Geology Reference
In-Depth Information
Figure 5.3. Cross-section of Mercury showing its
large iron-rich core (shaded) and a diagram of a
large impact. Such an impact is thought to have
generated seismic energy (both surface waves and
body waves through the planet) that was focused
in the antipode region, resulting in jostling of the
upper crust to form hilly and lineated terrain, or
“
although not shown here,
Mercury could have a mantle (courtesy of Peter
Schultz).
weird terrain;
”
that it re
ects some unknown event in its history that
might have generated a magnetic
field.
In 2007, planetary scientists Jean-Luc Margot, Stan
Peale, and their colleagues analyzed Earth-based radar
signals bounced off Mercury over a six-year period and
found that slight variations in the spin were more than
should be expected if Mercury were completely solid.
They suggested that the spin variation results from a litho-
sphere that is decoupled from the interior by a liquid zone,
meaning that at least part of the interior is molten today.
How could such a small planet as Mercury remain molten
since its formation? One suggestion is that Mercury
'
siron
core contains sulfur, which would lower the melting tem-
perature and keep the core molten. Although preliminary
MESSENGER results are not conclusive, data from the
flybys suggest that at least part of the magnetic signature
results from an active dynamo, which would be consistent
with a molten interior zone.
color-imaging data, coupled with Mariner 10 data and
Earth-based telescopic observations in the visible to mid-
IR parts of the electromagnetic spectrum (
Fig. 2.14
). In the
1990s, Mark Robinson recalibrated the Mariner 10 color
data and derived new insight into surface compositions;
those new insights are being con
rmed with MESSENGER
data. For example, some local heterogeneities in color and
albedo can be correlated with the geology,
including
inferred volcanic materials described below.
Mercury
'
s surface appears to be compositionally heter-
ogeneous, with a wide variety of silicate materials and
rocks such as low-iron basalt, as reviewed in the topic
Exploring Mercury: The Iron Planet by Robert Strom and
Ann Sprague (
2003
). MESSENGER data at global scales
con
rm this conclusion. In contrast with the lunar high-
lands, Mercury
'
s crust does not appear to be dominated by
anorthosite but instead seems to have olivine and low-iron
pyroxene. Thus, in comparison with the iron-rich lavas of
the other terrestrial planets, the FeO content of the mer-
curian surface is lower by factors of 4
-
12. However,
albedo and soil maturity related to space weathering in
u-
ence the spectral signatures of the regolith on the Moon,
and the same considerations must be taken into account
for remote sensing data for Mercury.
5.4 Surface composition
Information on the composition of Mercury
'
ssurfaceis
derived primarily from MESSENGER
'
s spectrometers and
