CHAPTER 22 | COMPARATIVE PLANETOLOGY OF VENUS AND MARS 487
on its surface. It is a desert world now, but someday an astronaut
may scramble down an ancient Martian streambed, turn over a
rock, and fi nd a fossil. It is even possible that life started on Mars
and has managed to persist as the planet gradually became less
hospitable. For example, life may have hung on by retreating to
limited warm and wet oases underground. You will learn in
Chapter 26 about possible evidence of ancient life in Mars mete-
orite ALH84001 and, even more intriguing, indications in spec-
tra of Mars’s atmosphere of current methane production that
could be biological.
A History of Mars
Th e history of Mars is like a play where all the exciting stuff hap-
pens in the fi rst act. After the fi rst two billion years on Mars,
most of the activity was over, and it has gone downhill ever
since.
Planetary scientists have good evidence that Mars diff erenti-
ated when it formed and had a hot, molten core. Some of the
evidence comes from exquisitely sensitive Doppler-shift measure-
ments of radio signals coming from spacecraft orbiting Mars.
Measuring the shifts allowed scientists to map the gravitational
fi eld and study the shape of Mars in such detail that they could
detect tides on Mars caused by the sun’s gravity. Th ose tides are
less than a centimeter high; but, by comparing them with models
of the interior of Mars, the scientists can show that Mars has a
very dense core, a less dense mantle, and a low-density crust.
Observations made from orbit show that Mars has no overall
magnetic fi eld, but it does have traces of magnetism frozen into
some sections of old crust. Th is shows that soon after Mars
formed, it had a hot metallic core in which the dynamo eff ect
generated a magnetic fi eld. Because Mars is small, it lost its heat
rapidly, and most of its core gradually froze solid. Th at may be
why the dynamo fi nally shut down. Today Mars probably has a
large solid core surrounded by a thin shell of liquid core material
in which the dynamo eff ect is unable to generate a magnetic
fi eld.
No one is sure what produced the dramatic diff erence
between the southern highlands and northern lowlands. Powerful
convection in the planet’s mantle may have pushed crust together
to form the southern highlands. Th e suggestion that a cata-
strophic impact produced the northern lowlands does not seem
to fi t with the structure of the highlands, but it is not
impossible.
Planetary scientists divide the history of Mars into three
periods. Th e fi rst, the Noachian period, extended from the for-
mation of the crust about 4.3 billion years ago until roughly
3.7 billion years ago. During this time the crust was battered by
the heavy bombardment as the last of the debris in the young
solar system was swept up. Th e old southern hemisphere survives
from this period. Th e largest impacts blasted out the great basins
like Hellas and Argyre very late in the cratering, and there is no
trace of magnetic fi eld in those basins. Evidently the dynamo had
shut down by then.
Th e Noachian period included fl ooding by great lava fl ows
that smoothed some regions. Volcanism in the Th arsis and
Elysium regions was very active, and the Th arsis rise grew into a■ Figure 22-21
(a) The impact crater Galle, known for obvious reasons as the Happy Face Crater, is 210 km (130 mi) in diameter. Such large impacts may
occasionally eject fragments of the crust into space. A few fragments from Mars have fallen to Earth as meteorites. (Malin Space Science
Systems/NASA) (b) Meteorite ALH84001, found in Antarctica, has been identifi ed as an ancient rock from Mars. Minerals found in the mete-
orite were deposited in water and thus suggest that the Martian crust was once richer in water than it is now. (NASA)aVisual-wavelength imageb