Most of the craters with bright ray systems were formed during this last period in lunar geological history, most notably Aristarchus and Tycho, formed around and million years ago respectively. The Copernican Period now shows the effects of surface erosion by innumerable meteoroids and micro- meteoroids - a relentless pummeling that has gradually rounded the lunar hills and produced a layer of fine soil. Geologists consider the Moon to be an essentially dead world. Beneath this lies a solid mantle km thick that between and km beneath the lunar surface produces moonquake foci. Moonquakes average less than a mild magnitude 2 on the Richter scale, and they have been found to occur in response to stresses in the crust, which are more prevalent when the Moon is closest to the Earth.
An average of 3, moonquakes takes place each year - annu- ally there are times as many quakes of similar magnitude on the Earth. Lunar seismic disturbances also occur as a result of random meteoroidal impacts. Since there is no seismic activity on the Moon other than that produced by natural causes, seismometers operated on the Moon can measure disturbances several orders of magnitude fainter than those detected on the Earth. Seismic amplitudes of one millionth of a centimeter can be measured on the Moon. Seismic measure- ments have given tantalizing clues as to the internal construction of our satellite.
The types of seismic waves detected include pressure waves and shear elastic waves which cannot travel through fluid, proving that the waves, had passed through a solid mantle. Long time delays in the arrival of seismic waves also suggest that the lunar crust is highly fractured and faulted. Cross-section through the Moon's megaregolith and upper crust not to scale.
Credit: Peter Grego dense. Partial melt zone Core Fig. C ross-section through the Moon the surface topography is not to scale. Maria almost entirely on the near side have average depths of several kilometers and overlie a crust several tens of kilometers thick. The highland crust is considerably thicker, having an average thickness of 70 km, and in places on the far side has a thickness exceeding 1 00 km. Beneath the crust lies the solid mantle, around km thick. Beneath the mantle lies a probable partial melt zone, around 1, km beneath the Moon's surface.
The boundary between the mantle and this Zone is the source of most moonquakes. The Moon's inner core is probably iron rich and smaller than km in diameter. Gravity maps of the Moon show that certain well-defined areas on the Moon possess a considerably higher gravitational pull than do their surroundings.
These patches of high-density material are called mascons mass concentrations , and most of them are located at the centers of large, lava-flooded impact basins. Mascons 14 indicate that the interior of the Moon must have been fairly rigid since the epoch of basin excavation; a hot ductile interior would not have been strong enough to bear the extra crustal mass for such a long period, and isostatic equilibrium would have resulted. Mare Orientale, a large basin in the western hemisphere that straddles the near and far-sides of the Moon, displays the most unusual gravity profile: its center is strongly positive, but the region between its inner ring and outer ring is strongly negative.
Negative anomalies are also found in the far-side basins of Hertzsprung, Mendeleev and Tsiolkovsky. Their presence indicates the greater thickness of the lunar crust on the far side, which prevented a post-impact uplift of denser mantle material, and caused the absence of significant flooding by lava.
Types of Lunar Rock The Moon has never possessed an appreciable atmosphere, and its surface has been subjected to none of the processes of erosion found on the Earth. The youngest lunar rocks - the basaltic lava that filled the near-side impact basins - date from around 3 billion years, which makes them about as old as the oldest rocks found on the Earth.
Regolith consists largely of fragments of the local rock broken up by meteoroidal impacts - plus a small proportion of material that originated in large impacts further afield - overlain with a few centimeters of lunar soil.
Lunar soil is comprised of rock fragments derived from the local material, various fragments of minerals, metals and glass agglutinates bound together by impact melt. In some locations, the soil contains tiny glass spheres, formed when high-viscosity lava was sprayed above the surface in fire fountains. This glass is composed of material that originated around km beneath the surface, magma that climbed through chan- nels in the lower mantle and crust relatively rapidly. Having experienced relatively little chemical interaction with the solid rocks surrounding it, the lunar volcanic glasses therefore represent the best samples of material from deep within the Moon.
Typical mare regolith consists of a layer of pulverized boulders, rocks and rubble from 2 to 8 meters deep, overlain with a few centimeters of soil. The regolith tends to be deeper in the highlands, ranging from 20 to 30 m in depth, but the regolith on the floors of some of the more recently formed large craters may be just a few centimeters deep.
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Larger boulders can also be dislodged from time to time, and as they roll downhill they leave clear tracks in the soil. Because there was never any substantial lunar atmosphere, and the surface was never covered with any expanses of water, not even the most primitive forms of life developed on the Moon.
Thus, many rocks commonly found on the Earth - the product of atmosphere, water and life in combination with geological processes - have no lunar counterparts. The Moon has no clastic sedimentary rocks, such as conglomerate or sandstone, nor has it any biogenic sedimentary rocks, such as coal 15 The Moon's Origin The Moon's Origin Fig.
The Moon's regolith is made up of fine-grained dust, mixed with rocks and larger boul- ders. This is a closeup view of the regolith in Hadley Rill, a lunar valley. The image shows a mixture of fine soil, rocks and substantial boulders that have rolled down the valley's slopes. Credit: NASA or chalk.
Because there has been no lunar tectonic activity or large-scale crustal movement, metamorphic rocks such as schist and gneiss, formed slowly through crustal pressure and heat, are not found on the Moon. Basalt, anorthosite and breccia make up the three main types of lunar rock. Basalt is a dark-colored volcanic rock that once flowed as lava and filled all the marial basins of the near side and some areas on the far side. Its grain size averages less than 1mm, an indication of its rapid cooling after extrusion. A basaltic rock taken from Mare Tranquillitatis. The lunar maria are all composed of this dark type of rock, which flowed onto the Moon's surface more than 3 billion years ago.
The most abundant Mg-suite rocks are norite plagioclase- pyroxene rock and troctolite plagioclase-olivine rock. The Mg-suite rocks date from around 4. Breccia is a composite rock made up of fragments created through the processes of impact shattering, mixing, melting and recrystallization of rock during the high energies released during meteoroid and asteroid impacts.
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Typical breccias contain coarse fragments embedded in a fine-grained crystal matrix. Breccias are described as monomict if they contain fragments of only one rock type; most breccias are polymict and are composed of more than one type of rock. All three rock types - mare basalt, anorthositic material and breccia - can be found in any location on the Moon, though the proportions will vary, depending on whether the site is located in the highlands or a mare. Anorthosite makes up the Moon's highland crust, and is the oldest rock type found on the Moon.
This sample is 4. A small proportion of material found in any spot will come from much further afield, debris thrown out by distant impacts hundreds of kilometers away. Shaping the Moon's Surface Impact Much of the Moon has been intensely sculpted by meteoroidal and asteroidal impact.
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Through the eons, the substantial atmospheres of Venus, the Earth and Mars have served as an effective impact buffer, allowing only a few of the biggest incoming objects to wreak large-scale damage. Atmosphereless Mercury and the Moon have been fully exposed to the harsh vacuum of space and subjected to bombardment by interplanetary dust, meteoroids, asteroids and comets, in addition to x-rays, gamma rays and cosmic rays.
A breccia taken from the valley of Taurus Littrow. Lunar breccias are made up of fused fragments of material derived from impacts. This sample consists of fragments of glass, minerals and rock cemented together in a glassy matrix. It has been esti- mated that around 3 trillion craters over a meter in diameter dot the lunar surface. Large tracts of the lunar surface have been modified by volcanic activity and other geological processes, including faulting.
The lunar crust has never been subjected to any appreciable tectonic activity, and large-impact features many billions of years old can be clearly traced; some of them were actually formed before life on Earth appeared. The overwhelming majority of lunar craters display the hallmarks of impact formation.
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Leaving aside the evidence of the lunar rocks themselves, there is a clear pattern of impact-crater morphology spanning all size ranges, from the tiniest micrometeorite impact pits to the vast asteroidal impact basins. The observed morphology of lunar craters perfectly matches computer studies of impacts, in addition to ballistic impact experiments performed in laboratories and field studies of terrestrial craters both natural impact features and manmade explosion craters. Many of the lunar rocks themselves could only have been formed as a result of the sudden high temperatures and pressures produced during impact events.
There is no evidence to support the idea that any of the larger craters repre- sent the rims of ancient lunar volcanoes, nor is there any evidence to suggest that any major craters were formed by violent crustal explosions or as a result of crustal 19 The Moon's Origin The Moon's Origin Fig. A small number just a few dozen of meteorites found on Earth are known to be mate- rial that originated on the Moon, having been thrown from the lunar surface by the explosive power of impacts. Only 0. This rare mate- rial may have been blasted from around 20 individual impact events during the past 10 million years.
These tiny fragments of lunar-highland breccia and mare basalt, owned by the author, are chips from the catalogued lunar meteorites. Credit: Peter Grego collapse after magmatic subsidence. If some craters did happen to be volcanic in origin, then a number of these features, frozen in various stages of formation, would likely exist, but this has not been not observed.
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