Thread: [Traveller_TNE] The Gas Giant in the Habitable Zone Thread

--- In , "paulsnowuk" <P.A.Snow@...> wrote:
>
> --- In , "Allen" <justacarpenter@>
> wrote:
> >
> > The more we search, the closer we get. Now we have found a star
> system
> > with a Gas Giant in the habitable zone. Can life be supported on a
> gas
> > giant moon? we might never know in our lifetimes
> > http://www.sciencedaily.com/releases/2007/11/071106133058.htm
> >
>
> Actually, I thought this was rather interesting too.
>
> http://www.nytimes.com/2007/11/06/science/space/07planetweb.html?
> _r=1&oref=slogin
>
> More chances of a goldilocks zone? Any real astrophycisits here to give
> us a briefing on the current thinking?
>
> Paulsnow
>
--- In , "Evil Dr Ganymede"
<evil_dr_ganymede@...> wrote:
>
> --- In , Bo <lorgryt@> wrote:
> > Can life be supported on a gas giant moon? Of course it can, and the
> > idea of a habitable zone is pretty much obsolete at this point.
> > Life exists in very different types of environs than we ever thought
> > it could.
>
> True, but that doesn't make the idea of a habitable zone obsolete. The
> fact remains that there's still a band around a star at a certain
> distance in which the temperatures are appropriate for organic life
> (in fact it's defined (according to Kastings 1993, Icarus, v101, p108)
> roughly speaking the definition is as follows:
>
> Inner boundary: Maximum distance from the star at which a runaway
> greenhouse effect would lead to the evaporation of all surface water.
>
> Outer boundary: Maximum distance at which a cloud-free CO2 atmosphere
> could maintain a surface temperature of 273 K.
>
> In practice that actually varies from star to star (it's not a
> constant times the root of the luminosity though as described in
> various Traveller books - details are in Jones/Sleep/Underwood 2003,
> ApJ, v649, p1010-1019) but for a G2 V star like the sun it's between
> 0.82 AU and 1.64 AU.
>
> That's not to say it can't form or survive elsewhere (underground or
> under ice where there is water, in cometary interiors, etc), but the
> habitable zone is still a valid concept.
>
>
> [quote]But, remember, life is just a single cell looking for love. And
> I believe that can and does exist around Jupiter, if not in its
> atmosphere.[/quote]
>
> If there's an ocean under the ice on Europa and Ganymede and Callisto
> (which seems likely) then I'd agree. I don't think we know enough to
> be certain quite yet - we don't even know how life forms to start
> with. Maybe it always forms whenever conditions are right, maybe it
> forms only very rarely even if conditions allow it.
>

--- In , "DED" <dedly@...> wrote:
>
> I was looking something up in TD19 and I caught the following out of
the corner of my eye. I don't know if it's of much use. Anyway,
someone wrote in asking what the proper way is to compute the seismic
stress factor if the subject world is a satellite of a gas giant.

The trouble with the SSF is that it's never actually meant anything -
it's just a number with no basis in reality.

This is what you get for the moons in our own solar system:

Luna: 0.90
Io (J): 2.28
Europa (J): 1.30
Ganymede (J): 1.32
Callisto (J): 0.70
Enceladus (S): 0.24
Titan (S): 0.50
Miranda (U): 0.34
Triton (N): 0.92

The Regina value that DED calculated may be fine using those source
numbers, but those are unfortunately wrong (you can't have a gas giant
in that situation with 240,000km diameter - when they start getting
more massive than Jupiter they just compact all the extra mass in the
same volume... the only way you can get a bigger radius is if it's
hugging a star and its atmosphere is very distended as a result of the
heat/tides). If we used realistic values for Assiniboia (from my
thread at http://www.traveller.comstar-games.com/viewtopic.php?t=1471
), and assumed that Regina was still in orbit 55 around it, then
Regina's SSF actually goes up to:

M = (148,240 x 1.60) / (55 x (148,240/11,920) x 64) = 5.42

Which is way higher than Io.

There's a couple of problems here:

1) The SSF numbers mean nothing. Our own moon has an SSF of 0.90. 0.90
of what though? What are the units? What is it even measuring - heat
flow? number of earthquakes? tidal height? Why is a value of 1.0
important as opposed to 1.5 or 0.5 or 2.0? And if my very rusty
dimensional analysis skills are correct, the units of M are
mass/length^2... which is kg/m2, and meaningless.

Even taken relatively they don't mean much. A value of 1.8 means that
the satellite has an SSF that is twice as high as that of our own
Moon... but there's no reference frame to put that in since the units
are meaningless.

Looking at the numbers, Europa, Io, and Ganymede are all over 1. Well,
Io is obviously hypervolcanic, but Europa isn't - though it is
geologically active and does have a deep ocean under its ice. And
Ganymede's probably got some geological activity (it certainly HAD
some, given all the faults and new terrain on its surface) but
nothing's really going on today. But while Enceladus' SSF is only
0.24, we know that it's blasting off tons of material every day from
massive geysers on its surface. And Triton's SSF is about the same as
Luna's, but it has geysers too (then again, it's made of ice, not
rock). So even in our own system, the numbers seem to be fairly
meaningless.

2) The SSF formula would have us believe that 'seimic stress' is down
to the size and density of the primary and the distance of the
satellite from its primary. It's not though - the composition of the
satellite (which we can say is illustrated by its density) is a major
factor - put the same amount of heat into a rocky body and an icy
body, and the icy body will respond a lot more spectacularly since the
ice will melt at lower temperatures (which can result in geysers, more
rapid internal differentiation, etc).

The eccentricity of its orbit is another very major factor since that
determines the extent of tidal dissipation - as I think I mentioned
earlier, if you keep something in even a slightly eccentric close
orbit around a large, massive planet then it'll heat up drastically
because the effect of those small variations in distance are amplified
greatly by the size of the primary. And if you have other moons nearby
to keep a body in an eccentric orbit through resonances, then we need
to know about them in a formula.

So right there we have the tidal dissipation being actually related to
the mass of the primary, the composition of the satellite, and the
current orbital properties of the satellite (primarily distance and
eccentricity and influence of nearby moons). And then you have
intrinsic properties like the age of the satellite (a young moon will
just be more geologically active because its still got its own
internal heat supply from radioactive decay). It's very difficult in
practice to boil that all down to a single SSF number - I do it more
qualitatively by seeing if it's in an orbital resonance, or an
eccentric orbit, or in a young system, and then go from there.

On 29 Nov 2007 at 13:43, DED wrote:

>
> Hence why I wrote: "I make no guarantees of the veracity of this
> formula (or my math). I'm just passing along the info." =)
>
> One question with regards to: "when they start getting more massive
> than Jupiter they just compact all the extra mass in the same
> volume... "
>
> How do we know this? Is it with computer models?

Probably. But given that the same sort of models also work quite well
for star sizes and are based on lab experiments with small volumes of
very high pressure.

Basically, once you reach a certain mass (around that of Jupiter),
the pressure at the core is enough to causing degenerate matter to
form.

That is, the electron shells of the atoms can no longer keep them as
far apart as in normal solids and density shoots up.

That higher density at the core makes the body smaller.

And they'll keep getting smaller as the mass goes up right until the
point where fusion starts happening.

As I recall, brown dwarfs have some deuterium fusion going on at the
core so they get a bit bigger or at least not smaller.

Once the mass hits the point where protium (H1) can fuse, you get a
large expansion as the body becomes a red dwarf star.

Going from small gas giant on thru the largest stars is all pretty
much one set of formulas and models.

--
Leonard Erickson (aka shadow)
shadow at shadowgard dot com

On 29 Nov 2007 at 13:43, DED wrote:

>
> Hence why I wrote: "I make no guarantees of the veracity of this
> formula (or my math). I'm just passing along the info." =)
>
> One question with regards to: "when they start getting more massive
> than Jupiter they just compact all the extra mass in the same
> volume... "
>
> How do we know this? Is it with computer models?

Probably. But given that the same sort of models also work quite well
for star sizes and are based on lab experiments with small volumes of
very high pressure.

Basically, once you reach a certain mass (around that of Jupiter),
the pressure at the core is enough to causing degenerate matter to
form.

That is, the electron shells of the atoms can no longer keep them as
far apart as in normal solids and density shoots up.

That higher density at the core makes the body smaller.

And they'll keep getting smaller as the mass goes up right until the
point where fusion starts happening.

As I recall, brown dwarfs have some deuterium fusion going on at the
core so they get a bit bigger or at least not smaller.

Once the mass hits the point where protium (H1) can fuse, you get a
large expansion as the body becomes a red dwarf star.

Going from small gas giant on thru the largest stars is all pretty
much one set of formulas and models.

--
Leonard Erickson (aka shadow)
shadow at shadowgard dot com

--- In , "DED" <dedly@...> wrote:
>
> Hence why I wrote: "I make no guarantees of the veracity of this
formula (or my math). I'm just passing along the info." =)

Yep. I was just clarifying why the formula wasn't very useful, is all :).

> One question with regards to: "when they start getting more massive
than Jupiter they just compact all the extra mass in the same volume... "
>
> How do we know this? Is it with computer models?

Yeah, with physics really. Once you start adding more mass, it
compacts the stuff in the middle more because the pressure is going up
in the central parts. So instead of increasing the radius, you end up
increasing the density of the planet. A massive brown dwarf packs up
to 60-70 Jupiter masses in a volume that's actually *smaller* than
Jupiter , so its density (and surface gravity) are ridiculously high.
If it crosses the BD/star boundary though then the fusion reactions in
the core make the object expand considerably (to about 200k-300k km
radius) and the density and gravity drop accordingly because of the
extra volume.

I don't think the densities inside brown dwarfs are anywhere near high
enough to form degenerate matter in their interiors - that does happen
in red giant stars, though. In BDs it's just normal gravitational
compaction of matter.