Geologic Processes on the Moon
I INTRODUCTION
This paper deals with the processes that formed the features we see on the moon. The primary geologic processes that shaped the moon are the formation of craters, volcanic activity, and tectonic activity. Each of these will be dealt with in their respective sections below.
II CRATERING ON THE MOON
Introduction
Craters
cover the surface of the moon. They are the result of hyper-velocity impacts by
meteorites. The velocity of meteorites upon impact varies, but is generally
between 10 and 40 km/sec. This number is a combination of the 'approach
velocity' and the 'escape velocity.' The approach velocity of objects refers to
the velocity of the object with respect to the moon. This varies with the type
of object (for example, long period comets generally have a higher approach
velocity than short period comets) and the direction with which it approaches
the moon (for example, if it approaching the moon 'head on,' it will have a
higher approach velocity than if it is 'catching up' with the moon in its
orbit). The escape velocity is a measure of the extra velocity an object gains
as it accelerates in the gravitational field of an airless moon/planet. For the
moon, this number is 2.4 km/sec
The velocity of a bolide (the technical name for a body that strikes any
planetary surface) is important for it is the major determinant of the amount of
energy released upon impact. Bolides possess 'kinetic energy', and the value of
this is proportional to the mass of the bolide multiplied by the square of the
velocity. Thus, if two meteorites of the same mass strike the lunar surface,
but one has twice the velocity of the other, than the faster one possessed four
times (not two times) the kinetic energy of the slower one.
Upon striking the moon, the kinetic energy is transferred to a massive shock
wave which both goes down into the moon's surface and rearward into the bolide
itself. The shock wave that goes rearward is so powerful that it excess the
strength of the rock--indeed, most of the bolide vaporizes. The shock wave that
goes forward into the moon vaporizes part of the surface of the moon (several
times the mass of the bolide), melts the layers of rock below this (up to 100
times the mass of the bolide), and shocks (fractures) the surface deeper yet.
This period in the cratering process is called the 'contact and compression'
phase.
The next period in the cratering process is called the 'excavation' phase. This
phase begins with the formation of a release (rarefaction) wave that develops at
the edges of the impact, and forms a route of escape for some of the
vaporized/melted/shocked rock. This escape of material produces the crater
itself, and the material that escapes forms the ejecta that goes outward onto
the moon surface. Finally, the decaying shock wave continues to travel through
the bedrock of the moon, creating effects further away (such as activating older
faults, creating landslides, etc.
The third period in the cratering process is called the 'modification' phase.
Here the liquid materials on the crater's sidewall (impact melt) and semi-stable
rim materials slip down to the crater's floor. Additionally, in larger craters,
this is the time that the central peaks and sidewall terracing occur.
From this brief description on the mechanics of crater formation, we will now
look at the types of craters and the unique morphology of each. While craters
are variously classified, based on their size and morphology, I am going to use
the most common classification: simple craters, complex craters, and basins.
Simple Craters
Simple
craters are bowl like depressions in the lunar surface. They occur from
submillimeter size to approximately 15 km in diameter (15-20 km is the
transition zone between simple and complex craters).
Simple craters form when small meteorites strike the moon at high velocities.
The bolide is vaporized along with the surface struck (the target). This
vaporized rock is injected into the floor of the crater, and follows the release
wave to escape to the outside where it will be emplaced as ejecta. As the shock
wave begins to dissipate, the next layer of target materials will not be
vaporized, but only melted (called 'impact melt'). This material is also
injected into the crater's floor and escapes to the outside as ejecta. As the
shock wave further dissipates, it is no longer able to melt the target
materials, but instead only fractures the rock. This fractured rock is again
pushed in both directions.
The crater itself is formed by decompression along the sides of the crater,
allowing these vaporized, melted, and shocked fragments to escape. This
material will lay itself down as the ejecta blanket, which has four distinct
parts. Just outside of the crater rim is the zone of continuous ejecta, which
is formed from the last material ejected from the impact. The next layer out is
the discontinuous ejecta, which interfingers with the surrounding lunar
surface. Further out yet is the bright ray system, which is formed from the
first material ejected. The fourth part of the ejecta is found in the area of
the discontinuous ejecta and just beyond it--this is the area of 'secondary
cratering', which results from 'chunks' of rock which are thrown out from the
crater. This secondary cratering typically forms a 'herringbone' pattern on the
lunar surface, with multiple craters in a line having small 'v' shaped lines
emanating from them.
Once the ejecta has exited, the remaining crater is called the transient crater,
for other processes will modify its final form. For simple craters, this final
'modification' involves the sliding down of impact materials (impact melt and
unstable rim/wall materials) onto the floor of the crater. For craters in this
size range, these materials generally fill the lower third to half of the
transient crater's depth. This will result in the crater's final form.
Observation of such a crater will reveal a bowl shaped depression with a sharp
rim, some rim deposits (blocks of material thrown out at the end of excavation),
a discrete ejecta blanket grading from continuous to discontinuous, and a bright
ray system. Across time, parts of this crater will degrade due to the erosive
rain of micrometeorite impacts. The first to go will be the ray system,
followed by the discontinuous ejecta and the sharp rim. This process will
continue until only a bowl shaped depression with a gentle slope remains.
Complex Craters
Complex
craters begin at 20 km (transition zone from simple to complex is 15-20 km in
diameter). They are characterized by the morphology of a bowl like depression
with a central uplift of one of more massifs (small, mountain like structures)
and terracing on the sidewalls.
Complex craters form when medium sized meteorites impact on the lunar surface.
The impact occurs as discussed in the simple crater above, though the energies
involved are much greater. The real differences begin after the formation of
the transient crater. At this point the rim is more massive than in a simple
crater. Because the subsurface rock is extensively fractured, this rim material
cannot be supported. It slides down these fractures (called 'slumping')
creating a series of 'terraces' on the crater's inner walls. Central peak or
peaks also form at this time. Peaks form because the impact compresses the
underlying rock, and this rock rebounds after the shock energy is
dissipated--much like a bedspring that is compressed and then released (the size
of the central peaks is also modified by slumping of the rim material, which
pushes rock towards the central uplift). At the same time this slumping and peak
formation occur, the impact melt on the sides of the crater is sliding down
along with other unstable side/rim material. This again covers the bottom of
the temporary crater and ponds in some of the terraces. This produces the
'final' form of the crater
The parts of the complex craters are, then, the central uplift, which can be one
or several peaks that may attain heights of over a 1000 meters. This is
followed outward by a flattened floor of impact melt which grades into the
terraced sidewalls. The rim occurs at the top of the crater and grades out into
the continuous ejecta, the discontinuous ejecta, the larger secondary craters
(which now can be seen by earth based telescopes; for e.g., see those around
Copernicus), and the bright ray system.
Degradation occurs in complex craters as in simple craters. First the ray
system goes, followed by discontinuous ejecta and the sharp rim. The continuous
ejecta erodes later along with the terracing and central peak. Across geologic
time, the crater will become a simple bowl like depression.
Basins
Basins
begin around 200 km in diameter (note: given the length of this article, I will
not further divide basins into peak-ring basins, central-peak basins, and
multi-ring basins; the reader is referred to more detailed texts here). They
are characterized by a series of rings (instead of a single rim). Multi-ring
basins are the largest cratering events on the moon, spanning up to 2500 km in
size. The formation of multi-ring basins is poorly understood, and competing
theories exist. The problem is that the amount of kinetic energy released is so
large that it is difficult to predict how a solid surface behaves under its
influence. The model we present assumes that the energy causes the solid lunar
surface to behave as a substance with little inherent strength (i.e., a fluid
surface), and so the rings form like a stone dropped into still water.
When a massive impact occurs on the moon, the transferred energy produces a
massive shock wave. This vaporizes most of the bolide and part of the moon's
surface. As in the simple crater, this material is both injected into the next
layer and allowed to escape out as ejecta. The next layers of melted rock and
shocked rock do the same. The transient crater which then forms is in the shape
of a shallow bowl. Next a central uplift occurs from rebound of the underlying
rock. This rebound cannot come into equilibrium in the fluid-like medium, and
so collapses, with the rebound-material forming a wave that propagates across
the transient crater's floor. The wave freezes in place as its kinetic energy
is dissipated by friction. Multiple rings may form in this fashion.
The morphology of a multiring basin is best illustrated by the Orientale basin.
While it is the most recent of the large basins, only a fraction of it can be
seen from earth. Fortunately, it is well photographed by spacecraft. The
center of the basin is flat, and probably covered with impact melt (it has since
been modified by volcanism). Further out, at a general spacing ratio, one comes
to each successive ring. Beyond the outer rim, we find the usual ejecta
blanket, with continuous/discontinuous/secondary impacts. However, here the
ejecta is much more massive and extensive (the secondary craters can be 10-20 km
across, and the continuous ejecta can be hundreds of meters thick). Also, note
that the ejecta forms a 'hummocky' terrain (examples of this can be seen around
the Imbrium basin as the Fra Maruo formation, and around the Nectaris Basin as
the Janssen Formation).
Across time even these massive basins are eroded away by the rain of
micrometeorites. Indeed, as the basins are all very old, this erosion has
already erased all evidence of their ray systems.
III OTHER EFFECTS OF CRATERING ON THE MOON
Impacts do more than just produce craters. To these effects we now turn our attention.
First, the
cratering event creates a shock wave that doesn't 'stop' in the impact's general
area, but continues to travel out across the moon. If this wave contains
sufficient energy, it will cause faulting in the bedrock (the Straight Wall is
an example of this). It can also activate faults that already exist. Finally,
it can loosen semi-stable materials on crater rims, producing landslides. An
example of this is the landslide in Copernicus that was caused (it is thought)
by the shock wave from the Tyco impact.
Upon impact, basins spread a thick ejecta blanket over a huge section of the
moon. These blankets accumulated into a layer several kilometers thick, called
the megaregolith. On top of it is a layer of fine, dusty material called the
regolith. This was produced by smaller meteorites/micrometeorites pulverizing
the upper layers of the megaregolith. The regolith can be over 15 meters thick
on the lunar highlands, and up to 8 meters thick on the mare. Because the
regolith is so thick, it acts as a protective shield to the underlying
structures (megaregolith, lava flows). Micrometeorites and small meteorites are
not able to pierce it. Only meteors around 3 meters in diameter can now reach
the megaregolith (depending on their velocity).
In earlier sections, we noted that micrometeorite impacts eroded the craters and
basins. This degradation occurs in an orderly fashion, so that one can detail
the age of a crater by examining its state of degradation. Thus medium sized
craters that have a sharp rim, rim deposits, terracing, a central peak, a
continuous and discontinuous ejecta and a bright ray pattern are the youngest.
These are in the 'Copernican Period', which extends from the present to 1.2
billion years of age. Medium sized craters that have all these parts except the
bright ray pattern are in the next in age. These are from the 'Eratosthenian
Period,' which extends from 1.2 to 3.2 billions years of age. Medium sized
craters that have lost their bright ray pattern and the discontinuous ejecta are
much older. They come from the Imbrium Period, which is from 3.2 to 3.85
billion years of age. Medium sized craters that have lost their continuous
ejecta and their 'sharp' rim are from the next period, called the 'Nectarian
Period.' This period extends from 3.85 to 3.92 billion years of age. Medium
sized craters that appear as simple bowl shaped objects without any rim or
ejecta are the most ancient of all. They come from the 'Pre-Nectarian Period,'
which extends from 3.92 billion years of age to the beginning of the moon's
solid surface.
Note, here, that crater dating has some limitations. First, small craters
degrade more quickly than larger ones. Second, ray systems degrade faster on
mare surfaces. Third, apparent degradation can occur when large ejecta sheets
or a volcanic flow obscure a crater's parts. However, even given these three
problems, we can still tell much about the age of craters from the amount of
erosion each one exhibits.
IV VOLCANISM
Volcanism
is the next major geologic force on the moon. Radioactive elements (such as
uranium, potassium, and thorium) reheated areas of the lower crust and upper
mantle, creating a series of partial melts. These melts were less dense than
the surrounding rock, and so began rising toward the surface. The eruption of
lava preferentially occurred in basins, and that for two main reasons: first,
these massive impacts sent faults deep into the moon's surface (tens of
kilometers), providing conduits for the rising lava. Second, the mantle
underneath the basins rose closer to the surface (isostatic compensation),
making the path to the surface much shorter.
As lava erupted into the basins, it sometimes flowed long distances before
finally 'emplacing'. It could do this because lava on the moon has a low
viscosity (it is very thin and runny). Indeed, when lava materials were melted
on earth, it was shown to have the consistency of motor oil. This is because
lunar lava is low in silicates ('mafic' lava). By contrast, the lava on earth's
shields is higher viscosity--making it more like toothpaste--as it is higher in
silicates ('felsic' lava). These lunar lavas generally erupted from fissures,
which poured out and ponded in the geographically lower plains. However when
erupted onto an inclined surface, the lava could flow downhill and even create
river-like channels from thermal erosion. On the moon, these formations are
called 'sinuous rilles'. Some run up to several hundred kilometers before
finally spilling their lava onto flatter surfaces.
This process of mare flooding resulted in large, flat lava sheets that covered
the basins. Because the basins were concave in shape, lava was thicker in the
center of the basin and thinner towards the edges. Now lava is denser (heavier)
than the surrounding crustal rock, so it 'compresses' the bedrock underneath (a
process generally called 'subsidence'). The thicker areas in the center do this
more than the thinner areas out at the edges. This changes the shape of the
basin from a 'flat' surface to a very gently sloped 'bowl' shaped surface. This
produces three unique formations.
First, it created unique, 'target like' surfaces. As the first lava flow
subsided, the center would 'sink' and the outer areas remain raised. The next
flows preferentially filled the lower central areas. Since each large eruptive
event(s) had a slightly different composition, the 'colors' of the flow would
also follow that pattern. This produced a 'target like' appearance to certain
of the maria, with the outer bands representing the older flows, and the inner
bands the younger ones. One of the best examples of this is seen in Mare
Serenitatis.
Second, lava subsidence produced stresses within the lava bed itself. As the
lava in the center sank, it produced a compressive force where the thicker lava
beds (on the sides of basin rings) met the thinner lava beds (on top of basin
rings). These forces caused the lava to 'buckle' (perhaps due to blind thrust
faulting) producing mare ridges over the basin rings. While there are several
types of mare ridges (discussed below), these are identified by forming a ring
within the mare, and are often associated with small peaks that represent the
highest points of the flooded basin ring (e.g. Mons Piton).
Third,
this process of subsidence put stresses on the lava bed and in the bedrock
underneath. This rock was already deeply fractured from the basin impacts, and
these new downward and inward stresses caused some of those faults to activate.
They opened up creating a series of 'grabens' (grabens occur where two parallel
faults are 'pulled apart,' with the center section falling down; this produces
a flat bottomed valley). On the moon, these are specifically called 'arcuate
rills'. These are only found around the edges of lava filled basins (the best
examples are those around the Mare Humorum).
To this point we have discussed the usual schemes for lava filling of the
basins, along with the formation of sinuous rilles, arcuate rilles, and mare
ridges. Next we need to examine a few other features produced by the volcanic
process.
The first of these are lunar volcanoes, which are called lunar 'domes' (not to
be confused with volcanic domes on earth, that have steep inclines). Lunar
domes are smooth sided with low levels of incline. This is because lunar lava
has such a low viscosity (as noted earlier). Most lunar domes are 5-20 km
across, and often have a small pit crater at their summit. Note that a few
lunar domes are steep sided (especially in the Marius Hills region), and thus
offer evidence for changes in the lava's characteristics--such as cooling and
lower rates of eruption.
The next features are called 'dark mantling' areas. These were formed by the
process of 'fire fountaining'. When lava is in the moon's mantle, it is under
considerable pressure. As it rises to the surface this pressure falls off,
allowing gasses trapped in the lava to escape (called degassing). These
gasses--thought to be carbon monoxide or carbon dioxide--act as propellants,
shooting the lava high above the lunar surface. There the lava cools as dark,
glassy beads. Upon falling back to the lunar surface, these beads produce large
patches of 'dark mantling'. The Apollo missions returned some of these glassy
volcanic beads (the first ones identified were dubbed 'orange glass').
Visually, these patches appear as large, very dark areas with low crater counts,
and occur around basin edges. Some excellent example can be seen around Mare
Serenitatis.
Finally, there are two unusual lunar features produced by volcanism. Endogenous
craters, such as Hyginus Rille, are interpreted as volcanic in origin, and
probably formed as collapse features ('collapse pits'). Only a return to the
moon with further geologic work will fully resolve their origin. The other
unusual feature is the 'dark halo' crater. Two types of 'dark halo' craters
occur, and both are associated with volcanic products. In the type found in
Crater Alphonsus, the halos are associated with rilles, and likely represent
places of eruptive degassing with fire fountaining. Thus it is no surprise that
their halos are reminiscent of dark mantling materials. The other type of dark
halo crater occur where a bright ejecta blanket covers an older lava flow. When
a recent impact occurs here, it pierces the thin veneer of the bright ejecta and
unearths the darker lava flow beneath it. The ejecta from this crater will
include those darker materials (an example of this was Crater Shorty, which was
visited by the Apollo missions).
V TECTONIC PROCESSES
Tectonism refer to those forces that deform the lunar surface. These can be endogenous (such as thrust faults) or external (such as the creation of faults by impact events).
Crater Induced Processes
Impacts
create a shock wave that propagates through the lunar surface. If of sufficient
energy, these waves can induce faulting in the subsurface bedrock, can
reactivate faults located elsewhere, and can induce local changes in semi-stable
materials (e.g.: produce landslides in crater walls).
Examples of faulting in the subsurface layers are seen around a variety of
basins. Such faulting can be radial (straight out from the basin's center) or
concentric (around the basin's sides). Examples of concentric faulting include
'arcuate rills' (discussed above). They were only later 'activated' by the
stresses of volcanism. Good examples of these are seen around Mare Humorum and
Mare Serenitatis.
Faulting radial to a basin was also caused by the initial basin impact. Here
the shock wave created faults in the subsurface rock at some distance from the
basin. While initially covered by ejecta, these were later reactivated by other
processes (such as volcanism). Examples of these include the Straight Wall, the
Cauchy Rilles, and the rilles in Lacus Mortis.
Semistable material can be made unstable by a shock wave, creating a landslide
in a crater. An example of this is the landslide in Copernicus, that was
thought to be triggered by the Tyco impact.
Volcanism as a Tectonic Process
Other
types of tectonic activity are found in association with volcanism. Lava, by
coming from the mantle, is denser than the overlying crust. As noted earlier,
this denser rock creates local stress fields in the underlying bedrock,
producing mare ridges and arcuate rilles as the lava subsides. Here, however,
we need to discuss the way other types of mare ridges form.
Mare ridges can also form over crater/basin rims. Such a situation occurs when
a lava bed fills and covers a crater/basin rim. Now we have a shallow shelf of
lava over the rim and a much deeper shelf where the rim falls off. The dense
lava will subside more over the deep area and less over the shallow area,
inducing local stress fields in the cooling, plastic lava. At such a point a
mare ridge will form. Indeed, it is by examining mare ridges that we can tell
where submerged basin rings exist!
Two other processes that form mare ridges are a volcanic intrusion just under a shelf of cooling lava and activation of a fault due to lava loading, with slippage and subsequent lava deformation. Thus, mare ridges are the end result of a variety of tectonic processes.
Tidal Interactions
Tidal
forces refer to the stresses induced by gravity between planetary bodies. For
example, the Earth's tides are caused by the tidal stress induced by the moon.
As earth is larger, it induces proportionally larger stresses on the moon. In
fact, the earth exerts sufficient force to distort the moon's shape, so that it
is not perfectly round. Before the moon was in locked rotation with respect to
earth (the same side of the moon always faces the earth), this distortion likely
produced moonquakes and subsurface faulting. However, this distortion also
caused tidal slowing--the friction of these events slowed the moon's spin.
Eventually, the moon locked into synchronous rotation. Interestingly, the moon
is also causing tidal slowing of the earth, and our spin is ever so minutely
slowing across time.
Now, if the moon were completely locked into rotation with earth, one might
expect little seismic activity on the moon. However, the seismic monitors left
by the Apollo missions revealed small moonquakes--Richter Scale 2-3. This is
because the moon still has some wobble (librations), which causes changing tidal
stresses, resulting in these continuing moonquakes (note that there are also
thermal causes from secular cooling of the moon).
Endogenous Forces
The only new endogenous tectonic force is that induced by the moon's continued secular cooling. With this cooling comes shrinkage of the more plastic mantle. However, the rigid crust cannot shrink with it. This creates local stress fields, which are eventually released by thrust faulting (the crust on one side of the fault slides up diagonally). Similar faults exist on Mercury where the shrinkage has been even greater. While these faults are small, there are many of them, and they are continuing to form (according to experts like Dr. Alan Binder).
VI CONCLUSION
In the end, we find that the moon's surface was formed through a diverse set of processes. While these are not as complex as the geologic forces on earth (the moon lacks plate tectonics, hydrological and aeolian forces, or a significant geochemical cycle), it is still a fascinating world. And precisely because it lacks this extra complexity, it allows us to study these simpler processes in isolation. While it might seem that we understand everything about the moon, let me remind the reader that there are still many mysteries about the moon that are unsolved, and that the simplified scheme presented here is bound to be exactly that--too simple! May we one day return to the moon and learn more about our daughter world.