Astronomy for Students
Lesson 6: The Moon
Earth is the first planet out from the sun with a moon. And, unlike any other planet, we have only one.
If something the size of the moon were put on the earth, it would cover most of the United States:
That picture is of a full moon, but that is not how we usually see it. The moon circles the earth, and when the moon is between the earth and the sun, or partly between the earth and the sun, then a lot of the sunlight is shining on the back side of the moon and we can't see the moon, or we can only see part of the moon.
The terms used for the different phases of the moon are pretty familiar to us, except for one, "gibbous." But if we start at the new moon, it is just starting to circle out of its own shadow and we see a tiny bit of light on the right hand side if we are in the Northern Hemisphere and on the left hand side if we are in the Southern Hemisphere.
Then what we see gets bigger and the moon becomes a 'waxing' (getting bigger) crescent. What's strange is that when we see half the moon, it is called the First Quarter. But if you understand that refers to the whole cycle of the moon from new moon to new moon, it makes sense. The moon is now a quarter of the way around the earth. Then comes that strange term and, before the moon is full, but after we see half of it, it is called a "waxing gibbous moon." As the moon circles behind the earth, we can see the full face of it because the sun is shining directly on the side we can see. This is the Full Moon. Then the moon goes into the 'waning' (getting smaller) gibbous phase, followed by the Third Quarter (when we see the left half of the moon in the Northern Hemisphere), then the waning crescent. As the moon continues to circle the earth, for about two days we don't see the moon at all. It's back side is getting all the sun and the side facing us is getting none.
Remember Galileo's drawing of what he thought the moon looked like?
He showed two sides. The light side shows the dark areas and some of the craters we see. But until we had spacecraft, no one knew what the other side of the moon looked like. Why not?
The moon does spin on its own axis. It spins once fully around every month. But it also circles the earth completely every month. That means that it is spinning at the same rate it is circling us and so we always see the same side of it.
Here is a picture of the side of the moon that we see, from some strong telescopes:
The bright spot at the bottom with streaks coming out of it is not the 'south pole' of the moon, but is the crater Tycho. It's about 56 miles across! It's also about 15,000 feet deep! That's almost three miles deep. Our Grand Canyon is only about a mile deep. So Tycho is three times deeper than Grand Canyon.
As we come closer, we can see something a little bit strange: there is a mountain in the middle of it:
We do not see mountains in the middle of craters here on Earth. For more information about these central peaks, please read the article Crater Origins.
So that is the bright spot on the moon. What about those dark areas? When people first tried to figure them out, they thought they were oceans of water, so they called them "mare" (pronounced mar' - ay). That is the Latin word for "sea." The plural is not with an "s" but is like the girl's name: maria. But the accent is on the first syllable.
At that time, it was thought that, if the moon had these enormous seas, there must also be life on the moon. And so many stories were started about 'moon men' and other forms of life that must live there -- especially on the dark side, that no one had ever seen!
The maria are not seas. They do not have water. They are great stretches of the moon's surface where dark lava flowed out and covered low areas. The lighter areas on the moon are highlands and the bright spots we see there are craters. The bright one in the middle of the left on the picture of the whole moon is called Copernicus. Both Tycho and Copernicus are named after famous astronomers. All the major craters are named after either astronomers or other major people in history.
So what about the 'dark side' of the moon -- the side we cannot see? Well, now that we have circled the moon, we have pictures of it:
That's strange -- it looks different from the other side. It does not have nearly as many maria, but is mostly craters -- a lot of craters. How come? We are not exactly sure why, but the maria on the other side are very low, and they are generally circular, meaning they are the places where there were some pretty large asteroid hits. These would have caused the crust to drop dramatically, sometimes over two miles down, in places. This allowed the molten rock from the interior -- the magma -- to leak out across the surface of the sunken areas. If these asteroids came mostly from one direction, that would explain why the side of the moon we don't see shows mostly craters with just a few of the maria. Another common explanation for the difference in the sides is that our side of the moon has a thinner crust. Why that might be, we don't know, but a lot of people like to guess.
Level 2 and Level 3
Impact Cratering on the Moon
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 impact 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 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.
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 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.
The crater being referred to here is the entire mare, which has the smaller craters inside it.
Basins begin around 200 km in diameter. 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. These were formed during the Late Heavy Bombardment. 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.
Another method of crater formation
Some recent research in plasma physics has produced some very interesting results. There have always been questions about some craters and some crater formations. First, some craters appear to be strung out like beads:
To understand what may have happened here, we need to look at something we are able to do with electricity. A strong current can be passed from either a negative to a positive pole or a positive to a negative, which is exactly what lightning does.
This is the effect:
This is called electrical discharge machining
Here are some of the effects:
If the electric charge is traveling, the craters will be strung out in a line
this chain is on one of the moons of Jupiter, Ganymede.
The standard explanation for these chains is of a series of fragments which hit all in a row. However, when we use electrical machining, we get exactly the same effects. If, in the early universe, there were electrical discharges which hit the planets and moons, we have the explanation for a lot of the craters which have puzzled us until now.
Including the mountains seen in craters like Tycho. It has been presumed that these mountains were caused by the impact being strong enough to push the center of the crater down so far that it rebounded. This can happen when the crater is between 15 and 80 miles wide, but for craters outside this size range, it cannot happen. So what is the explanation when we see this formation in very large or very small craters?
Take a look at the string of craters on our moon shown above:
In the center of several of them you can see peaks. These craters are much smaller than fifteen miles across. The peaks are also known effects of electrical machining. What impacted our moon here, and a number of other places, were not meteorites or asteroids, but electrical charges.
Other Effects of Cratering on the Moon
Impacts do more than just produce craters. 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.
When the impacts occurred, which formed the great circular basins, like Mare Imbrium, it 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).
The Nectarian Period (3.85-3.92 billion atomic years) corresponds to the time of the Late Heavy Bombardment (LHB). The Imbrium Period (3.2 -3.85 billion atomic years) corresponds to the time when the huge impact basins from the LHB sank about 2 miles on their circumferential fault lines and displaced the magma from the moon's interior so that it covered the surface. The Eratosthenian Period (from 1.2 to 3.2 billion atomic years) covers the time that the Maria became solidified. The Copernican Period (from the present back to 1.2 billion atomic years) covers the entire period from the solidification of the Mare basalts unto the present. This Period includes the three specific cratering events relating to the breakup of the original asteroid planet and its moon given by Cosmic Ray Exposure ages of meteorites and asteroids. These dates are 800 - 650 million atomic years, 255 million atomic years and 70 million atomic years. The crater Copernicus (after which the Period takes its name) dates from the first of these events, about 800 million atomic years old. There would have been a lot of impacts starting about that time, then tapering off. Similarly, there would have been a large number of impacts about 255 million atomic years and then tapering off. Finally, the crater Tycho dates from about 70 million atomic years, and would have had a large number of other craters formed concurrently. Then the impact rate tapered off again. Therefore, the break-up of the Kuiper Belt planet caused the LHB and formed the big basins and many craters in the southern uplands. The events associated with this take up the majority of the atomic time scale. But the brek-up of the asteroid planet and its moon are all covered by the craters formed in the Copernican period.
Craters are placed into these periods by the degree of degradation of their features.
Copernican Period craters are the yongest. Their medium sized craters that have a sharp rim, rim deposits, terracing, a central peak, a continuous and discontinuous ejecta and a bright ray pattern. Medium sized craters that lack ray systems are classed as Eratosthenian. Medium sized craters that have lost their ejecta ridges are classed as being from the Imbrium Period, while those that have lost their sharp rims as well are classified as Nectarian.
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
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 for three 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. Third, the great basins sank about two miles and displaced a huge amount of magma.
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. However, other features of the rilles suggest that they were formed by plasma-electric interactions. See the section on Crater Origins.
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.
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
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). They were only later 'activated' by the stresses of volcanism.
Faulting which radiates out from 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).
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
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 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. 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, so that the same side of the Moon always faces us, 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).
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, it is still a fascinating world. And precisely because it lacks this extra complexity, it allows us to study these simpler processes in isolation.
NOTE: while Helen was putting the above material together from Barry's old lecture notes, he was doing some experiments in the garage with cratering. Instead of incorporating that here, we have put it up as its own separate article in the Research Papers section, but it is written clearly enough for students in high school. Crater Origins.