Seasons on gas giant moons

Here's something I got to thinking about while considering Earth-like
moons around gas giants in the habitable zones around stars. What
would seasons be like? (I will use the term "year" for one orbit of
the gas giant, "month" for the time it takes for the gas giant to go
from "full" to "new" and back to "full", and "day" for the time
between sunrises on the moon). There are a number of effects to
consider:

1) the tilt of the moon's spin relative to the orbit around the gas
giant
2) the tilt of the moon's orbit relative to the gas giant's orbit

and less importantly, the eccentricities of the two orbits. Here are
a few things I would like to get a handle on:

what is the likely ranges of 1) and 2) ?

For a habitable moon around a sun-like star, the "year" will be ~1
Earth year; Titan is tidally locked with a 16 day "day/month".
Jupiter's major satellites range from a 2-16 days, with the largest
one having a 7 day "day/month". These are all significantly smaller
than Earth, though; would something Earth-sized also likely be
tidally locked?

Just think: somewhere out there, there could be a planet with normal
year long seasons, but where it's usually cold on Wednesday but warm
for the weekends

Though the fact that day and night last three and a half days could
be somewhat annoying ...

Hephaestus wrote:


For a habitable moon around a sun-like star, the "year" will be ~1
Earth year; Titan is tidally locked with a 16 day "day/month".
Jupiter's major satellites range from a 2-16 days, with the largest
one having a 7 day "day/month". These are all significantly smaller
than Earth, though; would something Earth-sized also likely be
tidally locked?

There is a peculiar resonance going on between Earth and Venus that
leads to Venus always having the same face displayed toward us at
closest approach; considering the great distance between the two
planets, to me this suggests that objects tend to get tidally locked
sooner or later despite great distances.
A quick check of my "Atlas Of The Solar System" shows that _all_ the
known moons of Jupiter and Saturn are presumed to be tidally locked.
Saturn's "shepherd moons" might make an interesting case in this regard,
it will be fun to see if Cassini finds them locked also.
One of the big problems in regard to habitable planets around a gas
giant is going to be the radiation fields; on the side facing the star
it orbits they will probably come out a fair distance, but they really
extend out behind the planet- if Jupiter is anything to go by- and might
sterilize the surface of any habitable moon as it passes through them in
its orbit: http://blueox.uoregon.edu/~courses/B...1/FG11_011.jpg

Pat

In article ,
says...

snip

One of the big problems in regard to habitable planets around a gas
giant is going to be the radiation fields; on the side facing the star
it orbits they will probably come out a fair distance, but they really
extend out behind the planet- if Jupiter is anything to go by- and might
sterilize the surface of any habitable moon as it passes through them in
its orbit: http://blueox.uoregon.edu/~courses/B...1/FG11_011.jpg

Well... that depends on whether or not such a moon had its own strong
magnetic field, wouldn't it? If you speculate that a gas giant is
within a star's habitable zone and that it has a moon that has similar
composition to Earth, then it would become a question of whether or not
it would retain enough rotation to generate a dynamo effect from its
molten core that would create a magnetic field.

If you *did* have a magnetic field comparable to Earth's, wouldn't that
serve to protect said moon from the radiation effects of the gas giant's
own magnetic field?

Doug

In article ,
Pat Flannery wrote:

One of the big problems in regard to habitable planets around a gas
giant is going to be the radiation fields; on the side facing the star
it orbits they will probably come out a fair distance, but they really
extend out behind the planet- if Jupiter is anything to go by- and might
sterilize the surface of any habitable moon as it passes through them in
its orbit: http://blueox.uoregon.edu/~courses/B...1/FG11_011.jpg

Sterilization is relative. What might be lethal doses of radiation to
Earth life may well be a handy source of energy to life forms that had
evolved in it. I can see no a priori reason why evolution couldn't cope
with quite high radiation levels.

(It reminds me of this other planet I heard of, where the whole
atmosphere and even the oceans were polluted with an extremely reactive,
toxic molecule, but in the end this just kick-started a whole new phase
of evolution.)

,------------------------------------------------------------------.
| Joseph J. Strout Check out the Mac Web Directory: |
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Doug... wrote:

Well... that depends on whether or not such a moon had its own strong
magnetic field, wouldn't it? If you speculate that a gas giant is
within a star's habitable zone and that it has a moon that has similar
composition to Earth, then it would become a question of whether or not
it would retain enough rotation to generate a dynamo effect from its
molten core that would create a magnetic field.

If you *did* have a magnetic field comparable to Earth's, wouldn't that
serve to protect said moon from the radiation effects of the gas giant's
own magnetic field?


That's a very interesting thought; but it implies a molten iron core in
the Earth-sized moon to generate the magnetic field.
Assuming we are dealing with a gas giant planet in a sun's habitable
zone, would such a planet/moon evolve? It's thought that all the major
satellites of the gas giants consist primarily of ices and the lighter
types of rock such as silicates, not the heavier elements, such as iron.
So to get this to work, you have to have a big and hot star whose
habitable zone was out at the distance where you would find gas giants,
and I would think that most of the ices and gases that would have made a
proto-planet into a gas giant would have boiled off during the planet's
accretion, so that you would end up with a huge rocky planet instead of
a gas giant. (between Earth to Neptune sized?)
Alternately you could have the proposed predator gas giant that spirals
in toward the sun- eating all the other planets in the system as it
goes- until it arrives at a Sun-type star's habitable zone- I imagine
it _might_ be possible (though I think highly unlikely) for such a
planet to grab an Earth-type world as a satellite on it inexorable way
in, but the whole situation sounds unstable and short lived in
biological evolutionary terms; I doubt the captured planet's orbit would
be anywhere near circular, and that could lead to some pretty
devastating tidal effects as it orbits the gas giant. besides that,
interaction with the gas giant's own moons could lead to either
collision with them, or a perturbation of the captured planet's orbit to
where it either collided with the gas giant or was hurled out of its orbit.
Frankly, I wouldn't like to look up in the sky and see something like
Jupiter hanging there, far larger than the full Moon.

Pat

In article ,
says...
Doug... wrote:

Well... that depends on whether or not such a moon had its own strong
magnetic field, wouldn't it? If you speculate that a gas giant is
within a star's habitable zone and that it has a moon that has similar
composition to Earth, then it would become a question of whether or not
it would retain enough rotation to generate a dynamo effect from its
molten core that would create a magnetic field.

If you *did* have a magnetic field comparable to Earth's, wouldn't that
serve to protect said moon from the radiation effects of the gas giant's
own magnetic field?


That's a very interesting thought; but it implies a molten iron core in
the Earth-sized moon to generate the magnetic field.
Assuming we are dealing with a gas giant planet in a sun's habitable
zone, would such a planet/moon evolve? It's thought that all the major
satellites of the gas giants consist primarily of ices and the lighter
types of rock such as silicates, not the heavier elements, such as iron.
So to get this to work, you have to have a big and hot star whose
habitable zone was out at the distance where you would find gas giants,
and I would think that most of the ices and gases that would have made a
proto-planet into a gas giant would have boiled off during the planet's
accretion, so that you would end up with a huge rocky planet instead of
a gas giant. (between Earth to Neptune sized?)
Alternately you could have the proposed predator gas giant that spirals
in toward the sun- eating all the other planets in the system as it
goes- until it arrives at a Sun-type star's habitable zone- I imagine
it _might_ be possible (though I think highly unlikely) for such a
planet to grab an Earth-type world as a satellite on it inexorable way
in, but the whole situation sounds unstable and short lived in
biological evolutionary terms; I doubt the captured planet's orbit would
be anywhere near circular, and that could lead to some pretty
devastating tidal effects as it orbits the gas giant. besides that,
interaction with the gas giant's own moons could lead to either
collision with them, or a perturbation of the captured planet's orbit to
where it either collided with the gas giant or was hurled out of its orbit.
Frankly, I wouldn't like to look up in the sky and see something like
Jupiter hanging there, far larger than the full Moon.

Actually, several of the extrasolar planets that have been discovered in
the past few years are gas giants that orbit their stars either as close
as one AU, or closer. They're called "hot Jupiters" in some of the
literature.

Now, the question is, could a hot Jupiter form in a second-generation
star's planetary system? The planetary nebula/disk of a second-
generation star incorporates the heavy elements formed in a massive star
that went kablooey, right? Perhaps the systems with hot Jupiters are
all first-generation stars, and the lack of heavier elements caused a
single gas giant to form in the close-in range.

But if a hot Jupiter *could* form in a second-generation star's system,
then it might make sense that some or all of its moons would be formed
of rock, not ice. That would mean that they could have roughly
terrestrial compositions.

The bigger problem in generating a strong magnetic field, as I see it,
would be the rotation issue. Every sizable moon of the gas giants in
our own solar system are tidally locked to their primary. I wonder just
how fast a moon with a molten nickel-iron core would have to rotate in
order to generate a magnetic field? That would define the maximum
distance at which such a moon could orbit its primary, since distance
determines orbital period, and with a tidally locked moon, rotation
would equal orbital period.

Can you imagine living on such a moon, though? On one side of the
thing, the primary would hang like a huge, baleful presence, dominating
the sky and everything under it. On the other side, there would just be
the alternation of the sun, stars and other moons. Such a dichotomy
would have profound effects on any civilization that arose there.

Doug

"Doug..." wrote:
But if a hot Jupiter *could* form in a second-generation star's
system,
then it might make sense that some or all of its moons would be
formed
of rock, not ice. That would mean that they could have roughly
terrestrial compositions.

The bigger problem in generating a strong magnetic field, as I see
it,
would be the rotation issue. Every sizable moon of the gas giants
in
our own solar system are tidally locked to their primary. I
wonder just
how fast a moon with a molten nickel-iron core would have to
rotate in
order to generate a magnetic field? That would define the maximum
distance at which such a moon could orbit its primary, since
distance
determines orbital period, and with a tidally locked moon,
rotation
would equal orbital period.

Can you imagine living on such a moon, though? On one side of the
thing, the primary would hang like a huge, baleful presence,
dominating
the sky and everything under it. On the other side, there would
just be
the alternation of the sun, stars and other moons. Such a
dichotomy
would have profound effects on any civilization that arose there.

Is there a mechanism that suggests that large terrestrial moons
would be more likely to form *close to* a gas giant? The Earth's
distance to the Sun varies by about five million km, and besides
that, the seasons have more to do with Earth's axial tilt than with
actual Earth-Sun distance.

I have no tools with which to analyze the likelihood of an
Earth-sized planet forming "far away" from a gas giant, though.
Still, if possible, it would remove some of the difficulties
associated with radiation from the gas giant itself and tidal
locking.

I could imagine a civilization working things out with geometry for
the first time, and shortly after their Eratosthenes-equivalent
figures out the size of their *own* planet, discovering with a
certain amount of shock how big that small disk in the sky *really
is*...

-- Best regards,
Matt Funke

In article ,
says...

snip

Is there a mechanism that suggests that large terrestrial moons
would be more likely to form *close to* a gas giant?

The only examples we can look at closely (at least comparatively
closely) are the gas giants in our own solar system, and their moons.
It's really impossible to say if the moons of a hot Jupiter would form
in the same manner as they have here, around the relatively cold gas
giants in Sol System, but it *is* true that the Jovian moon of any real
size closest to its primary, Io, is entirely made of rock. The rest of
the Galilean moons are also made mostly of rock -- they just have a
whole lot of ice (and maybe liquid water) on top of the rock.

Also, consider that Io does seem to have a molten core. Now, the Jovian
system is probably so depleted in heavy elements (relative to the inner
system) that Io's core is mostly silicate rocks and not primarily
metals, like iron or nickel, so it doesn't have a strong intrinsic
magnetic field. But if Jupiter was located one AU from Sol and it and
its moons collected most of the heavy metals out of the original solar
nebula, it might be possible that Io and the other large moons would
have been blessed with molten metal cores.

Of course, there is the problem that a hot Jupiter might just gather all
of the heaviest elements into itself, leaving only the lighter elements
for aggregation into its moons -- in which case, achieving gas giant
moons of terrestrial composition might be impossible. I'm sure you can
come up with models for both cases -- it will probably have to wait
until we can investigate other planetary systems far more closely before
we can know anything for certain.

And that isn't likely to happen in our lifetimes, unless we develop an
FTL drive in the next couple of years.

Doug

Pat Flannery wrote:
Hephaestus wrote:


For a habitable moon around a sun-like star, the "year" will be ~1
Earth year; Titan is tidally locked with a 16 day "day/month".
Jupiter's major satellites range from a 2-16 days, with the largest
one having a 7 day "day/month". These are all significantly smaller
than Earth, though; would something Earth-sized also likely be tidally
locked?

There is a peculiar resonance going on between Earth and Venus that
leads to Venus always having the same face displayed toward us at
closest approach; considering the great distance between the two
planets, to me this suggests that objects tend to get tidally locked
sooner or later despite great distances.
A quick check of my "Atlas Of The Solar System" shows that _all_ the
known moons of Jupiter and Saturn are presumed to be tidally locked.
Saturn's "shepherd moons" might make an interesting case in this regard,
it will be fun to see if Cassini finds them locked also.
One of the big problems in regard to habitable planets around a gas
giant is going to be the radiation fields; on the side facing the star
it orbits they will probably come out a fair distance, but they really
extend out behind the planet- if Jupiter is anything to go by- and might
sterilize the surface of any habitable moon as it passes through them in
its orbit: http://blueox.uoregon.edu/~courses/B...1/FG11_011.jpg

Pat


SFAIK Saturn doesn't have a powerful magnetic field like Jupiter's.
Which mystifies me as it would seem the Saturn also would have a rapidly
rotating ball of metallic hydrogen at it's core.


--
Hop David
http://clowder.net/hop/index.html

Doug... wrote:

Actually, several of the extrasolar planets that have been discovered in
the past few years are gas giants that orbit their stars either as close
as one AU, or closer. They're called "hot Jupiters" in some of the
literature.


From what I've read these are assumed to be the predatory gas giants in
the last stages of their descent into their suns.


Now, the question is, could a hot Jupiter form in a second-generation
star's planetary system? The planetary nebula/disk of a second-
generation star incorporates the heavy elements formed in a massive star
that went kablooey, right?


That would put the planet in orbit around a white dwarf, wouldn't it?

Perhaps the systems with hot Jupiters are
all first-generation stars, and the lack of heavier elements caused a
single gas giant to form in the close-in range.


There is obviously something different about what happened in these
systems and what our own is presently like


But if a hot Jupiter *could* form in a second-generation star's system,
then it might make sense that some or all of its moons would be formed
of rock, not ice. That would mean that they could have roughly
terrestrial compositions.

But now you have the problem of again having heavy elements to work with
for the moons, but a lack of light gases for the formation of the
proposed Jupiter-like planet; they would have been striped away from the
area near the star by the nova explosion as it entered old age.


The bigger problem in generating a strong magnetic field, as I see it,
would be the rotation issue. Every sizable moon of the gas giants in
our own solar system are tidally locked to their primary. I wonder just
how fast a moon with a molten nickel-iron core would have to rotate in
order to generate a magnetic field? That would define the maximum
distance at which such a moon could orbit its primary, since distance
determines orbital period, and with a tidally locked moon, rotation
would equal orbital period.


On something as big as a gas giant, that would be a very good distance
indeed; and I'm fairly sure that large bodies get tidally locked sooner
than small ones (maybe even during formation) so a Earth-sized one would
have to be pretty far out to escape the effect. The Venus/Earth
resonance is a good case in point- either it is one hell of a
coincidence, or there is some sort of a tidal lock caused by the gravity
fields of the two planets even at the great distance in comparison to
their masses that is involved.


Can you imagine living on such a moon, though? On one side of the
thing, the primary would hang like a huge, baleful presence, dominating
the sky and everything under it. On the other side, there would just be
the alternation of the sun, stars and other moons. Such a dichotomy
would have profound effects on any civilization that arose there.


But on the other hand, what an invitation to space travel, assuming that
the primary has other moons!
.....and the obvious orbiting of such moons around the primary as
indicated by its eclipsing of them would be an easily seen lesson in
celestial mechanics, and mean that there would probably be no rise of a
geocentric cosmological theory, but rather a early recognition of the
true nature of a solar system.

Pat