Chapter 7: Relative Ages on the Moon
( examine photos in this article)
(Notes from Seacrest School Moon Crew, 2012)
Note to readers: The Chapter 5 and 7 Selections in Moon 101 are really dense, academic discussions of
lunar geology. Yet, the content is really important and will help us understand what we will see in our
LROC images. The main idea of Chapter 7 is to learn to date Moon surfaces using crater sizes and
shapes.
Stratigraphic Units are layers of rock, anywhere – Earth, Moon, or Mars.
If these layers have not
been disrupted by “earth”quake activity, they are in chronological order: oldest is on the bottom,
youngest is on the top.
Look at the opening diagram on the title page for Chapter 7. Relative Ages.
1.At the top (left side) are Crater materials: The most recent (although over 3 billion years ago) is all
the junk, crater ejecta from all recent lunar impacts
2. Mare Materials are the lava flows from the 3.9 billion year old HUGE Mare impacts
3. Volcanic materials are older still, from a period when there were flows and eruptions
4. Oldest on the bottom is the crust materials, those lighter elements that rose to the surface of the
entirely molten moon, 4.5 billion years ago.
Figure 7.4:
good image of secondary impacts (impacts made by ejecta from other craters)
Note: up and down (somewhat blurry) crater chain is from Copernicus, 450 km south. The fresher
crater chains going across (side to side) is from Aristarchus (650 km west). The secondaries from
Aristarchus are on top of the Copernicus secondaries, meaning that Aristarchus is younger.
Also note peaks buried by the mare basalt flows (lower left of A)
Also note that all the craters in the images are made in Mare basalt lava flows, so they all are younger ,
(came after), the Mare was there.
Fig 7.6 and 7.7 Compare these with our assignment strips from LROC to see a layering of features and
events that causing them.
Orientale Basin image (7.6): (actually Orientale is NOT in the image it would be in the upper left)
The large crater in the upper image (Schickard, 227 km diam, 125 miles) was affected by Orientale
impact, so Schickard predates, is earlier, than Orientale. Crater chains (good indicator of secondary or
ejecta impacts) affects the lower part of image as well, (SC) indicating that those craters were there
before the Orientale Impact. Just by looking at the superposition, we can construct a time-line:
Time visible in 7.6: Initial cratering (Schickard and Schiller and surrounding craters)
Flooding of craters with lava (from somewhere) smoothing area
Orientale Impact (it was Huge)
Immediate impact of Secondaries (ejecta) from Orientale
Random new and secondary impacts on top of lava flows And Orientale Ejecta
Crater Dating: pg 129
Following the procedure in figure 7.6, we can construct a time-line or a chronology for a land surface
based on our observation of the position of cratering events. We can use craters and their debris to
mark time!
In addition to observing what is on top of other things; you may also observe the “freshness” of craters.
Sharp well defined craters must be newer than rounded, collapsed, and filled craters. One further
method of observing a surface is a crater count: the older the surface the more tiny craters it should
have, the new surface will have less. LROC NAC images allow this level of study.
Cratering and counting craters is complicated! There are more small impactors shooting around in
space than large objects. So we expect more small craters. Some craters are primary impacts and some
made by secondary ejecta debris; there must even be tertiary craters as well from bouncing objects on
the surface. Large impacts will make big holes and will obliterate smaller, earlier impacts. Really large
impacts will cover the entire Moon with ejecta debris and secondary (and tertiary) impacts. The surface
of the moon is a giant train wreck! The best we can do is to try to solve the puzzle of what’s there.
Figure 7.7: this is the skinny photo after page 127 with Pt, Al, Ar as well as other craters noted*. (NOTE:
this is a good study plot to use to decide what is old and what is new using LROC WAC images)
This image is 1400 km from the edge of the HUGE Imbrium impact that made an 800 mile wide crater on
our face of the Moon. Debris from Imbrium is found all over the moon, and in this image.
Notice the linear gouges on the Moon’s surface, going from upper left to lower right. This is Imbrium
ejecta. It happens to cover F, Pt, Al and their surroundings. So these areas and craters were there
previous to Imbrium. Also previous to Imbrium are PU, L, R (bottom image) because they all have round
impacts obliterating their walls. So the craters and their walls were there first, then secondary impacts
rained in from the Imbrium collision. These and the other pre Imbrium craters then flooded with lava
(smooth floors). The Imbrium gouge-features are covered over or obliterated by H, Ar, T and W. These
craters are post-Imbrium.
This Moon surface history: old surface with F, Pt, Al etc.
Imbrium Impact showers area with ejecta, gouging surface, secondary craters
Lava flooding of crater floors
Later impacgts of Ar, H, W, T on top of all previous stuff
*Crater names: Al: Alphonsus; AR: Arzachel; F: Flammarion; H: Herschel; L: La Caille;
Pt: Ptolemaeus; Pu: Purbach; T: Thebit; W: Werner
Tools to use in understanding time lines of Moon surface: (applies to small craters such as we see in the
LROC strips) or “let’s use crater shapes and sizes to date Moon surfaces”
Note: Contrary to popular belief there IS erosion on the moon. Surface features degrade over immense
periods of time.
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Older craters are rounded and may be flooded, younger craters are sharp and jagged. “craters
become more subdued and indistinct over time” “…become less sharp, more subdued by
erosion of the rim crest and infilling of the floors…” )p131)
Younger craters are seen overlaying older (p131) Younger craters modify older.
Small craters disappear sooner than larger
Older craters are shallower than younger. Older may have accumulated filler debris inside.
Older craters will have newer impacts inside, may have damaged walls. Large subdued craters
are found on older surfaces (unless filled or buried from new nearby impacts)
Older surfaces have more craters than younger surfaces and have more older appearing craters
Older surfaces have large craters
On the Maria, all craters degrade at the same rate
SECONDARY IMPACTS:
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Likely lower speed, possibly low angle impacts
Craters may be shallow, clustered, elongated, chained together (bounces)
See Image 7:12 page 134
The NAC images we are studying from LROC show mostly craters on the far right of this Figure, the
craters from 50 to 200 meters.
The oldest craters are at the bottom – the Imbrium “Time” . Note that they are flattened, rounded,
filled. They look eroded.
The Upper most craters are newest, youngest. They are sharp, have rays, and internal features visible.
See Image 7:14
Note the scale in the 4 panels, 5 Km. This is wider than our LROC NAC images.
Panel A: a young surface, sharp craters, level surface
Compare to B which has more eroded craters
C is near Apollo 11 (on Mare Tranquility), still older surface, peppered with eroded craters (lots of
time)
D. NOTE THIS ONE. Here is a really older surface, eroded craters, some beyond recognition. (Compare
with Taurus Littrow)