Exploring the Solar System
Lecture 3: Structure of the Moon
Professor Paul Sellin Department of Physics University of Surrey Guildford UK
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Lecture 3 Page 1 Overview
Physical properties of the Moon
Rotation and phases of the Moon
The Moon’s orbit
Observation of the Moon’s features from Earth
Formation of the Moon
Crater formation and the lunar regolith
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Lecture 3 Page 2 Page 3 Paul Sellin
Harrison Schmidt walking on the lunar surface – December 1972. Photographed by Eugene Cernan, Apollo 17. This was the last manned mission to the Moon. The Moon has only been visited by 12 people. The large boulder in the image is one of the most recent surface features on the Moon – it tumbled down a mountainside to its present location a mere 800,000 year ago. Unlike Earth, the surface of the Moon has remained essentially unaltered for billions of years.
Lecture 3 Page 3 Guiding Questions
What key findings resulted from the manned exploration of the Moon during the Apollo program in the 1970s? Does the Moon’s interior have a similar structure to the interior of the Earth? How do Moon rocks compare to rocks found on the Earth? How did the Moon form?
The image shows the Earth and Moon to scale, taken from the Galileo spacecraft Both objects are at the same distance of 6.2 million kilometres from the spacecraft
The moon shows no atmosphere, no oceans, and no evidence of plate tectonics
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Lecture 3 Page 4 Size of the Moon
Compared to planetary distances, the Moon is very close – just 384,400 km The angular diameter is only 1/2° the diameter of the moon is relatively small The orbital motion of the Moon around the Earth cannot be considered as simple rotation of an object around a fixed point: mass of the Moon is 1.23% of the mass of the Earth both objects orbit around a fixed centre of mass of the Earth-Moon system – this is inside the volume of the Earth. The centre of mass moves in an elliptical orbit around the Sun:
The actual value for the diameter of the moon is 3476 km, 27% of the Earth’s diameter
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Lecture 3 Page 6 Phases of the Moon
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The figure shows the Moon at 8 positions in its orbit, plus photographs of what the moon look like at each position as seen from Earth. The changes in phase occur because light from the Sun illuminates one half of the Moon, and as the Moon orbits the Earth we see varying amounts of the Moon’s illuminated half.
It takes 29.5 days (synodic period) to go through a complete cycle of phases.
Note that the phases of the moon are not due to the shadow of the Earth falling on the Moon’s surface – this is a popular misconception. When this occurs we get as lunar eclipse.
Notice that the Moon does rotate around its axis – see next slide
Lecture 3 Page 7 Rotation of the Moon
The phase of the Moon is always changing, but the Moon always keeps the same hemisphere towards the Earth the Moon rotates about its axis with the same period as its orbit around the Earth:
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Lecture 3 Page 8 The Dark Side of the Moon
“There is no dark side of the moon… matter of fact it’s all dark”
So said the final line of Pink Floyd’s famous 1973 concept album ‘Dark Side of the Moon’
Pink Floyd, clockwise from the top: Roger Waters, Nick Mason, Dave Gilmour, Rick Wright.
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Lecture 3 Page 9 The real ‘dark side of the moon’ – as seen by Apollo 16
Locked in synchronous rotation, the Moon always presents its well-known near side to Earth. But from lunar orbit, Apollo astronauts also observed the Moon's far side. This sharp picture from Apollo 16's mapping camera shows the eastern edge of the familiar near side (left) and the heavily cratered far side of the Moon. Surprisingly, the rough and battered surface of the far side looks very different from the near side which is covered with smooth dark lunar maria. The likely explanation is that the far side crust is thicker, making it harder for molten material from the interior to flow to the surface and form the smooth maria.
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Lecture 3 Page 10 Both sides of the Moon
Photo: our back garden, ~1960 Photo: NASA, Apollo 16, 1972
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Lecture 3 Page 11 Tidal forces (1)*
In the Earth-Moon system, tidal forces are the gravitational forces between these two bodies. The near and far sides of the Moon will experience different gravitational force, due to the varying distances
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Lecture 3 Page 12 Tidal forces (2)*
The tidal force experienced by the Moon, due to Earth, is proportional to R3, such that:
Problem: calculate the tidal force for two 1kg rocks on either side of the Moon’s surface
G = 6.67x10-11 N m2/kg2 M = 6x1024 kg d = 3476 km R = 384,400 km
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Lecture 3 Page 13 Tidal forces (3)*
Now calculate the gravitational force experienced by a 1kg mass on the Moon’s surface:
Thus the tidal force on the lunar surface is much less than the gravitational force But the tidal force due to Earth deforms the Moon’s surface, producing tidal bulges around the equatorial region These tidal forces produce a torque on the Moon which keeps the rotation locked into synchronisation with the orbit around the Earth
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Lecture 3 Page 14 The Sidereal and Synodic Months
The sidereal month is the time the Moon takes to complete one revolution of the Earth with respect to the background stars = 27.32 days Because the Earth is moving along its orbit, the Moon must travel through more than 360° to get from one new moon position to the next the synodic month is the time from one new moon to the next. This is longer than the sidereal month, = 29.53 days This is the period of the new moon and full moon
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The time for a complete lunar day (the time for the Moon to rotate once on its axis) is about 4 weeks. Because the Moon’s rotation is synchronous, it takes the same time for complete lunar orbit. Astronomers define two types of lunar month, depending on whether the Moon’s motion is measured relative to the stars or to the Sun: Sideral month – time for one complete orbit of the Earth, with respect to the stars (‘true’ orbital period) Synodic month, or lunar month – time for the Moon to complete one cycle of phases (eg. full moon to full moon), and hence return to the same position relative to the Sun. The synodic month is longer by about 2.2 days because the Earth is orbiting the Sun whilst the Moon goes through its phases. Thus the Moon must travel more than 360° around its orbit in order to complete a cycle of phases.
Lecture 3 Page 15 The Moon’s Orbit
The Moon’s orbit around the Earth is elliptical, not circular: The Moon is at perigee when it is nearest the Earth The Moon is at apogee when it is farthest from the Earth The line connecting these 2 points passes through the Earth and is called the line of apsides Average centre-centre distance of the Earth and Moon is 384,400 km Minimum distance is 356,410 km and maximum distance is 406,697 km Average speed of the Moon along its orbit is 1.02 km/s As seen from the Earth the Moon moves eastwards through the constellations from one day to the next: • the Moon’s daily eastern progress averages 13.2° (360°divided by 27.32 days in the sidereal month) •in 1 hour the moon moves slightly more than 1/2° which is a bit larger than its own diameter • moonrise is about 50 minutes later from one day to the next
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Lecture 3 Page 16 Inclination of the Moon’s orbit and the Line of Nodes
The image shows the Moon’s orbit around the Earth (yellow) and part of the Earth’s orbit around the Sun (red) The plane of the Moon’s orbit (brown) is tilted by ~5° wrt the plane of the Earth’s orbit (blue, plane of the ecliptic) The 2 planes intersect along the line of nodes
Lunar Eclipse An eclipse can only occur when the Sun and Moon are both very near to or on the line of nodes Only then can the Sun, Moon and Earth all lie along a straight line, required for the Moon’s shadow to fall on the Earth A solar eclipse occurs if the Moon is on the line of nodes at new moon A lunar eclipse occurs if the Moon is on the line of nodes at full moon
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Lecture 3 Page 17 Conditions for the Eclipses
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An eclipse requires that the Sun, Earth and Moon all fall on the same line. A lunar eclipse occurs when the Moon passes through the Earth’s shadow, ie. when the Earth is between the Sun and the Moon. At this position the surface of the Moon facing the Earth would normally be fully illuminated, but is instead lit by a dim glow. A solar eclipse occurs when the Earth passes through the Moon’s shadow. As seen from the Earth, the Moon moves in front of the Sun. For a solar eclipse to occur the Moon must be between the Earth and the Sun and so can only occur at a new moon. A similar effect is seen by the (rare) transit of Mercury and Venus across the Sun. Calculate the angle subtended by the Sun at Earth:
(arcsec)d(km) D(km) 206265 D(km) 206265 (arcsec) d(km) 8 Earth-Sun distance: d = 1AU = 1.49x10 km 11 5 1.4310 Diameter of the Sun: D (=arc 6.96x10sec)km 962 0.26 1.49108
Lecture 3 Page 18 Total solar eclipses on the Earth 1997 - 2020
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Lecture 3 Page 19 More complex features of the Moon’s orbit
Gravitational pull from the Sun further complicates the orbit of the Moon around the Earth, ie. the orientation of the Moon’s orbital plane constantly changes:
the plane of the Moon’s orbit always maintains a 5° tilt to the plane of the ecliptic the orientation of the orbit’s plane changes slowly, causing the line of nodes to slowly move westward with time it takes 18.61 yrs for the line of nodes to complete one full rotation
The orientation of the Moon’s elliptical orbit within its plane also changes due to the Sun the line of apsides moves slowly within the Moon’s orbital plane, called the rotation of the Moon’s orbit while the line of nodes is moving westward, the Sun’s gravity causes the line of apsides to move east it takes 8.85 yrs for one full rotation of the line of apsides
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Lecture 3 Page 20 Libration of the Moon (1)
The Moon’s orbital period (sidereal month) is equal to the period of rotation of the Moon However due to the elliptical orbit of the Moon the 2 motions are not completely synchronised
From the figure: in ¼ of the sidereal period (6.83 days) the Moon only travels 84° but has rotated by 90° when the Moon is at point 2, observers on the Earth can see 6° further west on the Moon’s surface than when at point 1 similarly when at point 4 observers can see 6° further east on the Moon’s surface The Moon appears to rock back and forth in an east-west direction by 6°. This is called libration in longitude
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If the Moon’s orbit was a perfect circle then the orbital motion and the rotation of the moon would be completely synchronised, eg. during the time for the Moon to complete 90° of its orbit it would also have rotated by exactly 90°. However because the orbit is elliptical, Kepler’s 2nd law tells us that the Moon moves fasted at perigee and slowest at apogee. The average time for the orbit (the period) is the same as for a perfectly circular orbit, but the Moon relatively lags or leads against its rotation by a small amount.
Lecture 3 Page 21 Libration of the Moon (2)
The axis of the Moon’s north and south poles (axis of rotation) is not exactly perpendicular to the plane of the Moon’s orbit the axis of the Moon is tipped by ~7° relative to the plane of the orbit we can see 7° over the Moon’s north pole when at point 1, and over the south pole when at point 2 this causes a ‘vertical’ nodding motion of the Moon, or libration in latitude
The 5° angle between the Moon’s orbital plane and the ecliptic means that the Moon’s poles have a net rotation of 2° from the ecliptic This is much less than the 23.5° rotation of the Earth’s poles from the ecliptic, and as a result the Moon does not have seasons astronomers predict that there may be regions at either pole on the Moon where the Sun’s light never shines, eg. at the bottom of craters. The temperature in these areas could be as low as 50K (-223°C), and are potential regions for the existence water ice
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Lecture 3 Page 22 The nature of the Moon
The Moon is a relatively small object, with a diameter 27% that of Earth. The Moon has no atmosphere, no global magnetic field, and no liquid water The average surface temperature is 130°C (day time) and -180°C (night time) The surface of the Moon has remained unaltered for billions of years
The most favoured theory of formation of the Moon was a collision between Earth and another planet (similar in size to Mars) 4.56 billion years ago: the interior of the Moon originally contained lava, and evidence of volcanic activity can be seen on the Moon’s surface the Moon contains a small iron-rich core which is now solid, approximately 700km in diameter ‘Moon-quakes’ occur with significantly less severity than on the Earth (eg. Richter scale up to 1.5).
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Lecture 3 Page 23 Internal structure of the Moon
Like Earth, the Moon has a crust, a mantle and a core. The partially liquid iron-rich core is roughly 700 km in diameter and contains ~3% of the total lunar mass. In contrast, Earth’s core contains 32% of its total mass the Moon has no magnetic field, hence the core is at least partially solidified the lithosphere is relatively thick (800km) compared to Earth (50- 100km). This is the limit of the solid part of the Moon the lunar asthenosphere extends right down to the core, and is composed of heavier elements. This layer is plastic and can flow to some extent the lunar crust is ~50km thick in the near side, and ~100km thick on the far side. In contrast the Earth’s crust can be only 5km thick
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Lecture 3 Page 24 The formation of the Moon
The Moon probably formed from debris cast into space when a huge proto-planet struck the early Earth This is called the collisional-ejection theory - that the Earth was struck by a Mars-sized object and that debris from this collision coalesced to form the Moon This theory successfully explains most properties of the Moon:
The Moon was molten in its early stages, and the anorthositic crust solidified from low-density magma that floated to the lunar surface The mare basins were created later by the impact of planetesimals and filled with lava from the lunar interior
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The image shows a simulation of the effect of this type of giant collision. The energy released during the collision produces a huge plume of vaporised rock that squirts out from the point of impact. This is the ejected material which cools to form the Moon. The collisional ejection theory is consistent with the low iron content of the Moon: • rock vaporised by the impact is depleted of volatile elements and water. If the collision with the Earth occurred after ‘chemical differentiation’ has occurred, then the Earth’s iron would have already moved to its core. • consequently little iron would be ejected, to form the Moon. This explains the Moon’s low density and small size of its iron core. • the South Pole Aitken basin on the moon is a very deep crater produced by an ancient impact. Measurements from the Clementine spacecraft show a very low iron concentration (~10%) at the bottom of this crater 12km deep, compared to 20-30% iron seen at a comparable depth on Earth.
Lecture 3 Page 25 Geological history of the Moon
To summarise the geological history of the Moon: the initial impact with Earth probably occurred about 4.5 billion years ago the newborn Moon’s surface was probably molten for a long period, due to heat released from the impact, and the decay of radioactive isotopes As the Moon cooled, low density lava floating on the surface solidified to form the current anorthositic crust A period of intense rock bombardment ended about 3.8 billion years ago, during which time the majority of the craters were formed At the end of the crater-making period the Moon was struck by several (~10?) massive asteroids, perhaps 100 km across. These made the vast mare or impact basins From 3.8 to 3.1 billion years heat from the radioactive decay of heavy elements (U, Th) melted the inside of the Moon, causing floods of molten rock to cover the surface and fill the impact basins, and forming the basaltic maria Very little futher major changes occurred on the Moon’s surface from about 3 billion years
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Lecture 3 Page 26 Geology of the Moon
Lunar rocks reveal a geologic history quite unlike that of Earth The anorthositic crust exposed in the highlands was formed between 4.0 and 4.3 billion years ago. During this period the Moon’s centre was molten – the less dense anorthosite rose to the surface The mare basalts solidified between 3.1 and 3.8 billion years ago The rate of surface change has decreased rapidly over time; estimated from the density of craters and collected rocks Imapact breccias are common on the moon – these are composite rocks which have fused together as a result of meteorite impacts The Moon’s surface has undergone very little change over the past 3 billion years – evidenced by the low crater density in the maria
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Lecture 3 Page 27 Lunar geology
High- lands anorth- Iron-rich osite mare basalt
Lunar rocks are totally dry. With the exception of possible ice at the poles, no evidence has been found that water ever existed on the moon. All of the lunar rock samples are igneous rocks Impact formed largely of minerals found in terrestrial breccia rocks. They differ from terrestrial rocks in being relatively enriched in the refractory elements and depleted in the volatile elements
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Mare basalt is a type of balsatic rock quite similar to dark coloured volcanic rocks on earth, such as lava found in Hawaii and Iceland. On the moon the rock from the low lying plains is called mare basalt. Mare basalt is rich in heavy elements such as Fe, Mn and Ti. The lunar highlands are made up of light-coloured rock called anorthosite. On Earth anorthosite is only found in very old mountain ranges, and is rich in Si, Ca and Al. Anorthosite is less dense than basalt, and rose to the Moon’s surface during the liquid stage of the Moon’s formation. In this way anorthosite formed the lunar terrae that make up the majority of the Moon’s surface. Impact breccias are composites of different rock types that were broken apart, mixed, and fused together by meteoric impact.
Lecture 3 Page 28 Moon features observed from Earth
Since the Moon has no atmosphere the surface features can be observed easily, typically even with binoculars: craters due to the many impacts on the Moon by space debris over hundreds of millions of years. Over 30,000 crates can be observed from Earth, with diameters from 1-100’s km. maria, (from ‘mare’, Sea) the dark coloured features which are massive solidified lava flows terrae, the light-coloured lunar highlands
The largest craters were named in the 17th century after famous philosophers, mathematicians and scientists, hence: Plato, Aristotle, Pythagoras, Copernicus and Kepler Close-up photographs from lunar orbit have shown millions of craters too small to be seen from Earth The picture is a composite image of photographs taken at the 1st quarter and 3rd quarter when long shadows enhance the surface features
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This composite images shows the near and far sides of the moon, taken from Galileo. The Earth-facing side of the Moon is on the right, composed mainly of large dark flat maria. On the left is the lunar far side, composed almost entirely of light-coloured lunar highlands. Mare Orientale lies on the boundary between the near and far sides. The broad dark region at the lower left is the South Pole Aitken basin, an impact scar 2100 km across and 12 km deep
Lecture 3 Page 29 Impact craters
Craters are caused by impacts of meteoric material, also seen on Earth and the rocky planets The circular crater is caused by the shock wave from the impact – irrespective of the direction or angle of the incoming trajectory Many craters also have a central peak, characteristic of a high speed impact Planets which are geologically active tend to erase evidence of craters, eg by volcanic activity or motion of tectonic plates. This requires an interior which is at least partially molten. Furthermore the existence of an atmosphere causes erosion which further reduces surface features. Smaller planets lose their internal heat most quickly, and so are less geologically active, eg. Mercury has a heavily cratered surface, whereas as Mars still has intense volcanos
A crater on (left) the MoonPage (centre)30 the Earth, (right) Mars Paul Sellin
In 1824 German astronomer Franz Gruithuisen proposed that lunar craters were the result of impacts. A major problem raised at the time was why craters were all circular when meteorites would have a range of shapes, and would impact the Moon at a wide range of angles. A century later it was realised that the circular form of a crater was made by the shockwave of the impact, which will be circular for all types of trajectory. In a similar way the craters made by artillery shells are almost always circular.
Lecture 3 Page 30 Meteor impact on Earth
Meteor Crater in Arizona is a well-known terrestrial example of a crater formed by impact of a large meteor. The crater is 1.2 kilometers in diameter
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Meteor Crater (also known as Barringer Crater), Arizona, with a diameter of approximately 1.2 kilometers, was the first terrestrial impact crater to be recognized as such. Its impact origin was first suspected late in the nineteenth century, when abundant iron meteorite fragments were discovered in the immediate vicinity of the crater. This finding led the mining engineer Daniel Moreau Barringer to embark, between about 1905 and 1928, on a drilling project to find a suspected large iron meteorite body underneath the crater floor. At this time, however, researchers did not yet have a clear understanding of the immense energy that is liberated when an extraterrestrial body hits the surface of the Earth with cosmic velocity. It was only in the 1920s that the first quantitative studies revealed the explosive nature of meteorite impact. Under impact conditions, tremendous amounts of energy are released instantaneously, completely destroying the cosmic projectile and generating a crater that is many times larger than the original meteoroid. In the case of Meteor Crater, an iron meteorite body only about 30–50 meters in diameter was sufficient to create a crater 1.2 kilometers in diameter.
http://www.lpi.usra.edu/publications/slidesets/craters/
Lecture 3 Page 31 Crater formation and structure
Comets and asteroids strike the Moon at a wide range of impact speeds, with 20 km/sec being typical. Such a high-speed impact will produce a crater that is 10 to 20 times larger in diameter than the impacting object. The detailed form of the crater depends on its size:
Figure shows idealised cross-sections of the structure of small, simple craters (top) and of larger, more complex craters (bottom). Simple craters have bowl-shaped depressions and are the typical crater form for structures on the Moon with rim diameters (D in the figure) of less than about 15 kilometers. Moon craters with diameters >15 km have more complex forms, including shallow, relatively flat floors, central uplifts, and slump blocks and terraces on the inner wall of the crater rim. Near the surface is a layer of breccia (a type of rock composed of coarse, angular fragments of broken-up, older rocks).
http://www.lpi.usra.edu/expmoon/science/craterstructure.htmlPage 32 Paul Sellin
In craters on the Moon with diameters between about 20 and 175 kilometers, the central uplift is typically a single peak or small group of peaks. Craters on the Moon with diameters larger than about 175 kilometers can have complex, ring- shaped uplifts. When impact structures exceed 300 kilometers in diameter, they are termed impact basins rather than craters. More than 40 such basins are known on the Moon, and they have an important control on the regional geology of the Moon. Much of the material ejected from the crater is deposited in the area surrounding the crater. Close to the crater, the ejecta typically forms a thick, continuous layer. At larger distances, the ejecta may occur as discontinuous clumps of material. Some material that is ejected is large enough to create a new crater when it comes back down. These new craters are termed secondary craters and frequently occur as lines of craters that point back to the original crater. Material below the surface of the crater is significantly disrupted by the shock of the impact event. Near the surface is a layer of breccia (a type of rock composed of coarse, angular fragments of broken-up, older rocks). Rocks at deeper depths remain in place (and are termed bedrock) but are highly fractured by the impact. The amount of fracturing decreases as the depth below the surface increases. The energy of the impact typically causes some material to melt. In small craters, this impact melt occurs as small blobs of material within the breccia layer. In larger craters, the impact melt may occur as sheets of material.
Lecture 3 Page 32 Lunar Craters (1)
Bessel Crater, 16 km diameter, 2 km deep
Moltke Crater, 7 km diameter
King Crater, 77 km diameter, more than 5 km deep
http://www.lpi.usra.edu/expmoon/scie nce/craterstructure.html
Page 33 Paul Sellin Euler Crater, 28 km diameter, about 2.5 km deep
Moltke Crater, 7 kilometers in diameter, is an excellent example of a simple crater with a bowl-shaped interior and smooth walls. Such craters typically have depths that are about 20 percent of their diameters. The hummocky material surrounding the crater is Moltke's ejecta deposit. (Apollo 10 photograph AS10- 29-4324.) Bessel Crater, 16 kilometers in diameter and 2 kilometers deep, is an example of a transitional crater between simple and complex craters. Slumping of material from the inner part of the crater rim destroyed the bowl-shaped structure seen in smaller craters and produced a flatter, shallower floor. However, wall terraces and a central peak have not developed. (Part of Apollo 15 Panoramic photograph AS15-9328.) Euler Crater, 28 kilometers in diameter and about 2.5 kilometers deep, is a good example of complex crater morphology. It has a flattened floor, a small central peak, and material that has slumped off the inner crater rim. The blanket of ejecta surrounding the crater is quite clear. (Part of Apollo 17 Metric photograph AS17- 2923.) King Crater, on the Moon's farside, is 77 kilometers in diameter and more than 5 kilometers deep. The terraces and slump blocks on the inside of the crater rim and the relatively flat floor are both typical of large lunar craters. However, the central peak is much larger at King Crater than at other lunar craters of similar size, such as Copernicus or Tycho. (Apollo 16 Metric photograph AS16-1580.)
http://www.lpi.usra.edu/expmoon/science/craterstructure.html
Lecture 3 Page 33 Lunar craters (2)
Copernicus Crater, 93 km diameter Schrodinger Impact Basin, 320 km in diameter (inner ring 150km diameter)
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Copernicus Crater, 93 kilometers in diameter, is one of the youngest and freshest impact craters on the nearside of the Moon. Like King Crater, Copernicus is a well-developed complex crater, with a prominent central peak and a relatively flat floor. This oblique photograph clearly shows the terracing and slump blocks on the inside of the crater rim and the rough ejecta deposit outside the crater. (Apollo 17 photograph AS17-151-23260.) Schrodinger is 320 kilometers in diameter, large enough to be considered an impact basin rather than a crater. In addition to the main, outer rim, Schrodinger also has an inner ring that is 150 kilometers in diameter and about 75 percent complete. Schrodinger is one of the youngest, freshest impact basins on the Moon. (Mosaic of Clementine images. Image processing by Ben Bussey, LPI.)
http://www.lpi.usra.edu/expmoon/science/craterstructure.html
Lecture 3 Page 34 The crater Clavius
Clavius is one of the largest craters on the Moon, with a diameter of 232 km and a depth of 4.9 km measured from the crater’s floor to the top of the surrounding rim This photograph was taken from Earth at the 5m Mount Palomar observatory The crater was probably produced by a fast moving meteorite only a few kilometres in radius
The Alphonsus craterPage 35photographed by Ranger 9 shortly before impactPaul Sellin
Lecture 3 Page 35 The Moon’s Maria and Terrae
The ‘man on the moon’ pattern observed on the Moon’s surface is due to the large dark coloured maria. Early astronomers thought these regions were seas of water, and hence named them ‘Sea of tranquillity’, ‘Sea of clouds’ etc Maria are due to large flows of solidified larva, and tend to be circular in shape These very large depressions in the lunar surface were caused by very early impacts with large (tens of kilometres) meteoroids which formed basins that flooded with larva due to cracking of the lunar crust
Such massive impacts produced mountain ranges around the edges of the basins, for example around the edge of the Mare Imbrium (Sea of showers) The older light-coloured terrae are often called the lunar highlands, and are heavily cratered The maria cover 15% of the lunar surface and the terrae cover the remaining 85%
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Lecture 3 Page 36 Mare and Terrae (2)
The Earth-facing side of the Moon displays light-coloured, Photograph from lunar orbit shows numerous tiny craters heavily cratered highlands and dark-coloured, smooth- and cracks in the surface of a typical mare. The Apollo 11 surfaced maria. The Moon’s far side has almost no maria landing site is near the centre of the top edge of the image
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Lecture 3 Page 37 The Lunar Regolith
The lunar regolith, or "soil," consists of tiny rock fragments that have been broken up by the bombardment of meteorites on the lunar surface over the eons. Most regolith particles are the size of fine-grained silt or sand. However, there are also larger fragments, including pebble-sized objects and even some boulders. At the surface, the regolith is somewhat porous, but it becomes more densely packed as the depth below the surface increases. One important objective of the Apollo 11 mission was to observe the properties of the regolith and assess how these properties affected the crew's ability to move about on the lunar surface.
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"The surface is fine and powdery. I can kick it up loosely with my toe. It does adhere in fine layers like powdered charcoal to the sole and sides of my boots. I only go in a small fraction of an inch, maybe an eighth of an inch, but I can see the footprints of my boots and the treads in the fine sandy particles." - Neil Armstrong (Apollo 11 photograph AS11-40-5977.) Left figure: The footpads on the lunar module's legs penetrated 2-8 centimeters into the regolith. This is about the amount of penetration that was predicted prior to the mission. The rod-shaped object protruding from beneath the footpad is a contact probe. These probes originally extended about 1.5 meters vertically below the footpads and were used to indicate when the lunar module was approaching the Moon's surface. This allowed the descent rocket engine to be turned off just prior to landing. (Apollo 11 photograph AS11-40-5925.) Right figure: The exhaust gas from the lunar module's descent engine caused some scouring of dust on the surface during landing. On Apollo 11, a significant dust cloud was visible when the lunar module was still 30 meters above the surface. The scouring of the surface is visible in this photograph of the region below the descent engine, although the scouring did not form any sort of hole in the surface. (Apollo 11 photograph AS11-40-5921.) http://www.lpi.usra.edu/expmoon/Apollo11/A11_LandingSite_viewsfrom.html
Lecture 3 Page 38 Conclusions
Appearance: the Earth-side facing surface contains dark coloured Maria and light coloured heavily cratered highlands. The far side contains almost not Maria. There is no evidence for plate tectonics or volcanism Internal structure: the Moon contains a thick crust, a large mantle, and a small iron core. The crust is thicker on the far side, evidenced by the lack of flooded maria. The Moon’s crust and lithosphere are both much thicker than Earth. There is no global magnetic field Geological history: the anorthositic crust exposed in the highlands was formed 4.0 - 4.3 billion years ago. The mare basalts solidified 3.1 - 3.8 billion years ago. There has been little major change to the surface in the last 3 billion years. Lunar rocks are formed of minerals similar to Earth, although depleted in the volatile elements. Lunar rock contains no water. Cratering: crater damage and large impacts from meteoroids are the only significant source of weathering of the Moon’s surface. The period of intense bombardment was 4.0 – 4.6 billion years ago. The regolith on the Moon’s surface is a fine rock powder formed by meteoritic action Formation of the Moon: formed by collision between Earth and a Mars-sized planet. Initially molten, the Moon solidified to form an anorthositic crust, with the lower density lava floating to the surface. Tidal interactions with the Earth have deformed the Moon, and lock it into a synchronous orbit
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