Earth’s Moon - Introduction to Planetary Geomorphology - page 72

other terrestrial planets. It must be remembered that the
Apollo network was located in the low-latitude, near-
side band dictated by the constraints of the landing sites
and was not ideal. Re nements to the model of the lunar
interior must await new measurements obtained from
better-sited stations.
One of the unexpected bene ts of the Lunar Orbiter
missions came from careful tracking of the spacecraft in
their orbits around the Moon. It was found that in some
places the spacecraft dipped closer to the surface and
speeded up, while in other places the spacecraft rose in
altitude and slowed down. Although such changes in orbit
were small (changes in velocity as small as 1mm/s could
be detected), by applying the laws of physics to these
motions, it was inferred that some locations on the
Moon consist of higher-density materials, causing the
spacecraft to be gravitationally accelerated, or “ pulled, ”
toward them. These areas were dubbed mascons (short for
mass concentrations) for positive gravity anomalies.
Negative gravity anomalies were identi ed over areas
where the spacecraft rose in altitude.
When later compared with terrains and geologic maps,
most mascons were found to correlate with circular
patches of maria in lunar impact basins, such as
Imbrium, Serenitatis, and Crisium, as well as mare depos-
its in some smaller craters. They also occur in association
with some large mountain ranges around basins, as well as
in the volcanic Marius Hills ( Fig. 4.9 ). Initially, it was
thought that mascons re ected buried impact projectiles,
but it was realized that bolides are generally fragmented
upon impact and mostly disbursed with the ejecta. High-
resolution gravity data from the Kaguya mission support
the earlier idea that mascons result from a combination of
mare flooding (perhaps accompanied by high-density iron
and titanium oxide minerals that settled to the bottom of
flood-lava lakes) and uplift of high-density mantle mate-
rial following the impact. As described by Namiki et al.
( 2009 ), gravity data indicate that the basins on the lunar
far side are supported by a rigid lithosphere, whereas the
near-side basins deformed during the eruption and
emplacement of the mare lavas.
One of the early discoveries of the pre-Apollo landings
was the existence of a very weak magnetic field.
Subsequent measurements both from orbit and from the
surface showed that the field is not uniform on the Moon.
The source of the magnetic signature remains debatable,
but some suggest that it was somehow imposed externally
by the Earth or the Sun or that it represents an ancient
intrinsic field from a time when the Moon ' s interior was

suf ciently molten to enable the operation of a dynamo
similar to that responsible for Earth ' s magnetic field.
Others have noted that many of the circular basins show
stronger magnetic signatures and have suggested that
massive impacts can generate local magnetic fields. This
idea is at least partly supported by laboratory impact
experiments by Pete Schultz that have recorded slight
transient magnetic fields.
On Earth, measurements of the amount of heat reaching
the surface from the interior provide important insight into
the interior of our planet. Such measurements were
attempted on Apollos 15, 16, and 17. Unfortunately,
when the heat- ow experiment was deployed on Apollo
16, an astronaut tripped over the wire and disconnected it
from the main station. It probably did not matter, however,
as the results from Apollos 15 and 17 are now generally
regarded as inconclusive. Heat- ow experiments require
deployment into the subsurface (the deeper, the better) and
suf cient time for the surrounding rock and soil to stabi-
lize thermally, after the disturbances caused by their
emplacement. The Apollo experiments were placed
< 3m below the surface and did not record for a suf cient
length of time for thermal stabilization.

4.4 Surface composition

Information on the composition of the lunar surface comes
from in situ measurements made from unmanned landers,
remote sensing from orbiters and Earth-based observato-
ries, samples returned to Earth from the Apollo and Soviet
missions, and from dozens of meteorites considered to
have been blasted from the Moon by impacts and sent
on trajectories to Earth.
NASA ' s Surveyor landers revealed iron-rich (ma c)
compositions for the mare deposits, which, when com-
bined with images of features suggestive of ows, were
correctly identified as basaltic lavas. The spectral signa-
tures of the titanium-rich lavas of Apollo 11 were later
mapped in remote sensing data and extrapolated to other
parts of the Moon, but they were found to be relatively
restricted in distribution in comparison with most of the
mare basalts. In fact, Paul Spudis of the Lunar and
Planetary Institute has noted that the basalts in the
returned samples ( Fig. 4.20(a) ) represent only about
one-third of the total number of varieties of mare basalts
suggested in the Clementine data.
The so-called highland rock from the Apollo 15

site was found to be nearly pure anorthosite ( Fig. 4.20(b) ),

“ genesis ”