From: Robert J. Bradbury (bradbury@www.aeiveos.com)
Date: Thu Nov 25 1999 - 12:39:29 MST
... Eric Watt Forste <arkuat@idiom.com> wrote:
> Right, but biota aren't based on heavy metals. Most carbon and oxygen
> are produced by *much* slower processes, principally planetary
> nebula ejection and white-dwarf novas. These events happen
> gigayears or tens of gigayears after star formation.
Its an ongoing process, and the lighter elements are produced in
much greater abundances than the heavy elements.
Astronomy Today, Table 21-1, Cosmic Abundances (current era):
Elemental Group % abundance by number
H 90
He 9
Li Group (7-11 (p+n)) 0.000001
C group (12-20 (p+n)) 0.2
Si group (23-48 (p+n)) 0.01
Fe group (50-62 (p+n)) 0.01
Middle wt. group (63-100 (p+n)) 0.00000001
Heavy wt. group (>100 (p+n)) 0.000000001
Heavy stars (> 8 M_sun), have a layered structure (from the outside) of:
H -> He -> C -> O -> Ne -> Mg -> Si -> Fe
Presumably much of the Fe gets dumped into the black holes or the
neutron star for lighter stars, but not all of it since some of it
gets released into the interstellar media. Since these large
stars only have lives of millions to tens of millions of
years the interstellar dust gets enriched with the materials
of life *very* rapidly after the formation of the galaxy.
While the distinguishing feature between population I and II
stars tends to be their location and velocity (II are found
more in the halo and have higher velocities, probably due to
their being older and having had more change sling-shot
encounters), they also differ in metal content. Pop. II stars
are very deficient in elements heavier than He, while Pop. I have
approximately solar abundances of metals.
So I would propose that almost all population I stars should
be potentially capable of forming planetary systems on which life
could evolve.
[Now, of course it goes without saying that the astronomers
would start pulling out their hair if one were to suggest
the population II stars aren't old, but are instead leftovers
from SI mining activities.]
On Wed, 24 Nov 1999 CurtAdams@aol.com wrote:
>
> More relevant might be concentrations of elements whose concentrations often
> limit on earth; nitrogen and phosphorus spring to mind. (Iron can be
> limiting in the ocean but that's more of a solubility issue.) Life uses a
> lot of trace elements: I wouldn't think concentrations of things like
> selenium would be limiting but I'm not really sure.
>
> I think nitrogen is the most limiting element since life uses a lot of it,
> it's relatively rare, and since it's mostly gas it's very prone to get lost.
>
N2 is an interesting element to look at for rarity, particulary
from the perspective of why you don't find much on Mars or Venus
(relative to say CO2). On Mars, I can understand losing N2 before
CO2, but on Venus there ought to be a fair amount.
An article (Terraforming with Nanotechnology, JBIS, 47:311-318, 1994),
discusses this: "The availability of nitrogen seems to be the most
serious constraint on the eventual establishment of a completely
Earth-like environment on Mars." [That and the complete dismantlement
of the planet....]. They also observe, "If underground stores of
nitrogen do exist on Mars, just what form they take is a mystery.
Nitrate minerals are one possible store. Although nitrate minerals
do exist on earth, they are rare. Due to their solubility they are only
found in the most arid regions of Earth such as the deserts of Chile.
Even though Mars is now in a desert state, its history as a water planet
makes the existence of large stores of nitrates doubtful." [I'm not
sure why this would be true, if N2 gets locked up in soluble nitrates,
it would seem that it would be deposited as the H2O evaporates or
still be present in any frozen ice.]
This gets particularly interesting in light of the abundance of
elements in the solar system (from Physics & Chemistry of the
Solar System, J.S. Lewis, after Grevesse & Anders (1988)) and
Nanomedicine, Chapter 3, Table 3.1:
Element Solar Abundance Atoms in Body
H 2.8 x 10^10 4.22 x 10^27
O 2.3 x 10^7 1.61 x 10^27
C 1.0 x 10^7 8.03 x 10^26
N 3.1 x 10^6 3.9 x 10^25
Fe 9.0 x 10^5 4.5 x 10^22
S 5.2 x 10^5 2.6 x 10^24
Ca 6.1 x 10^4 1.6 x 10^25
Na 5.7 x 10^4 2.5 x 10^24
P 1.0 x 10^4 9.6 x 10^24
K 3.8 x 10^3 2.2 x 10^24
Zn 1.3 x 10^3 2.1 x 10^22
Cu 5.2 x 10^2 7 x 10^20
Se 6.2 x 10^1 3 x 10^18
At any rate, after going through the data entry and calculations...
Relative body enrichment: P > K > Rb > Cl > Ca > Cd > Br ...
Relative body depletion: Cr < Ni < V < Co < Mn < Al < Se < Fe ...
Relatively equivalent: C ~= O ~= I ~= Na ~= Zn ~= N
(decreasing order but within a factor of 6.3 of each other)
I'll have to dig a little further to do the calculation relative
to the Earth's crustal or salt water abundances (which may make
more sense).
Of course, the ratios will probably vary from species to species,
particularly in widely separated branches of the animal & plant kingdoms.
Now, the interesting thing about most of the heavier elements
is that they are being used in small quantities for specific
purposes. S gets used for disulfide bridges, P is used as an
energy carrier, Fe as an O2 carrier or catalyst, Zn as a
protein "cross-linker" (Zn-finger transcription factors)
and catalyst, Cu & Se as catalysts, etc. I've never heard anyone
suggest that the heavier elements are *required* for life. I've
given some thought to whether we could re-engineer life to remove
the metals (you especially want to remove Cu & Fe if you want to
minimize the free radical damage to DNA in the long term).
So far, I haven't come across any strong reasons why the
functions done by heavy "metals" in our body could not
replaced by protein only constructs. I will admit that
you may suffer some reductions in reaction rates, but
it isn't clear that there are reactions that simply
"cannot" occur without the presence of metals.
So even though we require heavy metals (from supernovas)
it isn't clear that all forms of life must. I suspect that
life gets started without using metals (other than HOCN)
and then incorporates them for specific functions. To
use "heavy" metals effectively, you have to evolve
transport and storage management systems. Given some
really big computers for molecular dynamics simulations,
I'm pretty sure we could rapidly develop proteins/enzymes
that did not require metals.
So, in solar systems where heavy metals are not as abundant
(very early population I stars) life might certainly evolve.
It might take longer, meaning it might take 5-6 billion years
instead of 4 billion, implying that it would want to occur
around a smaller, longer lived star, but those stars are
more abundant so overall, so things probably average out.
A more critical problem, IMO, is whether early population I
stars would have enough heavy material to form solid planets as
are found in our solar system. If not, then you have to
discuss evolution of life on moons around gas giants or
on comets which rapidly gets highly speculative.
Robert
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