ASTR 330: The Solar System
Announcements
• Homework #3 out today.
• Due back in 1 week (October 17th).
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lecture 12:
The Moon
Photo credit: Bruce L Gary
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Overview
• In this lecture we will discuss the general properties of the
lunar surface, including:
•
Geology
•
Impact craters.
•
Highlands and lowlands.
• In the next lecture after the exam, we will discuss the Moon and
Mercury together, and examine topics such as:
•
Formation of the Moon.
•
Tidal interactions between the Moon and the Earth.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
The Moon From The Earth
• The Moon is the second brightest object in the sky, and the only
planetary body that can be seen with the naked eye as a globe
rather than a point.
• We observe the Moon to go through different phases as it orbits
around the Earth, caused by our changing view of the Moon
illuminated by the Sun.
• To the naked eye, we can distinguish light and dark regions.
• The dark regions were named “maria” (seas) by early astronomers,
by analogy to the Earth. Of course, we now know that there is no
liquid water on the surface of the Moon.
• We always see the same side of the Moon from the Earth; we will
discuss the reason for that in the second lecture.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Moon Map
• Do you see any major differences between the near and far side?
Map: Jim Loy
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Named Features
• The feature types all have Latin names: many dark features are
named in parallel to water features of the Earth, such as: mare (sea),
oceanus (ocean), lacus (lake), sinus (bay) and swamp (pallus).
• Some examples are: Mare Tranquillitatis (Sea of Tranquility) and
Oceanus Procellarum (Ocean of Storms), Pallus Putredinis (Marsh of
Decay).
• Then we have the ‘land’ features: planitia (plain), vallis (valley), dorsa
(ridge), mons (mountain), and catena (crater chain).
• Examples include: Rupes Kelvin and Mons Huygens: most are
named after prominent scientists and astronomers.
• These names lend the dry, arid moonscape a colorful, terrestrial
charm. However we must remember that the ‘water’ names at least
are pure imagination!
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
General Properties
• Let us now state some bulk properties of the Moon:
• Diameter: 3476 km, just over quarter the Earth.
• Mass: 7.3x1022 kg, just over 1% of the Earth!
Why is it not closer to 1/64= 1.5% of Earth’s mass?
• Density: 3.3 g/cm3, lacks an iron core, but solid
throughout (unlike asteroids).
• Albedo:
15% for bright terrain (highlands)
8% for dark terrain (lowlands, maria).
• Only 17% of the Moon is maria: almost none on far side. Note,
the so-called ‘dark side’ is bright!
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Highlands and Lowlands
• Here is the Moon seen at half phase and full. Can you see any
advantages to viewing the Moon at half phase instead of full?
Photos: John French, Abrams Planetarium, Michigan State Univ.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Highlands and Lowlands
• The bright terrain has been shown to be at higher elevation than the
dark terrain: we can think of mountains and plains.
• The lunar highlands and lowlands also show radically different
densities of cratering.
• In the highlands, the craters are layered many on top of each other,
with well-formed, younger craters sitting on top of older, battered
ones. In the lowlands (maria), the craters are more widely spaced.
• At the largest scale, we see circular seas such as Mare Imbrium,
which are sometimes called basins. In fact, these are also a type of
craters, but unlike the smaller craters have been filled with lava flow.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Example
Craters
• (left) old Hipparchus
seen by Apollo 16,
138 km diameter.
• (right) Letronne, 116
km, from Apollo 16.
• (left) Archimedes, 85
km from Apollo 15.
• (right) Davy crater
and crater chain,
from Apollo 12.
Images: NASA/JSC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Could a lunar crater be a volcano?
• In the 19th century, the origin of lunar craters caused a great debate
among scientists. While some thought that craters could be caused
by impacts, many disagreed and believed they must be volcanoes.
What evidence was there for this point of view?
VOLCANO THEORY:
• No impact craters were known on Earth: Barringer crater not yet
attributed to a meteorite impact!
• Known meteorites were all small, and no large bodies near the Earth
(NEAs) were known.
• Volcanoes were well known and studied, and had craters on top.
Picture: Earth Science Australia/Cliff Ollier
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
New Evidence For Impacts
• What happened to overturn the volcano theory?
• The man in the picture below, Grove Gilbert (1843-1918) of the
USGS, was the one of the first to argue that lunar craters did not
occur on top of mountains, as would be expected for volcanos.
• In fact, the floors of lunar craters lie below the
level of the surrounding plains.
• In the 1890s Gilbert made a strong case for
impacts, but was unable to explain one curious
fact: lunar craters were all round.
• It was believed at the time that only ‘normal’
impacts would produce round craters: oblique
impacts were thought to produce oval craters.
Picture: National Academy Of Sciences
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
A Resolution to the Crater Problem
• In fact, the impact cratering idea turned out to be right after all! The
problem was that the analogies used to think about cratering (e.g. a
stone thrown into sand-box), were not correct at high impact speeds.
• When a very high-speed projectile impacts, the impactor is totally
vaporized and explodes not unlike a bomb: the resulting crater is
always spherical, regardless of the direction of the bolide.
• E.g. for the Moon, the velocity of impact is at least equal to 2.4 km/s
(the escape velocity), and to that is added any orbital velocity relative
to the Moon.
• Most of the energy of impact goes into heating and vaporizing the
projectile, which hence expands forcibly, excavating the crater and
throwing material into an ejecta blanket.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Cratering Step-by-Step
1. Impact: the meteorite penetrates to
2-3 times its own diameter, before
being stopped and vaporized.
Surface rocks are melted and
shattered and seismic waves travel
through the Moon. Pressures may
reach 1 million bar.
2. The material vaporized and
excavated (up to several hundred
times the mass of the impactor) is
thrown upwards and outwards by
the explosion. The ejecta or debris
falls over the surrounding area,
possibly forming secondary craters.
Figures: NASA Ames Research Center/CME
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Later Processes
3. Later adjustments to the crater
shape may occur. These may
include:
• The slumping of craters walls, to
form step-like ‘terraces’.
• For large craters, the weight of
material removed from the center
may be enough to cause a
rebound, forming a central peak.
• In-filling by lava flows, causing a
flat floor or plain. This seems to
be true for the largest craters.
Figures: NASA Ames Research Center/CME
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Crater Types
Figure: Walter S Kiefer, LPI
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Craters Large and Small
• The exact form of the crater depends on (i) the impact velocity, (ii) the
impact size, (iii) the target material (e.g. Mars produces different
ejecta blankets to the Moon).
• Different crater sizes have different characteristics:
1. Simple craters (<15 km). Bowl-shaped depressions with wellformed sides, and depth to diameter ratios of 20%.
2. Complex (>20 km). Have central uplifts, flat floors, slump blocks,
and terraces. In addition, craters:
• 20-175 km typically have central peaks.
• 175-300 km have ring-shaped central uplifts.
• >300 km are called impact basins: affect regional geology.
• Now let’s try to identify some craters.
Source: Walter S Kiefer, LPI
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Crater Identification Quiz: #1
• Try to identify
what type this
crater is: look
carefully for tell-tale
signs.
• Moltke Crater
(Apollo 10)
Answer: Simple crater: 7 km
diameter. Note smooth steep
walls.
Concept: Walter S Kiefer, LPI
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Crater Identification Quiz: #2
• Bessel Crater
(Apollo 15)
Intermediate type: 16 km wide, 2 km deep. Some slumping,
but no terraces or central uplift.
Concept: Walter S Kiefer, LPI
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Crater Idenification
Quiz: #3
• Euler Crater (Apollo 17)
Complex type crater: 28 km
diameter, 2.5 km deep.
Note slumping and central
peak. Also rays of ejected
material.
Concept: Walter S Kiefer, LPI
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Crater Identification Quiz: #4
• Schrodinger
Crater
(Clementine
Spacecraft
mosaic)
Impact Basin: 320 km wide. The inner ring is 150 km wide.
Concept: Walter S Kiefer, LPI
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lunar Stratigraphy
• How did we determine the age of lunar features before we sent
sample-return missions?
• Stratigraphy is a sort of poor-man’s version of age determination. It
is the best we can do in the absence of actual samples for radioactive
dating.
• Stratigraphy is based on the observation that younger features lie on
top of older ones. We can use this to establish the relative series of
events, by making careful observations from afar (i.e. the Earth).
• E.g. a crater which overlaps the rim of another crater must be
younger. Or, a colored deposit which overlays another deposit must
be younger (imagine spilling fresh paint on top of a dried color patch).
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Imbrium Basin and Fra Mauro
• Using stratigraphy, we learned that the formation of the Imbrium Basin
was a benchmark event, covering the near side in a recognizable
deposit called Fra Mauro formation.
• Events which are partially covered by Fra Mauro material are preImbrium; events which overlie Fra Mauro are post-Imbrium.
• We further deduced that there are younger craters in the Mare
Imbrium itself, and that these were subsequently flooded by lavas.
Therefore, the lava flows cannot have been caused by the Imbrium
impact: there must have been time for the overlying craters to be
produced.
• We now know, using rocks returned by Apollo, that the Imbrium Basin
was formed 3.8 billion years ago. The space program greatly
expanded our knowledge of the Moon.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Crater Retention Age
• The crater retention age refers to the time since a piece of terrain was
last re-surfaced.
• Do you think the typical crater retention age is longer on the Earth, or
on the Moon?
• On the Moon, the crater retention age is usually the time since either:
1. A lava last flowed on the surface (maria).
2. Since newer craters completely obliterated older craters
(highlands).
• To determine crater retention age we must first count the area density
of craters. We are not talking mass/volume here: rather we mean
number of craters per unit area, usually per million square km.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Meteorite and Crater Size Distributions
• We saw in our earlier lecture regarding asteroids, that the number of
objects per unit size follows approximately a D2 distribution (rather
than D3 as expected).
• I.e., the number of asteroid fragments/meteorites that are 1 km in size
is about 100 times more than the number which are 10 km in size.
• Now, each crater has a size roughly ten times that of the impactor.
So, we preserve the distribution of impacting objects in the crater size
distribution.
• Roughly, if we find N craters 10 km in size (produced by 1 km size
meteorites) then we expect 100 x N craters which are 1 km in size,
produced by 100 m size objects.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Crater Densities
• This figure shows the crater densities for the lunar maria (dots and
dashed line).
• The upturn at small sizes is
thought to be due to
secondary cratering:
fragments blown off from
large impacts.
• The solid line shows the
crater density for lunar
highlands. This is also a
saturation density line: if any
more craters were added,
the density we see would be
about the same, why?
Figure: Bill Hartmann, PSI.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Relative Crater Ages for Lunar Terrains
• Clearly, the greater the crater density, the older the terrain. But how
much older?
• Lunar maria (lowlands) have only 1/20 the crater density of the lunar
highlands, therefore, they should be 1/20 the age, right?
• So, by this reasoning, if the highlands were 4.5 billion years old, as
old as the Earth, then the maria would be just 200 million years old.
• A problem arose in the 1960s when scientists began to measure
near-Earth objects. They calculated how long it would take for the
maria to accumulate their craters, and arrived at a figure of 4 billion
years!
• But then, how come the highlands had 20 times more craters? What
age would that imply?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Absolute Ages
• The problem was not resolved until the 1960s, when Apollo samples
were returned and dated. The Mare Tranquillitatis rocks turned out to
be 3.7 Gyr.
• Subsequent maria samples turned out to be 3.2 to 3.9 Gyr, and
highland samples were aged at >4.0 Gyr.
• How do we then explain the cratering record?
• We must postulate that the impact rate was not constant: in fact, it
must have been very much higher prior to 3.9 billion years ago, to
form the densely cratered highlands.
• In fact, the impact rate 4.0 billion years ago must have been 1000
times higher than the rate 3.8 billion years ago. Any ideas why?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Measured Crater Densities and Ages
Lunar Region
Crater Density
Age
(10 km craters per (billion years)
million square km)
Highlands
1000
>4.0
Fra Mauro (Apollo 14)
130
3.9
Apennine (Apollo 15)
95
3.9
Tranquillitatis (Apollo 11)
50
3.7
Fecunditatis (Luna 16)
30
3.4
Putredinis (Apollo 15)
25
3.3
Copernicus Crater
10
0.9
Tycho Crater
2
0.2
Credit: table 7.1 of Morrison and Owen
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Early Lunar History: Highlands
• The lunar highlands have apparently reached a state of saturation
cratering: no more craters can be added without replacing existing
ones.
• This puts a limit on our knowledge of the early cratering rate: if the
cratering was 10 or 100 times higher, we may never know.
• We can however gain some insight from the depth of shattered
material.
• Using seismic detectors called ALSEPs left by the Apollo missions,
we can tell that the highlands are shattered to a depth of 25 km, with
a rubble depth of 100s of meters. In contrast, the rubble on the maria
is only 10 m deep.
• What sort of rocks might we expect to find in the highlands?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lunar Breccia
• This picture (right) shows a piece
of lunar highland breccia.
• Below is a close-up of a breccia
at 40x magnification, showing
grains (Apollo 15, Apennine mountains).
Pictures: (i) NASA (ii) Kurt Hollocher, Union College
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Highland Samples
• Lunar breccias may be extremely complex, often showing evidence of
3 or 4 actual impacts. They are much more complex than meteoric
breccias.
• The oldest highland samples fragments are found within breccias,
dated at 4.2-4.4 Gyr
• The primary material of the highland crust is a class of igneous silicate
rocks called anorthosites.
• These rocks contain oxides of Si, Al, Ca, and Mg, but are depleted in
lava volatiles: elements which can be evaporated below 1000 C.
• Lava or geologic volatiles include N, C, S, Cl and K, and of course
water. Hence there are no clays or other soils which incorporate water.
• Anorthosites tell us that the Moon differentiated early on, before the
crust solidified.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lunar Basins
• Basins are impact features larger than 300 km, formed by impactors
30-100 km in size. The largest basin, at the south pole (Aitken) is
2200 km across.
• The youngest basins are Orientalis and Imbrium. Imbrium was formed
earlier (3.9 Gyr ago) and Orientalis slightly later (3.8-3.9 Gyr ago).
Imbrium was later flooded by lavas, forming the Mare Imbrium, but
Orientalis was not.
• Imbrium is ringed by mountains which rise to 9 km high, similar to
Mauna Loa in Hawaii. The outer ring is 1200 km in diameter.
Orientalis produced rings of 600 km and 900 km diameter.
• Basin-rim mountain formation is not well understood, but seem to
involve uplift due to the original impact, followed by later subsidence
along concentric cracks.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Basin Images
Mare Orientalis and Mare Imbrium
Image credit: NASA Lunar Orbiter IV Spacecraft
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lunar Volcanism
• The major period of lunar volcanism was apparently not during the
Late Heavy Bombardment of 3.9 Gyr ago, but between 3.8 and 3.3
Gyr ago, after the formation of the Imbrium basin.
• During the volcanic episodes, dark basalt poured out onto the
surface, filling many of the craters, and causing the maria we see
today.
• Lunar basalts have a higher iron content than Earth basalts. Also,
they have no water or oxygen (of course). Hence, they are simpler.
• In fact, only about 100 different minerals are found on the Moon,
compared to 2000 or so on the Earth.
• The oldest basalts are 3.8 to 3.9 Gyr, found by Apollo 14 and 17. The
youngest are found on Oceanus Procellarum, solidifying just 3.2 Gyr
ago.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lunar Eruptions
• Lunar eruptions did not form the typical volcano cones seen on the
Earth, but instead, broad flat plains.
• The last major flow on Mare Imbrium deposited a layer 30-50 m thick.
100 flows like this are required to account for the depth of mare
basalt, estimated at 5 km, with maybe 1 million years between flows.
• We see features left by the settling and cooling lava, such as:
• ‘High-tide’ marks, where the lava reached and then retreated.
• Cooling cracks, often concentric.
• Wrinkle ridges, formed by compression.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Volcanic Valleys and Mountains
• The Moon also exhibits snake-like channels called ‘rilles’ (rima).
• Originally thought to have been carved by running water, like a river
valley, these are now known to have been caused by lava rivers.
• The Apollo 15 astronauts visted the Hadley Rille on the edge of Mare
Imbrium, finding a valley 1200 m wide and 370 m deep, filled with
boulders.
• There are also hints at several true volcanic mountains, such as the
Marius Mountains in the Oceanus Procellarum.
• These were due to have been visited by a later Apollo flight, but the
program was cancelled early, in 1972.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Hadley Rille
• Below left, the Hadley Rille seen from orbit by Apollo 15. The Rille is
120 km long and 3.3 billion years old. In terms of width and length it is
about 10 times bigger than lava channels seen on Hawaii. Why?
• Below right, the same Rille seen on the surface by Apollo 15 crew.
Image credit: NASA/JSC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Sources of lunar eruptions
• Why are the maria all concentrated on the near side of the Moon?
• The main reason seems to be elevation: the lowlands are all on the
near-side, and the higher pressures there caused the eruptions.
• Maria lava samples show that the original magma is separated from
primordial material by three processes:
1. Once during the original loss of volatiles when the Moon formed.
2. Again when the Moon originally differentiated into core, mantle
and crust.
3. Finally during a partial re-melted episode, creating vast magma
reservoirs.
• The volcanism ceased at about 3 Gyr ago.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-summary
1. What are the lunar maria?
2. The bright and dark regions of the Moon have what differences? (e.g.
in topography, age, history).
3. Name 2 types of lunar features, other than maria and describe them.
4. Why is the Moon 1% of the Earth’s mass, not 1.5% ?
5. What two theories of crater formation competed in the 19th century,
and give an argument for and against each.
6. Describe the three steps of crater formation. What processes occur in
step 3 to alter the original morphology?
7. What features distinguish between a simple and complex crater?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-summary
8. What is meant by lunar stratigraphy, and what is the significance of
Fra Mauro material?
9. What two processes set a limit on crater retention age?
10. The lunar maria have only 1/20 the crater densities of the highlands,
yet are not 1/20 the age. Why?
11. What is meant by an anorthosite, and give some properties.
12. Name one prominent impact basin on the Moon, and say when it was
formed.
13. What caused the lunar rilles?
14. When was the main period of lunar lava flows?
Dr Conor Nixon Fall 2006