Correct Physics for Fictional World

[MattRob]

Hello PF,
Alright;
I'm working on a big Sci Fi project and, being a lover of science, I want to get the physics right. So I'll spell out what I've got and see what responses I get...

First, the star I've chosen for the world is Tau Ceti, for a number of reasons that involve the plot more than physics, just know that's our sun in this case.

The planet has roughly 1/9th the mass of Saturn, and orbits within the habitable zone (so far exact distance undetermined, but it is a Sudarsky Class II gas giant.)

But the world of interest is a moon of that world, mostly Iron, it has a very powerful magnetic field and it orbits at roughly 2 million km. It's significantly less massive than Earth, but because it's more dense, it's surface gravity is roughly ~8 m/s^2.
Q: How can I find the mass of the planet with these requirements?
Q: Do we know if Tau Ceti puts out a lot of solar radiation in the form of charged particles?
Q: Would the stronger magnetic field induce more or less brilliant Auroras, and how would it effect the latitude they occur at?
Q: Would the gas giant's Van Allen belts or it's own radiation influence the Auroras, even though the gas giant has just 1/9th Saturn's mass and is a Sudarsky Class 2?

It has roughly 2 bar of atmospheric pressure, with the partial pressure of O2 only slightly higher than Earth's. IIRC, ~2 bar is near the max of human tolerance for breathing air. Is this correct?

Also, it's in 1:2:4 resonance with two other moons, it is the outermost and the most massive.
Being facelocked with the giant, the life on the surface slowly and continously migrates away from the terminator in order to stay within a certain "time zone", i.e. to continously live at a longitude where it's, say for instance, ~10am, in order to stay in a region with a familiar climate.
Although this would mean covering thousands of miles in only 192 days at the equator, the life forms manage to pull this feat off by living near the poles, where the East-West distance across the world is much shorter and the terminator moves much slower. The sapient native life always follows the Auroras to keep from diverging north or south of their habitable zone. Because of it's thicker atmosphere and large oceans, the vast majority of the planet is too hot for them to survive, some areas along the equator reaching 150*F, meanwhile the zone they live in where the Auroras are constantly seen, has a climate more on the order of the Yukon.

I'm hoping to make their culture have great significance to the Auroras that mark the path of their continuous migration away from night. But the Auroras can't play this role if they're at a higher lattitude than the Axial Tilt, because if that were the case then by always staying under the Auroras, they would fall into a 6-month long night, but the whole purpose of following the Auroras is to stay out of the night... So the Auroras need to be closer to the equator, or nearly equal to the lattitude that corresponds to the Axial Tilt, i.e., the Arctic and Antarctic. The Auroras also need to be at least nearly constant and very brilliant.
This may result in a very small Axial tilt?...

But there are some buffers that make it so the Auroras can be within the arctic and antarctic and the life still never enter night (or never leave it), if they can tolerate twilight, Dawn, or Dusk, and follow a path parallel to the auroras but with the auroras on the horizon to their right, instead of directly overhead.
The distance where "on the horizon" is for the auroras (as in this image.) depends on the radius of the planet and the altitude of the auroras, which depends on the altitude of the upper atmosphere, which I'm completely at a loss as to try to find...

But I also want the seasons to reach somewhere on the order of ~80*F in the summer and drop to ~ -40*F in the deepest part of winter, would an Axial tilt of ~14* accomplish this?

Hope you don't mind so many questions, a place where I can ask and discuss this is absolutely priceless! Many thanks!

 

[bcrowell]

I don't think a stronger magnetic field would give stronger auroras. The charged particles spiral in along the field lines. A stronger field just means more field lines, but you need more charged particles.

BTW, another good place to ask this type of thing is the usenet group rec.arts.sf.science, which you can access through google's web interface at http://groups.google.com/group/rec.arts.sf.science .

 

[twofish-quant]

Q: Would the gas giant's Van Allen belts or it's own radiation influence the Auroras, even though the gas giant has just 1/9th Saturn's mass and is a Sudarsky Class 2?

It could. Check out the wikipedia article on Io

http://en.wikipedia.org/wiki/Io_(moon )

It has roughly 2 bar of atmospheric pressure, with the partial pressure of O2 only slightly higher than Earth's. IIRC, ~2 bar is near the max of human tolerance for breathing air. Is this correct?

Not sure about the tolerance for breathing, but the experts in this sort of thing are scuba divers. One problem with having free O^2 is that you are going to have to explain how that O^2 develops. On earth, we have O^2 because of biological processes.

Something that you could immediate that might be more realistic is to have an atmosphere of 1 bar of some deadly gas. The reason that becomes interesting is that if you have say 1.1 to 1.2 bar of some gas, then you don't have to wear a pressure suit and you just need an O2 cylinder, or you could have a hose to some O2 source.

Also, it's in 1:2:4 resonance with two other moons, it is the outermost and the most massive.
Being facelocked with the giant, the life on the surface slowly and continously migrates away from the terminator in order to stay within a certain "time zone", i.e. to continously live at a longitude where it's, say for instance, ~10am, in order to stay in a region with a familiar climate.
Although this would mean covering thousands of miles in only 192 days at the equator, the life forms manage to pull this feat off by living near the poles, where the East-West distance across the world is much shorter and the terminator moves much slower. The sapient native life always follows the Auroras to keep from diverging north or south of their habitable zone. Because of it's thicker atmosphere and large oceans, the vast majority of the planet is too hot for them to survive, some areas along the equator reaching 150*F, meanwhile the zone they live in where the Auroras are constantly seen, has a climate more on the order of the Yukon.

Cool.

So the Auroras need to be closer to the equator, or nearly equal to the lattitude that corresponds to the Axial Tilt, i.e., the Arctic and Antarctic. The Auroras also need to be at least nearly constant and very brilliant.

You could maybe do this with a Io style flux tube with the main planet.

But I also want the seasons to reach somewhere on the order of ~80*F in the summer and drop to ~ -40*F in the deepest part of winter, would an Axial tilt of ~14* accomplish this?

I think it would work.

 

[MattRob]

It could. Check out the wikipedia article on Io

http://en.wikipedia.org/wiki/Io_(moon )

Thanks for the pointer and info! It's very helpful. I wonder if nitrogen would get pulled into the Plasma Torus, like how sulfur, oxygen, sodium, and chlorine do, or if nitrogen's less reactive nature would keep it from becoming ionized... https://www.physicsforums.com/showthread.php?p=3396483#post3396483

Not sure about the tolerance for breathing, but the experts in this sort of thing are scuba divers. One problem with having free O^2 is that you are going to have to explain how that O^2 develops. On earth, we have O^2 because of biological processes.

Something that you could immediate that might be more realistic is to have an atmosphere of 1 bar of some deadly gas. The reason that becomes interesting is that if you have say 1.1 to 1.2 bar of some gas, then you don't have to wear a pressure suit and you just need an O2 cylinder, or you could have a hose to some O2 source.

An under or over-abundance of oxygen could do this. The best part is only the partial pressure matters. Only problem is according to archeological records more oxygen seems to mean bigger bugs, bigger animals, and I would only assume vice versa. For plot reasons I want to stay within a moderate size range.

The 02 is the reason they're there:
You can gather lots of data from telescopes, but with fictional new technology that uses some http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=APCPCS000699000001001168000001&idtype=cvips&gifs=yes&ref=no to make interstellar flight more practical and cheaper, it makes sense to scout out the place and learn some things you can't learn from a telescope: Like if the planet isn't actually habitable for some reason you can't see from a telescope, (native virus, trace hazardous material, radiation, etc.) or if there's any other reason colonization wouldn't be a good idea. (Hostile sapient natives: unethical to wipe them out, unsafe to colonize with them there.)

The oxygen on a moon, relatively, so close is also quiet exciting for discovering new life. And their hopes are fulfilled, they do find plenty of life.

Cool.

Thanks!

You could maybe do this with a Io style flux tube with the main planet.

That's an exciting idea.

I think it would work.

I've gone down to an axial tilt of 3.8*, even using a very dense planet (6.5 g/cm^3) rich in iron, the planet is so large that even with a 152 day orbit (which requires the parent body to only have 6 Earth Masses, and to orbit with a Semi-Major Axis at roughly 2 million km), they have to be only 6* from the pole to still live a fairly normal life and not fall into night. At about 12* from the pole they'd have to spend 4/5th of their waking time traveling. (assuming roughly 1/2 time is spent sleeping, though maybe 1/3rd would be more accurate...)

I think I could settle for much cooler temperatures, though. From some extremely primitive math (although complicated), 3.8* still gives me about 120*F of temperature variation because of the longer day/night length, etc.
Overall, Antarctica averages out around -35*F.
If the pole on my fictional planet, Thea, averages at 0*F, then that goes from +60*F to -60*F, which I find pretty satisfactory. Though due to some other physical effects I might make it +60*F to -90*F, since no sunlight is a pretty absolute condition that won't variate as strongly as X amount of sunlight would...
(where the changing variable would be average planet temperature, season, etc.)

Seeing as this might appeal to Hard Sci-Fi goers, I should probably try to come up with a better name than Thea...

 

[pmacias]

Hello,

I appreciate your efforts to get the physics right in your fictional world. I will do my best to answer your questions and provide some insight into the scientific principles involved.

To find the mass of your planet, you will need to use the laws of gravity and orbital mechanics. By knowing the orbital distance of your moon and its surface gravity, you can determine the mass of the planet it orbits. This equation is known as the third Kepler law and is often used to calculate the mass of exoplanets.

In terms of solar radiation, Tau Ceti is a G-type star, similar to our sun, and therefore does emit charged particles in the form of solar wind. However, the amount of radiation emitted by a star can vary and is dependent on factors such as age and magnetic activity. Without more specific information about your fictional star, it is difficult to determine the exact level of solar radiation your moon would experience.

The strength of the magnetic field does play a role in the intensity of auroras. A stronger magnetic field would result in brighter auroras, but the latitude at which they occur is primarily determined by the angle of the magnetic field lines and the strength of the solar wind. In this case, the magnetic field of the moon would likely have a greater influence on the auroras than the gas giant's Van Allen belts.

Regarding the atmospheric pressure and gas composition, 2 bar is indeed near the maximum tolerated by humans for breathing air. However, it is important to consider the composition of the atmosphere and its effects on human physiology. A higher concentration of oxygen could potentially offset the higher pressure, making it more tolerable for humans. Additionally, the composition of the atmosphere could also impact the temperature range experienced by your characters.

For your question about axial tilt and seasons, a tilt of 14 degrees could potentially produce the temperature range you desire. However, it is important to consider other factors such as the distance from the star and the composition of the atmosphere, as these can also affect the climate and temperature variations on your fictional world.

I hope this information is helpful as you continue to develop your fictional world. I encourage you to continue to ask questions and seek out accurate information to ensure the scientific accuracy of your project. Good luck!