Chapter 3 Astro (Moon)

Simple craters

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

Bright rays

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

wide-spread volcanic activity

---> lava floods filled in the impact basins on the lunar ---> impact basins were flooded with relatively dark, iron-rich basaltic lavas, hence the maria are darker in color than the highlands.

Breccia

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.

Highlands:

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.

Secondary craters

- these are 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.

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.

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. E.g., Copernicus - 93 km diameter - near Mare Imbrium Tycho - 85 km diameter - in southern highlands -- 20-180 kilometers in diameter.

Peak-ring basins

-- these are distinctive for having a 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

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

properties of the interior of the Moon

-the Moon is much less seismically active than the Earth - " The lithosphere extends from the surface down to a depth of about 1000 km. - the lithosphere can be divided into two regions: - outer "crust" which is composed of anorthosite-like rocks. The highlands are composed of such crustal rocks. - deeper "mantle" which is located below the crust and composed of darker rocks that have a higher iron content. When the maria were being flooded with lava, the mantle was not hard and brittle as it is today. At that time it contained substantial amounts of molten rock, some of which rose up through fractures in the crust to flood the nearside impact basins. Below the lithosphere is the "asthenosphere" The rocks in this region are "plastic" - they will deform without breaking if pressure is exerted on them. Thus, the asthenosphere is not perfectly rigid as is the lithosphere --> stresses in the asthenosphere cause rock deformation but not breakage. Interior to the asthenosphere is the "core" of the Moon - composed of iron-rich and nickel-rich material - due to high temperatures the core may be partially molten over part of its volume, but this is uncertain.

volcanic activity stopped

Exactly when mare volcanic activity stopped entirely on the lunar surface is not known, some scientists conclude that it continued on in limited fashion in some places as late as 2 billion years ago.

impact basins with lava

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 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.

Straight rilles v sinuous rilles

Straight rilles - have a different origin to the sinuous rilles Straight rilles are features produced by forces within the lunar crust. They are associated with fracturing - breaking - of the lunar crust.

Accretion

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." "chemical differentiation"

"flow fronts"

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.

heavy bombardment

The lunar crust solidified during the era of heavy bombardment The heavy bombardment era ended about 3.8 billion years ago The heavy bombardment modified the original highlands rocks and caused them to become heavily brecciated. Consequently, pristine volcanic rocks that solidified from the magma ocean may now be very rare.

basalts

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.

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. (formed by compression of the mare surface) 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 in the mare basalt deposits that filled the giant impact basins on the Moon. The basin floors are covered over by multiple eruptions of lava producing layer upon layer of basalt rocks. The great weight of the newer basalt layers causes the center of the flooded basin to sag. As the basin center sags, the basalts slide inward, causing compression of the inner regions of the basin, which results in folding and buckling of the surface layers to form wrinkle ridges.

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.

"flood basalt"

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. 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.

Sinuous rilles

broad meandering channels - are found mostly on the maria. Notable example: Hadley Rille near edge of Mare Imbrium. Visited by Apollo 15 astronauts. 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.

"chemical differentiation"

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"

Highlands v lowlands

highlands heavily crated lowlands--less craters because less exposed

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.