Geology Reference
In-Depth Information
(b)
Figure 4.43. The mare ridge (also called
)inMare
Serenitatis, showing the typical broad basal component and upper,
sharper ridge component; note that the ridge system cuts across
both higher-and lower-albedo mare units. Mare ridges are
considered to be primarily structural features resulting from
tectonic processes, although they might also be accompanied by
igneous activity in some cases; the area shown is 150 km by 170 km
(NASA AS 17
“
wrinkle ridge
”
-
0451).
Moon had regolith thicknesses of about 2m, while the
older surfaces were as thick as 16 m.
It is important to note that the termmegaregolith is also
used. This refers to the part of the lunar crust that was
deeply fractured during heavy bombardment in the
final
stages of Solar System formation and after the lunar crust
had solidi
ed. The megaregolith is thought to be many
kilometers thick.
Figure 4.42. (cont.)
remote sensing data for the Moon because it in
uences the
spectral signatures.
Impact-generated debris, or regolith, is pervasive on the
lunar surface and increases with time in response to the
frequency and size of impacts. Verne Oberbeck and Bill
Quaide of NASA studied the morphology of small impact
craters that formed on unconsolidated debris overlying
more coherent bedrock. As shown in
Fig. 4.47
, they
found that normal bowl-shaped craters formed in thick
debris layers (through which the transient crater did not
penetrate),
at-
oored craters formed in intermediate
thicknesses of debris, and concentric craters formed in
thin debris layers. Analysis enabled derivation of equa-
tions relating the geometries of the crater diameters to the
thickness of the debris layer. They applied this technique
by analyzing the geometry of craters at the Apollo landing
sites and found that the youngest mare surfaces on the
4.6 Geologic history of the Moon
After more than four decades of intensive study, a general
hypothesis for the origin of the Moon has emerged.
Sometimes referred to as the
model, the
concept involves the collision of a Mars-size object with
the proto-Earth in the
final stages of Solar System forma-
tion. Both objects had already differentiated to form early
mantles and cores. Computer models developed by plan-
etologist Jay Melosh suggest that the Mars-size object
collided with the proto-Earth at an oblique angle, ejecting
“
Big Whack
”
