astr 330: the solar system

ASTR 330: The Solar System

Announcements

Dr Conor Nixon Fall 2006

• Homework #3 out today.

• Due back in 1 week (October 17th).


Lecture 12:

The Moon

Dr Conor Nixon Fall 2006Photo credit: Bruce L Gary


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.

• 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.


The Moon From The Earth



Moon Map


• Do you see any major differences between the near and far side? Map: Jim Loy


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!


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!


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.


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.


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


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


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.

Picture: National Academy Of Sciences

• 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.


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.


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.

Figures: NASA Ames Research Center/CME

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.


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


Crater Types

Dr Conor Nixon Fall 2006Figure: Walter S Kiefer, LPI


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 well-formed 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


Crater Identification Quiz: #1


Answer: Simple crater: 7 km diameter. Note smooth steep walls.

• Try to identify what type this crater is: look carefully for tell-tale signs.

• Moltke Crater (Apollo 10)

Concept: Walter S Kiefer, LPI




Intermediate type: 16 km wide, 2 km deep. Some slumping, but no terraces or central uplift.

• Bessel Crater (Apollo 15)



Crater Idenification Quiz: #3


Complex type crater: 28 km diameter, 2.5 km deep. Note slumping and central peak. Also rays of ejected material.

• Euler Crater (Apollo 17)





Impact Basin: 320 km wide. The inner ring is 150 km wide.

• Schrodinger Crater (Clementine Spacecraft mosaic)



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).


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 pre-Imbrium; 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.


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.


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.


Crater Densities


• This figure shows the crater densities for the lunar maria (dots and dashed line).

Figure: Bill Hartmann, PSI.

• 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?


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?


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?


Measured Crater Densities and Ages

Dr Conor Nixon Fall 2006Credit: table 7.1 of Morrison and Owen

Lunar Region Crater Density Age(10 km craters per (billion years)

million square km)Highlands 1000 >4.0Fra Mauro (Apollo 14) 130 3.9Apennine (Apollo 15) 95 3.9Tranquillitatis (Apollo 11) 50 3.7Fecunditatis (Luna 16) 30 3.4Putredinis (Apollo 15) 25 3.3Copernicus Crater 10 0.9Tycho Crater 2 0.2


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?


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


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.


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.


Basin Images


Mare Orientalis and Mare Imbrium

Image credit: NASA Lunar Orbiter IV Spacecraft


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.


Lunar Eruptions


• Lunar eruptions did notnot 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.


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.


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


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.


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?


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?

astr 330: the solar system

Documents