Astro CH 3

Anorthosite

A characteristic highlands rock is a type known as anorthosite it has little or no iron content but is enriched in the lighter elements aluminum (Al), calcium (Ca), silicon (Si), and oxygen (O). Anorthosite is a volcanic rock, it forms from the solidification of molten lava. The highlands solidified from molten material at a very early phase in lunar history, well before the volcanic events which produced the maria. Many of the anorthosite rocks of the highlands, however, have been modified to various degrees by impact events. Thus anorthosite samples are most often obtained as fragments from within larger samples of breccia.

LUNAR BASINS

A good example of a multi-ring basin is the Orientale Basin on the lunar farside: This basin has two inner rings of mountains plus a mountainous rim these rings give the basin a "bull's eye" appearance The basin has not been flooded by lava to the extent of the nearside maria As a result we can see large areas of relatively unmodified basin floor, although the central regions have been partly lava covered. The outer mountain ring, which has a diameter of 930 km, is named the Cordillera Mountains, which rise to heights of 7 km and form the rim of the basin. There are two interior mountain rings called the Inner and Outer Rook mountains. The Orientale Basin is surrounded by an ejecta blanket that is about 1,000 km wide.

Peak-ring basins

According to one idea the central region of such a basin rebounds upward following an impact, analogous to the formation of the central mountain of a complex crater. However, with a large basin the rebounding crust covers a great area and rises up so high that the central parts of the rebound begin to collapse back downwards under gravity, pushing the outer parts of the rebounding crust outward and upward to form a single ring of mountains on the interior floor of the basin (within the outer rim of the basin).

Lunar Volcanoes

Although large volcano-like constructs are not found on the Moon, some small volcanic hills - known as domes - are found in a few locations. A notable example is the Marius Hills in the Ocean of Storms. These domes appear to be the lunar equivalent of small volcanoes.

Lava Flows

During the era in which lavas were flooding the maria, some lavas may have flowed across the surface in channels, or beneath the surface in "tubes" or tunnels. Lava channel origin: According to this theory lavas carved out river-like channels directly across the surface of the maria - Hadley Rille may have formed in such a way. Lava tube origin (an alternative idea): When the uppermost layers of a lava field cooled and solidified, some molten lava may have continued to flow through tubes or tunnels below the surface. When the source of the lavas dried up the sub-surface tunnel was left empty. At a later time the roof of the tunnel collapsed (possibly due to impacts), exposing a sinuous rille. Relatively small sub-surface tubes have been found in terrestrial lava fields, but they are much smaller than lunar rilles.

Multi-ring basins

Following a very large impact crustal rocks are shattered and heated to such a great degree that the crust becomes very weak, and behaves partly like a fluid, allowing major structural modification to occur. The innermost ring of mountains may form by rebound in a similar manner to the mountain ring within a peak-ring basin. However, what now goes on beyond this innermost mountain ring is rather different. In regions around the innermost ring the softened and weakened rocks beneath the lunar surface may flow like a fluid inward towards the central impact site (from which vast amounts of lunar crust have been ejected). As the underlying weakened sub-surface crust migrates inward, the overlying harder and more brittle rocks near the surface are also pulled inwards. This causes the surface rocks to crack and split apart, producing concentric circular rings of deep ''fractures'' within the lunar crust. The fractures may extend to depths of 20-50 kilometers. When the lunar surface adjusts to a stable configuration, crust on one side of a fracture ring gets ''uplifted'', i.e., pushed upwards relative to crust on the opposite side. This can result in the formation of two additional rings of mountains that surround the innermost ring. It is the outermost ring of fractures that become the mountainous ''rim'' of a multi-ring basin. On top of these mountains are deposited fragments of material from an ejecta blanket. This entire process may take only 2-3 hours.

There is a striking contrast between the history of the Earth and the Moon. Earth

Higher forms of life developed less than 600 million years (0.6 billion years) ago, while the continents acquired their current shapes less than 200 million years ago. Moon: Surface topography has hardly changed in the last 600 million years, and most features were formed more than 2,500 million years ago.

Impact Melt Rock

In the case of very big craters or impact basins some of the crust at the impact site may be melted. When this molten material cools it produces a new type of rock known as an "impact melt rock" which may cover part of the floor of the crater or basin. Some of the molten impact-melted material may be flung outwards and fall onto the ejecta blanket. The cavity that is initially excavated by the explosion next undergoes adjustment and the crater shape changes and settles under the force of gravity into a more stable configuration. For small craters the adjustment may be relatively minor. For intermediate-sized craters, however, the adjustment produces significant change in the shape of the initial crater. Parts of the walls and rim of the initial crater are very steep and unstable under gravity, such that they soon collapse and slump down towards the crater interior. This produces the terraced walls that make the rim of a complex crater so irregular.

Impact basins exist on both the nearside and farside of the Moon. But it was mainly just the nearside basins that got inundated with basaltic lava. Why was this? A clue may lie with the internal structure of the Moon. Based on chemical composition, the outer 800 km of the Moon can be divided into two regions

An outer "crust" which varies from 60-100 km thick, being thicker on the lunar farside than the nearside. The crust is composed of rocks such as make up the lunar highlands. This crust cooled during the heavy bombardment period from the early magma ocean of the Moon. Below the crust is a deeper region known as the "mantle", which is composed of rocks that have a higher iron content (because of chemical differentiation within the Moon). When the maria were being formed it was magma from the mantle that was flooding into the nearside impact basins. At these times there were hot magma sources present within the mantle. But in order to flood out onto the lunar surface, molten magma originating from the mantle had to rise up through the solid overlying crust. On the nearside of the Moon the crust is about 60-70 km thick, but on the farside it may be 100 km or more thick. The farside crust may have been too thick to permit magma from the mantle to pass all the way through, even in the fractured regions below impact basins.

Rock Samples

Imbrium and Orientale are examples of the youngest basins on the Moon. Basin formation dates can be obtained by the analysis of rocks such as breccias and impact melts from the ejecta blankets that surround the basins. Samples of rock that was ejected by the formation of the Imbrium Basin were returned by astronauts of Apollo 14, who landed in the Fra Mauro hills formation, about 600 km south of Mare Imbrium, on material ejected from the Imbrium Basin. These rocks showed that the impact that formed the Imbrium Basin occurred 3.9 billion years ago. Imbrium (on the lunar nearside) and Orientale (on the lunar farside) are among the youngest basins on the Moon, and the ejecta from them has spread over large fractions of the lunar surface. This is one of the reasons why determining the age of the Imbrium Basin was a key objective of Apollo 14. A number of other impact basins on the lunar nearside also appear to have formed around this time, in the period 3.8-3.9 billion years ago, while other basins appear to be older. The era of large basin formation on the Moon ceased 3.8 billion years ago. This was an important scientific conclusion from the Apollo 14 rock samples.

Imbrium Basin

Like Orientale, many lunar basins and maria are bordered by rings of mountains. For example, Mare Imbrium is flanked by a mountainous rim comprising the Apennine, Carpathian, and Caucasus ranges, as well as the lunar Alps. Like Orientale, the Imbrium Basin (i.e., the basin that was flooded to form Mare Imbrium), which has a diameter of over 1100 km, is thought to have had two inner mountain rings, but these were mostly covered over by the lavas that eventually flooded the basin.

Mare Basalts

Mare basalts have a higher content of the element iron, and sometimes titanium (Ti), than highland rocks. These elements tend to be dark in color and are heavier than elements like Al and Ca. Consequently, mare basalts are both heavier in weight and darker in color than highlands rocks. The darkness and color differences between the highlands and the maria are primarily a consequence of the different chemical compositions of their surface rocks.

THE MARIA

Maria (plural); Mare (singular - Latin for "sea") Found predominantly on the nearside of the Moon. They were thought by some 17th century telescopic observers to be water-covered seas or oceans, due to their relatively smooth appearance as seen through a small telescope. Many maria are approximately circular in shape, such as Mare Imbrium, the Sea of Rains, which is about 1,100 km in diameter. Maria were formed by the flooding of dark lava into large impact basins on the Moon's ancient surface. The formation of an impact basin causes extreme fracturing (cracking) of the crust below the basin. In the case of basins on the lunar nearside, these fractures allowed molten rock from deep within the Moon to make its way up to the lunar surface at a later time. Many of the giant impact basins on the nearside became flooded with lava to form the maria. Thus Mare Imbrium was produced when lavas flooded the original Imbrium Basin, which had been formed at an earlier time by a giant impact. Large impact basins are found on both the nearside and farside of the Moon. However, very few of the farside impact basins were flooded with lava, and as a consequence, there are very few maria on the lunar farside. Note: molten rock which is below the surface of a planet is called "magma" molten rock that has flowed onto the surface is called "lava."

Green Glass Beads

Some glassy beads found in lunar regolith, such as in the green soil from Apollo 15 and the orange soil from Apollo 17, are thought to have been produced by geyser-like eruptions of lava known as "fire fountains. Although these eruptions are rather explosive in nature, they are of a small scale, and do not eject the amounts of lava that typically issue from a large volcano. There are also some small dark areas on the Moon (often located around craters known as "dark halo craters") that could be sites where volcanic ash was erupted and deposited by moderately explosive volcanic activity, although no volcanoes were formed by such eruptions.

THE HISTORY OF THE MOON

The Moon formed about 4.55 billion years ago by the gravitational accumulation of a variety of rocky bodies of various sizes - a process known as "accretion." Accretion ---> start off with a system consisting of a vast number of small rocky particles ---> as these particles move around, some collide and combine to produce larger particles ---> larger particles next collide to form even bigger particles ---> this process continues until one object (a ''proto-Moon'') becomes much larger than the other objects in the system ---> the proto-Moon proceeds to sweep up and accrete all of the other smaller bodies in the system ---> considerable heat is generated (the kinetic energy of an object that is accreted onto the surface of the proto-Moon is converted to heat upon impact) ---> it is possible that the forming Moon was in a molten state a large "magma ocean" may initially have covered the surface ---> heavy metallic elements such as iron (Fe), titanium (Ti), and nickel (Ni), sank down through the interior of the molten Moon and lighter elements like silicon (Si) rose towards the surface this is a process known as "chemical differentiation" it caused the surface regions of the Moon to develop a different chemical composition from deeper regions.

Lava Floods

The depth of flood lavas on the lunar maria may range from about 1-5 kilometers. Most maria were formed not by a single major eruption, but by many episodes of flooding at different times, each episode producing a new layer of lava that is known as a "flow." Individual flows may be about 50 to several hundred meters thick. New eruptions of flood lava not only covered over previous flows, but also tended to cover over the vents and fissures from which earlier lavas had poured. On some maria "flow fronts" are apparent; these are smooth scarps which mark the edges of old solidified lava flows.

Other features associated with impact craters

The ejecta blankets, secondary craters, and bright rays associated with craters are all consistent with an impact origin. They are the types of features that would be formed by material that has been flung outwards from the site of an explosion. On the Moon explosions occur when a meteoroid (a rocky object from interplanetary space) impacts onto the lunar surface.

THE FORMATION OF LUNAR CRATERS BY IMPACT EVENTS

The high speed impact of a meteoroid onto the lunar surface produces an explosion (i.e., a sudden release of energy). This explosion digs out (excavates) the crater, and obliterates the original surface features. High-speed meteoroids impacting onto the Moon have speeds greater than 2.4 kilometers per second. These meteoroids have a large amount of kinetic energy, i.e., energy associated with motion. An impacting meteoroid will penetrate the lunar surface to a depth equal to several times the diameter of the meteoroid. The lunar crust is shattered and compressed by the very high pressures produced by the impact. As the meteoroid is brought to a halt - it releases its kinetic energy a fraction of the kinetic energy of the meteoroid is converted to heat energy most of the meteoroid and some of the surrounding lunar crust are vaporised into a superheated ball of high-temperature gas this hot ball of gas is known as a "fireball" the fireball expands explosively and excavates a crater material is excavated from the lunar crust, broken apart, and thrown upwards and outwards. Note that the rapid expansion of the fireball constitutes an explosion; the excavation of a lunar crater is an explosive event. The initial compression of the crust by the impacting object, followed by the expansion of the fireball, also generates a "moonquake", i.e., seismic waves that travel through the lunar crust, further disrupting the crust in the vicinity of the impact. A mass of lunar crust several hundred times greater than the mass of the meteoroid itself (most of which is vaporised) is excavated and ejected. The bulk of this material falls back to the lunar surface fairly close to the rim of the new crater (within a distance from the rim comparable to the diameter of the new crater). produces the rough, hilly deposits which constitute the ejecta blanket. Note: the material which makes up the ejecta blanket has been excavated from below the original lunar surface. The rocks in this material will have been shattered and perhaps partially melted. Fragments of rock that rain down after most of the ejecta blanket has been deposited will produce their own craters these are the secondary craters that are formed on and around the ejecta blanket. The highest speed debris that is thrown out may travel great distances before falling back to the surface, and will produce long radial streamers of deposited material and disrupted soil known as bright rays. These rays extend radially away from the crater over distances well beyond the ejecta blanket. In the case of large impacts, the debris which produces the bright rays may travel for many hundreds of kilometers. The explosive fireball which propagates outwards from an impact site not only ejects large amounts of rocky material from the lunar crust, but modifies this material as well by shattering, crushing, heating, and compressing it. These processes can produce two new types of rock in the vicinity of craters

Basalt

The maria are covered by a type of rock known as basalt, a volcanic rock which is relatively rich in the heavy metallic element iron (Fe). The basalts of the lunar maria cooled from molten lava flows that had originated at depths of about 100-400 km below the lunar surface (precise depths still uncertain). These lavas made their way to the surface through extensive cracks and fractures in the lunar crust that had been produced under the large nearside impact basins; this lead to the preferential flooding of the low-lying floors of nearside impact basins.

THE HIGHLANDS AND THE EARLY CRATERING HISTORY OF THE MOON

The oldest rocks in our Solar System are certain types of meteorites, the ages of which are 4.55 billion years. Mathematical calculations indicate that the Sun is also 4.55 billion years old. The Moon is therefore also likely to be 4.55 billion years old.

Wrinkle Ridges

The origin of wrinkle ridges, which are located mainly on the maria and are much less common on the highlands, is still a puzzle. The most widely accepted theory is that they were produced by horizontal compression of the surface of a mare. What caused such compression is still a subject of debate. One idea is as follows: Lunar wrinkle ridges are found on the surfaces of mare basalt deposits that filled the giant impact basins on the nearside of the Moon. The basin floors were covered over by multiple eruptions of lava producing layer upon layer of basalt rocks. The total thickness of these combined layers is greatest near basin center and thinnest around the margins. The great weight of the basalt layers caused the center of the flooded basin to sag. As the basin center sagged, the basalts slid inward, causing compression of the inner regions of the basin, which resulted in folding and buckling of the surface layers to form wrinkle ridges.

THE LUNAR REGOLITH

The surfaces of both the lunar highlands and the maria are covered with a very rubbly and powdery layer of material known as regolith. The bulk of the regolith consists of shattered fragments of rock and dust particles of various sizes that have been produced by the breakup of the lunar surface by impact events, and by the collision of atomic particles in the solar wind with lunar rocks. The depth of the regolith layer is greater over the highlands than the maria. This is because the highlands have been exposed to more impact events than the lowlands. The regolith may vary from 15-40 meters deep over highlands areas, to about 1-10 meters deep on the maria. When a cratering event occurs and excavates fresh subsurface material and deposits it in an ejecta blanket or system of rays, the deposited material will often be brighter than the surrounding regolith - hence the formation of bright rays. This ray material darkens with time and becomes less discernible, so that bright rays tend to be a sign of a relatively young crater.

Basalt

The surfaces of the maria are composed of a dark volcanic rock called basalt, which was formed by the cooling of lava that had erupted onto the lunar surface. Most mare volcanism occurred via fairly quiescent outpourings of vast volumes of lava from extensive fissures (vents or cracks) in the rocks below impact basins. Note: mare volcanism was of a different type to the explosive volcanism that is associated with some of Earth's large volcanoes, such as Mount St Helens and Krakatoa. There are no large extinct volcanoes on the Moon. However, lunar-like volcanism has occurred on Earth, for example, "flood basalt" eruptions have produced large-scale lava plains in eastern Washington state and Central India.

concentric rings of mountains interior to the main rim of the basin

The very largest meteoroid impacts produced big impact basins that have one or more concentric rings of mountains interior to the main rim of the basin. The mountain rings within impact basins are formed as part of the readjustment of the crust following an impact, but exactly what takes place is still the subject of active research.

Breccia

a rock that is made up of fragments of smaller rocks and material which has been cemented together under conditions of high heat and pressure. By shattering the rocks near an explosion site, and then heating and compressing the fragments back together, an impact event will produce large amounts of brecciated rock. Some of these breccias are flung upwards and outwards from the explosion site: the material which is flung upwards will fall back onto the floor of the new crater, while material which is flung outwards will be deposited in the ejecta blanket. Thus the ejecta blanket will contain rocks and material that has been brecciated, while a layer of "fallback" breccia will be scattered across the crater floor.

The formation of the lunar maria began

at least 3.9 billion years ago. The oldest mare basalts collected during the Apollo missions were obtained by Apollo 14 and 17 astronauts. They have ages of 3.8-3.9 billion years. Apollo 11 basalts from the Sea of Tranquillity have ages of 3.6-3.7 billion years. Luna 16 basalts from the Sea of Fertility have ages of 3.4 billion years. Mare Imbrium basalts returned by Apollo 15 astronauts have ages of 3.3 billion years. Basalts returned from Oceanus Procellarum by Apollo 12 astronauts have ages of 3.2 billion years.

Simple craters

bowl-shaped with gently sloping floor smooth inner walls and rim (the high edge of the crater) smallest lunar impact craters - less than 20 kilometers in diameter (i.e., less than 20 km from one side of the rim to the opposite side).

Sinuous Rilles

broad meandering channels found mostly on the maria. Notable example: Hadley Rille near edge of Mare Imbrium. Visited by Apollo 15 astronauts. A meandering valley about 1.2 km wide, 400 meters deep. Floor and slopes of rille are littered with boulders. Layers of solidified lava are exposed on walls of this rille. Sinuous rilles were carved by ancient "rivers" of lava.

Straight Rilles

different origin than sinuous rilles they are features produced by forces within the lunar crust. associated with fracturing - breaking - of the lunar crust. A straight rille is a valley that is bounded on both sides by long, parallel walls where the lunar surface has been fractured and split. This fracturing occurred when the crust on either side of the valley was pulled apart by forces originating beneath the lunar surface. The central section of the rille has dropped downwards relative to the terrain on either side to produce a long valley. Such valleys are known as "grabens." They are also found on Earth. Where a fracture separates two adjacent segments of crust that have moved relative to one another, that fracture is known as a fault: the crust on one side of the fault is displaced (shifted) relative to the crust on the other side. In a straight rille the valley floor is bordered on both sides by parallel faults.

The central peak of a complex crater

is also produced by readjustment of the crust. The rocks near the center of the initial crater were formerly located deep below the surface and were once under great downwards pressure from overlying rock. In addition, when an impact occurred this downwards pressure was increased even further. Following the impact and the excavation of the overlying material, these downward pressures were removed. The rocks near the crater center respond to the removal of this downwards pressure by ''rebounding,'' i.e., rising upward, to form a central mountain peak. The collapse of material from the walls of the crater into the interior may also contribute to pushing up the central mountain peak.

Era of Volcanism on the Moon

lasted more than 700 million years. It is thought that most large-scale volcanism associated with the maria had ceased by 3.1 billion years ago, although it may have continued on longer in some areas, perhaps until 2 billion years ago according to some studies.

Secondary craters

small craters which are found scattered amongst the ejecta blanket they are much more numerous on the outer part of the ejecta blanket than on the inner part in some locations groupings or even chains of secondary craters are seen.

Complex craters

the inner walls of the crater interior to the rim have a terraced (platformed) or rough appearance there is a mountain peak, or a range of peaks, near the center of the crater. These are surrounded by a flat floor. Copernicus - 93 km diameter - near Mare Imbrium Tycho - 85 km diameter - in southern highlands 20-180 kilometers in diameter.

Maria

the oldest Apollo mare basalts have ages of 3.9 billion years. The oldest maria are therefore comparable in age to the youngest highland areas. However, these two different types of terrain are cratered to very different degrees. If we compare a typical region on the lunar highlands with another region of the same area on the lunar lowlands, then typically there will be 10-20 times more craters in the highlands region than on the comparison lowlands (mare) region.

Highlands

the oldest surviving part of the lunar crust - they predate the maria. The highland surface is typically more than 4.0 billion years old, as measured from rock samples returned by the Apollo 16 astronauts from a nearside highland area, as well as highland rocks collected at the Apollo 14, 15, and 17 landing sites. The oldest minerals found among the Apollo highlands rock samples have ages of more than 4.4 billion years.

Peak-ring basins

there is no central peak, instead these structures have one concentric ring of mountains interior to the basin rim diameters of 180-400 kilometers the smaller examples of these types of structures may also be referred to as peak-ring craters

Bright rays

these are bright streaks of material that radiate outwards from a crater over distances much larger than the crater diameter associated only with very young, fresh-looking craters that have well-defined rims.

Multi-ring basins

these are the largest impact structures on the Moon they have two or more inner rings of mountains interior to the rim of the basin the basin rim itself has the appearance of an extended circular range of mountains excellent example: Orientale Basin on the lunar farside. multi-ring basins on the Moon tend to have diameters greater than 400 kilometers

Ejecta blanket

this term refers to an exterior deposit of material that surrounds a crater and extends outwards from the crater rim the surface is often "hummocky" and uneven in texture close to the crater rim the ejecta blanket is thickest and forms a continuous cover of material, while farther out from the rim it breaks up to form a thinner more diffuse cover that merges into the surrounding terrain.