Encyclopedia of the Solar System 2nd ed

Astrobiology 857

TABLE 5 Sources of Prebiotic Organics on Early Earth

Organic Production

Energy Dissipation (in a reducing atmosphere)

Source (J yr−^1 ) (kg yr−^1 )

Lightning 1 × 1018 3 × 109

Coronal discharge 5 × 1017 2 × 108

Ultraviolet light (λ<270 nm) 1 × 1022 2 × 1011

Ultraviolet light (λ<200 nm) 6 × 1020 3 × 109

Meteor entry shocks 1 × 1017 1 × 109

Meteor post-impact plumes 1 × 1020 2 × 1010

Interplanetary Dust – 6 × 107

Hydrothermal vent environments have been suggested
for the subsurface origin of chemotrophic life. In the ab-
sence of sunlight, these organisms must utilize chemical
energy (e.g., CO 2 +4H 2 →CH 4 +2H 2 O+energy). Alter-
natively, phototrophic life utilizes solar radiation from the
surface for prebiotic synthesis. These organisms with the
ability to chemosynthesize and photosynthesize can assim-
ilate their own energy from materials in their environment.
One feature that the various theories for the origin of life
have in common is the requirement for liquid water because
the chemistry of even the earliest life requires a liquid water
medium. This is true if the primal organism appears fully
developed (panspermia), if it engages in organic chemistry,
and for the clay inorganic theories.

For many years the standard theory for the origin of life

posited a terrestrial organic origin requiring endogenous

production of organics leading to the development of het-

erotrophic organisms, generally known as the primordial

“soup” theory. Recently there has been serious considera-

tion for the chemotrophic origin of life, and at the present

time the scientific community is split between these two

views.

5. Limits to Life

In considering the existence of life beyond the Earth, it is

useful to quantitatively determine the limits that life has

TABLE 6 Comparison of the Amino Acids in Murchison Meteorite and

in an Electric Discharge Synthesis, Normalized to Glycine

Amino Acid Murchison Meteorite Electric Synthesis

Glycine 100 100

Alanine > 50 > 50

α-Amino-n-butyric acid > 50 > 50

α-Aminoisobutyric acid 10 > 50

Valine 10 1

Norvaline 10 10

Isovaline 1 1

Proline 10 0. 1

Pipecolic acid 0. 1 < 1

Aspartic acid 10 10

Glutamic acid 10 1

β-Alanine 1 1

β-Amino-n-butyric acid 0. 10. 1

δ-Aminoisobutyric acid 0. 10. 1

γ-Aminobutyric acid 0. 11

Sarcosine 1 10

N-Ethyl glycine 1 10

N-Methyl alanine 1 1