866 Encyclopedia of the Solar System
6.5 Asteroids
Asteroids seem unlikely locations for life to have originated.
Certainly they are too small to support an atmosphere suf-
ficient to allow for the presence of liquid water at the
present time. However, asteroids, particularly the so-called
carbonaceous type, are thought to contain organic mate-
rial, thereby playing a role in the delivery of organics to
the prebiotic Earth. A more intriguing aspect of some as-
teroids is the presence of hydrothermally altered materials,
which seems to indicate that the asteroids were once part
of a larger parent body. Furthermore, conditions on this
larger parent body were such that liquid water was present,
at least in thin films. Containing both organic material and
liquid water, the parent bodies of these asteroids are in-
teresting targets in the search for extraterrestrial lifeforms.
However, a thorough assessment of this possibility will re-
quire a more detailed study of carbonaceous asteroids in the
asteroid belt. Meteorites found on the Earth provide only a
glimpse of small fragments of these objects and no signs of
extraterrestrial life have been found. But the samples are
small and the potential for contamination by Earth life is
great.
6.6 Comets
Comets are also known to be rich in organic material. How-
ever, unlike asteroids, comets also contain a large fraction of
water. In their typical state this water is frozen as ice, which
is unsuitable for life processes. As a comet approaches the
sun, its surface is warmed considerably, but this leads only
to the sublimation of the water ice. Liquid does not form
because the pressure at the surface of the comet is much
too low.
It has been suggested that soon after their formation the
interior of large comets would have been heated by short-
lived radioactive elements (^26 Al) to such an extent that the
core would have melted. In this case, there would have
been a subsurface liquid water environment similar to that
postulated for the present day Europa. Again the question
of the origin of life in such an environment rests on the
assumption that life can originate in an isolated deep dark
underwater setting.
7. How to Search for Life on Mars, Europa,
or Enceladus
If we were to find organic material in the subsurface of
Mars, or in the ice of Europa, or entrained in the geysers of
Enceladus, how could we determine if it was the product
of a system of biology or merely abiotic organic material
from meteorites or photochemistry? If that life is related
to Earth life, it should be easy to detect. We now have
very sensitive methods, such as the amplification of DNA
and fluorescent antibody markers, for detecting life from
Earth. The case of Earth-like life is the easiest but it is also
the least interesting. If the life is not Earth-like, then the
probes specific to our biology are unlikely to work. We need
a general way to determine a biological origin. The question
is open and possibly urgent. As we plan missions to Mars
and Europa, we may have the opportunity to analyze the
remains of alien biology.
One practical approach makes use of the distinction be-
tween biochemicals and organic matter that is not depen-
dent on a particular organic molecule but results from con-
sidering the pattern of the organics in a sample. Abiotic
processes will generate a smooth distribution in molecular
types without sharp distinctions between similar molecules,
isotopes, or chemical chirality. If we consider a generalized
phase space of all possible organic molecules, then for an
abiotic production mechanism the relative concentration
of different types will be a smooth function. In contrast to
abiotic mechanisms, biological production will not involve
a wide range of possible types. Instead, biology will select a
few types of molecules and build biochemistry up from this
restricted set. Thus organic molecules that are chemically
very similar may have widely different concentrations in a
sample of biological organics. An example of this on Earth is
the 20amino acidsused in proteins and the selection of life
for the left-handed version of these amino acids. To maxi-
mize efficiency, life everywhere is likely to evolve this strat-
egy of using a few molecules repeatedly. It may be that other
life forms discover the same set of biomolecules that Earth
life uses because these are absolutely the most efficient and
effective set under any planetary conditions. But it may
also be that life elsewhere uses a different set that is opti-
mal given the specific history and conditions of that world.
We can search for the repeated use of a set of molecules
without knowing in advance what the members of that set
will be.
We can apply this approach to the search for biochem-
istry in the Solar System. Samples of organic material
collected from Mars and Europa can be tested for the preva-
lence of one chirality of amino acid over the other. More
generally, a complete analysis of the relative concentration
of different types of organic molecules might reveal a pat-
tern that is biological even if that pattern does not involve
any of the biomolecules familiar from Earth life. Interest-
ingly, if a sample of organics from Mars or Europa shows a
preponderance of D amino acids, this will suggest the pres-
ence of extant or extinct life and at the same time show that
this life is distinct from Earth life. This same conclusion
would apply to any clearly biological pattern that is distinct
from the pattern of Earth life. The pattern of biological ori-
gin in organic material can potentially persist long after the
organisms themselves are dead. Eventually this distinctive
pattern will be destroyed as a result of thermal and radia-
tion effects. Below the surface of Mars, both temperature
and radiation are low, so this degradation should not be sig-
nificant. On Europa the intense radiation may destroy the