Re: Help with worldbuilding


brdavis@xxxxxxxx wrote:
> Denni wrote:
>
> > I still have to <i>read</i> ["Worldbuilding" by Gillet]
>
> Do, it's well worth the time. And don't be scared, or embarassed, or
> anything, in this forum - this is a GREAT place to ask this sort of
> thing, and there's a good number of informed folks here who love
> helping out.
>
> > at 0.85 solar masses as an example, with my
> > world circling at 0.4 AU, about 0.5 Terra years orbit
>
> The orbital period at 0.4 AU from a 0.85 solar mass star should be
> sqrt(a^3/M) = sqrt( 0.4^3 / 0.85 ) = 0.27 years, or about 100
> terrestrial days. And yes, "Worldbuilding" will show you how to do this
> & make it simple.
> Where did you get the 0.85 solar masses from? And no, that's not at
> all a criticism - there are so many different references for this stuff
> I'm not sure who's "right". I've got written down 0.71 solar masses (&
> I didn't note where I got that, either - my bad).
> I also have listed the luminosity at about 0.292 solar, meaning at
> 0.4 AU from the star, the planet will receive 1.8 times the insolation
> (amount of light) as the Earth - it's going to be really, really warm,
> like warmer than Venus, by my rough estimate.
> If you still want to use Eps Eri, I'll assume a luminosity of 0.292
> solar and a mass of 0.71 solar. To get Earth-normal insolation, put the
> planet 0.54 AU out from the star. At 0.54 AU from a star with 0.71
> solar mases, the orbital period would be 0.47 terrestrial years.
>
> > The planet is a low gravity world, about 1/4
> > Terra's, half its size...
>
> I'll assume here by "half its size" you mean half the radius. That
> gives use enough to figure the planets density at about half Earth's.
> Earth is at 5,520 kg/m^3; this 0.25 G half-the-Earth's-radius planet
> would have a density of 2,760 kg/m^3, a little on the light side for
> comfort. Even the Earth's Moon is denser, at 3,340. Probably the only
> way to get the density down that low is to have a significant
> percentage of low density ices in the mix, and that's not going to make
> for a very stable bulk planet (if it's that warm).

For comparison:
The data of OP: diametre about 6400 km, density 2,76.

Mars: diametre 7000 km, density 3,94 or so.
Moon: diametre 3500 km, density 3,34

Ganymede: diametre 5260 km, density 1,94
Titan: diametre 5150 km, density 1,88

Ceres: diametre 1000 km, density 2,1

So. The OP-s planet would probably be appreciably denser than a body
with composition of Ganymede or Titan, even after the gravity
compression.

Ceres, despite low density, does not look like having an ocean like
Europa.

Are there any low-density, but rocky, materials available? Like
carbonates, carbon itself, quartz, salts... I think that the
sedimentary rocks of Earth are generally less dense than Moon. Is 2,76
in bulk doable? Phobos and Deimos have densities of around 2. They are
small, but Ceres is not.

> To keep the 0.25 G surface gravity, but using a reasonable density
> (say, that of Mars; 3,940 kg/m^3), the planet would be 0.35 Earth's
> size, or a radius of 2,234 km. Really rather small, smaller than Mars.
>
Would be unwise. You want all the possible escape speed.

Titan holds a massive atmosphere with escape speed of 2,65 km/s, but it
is not far from marginal. Ganymede has none. So, an escape speed of 4
km/s (as follows from OP-s specifications) might be just enough.

> > but with a similar magnetic field...
>
> As another poster mentioned, that generally requires a conducting,
> convecting fluid core - i.e., the planet still needs to have internal
> heat production due to radioisotopes. Do you *need* a magnetic field?
> It's not needed (IMS) for biology, nor to protect the atmosphere.
>
> > and a breathable, <i>much</i> denser,
> > atmosphere (gliding plays a part in the story)
>
> Gliding will be easier on such a low-gravity world. If you want it
> human-breathable, 400 mb pO2 (that's "partial pressure of oxygen", one
> atmosphere is 1013 mb) is about the upper limit,

Has been argued about. Some are more optimistic, and talk of 500-600
mb.

> while for pN2 higher than about 3,100 mb isn't a good idea (unless you *want* > your
> characters to be "narked out" with nitrogen narcosis). Combining those,
> the total surface pressure for a N2/O2 atmosphere would end up about
> 3,500 mb (just under 3.5 Atm) with around 11% oxygen (fires would be
> much tougher to make on this world).

Indeed. And this could thwart attempts at making engines.

> As a fun story note, the humans
> might want to stay near the mountaintops; going down (a lot; see below)
> to the lowlands would increase the total pressure (not a problem) as
> well as the partial pressure of N2 (a problem; narcosis again).

And oxygen partial pressure. Can be a problem with lung irritation.

How much pCO2 is acceptable? I think the limit is somewhere about 20-50
mb (carbonic acid in blood...)

> The gas density (what you want to worry about for gliding) will
> depend on temperature, but we can rough it out assuming a nice 293 K
> (about room temperature) at around 4.1 kg/m^3; Earth-normal is about
> 1.4 kg/m^3, so flyers are in *great* shape on this world. Due to the
> low gravity, they only need to produce 1/4th the lift of an Earthly
> bird, but each patch of wing can now deflect 2.9 times the mass of that
> same Earthly-analog bird; assuming other things being equal*, wings
> could work with less than 10% of the surface area of their terrestrial
> counterparts.
> Note that ballooning would be even easier here; on Earth, the
> greatest density contrast you could get is 1.4 kg/m^3 (vacuum inside
> the balloon, normal atmosphere outside). Here you could, again, get
> nearly three times that density contrast, and, again, the gravity being
> lower... fun.
> This was essentially the setup some folks (including for a while me)
> were trying to use to flesh out a world called "Velvet", with a mostly
> airborn biosphere. It was great fun while it lasted (although, it turns
> out hailstones were a *serious* problem).
> This would also be an amazingly thick atmosphere, in that the
> pressure would not drop as rapidly with altitude, nor would the
> temperature drop as rapidly with altitude, as on Earth. Both these
> things, again, are tied to the gravity. Convective storms are usually
> only as wide as they are high, with their height limited by the
> tropopause; on your world, the tropopause will be very high, resulting
> in very very wide, very tall convective complexes... think
> "superthunderstorms", black as night beneath and extending for perhaps
> hundreds of kilometers wide, with denser air in the longer updrafts. A
> rough calculation predicts hailstones could be 14 times as wide, or
> more than 2000 times as massive as Earthly counterparts... ouch.
>
> > what proportion of it has to be land?
>
> What proportion do you want to be land? More ocean means storms have
> a greater potential to grow in many cases, and the wind-driven waves
> will be higher (more fetch, more time to build waves). Some oceans are
> probably a good idea.
>
> > It is very poor in radioisotopes
>
> Why? Is that needed for the story? Radioisotopes buy you an internal
> heat source, which allows magnetic fields, volcanic and
> mountain-building activity, and lots of other useful things. The fact
> that this planet is so small means that given an Earth-normal
> concentration of radioisotopes the heat per unti surface area radiated
> is going to be *less* than Earth's, so you might want to go with a
> "radioactively-enriched" planet in some cases.
>
> > The dominant colour of the vegetation in
> > the dimmer light might be red/purple.
>
> It probably will be dimmer near the bottom of this atmosphere, but
> remember I set up this "thought experiment" by assuming (ass-u-me) an
> Earth-normal insolation level.

Possible. But 1) the star is redder - therefore for a given total
energy flux, there is less shortwave light for human vision or plant
photosynthesis, 2) there may be strong greenhouse effects by carbon
dioxide or thick layers of water vapour, allowing Earthlike
temperatures at lower total energy flux and 3) thick clouds were
assumed, so much of the light is reflected.

> Also, planets don't have to be very
> efficient (Earthly photosynthesis is terrible, for instance, only about
> 3-5%), they just have to beat out the neighbors. If you just want
> "other-worldly", I'd go with chlorophyll-based with phycobilins as
> accessory pigments. Or even a rhodopsin-based system. But the color is
> probably up to your imagination here, not biochemistry.
>
Hm. Why not biochemistry?

Plants on Earth normally use blue and red light and discard the green.

They have 2 circumstances of nonstandard spectrum of light:

In deep, clear water of clear seas and lakes, water absorbs the red.
The plants must make do with blue light alone, and have special
pigments to deal with it (brown and red seaweeds).

In understory of thick vegetations, plants must make use of light
already rejected by other plants. Mostly green. What is the response?

Now, we have a different case: starlight where blue light is in short
supply, but red is abundant. What will plants do?

It might make sense for them to use all the blue light they can, while
rejecting part of the unneeded red. But I am not sure about what the
photosyntetic machinery actually does.

> > massive trees, any idea how high they'd
> > grow?
>
> There was some nice work on this recently published in Nature
> (within the last couple years?). On Earth, tree height is limited by
> water stress, with the very tops of the trees looking for all the world
> like desert plants. IMS, the conclusion was that with current system
> redwoods etc. are near the limit.

Redwoods also grow in a definitely seasonal climate that has some water
stress, and little actual rainfall, in summer.

In tropical jungles, there are the epiphyts, many of which tap neither
the water in the host nor the soil with their own roots. They also
often are xerophyts.

Could one have a tree that does not depend on the water rising through
the trunk, because the upper branches capture water from atmosphere and
rain the way epiphyts do?

> Drop the surface gravity and you can
> get a lot higher (at 0.25 G less than 4x as high, due to capilary
> forces and friction), but nowhere near the ultimate tensile strength of
> water (equivilent to around 2 km on Earth). So maybe 3x times redwoods,
> or in the 1 km range (wow... that seems high).
>
> > It turns out that every couple of 100 standard
> > years or so, an outside event (comet?
> > Another star coming close?) triggers a complete
> > turnover.
>
> Hmm. 100 years or a very very short time (even 1000 years, in this
> context). A comet wouldn't do it, because a comet is only going to last
> 10-100 (being really really optimistic) pericentron passes before
> becoming inactive, if not completely vaporising. That's "only" 0.1
> million years for these events to have been happening, *VERY* short in
> terms of evolution.
> A passing star is worse. To get something to come by every, say,
> 1000 years, the semi-major axis of the orbit would be less than 9 AU.
> Assuming a really eccentric, cometary-type orbit (e = 0.99) means it
> would only get at *most* less than 18 AU from the main star. Heck,
> Uranus is further than that from the Earth, and from a truely dark-sky
> site it's naked eye visible. There's NO WAY to hide a star, or even a
> moderately sized planet, at those distances.
> How about episodic vulcanism?
>
> > I would be grateful for any advise.
>
> Oh, I don't have any of that handy ;-)
>
> > I find this world-building lark vaguely scary :}
>
> That's OK - you just posted to a place where most of us find it at
> least as facinating as our day jobs (speaking of which...).
>
> --
> Brian Davis
>
> *they aren't; drag is going to go up drasticly as well in this
> atmosphere, due to the increased density, but we'll be foolishly
> optomistic ;-)

.

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