Geoscience Reference
In-Depth Information
Mercury -- first rock from the Sun
it nevertheless contains a large fraction of the
terrestrial inventory of incompatible elements.
The thin crust on Earth can be explained by
crustal recycling and the shallowness of the
basalt--eclogite boundary in the Earth. Most of
Earth's 'crust' probably resides in the transition
region of the mantle. Estimates of bulk Earth
chemistry can yield a basaltic layer of about 10%
of the mass of the mantle.
The crust of the Earth is enriched in Ca,
Al, K and Na in comparison to the mantle,
and ionic-radii considerations and experimental
petrology suggest that the crust of any planet
will be enriched in these constituents. A max-
imum average crustal thickness for a fully dif-
ferentiated chondritic planet can be obtained by
removing all of the CaO, with the available Al
2
O
3
,
as anorthite to the surface. This operation gives
a crustal thickness of about 100 km for Mars.
Incomplete differentiation and retention of CaO
and Al
2
O
3
in the mantle will reduce this value,
which is likely to be the absolute upper bound
(Earth's crust is much thinner due to crustal
recycling, delamination and the basalt--eclogite
phase change). In the case of the Earth, up to
60--70% of some large-ion elements are in the
crust, implying that about 30--40% of the crustal
elements are in the mantle.
This does not require
that 30--40% of the mantle is still in a
primor-
dial undegassed state
as some geochemists
believe.
The average thickness of the crust of the
Earth is only 15 km, which amounts to 0.4% of
the mass of the Earth. The crustal thickness is
5--10 km under oceans and 30--50 km under older
continental shields. The thickest crust on Earth --
about 80 km -- is under young actively converg-
ing mountain belts. The parts deeper than about
50 km may eventually convert to eclogite, and fall
off. The situation on the Earth is complicated,
since new crust is constantly being created at
midoceanic ridges and consumed at island arcs.
The continental crust loses mass by erosion and
by delamination of the lower eclogitic portions.
Continental crust is recycled but its total volume
is roughly constant with time. Both the Moon
and Mars have crustal thicknesses greater than
that of the Earth in spite of their much smaller
sizes, and probable less efficient differentiation.
Mercury is 5.5% of the mass of the Earth, but
it has a very similar density, 5.43 g/cm
3
.Its
radius is 2444 km. Any plausible bulk composi-
tion is about 60% iron and this iron must be
largely differentiated into a core. Mercury has
a perceptible magnetic field, appreciably more
than either Venus or Mars, probably implying
that the core is molten.
Mercury's surface is pre-
dominantly silicate, but apparently not basaltic
.A
further inference is that the iron core existed
early in its history; a late core-formation event
would have resulted in a significant expansion of
Mercury.
Mercury's shape may have significantly
changed over the history of the planet. Tidal de-
spinning results in a less oblate planet and com-
pressional tectonics in the equatorial regions.
Cooling and formation of a core cause a change
in the mean density and radius. A widespread
system of arcuate scarps on Mercury, which
appear to be thrust faults, provides evidence for
compressional stresses in the crust. The absence
of normal faults suggests that Mercury has
contracted. This is evidence for cooling of the
interior.
One factor affecting the bulk composition
of Mercury is the probable high temperature
in its zone of the solar nebula; it may have
formed from predominantly high-temperature
condensates. If the temperature was held around
1300 K until most of the uncondensed mate-
rial was blown away, then a composition satis-
fying Mercury's mean density can be obtained,
since most of the iron will be condensed, but
only a minor part of the magnesian silicates.
Since the band of temperatures at which this
condition prevails is quite narrow, other fac-
tors must be considered. Two of these are (1)
dynamical interaction among the material in
the terrestrial planet zones, leading to compo-
sitional mixing, and (2) collisional differentia-
tion. A large impact after core formation may
have blasted away much of the silicate crust and
mantle. Our Moon may have been the result of
such an impact on proto-Earth. On Mars, the
crust is locally thinner under the large impact
basins.
