Re: Help with worldbuilding

Chornedsnork wrote:

> Are there any low-density, but rocky, materials available?

Sure, but it's a question of getting them together in planetary
quanities *without* the dense stuff. Carbonates, for instance are OK...
but require water (generally), and only occur in planetismals in
well-mixed trace quanities.

> Titan holds a massive atmosphere with escape speed
> of 2,65 km/s, but it is not far from marginal. Ganymede
> has none

A reasonable way to compare is the calculate what Hartmann ("Moons &
Planets, 4th ed") has called the "Retentivity", or v_esc / v_RMS. For
the gas balls, this is over 100, while for Earth it's 30, Venus is 25,
Mars is 15 & Titan is 12 (Ganymede is 11, Triton & Io 10, etc.). So a
rough rule of thumb is a relative retentivity above 10 (or, to be on
the safe side, 20) you get to keep an atmosphere.

>> human-breathable, 400 mb pO2... is about the upper limit,
>
> Some are more optimistic, and talk of 500-600 mb

I'd call that the same ;-). The figure I quoted is from medical
texts (my wife, the physician, not I) that studies show 40% O2
(roughly, 400 mb) can be used "indefinately", while 60% O2 (600 mb,
more or less) can be "tolerated for weeks at a time". Like all human
atmospheric limits, this is highly dependant on what you consider
"tolerable" (is, say, 10% of the population dying of pneumonia
"acceptable"? Then you can probably use the upper limit) and the
ever-present variation within the population (Everest has been summited
without supplemental O2, but most people would be unconcious within a
few minutes, and likely die).

> the star is redder - therefore for a given total energy
> flux, there is less shortwave light for human vision or plant
> photosynthesis

In a very small, trivial sense, yes. But keep in mind human vision
works remarkably well (one reference I found mentions the limit for
reading is around 0.01% of noon-time insolation given the Sun's
frequency distribution. For photosynthesis, (again, given solar
spectrum, which as you correctly point out would be shortward biased
relative to GJ876), it can occur at 1% non-time insolation. I can't see
the shortwards shift in spectrum really causing a lot of problems,
given these very low limits.

> there may be strong greenhouse effects by carbon
> dioxide or thick layers of water vapour, allowing Earthlike
> temperatures at lower total energy flux

Yep, but a minimum insolation that the greenhouse effect can "fix"
(correct for) is about 0.36 times Earth's. See below.

> thick clouds were assumed, so much of the light is
> reflected

But if too much is reflected, no ground-level greenhouse warming, so
there's a limit there as well. Venus light levels at the surface have
been compared to "a very cloudy day on Earth". One of my sources
("Worldbuilding" by Gillett, I think) list "a very cloudy day" at 110
W/m^2, while photosynthesis limit is around 10 W/m^2, and enough light
to read by is listed at 0.1 W/m^2. So, assume a base insolation of 0.36
Earth's (1300 * 0.36 = 470 W/m^2) and only half the effective shortwave
"needed" radiation due to the redder spectrum (so we're down to 230
W/m^2), and further that the maximum clouds will reduce this by a
factor of 10, lowering the effective shortwave light to 23 W/m^2, still
well above the limits quoted above. Even some terrestrial photosystems
will still work, and vision won't even notice (moreover, human vision
is not nearly the limit in this regard).

>> color is probably up to your imagination here,
>> not biochemistry.
>
> Hm. Why not biochemistry?

Because terrestrial photosystems already have a wide range of
pigments that alter their color, and there's not (with a few
exceptions) a lot of rhyme or reason to it. You mention:

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

Whatever works, *not* "what works best" (evolution doesn't work that
way). Consider, as you note, most plants reflect green light - which is
actually one of the most intense wavelengths at the Earth's surface.
they reject the single most plentiful type of photon. What "plants will
do" under a slightly longer wavelength spectrum is probably make due
with whatever they initally came up with, perhaps with the help of
accessory pigments. I would not assume that the slight longwards shift
in radiation would really alter much in the way of evolutionary
pressures. For instance in deep water enviroments, where the red
extinction is total and even the blue end is very significantly
reduced, and every photon counts (look up "Green Sulfur Bacteria").

> 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?

A very interesting suggestion. My first knee-jerk reaction is "no
way", because of the amazing amount of water that most plants (around
me) transpire. Than I think of desert communities and things like CAM
photosynthesis, and wonder if you couldn't get away with at least slow
growth based on water captured "at altitude". This might end up being
trees growing on trees, daughters growing on parents, perhaps in a
familial symbiosis (daughters produce food, some of which is fed into
the supporting understory parent (somehow, one way) to allow it to grow
and continnue to support the attic communities). Hmm. Could trees
actually end up with condensation traps? Actively cooled would be
interesting, but tough to justify evolutionarily. However, leaves are
great radiators, and with large upper surfaces exposed to a cloud-free
night sky, radiative cooling could be very efficient... oh, I'm liking
this. Nice idea, Chornedsnork!

--
Brian Davis

.

Relevant Pages

  • Re: Gliese 581c -- reviewing the bidding
    ... It is most odd that photosynthesis is not. ... have an aquarium and try to grow some plants. ... Furtermore on Earth the places and the seasons where light is very dim ... I understand that trees are unable to reduce carbon dioxide ...
    (rec.arts.sf.science)
  • Re: Mysterious microbe plays important role in ocean ecology.
    ... Leads scientists to rethink ocean's carbon and nitrogen cycles ... Someone told me Noah's flood was impossible because plants needed ... photosynthesis that could not be recieved under water. ...
    (talk.origins)
  • Re: Mysterious microbe plays important role in ocean ecology.
    ... Leads scientists to rethink ocean's carbon and nitrogen cycles ... Someone told me Noah's flood was impossible because plants needed ... photosynthesis that could not be recieved under water. ...
    (talk.origins)
  • Re: color of amur maples
    ... > I need to know something about trees and plants taking in water ... have wet autumns, the plants that grow there "like" wet autumns. ...
    (sci.chem)
  • Re: Global dimming masking greenhouse effect
    ... At 45C or higher they may not have enough water to do ... and photosynthesis may stop as enzymes stop working. ... Trees will ...
    (sci.physics)