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would be 0.36 of Mars' radius, or about 8% of its
mass. To determine a lower limit to the density,
one must consider possible major components of
the core. Of the potential core-forming materi-
als, iron, sulfur, oxygen and nickel are by far the
most abundant elements. The assumption of a
chondritic composition for Mars leads to values
of the relative radius and mass of the core:
R
c
/
a core of 15% of the mass of the planet, with pro-
portions of 12:1:2. Enstatite chondrites contain
between 15--25 weight% free iron. The oxidation
stateandtheoxygenisotopesofenstatitechon-
drites make them an attractive major component
for forming the Earth. These are not trivial con-
siderations since oxygen is the major elemnt, by
volume, in a terrestrial planet.
Carbonaceous chondrites have little or no free
iron but contain 7--25% by weight FeS and about
1.5% nickel. The average core size for a planet
made of carbonaceous chondrites would be 15%
by mass, Fe: S: Ni being in the proportions 18:10:3.
An absolute minimum core density can proba-
bly be taken as 4.8 g/cm
3
, corresponding to a
pure FeS core with a fractional core radius of 0.6
and a fractional mass of 26%. On these grounds,
the mass of the martian core can be considered
to lie between 8 and 24% of the mass of the
planet.
A third possibility would be to assemble Mars
from a mixture of meteorites that fall above and
below the curve. The meteorites below the curve
are relatively rare, although Earth may not be
collecting a representative sample.
The size of the core and its density can be
traded off. By using the density of pure iron and
the density of pure troilite (FeS) as reasonable
upper and lower bounds for the density of the
core, its radius can be considered to lie between
0.36 and 0.60 of the radius of the planet. It is
probably that there is sulfur in the core. There-
fore, a core about half the radius of the planet,
with a low melting point, is a distinct possibility.
The zero-pressure density of the mantle
implies an FeO content of 21--24 wt.% unless some
free iron has been retained by the mantle. The
presence of CO
2
and H
2
O, rather than CO and
H
2
, in the martian atmosphere suggests that free
iron is not present in the mantle. Chondrites
may therefore be an appropriate guide to the
major-element composition of Mars. The core of
Mars is smaller and less dense than the core of
Earth, and the mantle of Mars is denser than
that of Earth. Mars contains 25--28% iron, inde-
pendent of assumptions about the overall compo-
sition or distribution of the iron. Earth is clearly
enriched in iron or less oxidized than Mars and
most classes of chondritic meteorites.
=
R
0.50 and
M
c
/
0.21. The inferred density of
the mantle is less than the density of the silicate
phase of most ordinary chondrites.
Three kinds of chondrites, HL (high iron,
low metal), LL (low iron, low metal) and L (low
iron or hypersthene--olivine) chondrites, all have
lower amounts of potentially core-forming mate-
rial than is implied for Mars, although HL and LL
have about the right silicate density (3.38 g/cm
3
).
If completely differentiated, H (high iron) chon-
drites have too much core and too low a silicate
density (3.26--3.29 g/cm
3
). We can match the prop-
erties of H chondrites with Mars if we assume
that the planet is incompletely differentiated. If
the composition of the core-forming material is
on the Fe side of the Fe--FeS eutectic, and temper-
atures in the mantle are above the eutectic com-
position, but below the liquidus, then the core
will be more sulfur-rich and therefore less dense
than the potential core-forming material.
Carbonaceous chondrites are extremely rich
in low-temperature condensates, as well as car-
bon and 'organic matter.' If we ignore the water
and carbon components, a fully differentiated
planet of this composition would have a core of
15% by mass, composed mainly of FeS (13.6% FeS,
1.4% Ni), and a mantle with a density of about
3.5 g/cm
3
. However, these meteorites also con-
tain about 19% water, most of which must have
escaped if Mars is to be made up primarily of
this material. Otherwise, the mantle would not
be dense enough. But there is abundant evidence
for water near the surface of Mars in the past.
In ordinary high-iron chondrites, the free iron
content averages 17.2% by weight. The FeS content
is approximately 5.4% (3.4% Fe, 2.0% S) and the
nickel content is 1.6%. A planet assembled from
such material, if completely differentiated, would
yield a core of 24% of the mass of the planet,
with Fe: S: Ni in the approximate proportions of
21:2:2 by weight. Low-iron chondrites would yield
M
=
