318 Encyclopedia of the Solar System
The stability of water is of profound importance for
understanding martian geology. Under the conditions just
described, the planet has a thickpermafrost zonethat
extends a few kilometers deep at the equator and several
kilometers deep at the poles. Any unbound water present
will exist as ice in this zone. There may be liquid water
beneath the permafrost. Water ice caps are present at both
poles, although that at the South Pole is largely masked by a
remnant summer CO 2 cap. At latitudes between about 40◦
and the edge of the water ice cap, abundant ice has been de-
tected just below a dehydrated zone a few tens of centime-
ters thick. At latitudes less than about 40◦, ice is unstable at
all depths. A block of ice placed in the ground at these lati-
tudes will slowly sublimate into the atmosphere. The small
amounts of water that have been detected at low latitudes
may be water bound in minerals or water inherited from an
earlier era when water ice was stable at these latitudes.
2.3 Planet Formation and Global Structure
Like the other planets, Mars formed from materials that
condensed out of the early solar nebula, a disc of gas
and dust that surrounded the early Sun. Carbonaceous
chondrites, a class of meteorites that is almost identical in
composition to the photosphere of the Sun, are believed to
resemble closely the composition of the early nebula. Ra-
dioisotopes date the formation of the nebula at 4.567 bil-
lion years ago. The planets formed as the dust and gas
accumulated into discrete bodies, and gravitational attrac-
tion favored growth of larger bodies over smaller bodies.
Mars formed remarkably quickly. The evidence is from
short-lived radioisotopes. The high rate of accretion re-
sulted in global melting. Melting enabled settling of heavy
iron-rich melts to the center of the planet to form a core
separated from the silicate-rich mantle. During this pro-
cess siderophile elements, which dissolve preferentially in
iron-rich melts over coexisting silicate-rich melts, became
depleted in the mantle and enriched in the core. As a result,
formation of the core can be dated because the daughter
products of some short-lived, strongly siderophile elements
are present in the mantle, as indicated by the composition
of martian meteorites. For example,^182 Hf decays to^182 W
with a half-life of 9 million years. W is highly siderophilic so
should mostly enter the core, yet there is an excess of^182 W
in the mantle. Not all the Hf had decayed before the core
formed. This and other isotopic evidence indicate that the
core formed within 20 million years of the formation of the
elements that comprise the solar system. Global melting
may also have enabled some crust to form very early. This
is supported both by isotopic evidence and by the finding
of a martian meteorite (ALH84001) that has a 4.5-billion-
year age. New crust, of course, has continued to form as
indicated by volcanoes and extensive volcanic plains.
The Earth’s core is inferred to be iron-rich from (1)
the core’s density as deduced from the core’s size and the
planet’s moment of inertia, (2) modeling the bulk composi-
tion of the Earth and comparing it with the chondritic me-
teorites from which the Earth formed, and (3) depletion of
siderophile elements in mantle-derived rocks as compared
with chondritic meteorites. We can do similar reasoning
for Mars except that we know the size of the Earth’s core
from seismic data but must infer the size of Mars’ core. The
best estimate is that the core radius is between 1300 and
1500 km. In addition, the martian core may be more sulfur-
rich than the Earth’s core because the Mars mantle is more
depleted in chalcophile elements (those that preferentially
dissolve in sulfur-rich melts) than is the Earth’s.
One of the more surprising results of theMars Global
Surveyormission was discovery of large magnetic anoma-
lies in the crust despite the absence of a magnetic field
today. Their presence indicates that Mars had a magnetic
field in the past, but that it switched off at some time. The
size of the anomalies suggests that they must result from
sources in the outer few tens to several tens of kilometers
of the crust and that their magnetizations are higher by an
order of magnitude than magnetizations typically encoun-
tered in terrestrial rocks. The anomalies probably formed
when rocks, containing iron-bearing minerals, crystallized
in the presence of a magnetic field. Most of the anomalies,
and all the largest are in the southern uplands. They are
particularly prominent on either side of the 180◦longitude
where there are several broad, east-west stripes. One in-
terpretation of the linear anomalies is that they result from
injection of dikes or dike swarms several tens of kilome-
ters wide and hundred of kilometers long in the presence
of a strong magnetic field. Anomalies are mostly absent
around the youngest large impact basins, Utopia, Hellas,
Isidis, and Argyre. The simplest explanation is that there was
no longer a magnetic field when these basins formed, for-
mation of the basins destroyed any preexisting anomalies,
and no new ones formed when the affected materials cooled
after the basin-forming events. The ages of the basins are
not known, but, by analogy with the Moon, they are likely to
have formed toward the end of heavy bombardment around
3.8–4 billion years ago. Thus, the magnetic field may have
turned off by around 4 billion years ago.
Earth’s magnetic field is generated by convection within
its core. Mars’ early dynamo probably had a similar cause.
Possible causes for cessation of the dynamo are loss of core
heat, solidification of most of the core, and/or changes in
the mantle convection regime. Magnetization of minerals
within 3.9- to 4.1-billion-years-old carbonates in the mar-
tian meteorites ALH84001 suggests that there was still a
magnetic field at this time. If true, it implies that Mars had
a magnetic field for the first 500 million years of its history
and that the field turned off around 4 billion years ago, just
before formation of the youngest impact basins.
Like the Earth’s, Mars’ mantle is chondritic in composi-
tion except for the depletion of siderophile and chalcophile
elements as noted earlier and the depletion of volatile