Astrobiology 867
biological signature after several million years at depths to
about 1 m below the surface ice.
8. Life about Other Stars
In the Solar System, only our own planet has clear signs of
life. Mars, Europa, and Enceladus provide some hopes of
finding past or present liquid water but nothing comparable
to the richness of water and life on Earth. Our understand-
ing of life as a planetary phenomenon would clearly bene-
fit from finding another Earth-like planet, around another
Sun-like star, that harbored life.
One way of formulating the probability of life, and intel-
ligent life, elsewhere in the galaxy is the Drake equation,
named after Frank Drake, a pioneer in the search for ex-
traterrestrial intelligence. The equation and the terms used
with it are listed in Table 9. The most accurately determined
variable in the Drake equation at this time is R∗, the num-
ber of stars forming in the galaxy each year. Since we know
that there are about 10^11 stars in our galaxy and that their
average lifetime is about 10^10 years, then R∗∼10 stars per
year. All the other terms are uncertain and can be only es-
timated by extrapolating from what has occurred on Earth.
Estimates by different authors for N, the number of civi-
lizations in the galaxy capable of communicating by radios
waves, range from 1 to millions. Perhaps the most uncertain
term is L, the length of time that a technologically advanced
civilization can survive.
The primary criterion for determining whether a planet
can support life is the availability of water in the liquid
state. This in turn depends on the surface temperature of
the planet which is controlled primarily by the distance to a
central star. Life appeared so rapidly on Earth after its for-
mation that it is likely that other planets may only have had
to sustain liquid water for a short period of time for life to
originate. Planets orbiting a variety of star types could satisfy
this criterion at some time in their evolution. The develop-
ment of advanced life on Earth, and in particular intelligent
life, took much longer, almost 4 billion years. Earth main-
tained habitable conditions for the entire period of time.
Locations about stars in which temperatures are conducive
to liquid water for such a long period of time have been
called continuously habitable zones (CHZ). Calculations of
the CHZ about main sequence stars indicate that the mass
of the star must be less than 1.5 times the mass of our sun
for the CHZ to persist for more than 2 billion years.
An interesting result of these calculations is that the cur-
rent habitable zone for the sun has an inner limit at about
0.8 AU and extends out to between 1.3 and 1.6 AU, de-
pending on the way clouds are modeled. Thus, while Venus
is not in the habitable zone, Earth and Mars both are. This
calculation would suggest that Mars is currently habitable.
But we see no indication of life. This is owing to the fact that
the distance from the sun is not the only determinant for the
presence of liquid water on a planet’s surface. The presence
of a thick atmosphere and the resultant greenhouse effect
is required as well. On Earth the natural greenhouse effect
is responsible for warming the Earth by 30◦C; without the
greenhouse effect the temperature would average− 15 ◦C.
Mars does not have an appreciable greenhouse effect, and
hence its temperature averages− 60 ◦C. If Earth were at
the same distance from the sun than Mars, it would prob-
ably be habitable because of the thermostatic effect of the
long-term carbon cycle. This cycle is driven by the burial
of carbon in seafloor sediments as organic material and car-
bonates. The formation of carbonates is due to chemical
erosion of the surface rocks. Subduction carries this ma-
terial to depths where the high temperatures release the
sedimentary CO 2 gas, and these gases escape to the sur-
face in volcanoes that lie on the boundary arc of the sub-
duction zones. The thermostatic action of this cycle results
because the erosion rate is strongly dependent on temper-
ature. If the temperature were to drop, erosion would slow
down. Meanwhile the outgassing of CO 2 would result in a
buildup of this greenhouse gas and the temperature would
rise. Conversely, higher temperatures would result in higher
erosion rates and a lowering of CO 2 again stabilizing the
temperature.
Mars became uninhabitable because it lacks plate
tectonics and hence has no means of recycling the
carbon-containing sediments. As a result, the initial thickTABLE 9 The Probability of Life, and Intelligent Life, Elsewhere in GalaxyThe Drake Equation N=R∗×fp×ne×fl×fi×fc×LN The number of civilizations in the galaxy.
R∗ The number of stars forming each year in the galaxy.
fp The fraction of stars possessing planetary systems.
ne The average number of habitable planets in a planetary system.
fl The fraction of habitable planets on which life originates.
fi The fraction of life forms that develop intelligence.
fc The fraction of intelligent life forms that develop advanced technology.
L The length of time, in years, that a civilization survives.