Astrobiology 859
6.2 Venus
Venus currently has a surface that is clearly inhospitable
to life. There is no liquid water on the surface, and the
temperature is over 450◦C at an atmospheric pressure of
92 times the Earth’s. There is water on Venus but only in
the form of vapor and clouds in the atmosphere. The most
habitable zone on Venus is at the level in the atmosphere
where the pressure is about half of the sea level on Earth.
At that location, there are clouds composed of about 25%
water and 75% sulfuric acid at a temperature of about 25◦C;
these might be reasonable conditions for life. It is possible
therefore to speculate that life can be found, or survive if
implanted, in the clouds of Venus. What argues against this
possibility is the fact that clouds on Earth that are at similar
pressures and temperatures do not harbor life. We do not
know of any life forms that thrive in cloud environments.
Perhaps the essential elements are there but a stable envi-
ronment is required. [SeeVenus: Atmosphere]
Theoretical considerations suggest that Venus and Earth
may have initially had comparable levels of water. In this
case Venus may have had a liquid water surface early in its
history when it was cooler 4 billion years ago because of the
reduced brightness of the fainter early sun. Unfortunately,
all record of this early epoch has been erased on Venus and
the question of the origin of life during such a liquid wa-
ter period remains untestable. [SeeVenus: Surface and
Interior]
6.3 Mars
Of all the extraterrestrial planets and smaller objects in the
Solar System, Mars is the one that has held the most fas-
cination in terms of the existence of life. Early telescopic
observations revealed Earth-like seasonal patterns on Mars.
Large white polar caps that grew in the winter and shrunk in
the summer were clearly visible. Regions of the planet’s sur-
face near the polar caps appeared to darken beginning at the
start of each polar cap’s respective spring season and then
spread toward the equator. It was natural that these changes,
similar to patterns on the Earth, would be attributed to like
causes. Hence, the polar caps were thought to be water
ice and the wave of darkening was believed to have been
caused by the growth of vegetation. The 19th century ar-
guments for the existence of life, and even intelligent life,
on Mars culminated in the bookMars as the Abode of Life
by Percival Lowell in 1908 and in the investigations of the
celebrated canals. The Mars revealed by spacecraft explo-
ration is decidedly less alive than Lowell anticipated but its
standing as the most interesting object for biology outside
Earth still remains.
6.3.1 THE VIKING RESULTS
In 1976 theVikinglanders successfully reached the Martian
surface while the two orbiters circled the planet repeatedly
FIGURE 7 Schematic diagram of theVikingbiology
experiments.photographing and monitoring the surface. The primary
objective of theVikingmission was the search for micro-
bial life. Previous reconnaissance of Mars by theMariner
flyby spacecraft and the photographs returned from the
Mariner 9orbiter had already indicated that Mars was a cold
dry world with a thin atmosphere. There were intriguing
features indicative of past fluvial erosion but there was no
evidence for current liquid water. It was thought that any life
to be found on Mars would be microbial. TheVikingbiology
package consisted of three experiments shown schemati-
cally in Figure 7.
The Pyrolytic Release (PR) experiment searched for ev-
idence of photosynthesis as a sign of life. The PR was de-
signed to see if Martian microorganisms could incorporate
CO 2 under illumination. The experiment could be per-
formed under dry conditions similar to those on the Martian
surface or it could be run in a humidified mode. The CO 2
in the chamber was labeled with radioactive carbon which
could then be detected in any organic material synthesized
during the experiment. The very first run of the pyrolytic
release experiment produced a significant response. It was
well below the typical response observed when biotic soils
from Earth had been tested in the experiment, but it was
much larger than the noise level. Subsequent trials did not
reproduce this high result, and this initial response was at-
tributed to a startup anomaly, possibly some small prelaunch
contamination.
The Gas Exchange (GEx) experiment searched for het-
erotrophs, which are microorganisms capable of consum-
ing organic material. The GEx was designed to detect
any gases that the organisms released as a byproduct of
their metabolism, bacterial flatulence. After a sample was
placed in the chamber, the soil was first equilibrated with
water vapor and then combined with a nutrient solution. At