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

--- 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.