RE: [sl4] giant planets ignition - one more existential risk

From: Larry (
Date: Fri Dec 12 2008 - 09:22:03 MST

On Fri, 12 Dec 2008, William Hall wrote:

> Alexi,
> Let's get real here.
> To fuse, deuterium nuclei have to come together physically in an area the
> size of the atomic nucleus. This takes a great deal of pressure and heat.

This happens in an H-bomb.

> More likely than two deuterium nuclei fusing is that a deuterium nucleus and
> a hydrogen nucleus will fuse to form helium 3. I doubt that the pressures in
> a gas giant are anything like high enough to support detonation of the whole
> planet. Unlike nuclear fission, where fast neutron cause fission ahead of
> the shock front resulting from the release of energy by prior fission events
> and thus generate true detonations, fusion requires extremely high pressures
> and temperatures such as are found in the centers of stars.

Or in a fusion detonation wave, especially in a liquid hydrogen ocean.

> Even assuming that you could set of an H bomb in the center of a gas giant,
> the heat released would cause the surrounding material to expand and thereby
> reduce the probability of further fusion.

Actually the heating would come only after the detonation wave. High temp
plasma is opaque to light, neutrinos are the only thing that would out run
the wave. The wave travels faster than the speed of sound, the molecules
wouldn't "know" to move out of the way due to prior heating.

> As long as significant fusion
> continues, there is a homeostatic relationship between pressure and the
> release of heat. As the rate of fusion increases and more heat is released
> the star expands to reduce the rate of fusion.

Stars begin there initial nuclear ignition slowly, pressure slowly grows
until conditions are right, at that point it self regulates. Of course you
might not want to be right next to a star that just "turned on".

> Conceivably, if the temperature of a gas giant was raised locally by a bomb,
> perhaps there would be enough pressure in the core to sustain a continuing
> rate of fusion. With luck, you might end up with a short-lived brown dwarf.
> Even if the surface temperature of the planet reached the temperature of the
> Sun, several factors would mitigate any significant impact on the
> temperature and ecology of the earth.

I don't believe it would have sustained fusion, only a possible
detonation. Of course this doesn't need to be argued about, nuclear
physics can provide a definite answer. Regrettably I don't have
the background to do this math, the people who are most qualified
likely have very high security clearances. However I certainly can't
claim to rule it out, it certainly seems a liquid hydrogen ocean might
be able to support a fusion detonation wave.

> ...

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