The Solar System

CHAPTER 26 | ASTROBIOLOGY: LIFE ON OTHER WORLDS 593

layers under the surfaces of Ganymede and Callisto. Th at can
change as the moons interact gravitationally and their orbits
vary; Europa, Ganymede, and Callisto may have been frozen
solid at other times in their histories, which would probably
have destroyed any living organism that had developed there.

Saturn’s moon Titan is rich in organic molecules. You
learned in Chapter 23 that sunlight converts the methane in
Titan’s atmosphere into organic smog particles that settle to the
surface. Th e chemistry of life that could have evolved from those
molecules and survived in Titan’s lakes of methane is unknown.
It is fascinating to consider possibilities, but Titan’s extremely
low temperature of –180°C (–290°F) would make chemical reac-
tions so slow that life processes seem unlikely.
Water containing organic molecules has been observed
venting from the south polar region of Saturn’s moon Enceladus
(see Chapter 23). It is possible that life could exist in that
water under Enceladus’s crust, but the moon is very small, and
its tidal heating might operate only occasionally. Enceladus
may not have had plentiful liquid water for the extended time
necessary for the rise of life.
Mars is the most likely place for life to exist in the solar
system because, as you learned in Chapter 22, there is a great
deal of evidence that liquid water once fl owed on its surface.
Even so, results from searches for signs of life on Mars are not
encouraging. Th e robotic spacecraft Viking 1 and Viking 2
landed on Mars in 1976 and tested soil samples for living organ-
isms. Some of the tests had puzzling semipositive results that
scientists hypothesize were caused by nonbiological chemical
reactions in the soil. No evidence clearly indicates the presence
of life or even of organic molecules currently in the Martian soil.
If life survives on Mars, it may be hidden below ground where
there may be liquid water and where UV radiation from the sun
cannot penetrate.
Th ere was a splash of news stories in the 1990s regarding
supposed chemical and physical traces of life on Mars discovered
inside a Martian meteorite found in Antarctica (■ Figure 26-9).
Scientists were excited by the announcement, but they employed
professional skepticism and immediately began testing the evi-
dence. Th eir results suggest that the unusual chemical signatures
in the rock may have formed by processes that did not involve
life. Tiny features in the rock that were originally thought to be

c

Methane Concentration

parts per billion

02025305 10 15

b

a

■ Figure 26-9

(a) Meteorite ALH84001 is one of a dozen meteorites known to have origi-
nated on Mars. Its name means this meteorite was the fi rst one found in
1984 near Antarctica’s Allan Hills. (b) A research group claimed that the
meteorite contained chemical and physical traces of ancient life on Mars,
including what appear to be fossils of microscopic organisms. That evidence
has not been confi rmed, and the claim continues to be tested and debated.
(NASA) (c) A map of Mars color-coded to show methane concentration in the
atmosphere measured by spectrographs on Earth-based telescopes. Methane
is most abundant in locations apart from volcanic regions, indicating the
methane may be produced biologically. (NASA / M. Mumma)