Re: Help with worldbuilding

> 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.

Removing/smallifying the nickel-iron core would probably help a lot. That
accounts for most of the Earth's density. Also, increasing the ratio of
aluminum to magnesium.

> > 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.

Not really. You just want *enough* escape speed, and the lowest possible
exosphere temperature. Assuming that I didn't totally screw up my
calculation (and I'm pretty sure I didn't, since the spread*** I used has
worked right for everything else so far), you ought to be able to just
barely get by with 1/30 of an Earth mass and still hold on to a
nitrogen/oxygen atmosphere at the same density as Earth and a Venus-like
exosphere- but then you'll need a lot of CO2 or CO or some other convenient
gas to keep down the exosphere temperature, and of course the lower mass
limit gets higher as the density gets lower.

> 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.

Yup. 4 km/s is quite comfortable, as long as you can keep down that
exosphere temperature.

> > > 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.

Or, it just needs to be really big so that it hasn't yet lost the heat from
accretion. But, then, we're talking about a small planet, so that doesn't
really apply.

<snip>

> > > 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
> photosynthetic machinery actually does.

It might also make sense for them to discard the blue and use lots of
accessory pigments to lots of the red stuff, maybe concentrating the energy
derived from multiple quanta of far red light to do what one quantum of blue
light could do. Or, if the pigments they use are sufficiently versatile and
easily tunable (like the proteins in eyes, perhaps?), you might get black
plants that absorb darn near everything. You can probably justify nearly any
color you want.

> > > 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?

Yup. Blue Moon, from National Geographic's _Extraterrestrial_, has just such
plants. In this case, height is limited only by the mechanical strength of
the plant tissues.

-l.
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