Geology Reference
In-Depth Information
reactions of the solar wind with lunar minerals to form
water-bearing materials.
In addition to the search for water on the Moon, atten-
tion is also being given to characterizing the lunar interior.
In 2011, two small spacecraft that completed objectives to
study space weathering and solar processes were reposi-
tioned to collect geophysical data for the Moon. The
mission, termed ARTEMIS (Acceleration, Reconnection,
Turbulence and Electrodynamics of the Moon
'
s
Interaction with the Sun), places the spacecraft in orbit
around the Moon, coming as close as 100 km above the
surface, and will operate for seven to ten years. In parallel,
the Gravity Recovery and Interior Laboratory (GRAIL)
mission was launched in 2011. It involves two spacecraft
in near-circular polar orbits that are mapping the detailed
gravity of the Moon to determine the structure of the crust
and lithosphere, as well as to gain insight into the deep
interior. GRAIL is a Discovery project led by MIT geo-
physicist Maria Zuber.
RTG (radioisotope thermoelectric generator) power sources
that enabled longer lifetimes. In addition, Apollos 14, 16,
and 17 involved active experiments in which small explo-
sive charges were detonated to generate seismic events in
order to assess shallow subsurface structure.
The natural seismic events on the Moon are rather
different from those seen on Earth. Typically, they register
less than 3 on the Richter scale, and, although more than
3,000 events were seen each year, the total annual energy
was less than 2 × 10
13
ergs, compared with the annual
energy released on Earth by earthquakes of 10
24
10
25
ergs. Thus, the total annual energy generated from
“
moon-
quakes
”
is equivalent to only about 500 grams of TNT, or
slightly more than the detonation of about one pound of
explosives.
Moonquakes generate signals that have been
described as
“
ringing,
”
in which the magnitude is low
but of long duration. Except for seismic events from
impacts, natural moonquakes tend to occur in the same
areas on the Moon and are considered to result from
stresses generated by tidal interactions with Earth.
High-frequency teleseisms are shallow moonquakes
that manifest releases of energy in the lunar crust.
Recent applications of array-processing techniques by
Renee Weber et al.(
2011
) to the Apollo data suggest that
the Moon has a solid inner and a
fluid outer core overlain
by a zone of partial melt and a thick mantle (
Fig. 4.19
).
Although there is very little information on the Moon
'
s
core, the overall density of the Moon (
Tabl e 1 . 2
)sug-
geststhatitisdepletedinironincomparisonwiththe
-
4.3 Interior characteristics
Information on the interior of the Moon is derived primarily
from the Apollo network of seismometers, gravity measure-
ments, considerations of the Moon
'
s weak magnetic
field,
and other geophysical data. Each of the six Apollo landings
included placement of seismometers on the surface. The
Apollo 11 station used solar-powered batteries and was of
limited duration, whereas Apollos 12 and 14 through 17 used
Figure 4.19. The interior con
guration of the Moon
(not to scale), based on geophysical data and models;
anorthositic rocks compose a crust ranging in
thickness from 90 km on the far side to 40
50 km on
the near side, with relatively thin mare deposits
primarily on the near side; the greatest mass is in the
mantle, with shallow moonquakes (high-frequency
teleseisms) occurring near the crust
-
mantle boundary
and deep moonquakes near the lower mantle
boundary; the zone of partial melt has a radius of
~480 km, while the outer
fluid core has a radius of
~330 km; the inner solid core has a radius of ~240 km.
-
