Geoscience Reference
In-Depth Information
ten times the terrestrial value, suggests either a
high potassium-40 content plus efficient outgas-
sing, or a net depletion of argon-36 and, possibly,
other volatiles. If Mars is volatile-rich, compared
to Earth, it should have more K and hence more
argon-40.
11
MARS
10
cm
3
ρ
m
~ 3.54 g
/
METEORITES
9
Early
outgassed
argon-36
could
also
ρ
Sil.
>
ρ
m
have been removed from the planet.
SNC meteorites
have trapped rare-gas and
nitrogen contents that differ from other mete-
orites but closely match those in the martian
atmosphere. If SNC meteorites come from Mars,
then a relatively volatile-rich planet is implied,
and the atmospheric evidence for a low volatile
content for Mars would have to be rationalized
by the loss of the early accretional atmosphere.
Mars is more susceptible to atmospheric escape
than Venus or Earth owing to its low gravity. The
surface of Mars appears to be weathered basalt.
The dark materials at the surface contain basaltic
minerals and hematite and sulfur-rich material,
and there is evidence for the past action of liquid
water. The large volcanoes on Mars are similar in
form to shield volcanoes on Earth. Andesite -- a
possible indicator of plate tectonics -- has been
proposed as a component of martian soil but this
is controversial; weathered basalt can explain the
available data.
The topography and gravity field of Mars indi-
cate that parts of Mars are grossly out of hydro-
static equilibrium and that the crust is highly
variable in thickness. If variations in the grav-
ity field are attributed to variations in crustal
thickness, reasonable values of the density con-
trast imply that the average crustal thickness is
at least 45 km, and the maximum crustal thick-
ness may reach 100 km. Giant impacts may have
removedmostofthecrustbeneaththebasins,
replacing crustal material by uplifted mantle. If
so, the crust was in place in early martian his-
tory, consistent with other evidence throughout
the solar system for rapid early planetary differ-
entiation. On Earth, delamination of lower crust
produces a thinning but the whole crust is not
involved.
The only direct evidence concerning the inter-
nal structure of Mars is the mean density,
moment of inertia, topography and gravity field.
The mean density of Mars, corrected for pressure,
is less than that of Earth, Venus and Mercury
ρ
Sil.
>
ρ
m
8
~
Fe
7
Br.
Earth's Core
O.C.
Enst.
Hyp.C.
6
Amph
Eutectic
mix.
P
=
30 kb
CC3
P
0 kb
Fe S
=
5
CC2
CC1
4
0
0.2
0.4
0.6
Core radius (
R
c
/
R
)
Fig. 2.2
Radius of the core versus density of core for Mars
models. The points are for meteorites with all of the FeS and
free iron and nickel differentiated into the core. The dashed
line shows how core density is related to core size in the
Fe--FeS system.
but greater than that of the Moon. This implies
either that Mars has a small total Fe--Ni content
or that the FeO/Fe ratio varies among the plan-
ets. Plausible models for Mars can be constructed
that have solar or chondritic values for iron, if
most or all of it is taken to be oxidized. With
such broad chemical constraints, mean density
and moment of inertia and under the assump-
tion of a differentiated planet, it is possible to
trade off the size and density of the core and
density of the mantle.
The mantle of Mars is presumably composed
mainly of silicates, which can be expected to
undergo one or two major phase changes, each
involving a 10% increase in density. To a good
approximation, these phase changes will occur
at one-third and two-thirds of the radius of Mars.
The deeper phase change will not occur if the
radius of the core exceeds one-third of the radius
of the planet.
The curve in Figure 2.2 is the locus of possible
Mars models. Clearly, the data can accommodate
a small dense core or a large light core. The upper
limit to the density of the core is probably close
to the density of iron, in which case the core
