on Mar 8, 2000 at 19:52

Ken, no, I didn't know that. Io's volcanoes? How strange.

I'm some years out of date in planetary science. For example, you'll have a hard time convincing me that the Earth's oceans came from comets; I was told outgassing of volcanoes. And I STILL belive outgassing. The comet thing became popular shortly after the dinosaur killing asteroid. All of a sudden, collisions were popular. Until then, planetary science frowned on
catastrophism, as echoing biblical stories (e.g. Cuvier, Usher) I finally found a tiny link that at least mentioned fusion in Jupiter. It says Galileo found (unspecified) evidence that proved fusion does not occur. This was in 1996, I suppose, where
Voyager was in 1979, if I have my dates right.
Trying a link
wish me luck
only get one try
 
 

Message #1408 left by Eric Schulman on Mar 8, 2000 at 23:54

You're right, RSWebb, comets could not have produced the majority of the Earth's oceans because if they had then the hydrogen to deuterium ratio would be significantly different than it is (based on recent observations of passing comets). Outgassing (probably with some minor contribution from comets) is the most likely origin of the Earth's oceans.

Eric
 

Message #1412 left by Ruthie on Mar 9, 2000 at 4:36

Ken,RSWeb,Eric,

Thanks for all the advice. I'm working on it right now. That's why I haven't posted in a while because you've made it harder for me to understand despite all your good intentions. It makes me understand how difficult it must be to be a rocket scientist.

I would have completely missed the uranus joke had not someone pointed that out to me. Isn't uranus' axis horizontal? (seriously) I hope we can change the name of some of our planets because I won't even write about Pluto, Neptune or Uranus for obvious reasons.

Carolynn, I'll pick the sky color out after I get the geophysics right. (I think ganymede looks like pizza but not good for life)

I first asked an astromomer from Astromomy mag and he mentioned the radiation from my small star like planet Hades, (if you have better name idea's please post them)would seriously inhibit life on my planet sized moon, Despair.

Ken, I meant vertical rotational axis I think. Hades and Despair have to have the same rotational axis? What about the g or f class star?

Do stars have other that vertical rotational axis? Wouldn't Hades set and rise on Despair's horizon, like earth does on the moon? Can Despair be tidally locked if it's revolving.I can't find a definition for tidally locked. Is there any software for this?  I.E. If the mass of planet x and moon x at x a.u. for xtype star is this than the weather conditions are this etc.You all really have me thinking hard about this one. Thank you so much for taking the time to consider this for me.
 
 

Message #1429 left by Ruthie on Mar 11, 2000 at 11:39

Ken,

I've been hunting around for astrophysics software but all I could find was real astromomy sites and none of the psuedo stars fit my story discription. I did find some good sites for space organizations and graphing software. I'll post them for anyone interested next week.(I left them at work.)

I had to go have a look at the planet uranus and found a NASA site with the latest infrared images, color enhanced from the Hubble space telescope. I have an old book of the solar system with artist rendered images of our planets that do the REAL uranus no justice! 500kilom winds. Below 300 degrees in spring and that Ring! I'm soooo excited. Thank you, Ken!

Thank you Hubble space telescope.

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