Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization

First Edition

© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm

 

8.1.1  Temperature Chauvinism

Any life chemistry will inevitably be subject to a narrow, or at least specific, temperature range. This is because a successful biochemistry is based on large assemblages of complex, delicately balanced molecules. These molecules must walk the thin line between overstability and overreactivity. Too cold, and the system grows sluggish and grinds to a halt; too hot, and reactions become uncontrollably rapid and the metabolism destabilizes.

The dedicated temperature chauvinist wants to restrict the viable range of all lifeforms to less than 100 °C, hardly enough to cover the gamut of terrestrial organisms alone. More sophisticated arguments suggest that even unfamiliar carbon-based systems probably could not exist much above 500 °C, because large carbon macromolecules shake themselves to pieces long before things get even that hot.

At the cold end of the scale, carbon-based biochemistries may be much less successful below about -100 °C. Reaction rates become extremely low, and there are fewer and fewer solvents in which the life-chemistry may proceed.

But are these valid limits for all conceivable living systems?

Perhaps not.* Table 8.1 gives the energy of various chemical bonds that might possibly occur in biologically significant molecules. If a structure is given more than this energy, the bonds may snap and the molecule falls apart. The higher the bond energy, the more stable the molecular structure. And stability is essential for any chemistry that aspires to live.

 

Table 8.1 Chemical Bond Energies for Some Combinations of Xenobiological Interest
Bond Type
Energy
 Bond Type
Energy
 Bond Type
Energy
(eV)*
(eV)*
(eV)*
N = N
9.8
H - Br
3.8
S – Cl
2.6
C = N
9.4
Si – O
3.8
Cl – Cl
2.5
C = C
8.4
Si – Cl
3.8
S – S
2.2
C = O
7.4
C – O
3.7
O – Cl
2.2
C = C
6.4
C – C
3.6
N – Cl
2.1
H – F
5.9
S – H
3.5
Br – Br
2.0
Si – F
5.6
C – Cl
3.4
Si – Si
1.8
O – H
4.8
P – Cl
3.4
N – N
1.7
C – F
4.6
H – P
3.3
F – F
1.6
H – H
4.5
H – I
3.1
I – I
1.6
H – Cl
4.5
Si – H
3.1
O – O
1.4
N = N
4.4
Si – C
3.0
O2N – NO2
0.57
C – H
4.3
C – N
3.0
Hydrogen bonds
0.08-0.45
N – H
4.1
H – Se
2.9
van der Waals
0.04
 
* 1 eV = 1.60 × 10-19 Joules
Room temperature (25 °C) ~ 0.026 eV

 

Carl Sagan suggests that for life to exist, the fraction of bonds disrupted due to random thermal motions must be no larger than 0.0001%. If this is true, then lifeforms whose biochemistry is based solely on van der Waals forces (a weak attraction between atomic electrons and the nucleus of an adjacent atom) alone could survive at temperatures as high as 40 K. Biochemistries relying on hydrogen bonds alone could exist up to 400 K. Bonds of strength 2.0 eV or higher would suffer less than 0.0001% random breakage up to 2000 K, and for 5 eV bonds the molecules survive up to 5000 K.2358

This spans the range of temperatures from the coldest worlds to the surfaces of stars. Concludes Dr. Sagan: "There seem to exist chemical bonds of appropriate structural stability for life, and it would appear premature to exclude the possibility of life on any planet on grounds of temperature."

 

* Hal Clement’s two science fiction novels, Mission of Gravity (low temperature life)2069 and Iceworld (high temperature life),292 are highly entertaining.

 

Last updated on 6 December 2008