[Space] How much does a Gas Giant heat it's moons?
 

I've done a search of the forums (Via Google) and I can't seem to find anything that answers this question.

What sort of adjustment should I make to blackbody temperatures of moons of Gas Giants?

Re: [Space] How much does a Gas Giant heat it's moons?
 

Surface temperature? Negligible. As for core temperatures, that depends on how elliptical the orbit is. If it's elliptical enough, the moon's core can stay molten, and there can be active volcanism. Europa has a liquid ocean inder its icy crust for this very reason.

Re: [Space] How much does a Gas Giant heat it's moons?
 

Depends on how hot the gas giant is. Jupiter emits more heat than it absorbs. And the heating effect isn't from the elliptical nature of the orbit but friction from tidal stresses. Once it becomes tide locked those will not heat the moon more. And Io is a better example of a moon b ring heated.

Re: [Space] How much does a Gas Giant heat it's moons?
 

Actually, it's mostly because of tidal interactions with the other Jovian moons.

Re: [Space] How much does a Gas Giant heat it's moons?
 

And then there's the possibility that some sort of fusion is going on inside the planets core

Re: [Space] How much does a Gas Giant heat it's moons?
 

If there's fusion going on, it's not a gas giant, it's a small star (possibly a brown dwarf, if it's only D-D).

Re: [Space] How much does a Gas Giant heat it's moons?
 

Nope, Brown Dwarfs are defined by the type of fusion that occurs, or something along those lines.

There's also the amount of light reflected off the Gas Giant

Re: [Space] How much does a Gas Giant heat it's moons?
 

Early during the formation of a solar system, this might be significant. E.g the relative abundance of water on Io, Europa, Ganymede, and Callisto might be a direct result of how close they were/are to Jupiter. E.g. Io has no water because it was so hot due it's proximity to Jupiter that water would not condense out of the solar nebula.

Maybe search for how long it takes Gas Giants to cool. They warm as contract, differentiate, and accrete planetismals. My guess is you'll find they're cool enough to ignore as a significant source of heating their moons after a few million years. For systems that are younger than that, treat the still warm Gas Giants as a star of equivalent temperature.

Re: [Space] How much does a Gas Giant heat it's moons?
 

In Space, even brown dwarfs don't radiate significant heat for more than a few million years.

Re: [Space] How much does a Gas Giant heat it's moons?
 

Brown dwarfs are stars, not planets.

Re: [Space] How much does a Gas Giant heat it's moons?
 

They are in between stars and planets.

Re: [Space] How much does a Gas Giant heat it's moons?
 

Yep, and my point was that some specific sort of fusion, I can't recall off the top of my head, has to occur before a planet is classified as a Brown Dwarf, without that happening its still a planet

Re: [Space] How much does a Gas Giant heat it's moons?
 

Deuterium fusion. The line between red dwarf and brown dwarf is hydrogen (protium) fusion.

Re: [Space] How much does a Gas Giant heat it's moons?
 

And isn't modern astronomy always finding objects right in what we thought were nicely defined no man's lands between categories?

Re: [Space] How much does a Gas Giant heat it's moons?
 

It's not a "no man's land" in this case. It's not entirely certain what the critical mass is, but it can either fuse hydrogen or it can't.

Re: [Space] How much does a Gas Giant heat it's moons?
 

.....and there are no celestial bodies (star or planet) that are too small to fuse deuterium but can still fuse some other type of hydrogen.

Re: [Space] How much does a Gas Giant heat it's moons?
 

Well, in principle you might have something only big enough to fuse tritium, but with a half-life of 12 years you can't possibly have enough tritium to matter.

Re: [Space] How much does a Gas Giant heat it's moons?
 

For reference, there are two competing dividing lines between brown dwarfs and (really, REALLY) big gas giant planets.

1) Deuterium burning. This has some observational consequences for some objects, but is otherwise really, really boring. It just measures mass.

2) Formation mechanism (direct gravitational collapse, like a star, or core-accretion building up a rocky core, like a planet). While more conceptually useful than definition (1), it doesn't make any observational predictions…

Re: [Space] How much does a Gas Giant heat it's moons?
 

And it is not entirely obvious that there is any firm distinction between those two methods, or that you can't have two apparently identical objects that formed by different means.

Re: [Space] How much does a Gas Giant heat it's moons?
 

Direct gravitational collapse alone would result in an object with a solar nebula abundance. Core-accretion would result in an object with greater amounts of metals, rocky material, and possibly ices. It is possible that such a difference could be measurable or at least rule out certain formation models for certain objects.

To bring this back on topic: it seems based on everything I'm hearing that you can safely ignore the heat given off by a gas giant for it's moons, unless your system is still in the process of forming.

Re: [Space] How much does a Gas Giant heat it's moons?
 

And if it is, you have much bigger issues to worry about, as the massive cometary bombardment phase may not be over yet.

Re: [Space] How much does a Gas Giant heat it's moons?
 

OK, revisiting this, the idea has crawled back into my head so I decided to muck around with values from our solar system, using Jupiter and Ganymede. Now Jupiter is 5.2 AU from Sol, so I square that, and raise to -1 and multiply by Jupiter's albedo of ~0.5 tells me that Jupiter counts as a star of ~0.02 Luminosity. Is this the correct way to treat Jupiter?

Now Ganymede is a little more the 1mil KM from Jupiter, or 0.007 AU. Now when I plug those numbers into the formula on page 113 it tells me that Jupiter would heat Ganymede to over 1,200 Kelvin ALONE, that can't be right, so what am I doing wrong?

Re: [Space] How much does a Gas Giant heat it's moons?
 

Don't bother. It would be a fraction of a degree.

Re: [Space] How much does a Gas Giant heat it's moons?
 

For one, Jupiter's albedo is .343, not .5. Also, Space uses AU for measuring distances within solar systems, not kilometers. Try the math again and see what happens!

Re: [Space] How much does a Gas Giant heat it's moons?
 

Not quite, you need to calculate the fraction of the Sun's energy output that Jupiter captures.. Jupiter's radius is about 7 X 10^9 cm,
while its orbit is about 7.8 X 10^13 cm, so Jupiter intercepts about (pi(7X10^9 cm)^2)/(4pi(7.8X10^13 cm)^2) = 2X10^-9 of the solar energy. The factor of 4 is the difference between the area of a circle of radius r (the area that Jupiter covers), and the area of a shell of radius r (the area that the Sun is shining on). A few billionths of a solar luminosity don't really change things very much, and note that Jupiter's temperature isn't controlled by the Sun.

Re: [Space] How much does a Gas Giant heat it's moons?
 

Mostly. In the limiting case after you could imagine the moon always sat exactly between the sun and the gas giant, and the gas giant was a perfect mirror and so much bigger than the moon you could treat it as a flat one. This completely ridiculous arrangement maximizes the amount of light falling on the moon. It doubles it, which raises its temperature by the fourth root of 2 (i.e. about 19%).

The heating gas giants do of their moons isn't radiant, it's tidal. Maybe occasionally there'll be some meaningful magnetic effects too, at least on the air temperature of something with an atmosphere, but reflected light isn't going to contribute very much.

Re: [Space] How much does a Gas Giant heat it's moons?
 

Considering various factors and aspects, I would estimate* that it would be about

{----------------------------this much------------------------}.





*Depending on the units you choose, and the frame of reference.

Re: [Space] How much does a Gas Giant heat it's moons?
 

That last bit is actually part of the thing that makes this interesting. If Jupiter was bigger how much would it heat things up? If it was about as far away from Sol as Mars is how much brighter would it be?

Re: [Space] How much does a Gas Giant heat it's moons?
 

As a rule of thumb (this fails at very short distances), you can take the surface temperature of the object and multiply by sqrt( 0.5 * radius / distance ) to get the temperature it would apply to a spherical blackbody subject to no other heat sources. For example, take the sun (temperature 5778K, radius 696,342 km, distance 149,600,000 km) and we get a blackbody temperature of 279K for the Earth. For an object at the roche limit, the result is 45% of the temperature (and 4% of the heat per unit area). This is significantly affected by which face of the giant is visible, and for low orbits whether or not the moon slips through the shadow of the giant.

Re: [Space] How much does a Gas Giant heat it's moons?
 

Does all that mean that double planets would be warmer than initially assumed because they reflect light and heat onto each other?

Re: [Space] How much does a Gas Giant heat it's moons?
 

They also occasionally block sunlight from the other, so it's likely a wash, and in any case most of the time the effect is negligible.

Re: [Space] How much does a Gas Giant heat it's moons?
 

Cool, thanks. I was afraid my double planet system had even one more hassle than those I already knew about.

Re: [Space] How much does a Gas Giant heat it's moons?
 

So if I've got a moon of .56 Earth diameters (.23 radius) orbiting 61.07 Earth diameters from a Gas Giant the Blackbody temperature of the moon is increased by 4% of the Gas Giant's temperature? Is that correct? And how do I calculate the temperature of the gas giant?

Re: [Space] How much does a Gas Giant heat it's moons?
 

No, for several reasons. First, we need the radius of the gas giant. Second, you actually add T^4 from all sources and then take the 4th root, so if the secondary heat gave a temperature of 4% of the primary, the actual increase is (1 + .04 ^ 4)^1/4 - 1, or 0.000064%

Re: [Space] How much does a Gas Giant heat it's moons?
 

Not so often though, unless they're orbiting perfectly flatly in the equatorial plane. Look at how uncommon eclipses are for the Earth and moon to block sunlight from each other.

For a moon around a gas giant, however, it is more likely. You'd have to figure out the axial tilt of the gas giant and then the semi-major axis and eccentricity inclination of the moon's orbit to see how long it spends under occultation.