[ExI] Double-Earth (Was: kepler study says 8.8e9 earthlike planets)

[Eugen Leitl]

On Mon, Nov 25, 2013 at 08:27:01AM +0100, Anders Sandberg wrote:

> While we all know the anomalous properties of water and cherish
> them, it might be because we also live in a water-dominated
> environment where every little property has big effects on us. Had

Chemistry gives you an objective view on suitability of different
systems to support life. I was always interested in the origin
of life, ever since I've read about Oparin's coacervates as
a young kid. There is a tendency to see more possibilities the
less constraints you're aware of. Chemists routinely dwell in
objective views of how members of the PSE play together. It 
turns out, that most of the PSE is barren, and only few elements
play nice with each other, and elemental abundancies and
prebiotic chemistry (studied remotely by spectroscopy, and by
analyzing extraterrestrial samples, as well as laboratory
experiments simulating processes occuring in interstellar
space and accretion on planetary nebulas) we can rule a lot
of what ought to work but actually doesn't.

> we been living in a methane or high pressure water-ammonia

Methane is apolar and a solvent under deep cryogenic conditions, 
while kinetically stable is not stable under irradiation, and
will not coexist with oxidants. 

Lower hydrocarbons are of interest in prebiotic reactions, but
you're not going to find surface life in methane pools in the outer
solar system. No energy, no metabolism.

Water/ammonia is potentially fertile, especially as water-ammonia eutectic
and has interesting properties, but in general it has issues, and 
the stability of the nitrogen-nitrogen triple bond will tend to
drive towards decomposition to nitrogen and hydrogen (which reacts
with oxidants like oxygen, or is lost to space).

> environment we might have written the same number of papers about
> the anomalousness of methane and water-ammonia mixtures.

See the following interesting treatment of water/ammonia

http://www.nap.edu/openbook.php?record_id=11919&page=72

6.2.1
Polar Solvents That Are Not Water
6.2.1.1
Ammonia

Ammonia is analogous to water in many of its properties. Ammonia, like water, dissolves many organic compounds, including many polyelectrolytes. Preparative organic reactions are often done in ammonia in the laboratory. Ammonia, like water, is liquid over a wide range of temperatures (195 to 240 K at 1 atm). The liquid range is even broader at higher pressure. For example, at 60 atm ammonia is liquid from 196 to 371 K. Further, liquid ammonia may be abundant in the solar system. A large amount of the inventory of liquid ammonia in the solar system exists, for example, in clouds in the jovian atmosphere. However, as noted earlier by the committee, some view clouds as unlikely places to harbor life. However, if clouds are not transient and broken (as on Earth) but are rather more continuous (as on Venus), this view may need modification.

As compared with water, ammonia’s increased ability to dissolve hydrophobic organic molecules suggests an increased difficulty in using the hydrophobic effect to generate compartmentalization in ammonia, relative to water. This in turn implies that the liposome, a compartment that works in water, generally will not work in liquid ammonia. Hydrophobic phase separation is possible in ammonia, however, albeit at lower temperatures. For example, Brunner reported that liquid ammonia and hydrocarbons form two phases, where the hydrocarbon chain contains from 1 to 36 CH2 units.5 Different hydrocarbons become miscible with ammonia at different temperatures and pressures. Thus, formation of ammonia-phobic and ammonia-philic phases, analogous to the hydrophobic and hydrophilic phases in water, useful for isolation would be conceivable in liquid ammonia at temperatures well below its boiling point at standard pressures.

The greater basicity of liquid ammonia must also be considered. The species that serve as acid and base in pure water are H3O+ and HO−. In ammonia, NH4+ and NH2− are the acid and base, respectively. H3O+, with a pKa of −1.7, is about 11 orders of magnitude stronger (in water) as an acid than NH4+, with a pKa of 9.2 (in water). Likewise, NH2− is about 15 orders of magnitude stronger as a base than HO−.

The increased strength of the dominant base in ammonia,as well as the corresponding enhanced aggressivity of ammonia as a nucleophile, implies that ammonia would not support the metabolic chemistry found in terran life. Terran life exploits compounds containing the C=O carbonyl unit. In ammonia, carbonyl compounds are (at the very least) converted to compounds containing the corresponding C=N unit. Nevertheless, hypothetical reactions that exploit a C=N unit in ammonia can be proposed in analogy to the metabolic biochemistry that exploits the C=O unit in terran metabolism in water (Figure 6.1).6 Given this adjustment, metabolism in liquid ammonia is easily conceivable.

Most interestingly, ammonia is a potent antifreeze for water. Recently recovered data from Titan suggest that that moon is periodically being resurfaced by a liquid having a viscosity comparable to that of a water-ammonia eutectic, which is liquid even in an environment that experiences methane rain. Water-ammonia eutectics, which FIGURE 6.1 Different functional groups, but analogous mechanisms, could be used to form new C—C bonds in different solvents. In water, the C=O unit would provide the necessary reactivity. In ammonia, the C=N unit would provide the necessary reactivity. In sulfuric acid, the C=C unit is sufficient to provide the necessary reactivity.

are liquid even at the temperature of Titan, are a potential biosolvent. They are abundant in the cosmos, they have a wide temperature range of liquidity, and they are not bad as solvents.

etc., see the site for an interesting discussion of other
solvent systems.