Geology Reference
In-Depth Information
4.5.2 Highland plains
Many areas in the highlands display relatively smooth,
bright plains. Early in photogeologic mapping of the
Moon, these plains were interpreted as being either a
form of highland volcanism (such as silicic ash
ows) or
ejecta deposits from large impacts. Named the Cayley
Formation, this unit was given a high priority for Apollo
exploration to resolve the issue. For example, if the
plains represented silicic volcanism, then models for
magma evolution on the Moon would need substantial
revision from the favored basalt-dominated models.
With this goal in mind, Apollo 16 wassitedtolandon
the Cayley Formation (
Fig. 4.28
)neartherimofthe
Nectaris basin. This unit is now widely regarded as
ejecta deposits associated with one or more basin-
forming impact. Stimulated by the initial controversy
of the origin of the Cayley Formation, Verne Oberbeck
of NASA-Ames Research Center conducted impact
experiments and modeled the ejecta process. As
shown in
Fig. 4.29
, he demonstrated that ejecta deposits
consist of progressively higher percentages of locally
derived materials with increasing distance from the
primary impact. In considering impact cratering
mechanics, this makes sense; the blocks of ejecta
thrown the greatest distance impact at the highest
speeds; thus, they would transfer the highest energies
to the surface to form secondary craters and generate
local ejecta.
Figure 4.25. The 930 km in diameter Orientale basin, imaged by the
Lunar Reconnaissance Orbiter, is marked by the Inner Rook
Mountains (A), the Outer Rook Mountains (B), and the Cordillera
Mountains (C), which form concentric rings. The outer two rings rise
some 3 km above the surrounding terrain and are among the highest
features on the Moon (NASA PIA 13225).
as the rest of the Moon, from the Lunar (Reconnaissance)
Orbiter Laser Altimeter on the LRO spacecraft and from
the Kaguya mission enables accurate assessments of the
elevations of the mountains and the intervening low-
lying areas.
There has been considerable controversy regarding the
size of the original transient cavity of the Orientale impact,
as well as the mechanisms of basin-ring formation in
general. Many researchers favor the Outer Rook
Mountains to de
ne the Orientale transient cavity, on the
basis of observations that most of the ejecta lies beyond
the scarp formed by this range. The Cordillera Mountains
are thought to be a
“
mega-terrace,
”
formed by the inward
slump of material into the transient cavity. However, an
alternative model suggests that the Cordilleras represent
the transient cavity, because of the knobby texture of the
Montes Rook Formation, which lies beyond the
Cordilleras and is suggested to be the primary ejecta. In
either case, the inner rings and scarps probably represent
adjustments and rebound of the lunar crust following the
impact.
4.5.3 Mare terrains
Lunar maria and basins are often erroneously considered
to be synonymous, even by some planetary scientists. As
noted above, basins are impact structures, while maria
are lava
flows, most of which are younger than the
formation of the basins. Mare lavas tended to accumulate
in topographically low areas, which typically are in the
basins. Thus, the outlines of many maria are circular,
conforming to the shape of the impact basins that contain
them.
Maria cover about 17% of the surface of the Moon
and are found mostly on the near side, whereas basins
are randomly distributed over the Moon (
Fig. 4.23
). If
cryptomaria are taken into account, the areal extent of
mare lavas increases to about 20% of surface and near-
surface materials (Antonenko et al.,
1995
). Although
dif
cult to determine precisely, since estimates are
based on the degree of
flooding and partial
flooding of
