Theory Moon

Moon is our Earth's satellite

A Full Moon as seen in a small amateur telescope!

Moon Figures

Moon

Moon Figures

to tables values specifications

Moon's orbit around Earth is complex and affected by a number of variations. That is due to the perturbing gravitational influence of the Sun. Moon's position in a observer's sky however keeps inside the limits of the zodiac

(1) back line of apsides, which joins both the perigee and apogee, is rotating with a direct revolution of 8 years and 310 days as the revolution's motion is not uniform
(2) back inclination relative to the ecliptic is varying from 5° to 5° 18' with a period of 173 days
(3) back the range of the inclination relative to the terrestrial equator matches the fact that the line of nodes of Moon's orbit is regressing (moving clockwise) in 18.6 years
(4) back orbit's eccentricity varies from 0.045 to 0.065 along a cycle of 206 days, which is due to the gravitational influence of the Sun

Values Explanations

Moon

The Earth-Moon system appeared early in the history of Earth, as one of the theories about the Moon's origin is that a planetesimal the size of Mars came and hit the Earth about 4.5 billion, about 30 to 50 million years only after the Sun and the Earth were born. It ripped off the outer layers of the Earth or crust, sending a swarm of debris in orbit. Such debris eventually settled into a huge ring of debris, which lasted a year or so, as the ring further accreted back, forming the Moon. The reason why Moon rocks are dry is that the supposed impact which created our satellite from the Earth, heated and vaporized most of the water constituent of the lunar material and lost to space. That Mars-sized object which hit the Earth might have been another planet, at the time, in the solar system, called Theia. Asteroid-sized remains of Theia might be found, if any, at some Lagrangian points. The lopsided aspect of our Moon too, some think, might have resulted of a time when two satellites were orbiting about our Earth, about 4.4 billion years ago. Both moons would have result themselves from the collision of that massive object to Earth. What was to become our Moon was far more heavier than its sibling and eventually attracted it to crash unto. The crash occurred at the relatively low speed of 5,000 mph and creating no impact crater but the hilly mountainous far side surface instead. Theia could even have formed at the L4 or L5 Lagrangian points, as the balance of forces would have accumulated material. The growing size of the other, developing planets, like Venus, would have dislodged then the planet and sent it towards Earth. The collision would have occurred 4.5 billion years ago. Also specific to that theory is that a special type of rock at would have floated up to the Moon's crust soon after its theorized ocean of molten rock cooled, or soon after the Moon formed as a result of that spectacular crash. Most recent studies are questionning that theory somehow as it would imply that Theia should have contributed to the Moon rocks by 40 percent. A newest theory by 2012 is stating that both Earth and Moon were created together in a giant collision of two planetary bodies each five times the size of Mars, a scenario which accounts better for the similarity in composition of both. After colliding, the two similar-sized bodies then re-collided, forming an early Earth surrounded by a disk of material that combined to form the Moon. The re-collision and subsequent merger left the two bodies with the similar chemical compositions seen today. Considering the mass and lack of any significant iron core at the Moon bodes well with the giant impact hypothesis. To reconcile the fact that the material which made up the Moon resulted mainly from the impactor's but that lunar and Earth rocks have such similar compositions, astronomers necessitated that Earth and the planet that smacked into it resembled each other and, mostly, orbited at about the same distance from the Sun. That was proved plausible at 30 percent by a study in April 2015. Kilometer-sized fragments from that impact, now called 'Giant Impact,' which formed Moon hit main belt asteroids at much higher velocities than typical main belt collisions, enough to heat and degas target rocks. To reconcile giant-impact models with the compositional similarity of Moon and the Earth further, numerical simulations suggest that the Moon could instead be the product of a succession of a variety of smaller collisions, each forming a debris disk around the proto-Earth then accreting into a moonlet. Such moonlets eventually coalescend to form Moon. The multi-impact theory states that a half-dozen or dozen impactors over a period of 60 to 100 million years might have been at the origin of our Moon. Moon's late-accreted mass relative to that of the Earth is explained by a low impactor-retention ratio and a late retention of some elements in the lunar mantle, at 4.35 billion years ago a time when most of the lunar magma oceans were solidified. When the Moon and Earth are made of largely the same stuff, there is significantly less sodium and potassium in lunar regolith than in Earth soil due to solar activity. Solar particles further would have been deflected by the Moon's erstwhile magnetic field 4 billion years ago and deposited at the poles. The Moon was quickly locked into its orbit around Earth in that way that its orbital period matched its rotational period. This occurred in a few tens of millions of years. Like today, Moon orbited at the same time that it rotated, hence presenting always the same face towards Earth. This was due to the large mass of Earth and the larger rotational period of it. Earth-crossing planetesimals that were not yet accreted at the time of the Moon-forming event, impacted the forming Moon in terms of its polar axis inclination. The Theia collision likely yielded also the Earth's tilted axis contributing to the appearance of four seasons, and the Moon and its pull kept the iron core inside Earth from stiffening. Unimaginably violent impact events on the other hand, helped to build the lunar crust at the time of the Heavy Bombardment, forming the surface of the early Moon, which is believed to be 4.51 billion years old. The mixing of our satellite's inner and outer layers may have therefore been drawn by these meteorite impacts. Moon is thought to have gone through a magma-ocean phase. As the magma ocean solidified, dense mafic (rich in magnesium and iron) minerals such as olivine and low-calcium pyroxene crystallized at the ocean's base. After three-quarters of the ocean had solidified, less dense minerals such as plagioclase (aluminium silicate) floated to the surface, which led to the formation of a highland crust composed mainly of calcium-rich plagioclase. And at the end of the ocean's solidification, minerals enriched in elements that were the last to enter the solid phase crystallized beneath the crust. That early formation of the Moon also had like a consequence a dichotomy between the near, and the far side: as the early Moon had a ovoid shape due to Earth's gravitational attraction, that also rearranged the interior of our satellite with more magma pressing unto the near side despite that the Moon got distant. That magma eventually broke the lunar crust and expanded into the current lunar seas. The far side, as far as it was concerned was not affected by such a phenomenon as a thicker crust there was due to aluminum and calcium combining with elements in the mantle and surfacing. As the Moon cooled, its warped shape froze into place. Large-scale volcanism later occurred between 4 and 2 billion years ago contributed further to give the Moon a off-center distribution of mass. That, on a other hand, made that Earth’s gravity tugged and gradually triggered a shift in the Moon’s polar axis which originally was lying about 36° away from where it is now, at a latitude of 54°N, on the edge of Oceanus Procellarum. The lopsided shape of the Moon is one result of its gravitational tug-of-war with Earth. The mutual pulling of the two bodies is powerful enough to stretch them both, like two eggs with their ends pointing toward one another. On Earth, the tension has an especially strong effect on the oceans, and is the driving force behind tides. Earth’s distorting effect on the Moon, called the lunar body tide, is more difficult to detect as it is enough to raise a bulge about 20 inches high on the near side of the Moon and similar one on the far side. The position of the bulge actually shifts a few inches over time. Although the same side of the Moon constantly faces Earth the side facing Earth appears to wobble. The event of the kind which created the Moon also occurred at the other telluric planets of the solar system, with different consequences, like Caloris Basin at Mercury, the northern impact at Mars or the inverted rotation motion at Venus. Anecdotically, let's know too that the Earth-Moon gravitational system regularly captures small asteroids of the order of one or few meters in width, in orbit. Such tiny 'moons' to the Earth usually stay for about 10 months there only, or about 3 revolutions. Before the Apollo missions brought back lunar samples, astronomers were divided about the nature of the Moon. Some claimed it had formed by volcanism, others its structure owed mostlty to impacts of thousands of meteoroids as some more thought the Moon was a cold and dead world since the solar system formed 4.5 billion years ago and thuse the 'Rosetta Stone of the solar system.' That latter opinion proved false as the Moon was anything but a primordial body. The Great Bombardment Period indeed did melt the original lunar surface, extending down to at least 60 miles (96 km) or deeper, reaching to the interior magma which absorbed the primordial Moon's rock as new, lighter, white minerals crystallized, and forming the lunar highlands. After that only large impactors came, creating the large impact basins as the crust weakened that way gave way to dark lava coming from the inner layers and giving birth to lunar maria. It is why the Apollo missions revolutionarized the whole view of the Moon and even the solar system. The Great Bombardment Period also had comets maybe bringing water unto the surface as that water is maybe still extant today into the large, polar craters of the Moon. Water also remained like a vapor halo around the nascent body after the collision and eventually was dispersed into the void or embedded into lunar rocks! Impactors slamming on the far side, on the other hand, maybe, as far as they were concerned, also triggered through their shock waves volcanic eruptions of the near side which added to lava flows and maria. A neat dichotomy is extant as far as the Moon geology is concerned, with the far side mostly highlands and craters and the near side with maria. Widespread maria on the nearside are due to basaltic volcanism as battered highlands crust dominates where maria are not present. The crust on the farside is thicker, likely making it more difficult for magmas to erupt on the surface, limiting the amount of farside mare basalts. Although there are just as many impact basins on the lunar far side as the near, the extensive lunar volcanism which provided for lava flows on the near side has been lacking on the far side of the Moon, with Mare Moscoviense one of the few maria found there. With both sides of Moon different in thickness in their crust and that they probably weren't always perfectly coordinated with one side toward us and the other, that has turned out for the Moon to be the lowest energy position, with lunar poles moved in order to get it into the correct position. The bulk of the lunar crust is made of anorthosite forming the bright material that you see when you look at the Moon as the dark areas are basaltic. Lunar geology can be roughly broken down into two categories – the anorthositic highlands, rich in calcium and aluminum, and the basaltic maria, which are abundant in iron and magnesium, the direct result of crystallization from lunar partially molten mantle. During post-mission analysis, scientists determined that the Apollo 14 rock samples collected near the Fra Mauro Crater in the area of Mare Cognitum, were found to be generally richer in aluminum and sometimes richer in potassium than other lunar basalts. The Apollo 14 basalts were formed 4 to 4.3 billion years ago, older than the volcanism observed at any of the other locations studied during the Apollo program. Lunar highlands however, on a other hand, may be less homogenous than previously thought, with more sodium hinting to variations in the chemistry and cooling rate of the magma ocean which formed the early lunar crust, or they could be the result of secondary processing of the early lunar crust. In several locations, astronomers have detected too highly silicic minerals such as quartz,potassium-rich, and sodium-rich feldspar - minerals that are only ever found in association with highly evolved lithologies, rocks that have undergone extensive magmatic processing that is. Such areas had previously be found with anomalously high abundances of the element thorium, another sign for those highly evolved lithologies. Multiple processes have produced these rocks. Lunar geologists are looking too for pristine lunar mantle material, rich in iron and magnesium, that could be exposed at some places, with no success so far. Moon seems rich in iron and titanium as titanium is found in a rock called ilmenite which is composed with iron, titanium and oxygen. Moon's surface generally, is mostly made of oxygen, silicon, magnesium, iron, calcium, aluminum and titanium. Zinc, tin, cadmium, indium, and thulium are in low concentrations in the lunar mantle because they might have never re-condensed after the giant Moon-forming impact, staying in a gas phase that was subsequently separated from the material that ultimately formed the Moon. Titanium-rich minerals further should hold better such particles of the solar wind like helium and hydrogen, which should allow missions back to the Moon to crack open those minerals to provide for ressource

click to a gravitational map of Moon. Far side lies in the center and nearside at either side. Reds correspond to mass excesses which create areas of higher local gravity, and blues correspond to mass deficits which create areas of lower local gravity. courtesy NASA/ARC/MIT

A gravity field map made to NASA's GRAIL mission in the 2010's reveals that Moon's gravity field preserves the record of impact bombardment that characterized all terrestrial planetary bodies and reveals evidence for fracturing of the interior extending to the deep crust and possibly the mantle. Data also show the Moon's gravity field is unlike that of any terrestrial planet in our solar system. Any notable change in the gravity field can be sync up with surface topography features such as craters, rilles or mountains. The bulk density of the moon's highland crust is substantially lower than generally assumed, which matches data obtained during the final Apollo lunar missions in the early 1970s, indicating that local samples returned by astronauts are indicative of global processes. The average thickness of the moon's crust is between 21 and 27 miles (34 and 43 kilometers). As the bulk composition of the Moon is similar to that of Earth, this supports models where the Moon is derived from Earth materials that were ejected during a giant impact event early. Data also revealed a population of long, linear gravity anomalies, with lengths of hundreds of miles, crisscrossing the surface. These linear gravity anomalies indicate the presence of dikes, or long, thin, vertical bodies of solidified magma in the subsurface, among the oldest features on the Moon

->Does the Earth Feature More Than one Moon?
In terms of whether the Earth has more than one moon, astronomers have computed that, at any given instant, due to varied gravitational interactions, there likely is always one NEO -or two, or none- in orbit around the Earth, with a diameter 3 feet and a distance some hundreds of thousands miles. Such very diminutive moons usually keeps orbiting during 10 months, or they do several hundreds years. Astronomers, in any case, lately found that Earth features a Trojan satellite, a 1,000-foot (300-meter) asteroid, called 2010 TK7, orbiting at a distance no closer Earth than 15 million miles (24 million km), a value true for the next 100 years. Trojan are bodies which lie stable at a Lagrangian point in front of or behind their parent-body's orbit. Scientists had predicted Earth should have Trojans, but they were difficult to find because relatively small and daylight objects as they appear close to the Sun as seen from the Earth. The find occurred because the object has an unusual orbit that takes it farther away from the Sun than what is typical for Trojans, or at about 90 degrees instead of the usual 60. Asteroid 2010 TK7 has an extreme orbit that takes it far above and below the plane of Earth's orbit, a motion referred to as a 'epicycle.' In addition, 2010 TK7 also moves within the plane of Earth's orbit in what is called a libration pattern, as it circles horizontally around its Lagrangian stable point every 395 years

The Moon's surface is thought to be covered almost everywhere by a layer of regolith. Regolith is a term meaning soil produced by weathering of local rock. Regolith is made from oxidized metals like iron oxide, silicon oxide and aluminum oxide. Magnetized rocks near the lunar surface do create small, localized spots of magnetic field that extend anywhere from hundreds of yards to hundreds of miles. Under such bubles, lunar regolith is shielded from the solar wind thus from chemical reactions which turn it darker. When the solar wind careens onto the Moon's surface it enriches the Moon's surface in ingredients that could make water through a chemistry of its protons and regolith. On the Moon weathering is mostly caused by impacts, both large and small. In fact the smaller impacts, micrometeorite impacts, produce most of the upper portion of the regolith. An abundance of super fine and unconsolidated grains are produced by the considerable number of micro-impacts that have occurred over the past 4 billion years. About 10 percent of Moon's regolith has been melted or vaporized by meteoroid impacts, making that 50 to 60 percent of lunar soil is actually glass. Some sand grains on the Moon, or the Earth, are spherical and ellipsoidal in shape and likely formed from molten droplets that cooled to form glass after a explosive volcanoe event. In addition to rocks breaking apart into finer and finer fractions, over time space weathering makes rocks darker and redder. This effect is especially noticeable around steep surfaces where unweathered material slides down and stands in high contrast with its surroundings. Brightness, or albedo, of lunar soils is diminished over hundreds of millions of years due to solar wind and micrometeorite bombardment as the phenomenon is termed 'space weathering.' Over the eons, solar storms with their energetic particles may have significantly altered Moon's soil in the polar, coldest craters through the process of subsurface sparking at a depth of about one millimeter, eventually ejecting vaporized material from the surface. Large solar storms penetrate the Moon's frigid, polar regions and electrically charge the soil. By the frigid, permanently shadowed regions near the lunar poles, or 'permanently shadowed regions' (PSRs), sparks due to energetic solar events could vaporize and melt the soil, perhaps as much as meteoroid impacts. Solar ions and electrons accumulate in two layers beneath lunar surface because they cannot reach the same depth due to their mass. A ions are positively, and electrons negatively, charges accumulate in both layers until explosively released like a miniature lightning strike. And, as regolith in the area is so cold that it is a extremely poor conductor of electricity, it cannot avoid the destructive effects of those. As seen as for at least a thousand years, flashes were reported on the surface of the Moon as recent science only turned to characterize them better, under the name of 'transient lunar phenomena.' 8 such flashes might occur each hour at the Moon, likely hinting to meteoroids or micro-meteoroids

As far as Moon's interior is concerned, astronomers since the Apollo era, came to the conclusion that our Moon had a core as they differed about its size, state and composition. Recentest use of seismic Apollo-era data through recentest methodologies in terrestrial seismology, have brought a renewed knowledge about our Moon's interior and states that our satellite possesses a solid, iron-rich inner core with a radius of nearly 150 miles and a fluid, primarily liquid-iron outer core with a radius of roughly 205 miles as a partially molten boundary layer around the core estimated to have a radius of nearly 300 miles is then conceding to Moon's thick, solid mantle. Lunar core's smallness may be explained should the collisional model holds as our satellite formed from few dense materials which came from the Earth's outer layers. Moon, despite its small size, could retain enough warmth as it is pressure, at the center, which renders the core solid. A small percentage of light elements such as sulfur exist in the core, echoing new research on Earth that suggests the presence of light elements such as sulfur and oxygen in a layer around our own core. By November 2011 two theories about why the Moon might have had a magnetic field at some point of its history have surfaced as until now astronomers could not see any fluid motion inside our satellite to provide for such a field hence the remains found by Apollo missions in lunar rocks. A thinking goes that the core -liquid iron at some moment- and its frontier to the mantle, for cause of a slightly different axis, should not be spherical and generate some friction ans stirring hence magnetism. Earth gravitational tug also plays a role as the angle narrowed or Moon widened from Earth gradually and bringing no friction anymore. In that case, lunar magnetic field would have lasted about 1 billion years somewhere between around 2.7 billion and 4.2 billion years ago. A other theory thinks that the lunar magnetic field might have been caused by impacts which brought a rotational differential between the mantle and core and stirring. That brought, in that case either continuous a magnetic field over several hundred million years or a intermittent one after large impacts. A more recent thinking goes that Moon's magnetic field was actually powered by two separate phenomena, long term and weak core crystallisation, and the 'precession,' caused when the Moon orbited closer to Earth, with its solid outer shell wobbling and stirring up the molten fluid in the lunar core. Precession was dominant until about 2.5 billion years ago, when it was succeeded by core crystallisation phenomenon, which in turn died by 1 billion years ago. Moon does have local areas where there are magnetic fields, probably caused by very large impacts, which are trapped in the crust of the Moon and yielding, for example, something we call swirls, which are these patterns of light and dark markings in sort of swirly patterns

click to a illustration of the new concept of lunar basin, with the Humboldtianum Basin. map site 'Amateur Astronomy'

Lunar maria are giant impact basins with filled with lava flows, which later solidified. A common misconception is that those lava flows were triggered by the impacts, but the impact basins are indeed a bit older than the lava flows. Lavas came along hundreds of millions of years after the basins formed. Apollo era analysis of the decay of radioactive isotopes in the rocks dated giant impacts like Mare Imbrium, for example, at between 3.8 to 3.9 billion years old. A example of lunar mare, Mare Tranquillitatis (approximately 543 miles (873 km) in diameter) lies in the Tranquillitatis basin, which is thought to have been formed as a result of a very large impact in the Moon's early history, likely more than 3.9 million years ago, at the time of the Great Bombardment which also hit the planets of the solar system. The crater was then flooded with mare basalts, making it smooth and relatively flat. The mare has an irregular margin because several basins, including Serenitatis and Nectaris, intersect in this region. Some volcanic activity might have also occurred at such sites like the Apollo 11 landing site with broad, thin flows of lava flooding the region. Most other large lunar basins are resulting from giant impacts. Impact craters larger than about 180 miles (300 kilometers) in diameter are referred to as 'basins.' Resulting craters have increasingly complex structures, often with multiple concentric, raised rings as any central peak might not exist. Large, dark areas of solidified lava that include the Sea of Tranquility and the Sea of Serenity might also be termed basins. Early theories suggested the outline of Oceanus Procellarum, or the Ocean of Storms, was caused by an asteroid impact. Oceanus Procellarum, generally, is however underlain at its border by a quasi-rectangular pattern which is the frozen remnants of lava-filled rifts and the underlying feeder dykes through which the mare volcanism let the magma up. Over time, the region would cool and contract, pulling away from its surroundings and creating fractures. The Procellarum region possesses high surface concentrations of the heat-producing elements uranium, thorium, and potassium. Mare Crisium is a Nectarian-aged basin (about 3.9 billion years old) spanning 460 miles (740 km) with the floor approximately 1.1 miles (1.8 km) below 'lunar datum' -which is 'sea level,' while the outer rim is about 2.1 miles (3.34 km) above lunar datum. Lava flow features are prominent enough in this mare. A recent concept in lunar geology is the one of the lunar basin, which are vast, circular, multi-walled areas which often contain a mare. Mare Humboldtianum thus is lying inside the northeastern limb, 2.8-mile (4.5-km) deep, 404-mile (650-km) in diameter Humboldtianum Basin. That basin is estimated to have formed during the Moon’s Nectarian Period, approximately 3.92-3.85 billion years ago as many other multi-ring impact basins are also believed to have formed during this time period, including Crisium Basin. In the inner ring of Humboldtianum Basin is Mare Humboldtianum(Humboldt’s Sea) with a relatively smooth, flat floor of the mare. The younger mare is believed to have formed during the Late Imbrian, approximately 3.8-3.6 billion years ago. Also visible in the basin are smaller craters that were partially filled in by the mare lava. Another example of lunar basin is the famed multi-ring basin Orientale, the youngest of the large lunar basins, is only partially flooded by later eruptions of mare basalt, as its internal structure is still visible. A study of the basin Orientale might help to learn more about basin formation and the mechanics of how basins develop their concentric rings. Mare Crisium is a Nectarian-aged basin (about 3.9 billion years old) spanning 460 miles (740 km) with the floor approximately 1.1 miles (1.8 km) below 'lunar datum' -which is 'sea level,' while the outer rim is about 2.1 miles (3.34 km) above lunar datum. Lava flow features are prominent enough in this mare. With a neat and simple structure, Mre Crisium is not part of such basins with concentric rings. The larger impact basin on the Moon is the South Pole-Aitken (SPA) basin. It's too the oldest one. It has a diameter of 1,550 miles (2,500 km) thus stretching across nearly a quarter of the moon as 5 miles (8 km) of depth, which makes it the deepest lunar basin which might have penetrated into the mantle and distributing material like ejecta. The SPA has a relatively low reflectance. Schrödinger basin is only about 3.8 billion years old, the Moon's second-youngest large basin with 200 miles (320 kilometers) in diameter) which is located near Moon's south pole. It features a peak-ring, a mountainous region of crust that rose up after a huge object, probably measuring 21-25 miles (35-40 kilometers), hit the Moon, the oldest rocks in the basin and the only material that wasn't melted by the heat from the object's impact. The melted material was spewed in all directions and formed the plains as cooling occurred at different times. Fractures came in the basin floor with that cooling. Schrödinger Basin is one of the few areas near the Moon's south pole with evidence of recent volcanic activity. This includes lava flows from volcanic activity on the surface as well as explosive eruptions from a vent brought up dark material which blanketed the plains. Older volcanic material is spread over a wider range. Lunar samples suggest that most of the major basins on the moon formed around 3.9 billion years ago in a period called the late heavy bombardment when asteroids and comets, leftovers from the solar system formation came to impact young planets. Famed Sinus Iridum is a mare-filled impact crater that superposes the Imbrium basin, North of Oceanus Procellarum. The gulf is made of basalt as wrinkle ridges cross the mare, and in places families of boulders are perched on the ridges. Many small irregular shaped craters dots Sinus Iridum

As it features no protective atmosphere, our Moon has been dotted by a total of 3 million craters over the eons. Famed types of lunar craters are, for example, Copernicus or Tycho. Bright-rayed Copernicus, in Oceanus Procellarum was formed 800 million years ago from a impactor. It is 58-mile (93-km) wide as the rim is reaching almost 1,000 ft (300 m) above lunar datum and the floor resting by some minus 5,600 ft (minus 1,700 m). Three central peaks are remains from the impact. Southern Tycho, with its 53 miles (85 km) in width and its bright radiating streaks was created 108 million years only. Tycho’s average depth below the rim is 15,700 ft (4,700 meters) as a central peak rises 8,000 ft (2,400 meters) above the crater floor. Tycho and the craters surrounding it are part of the lunar southern highlands. Aristarchus crater is located on the edge of the Aristarchus Plateau, one of the most geologically interesting regions of the Moon. It is a 25 miles (40 km) wide, 2 miles (3.5 km) deep, complex impact crater, which probably formed about 175 million years ago. The impact straddled the boundary of the plateau and the surrounding mare, thus excavating both very different rock types, as well as underlying crustal rocks. Valles Schröteri is also located upon the Aristarchus Plateau. There are two types of impact craters on the Moon: primary and secondary. 'Primary craters' form as the result of the original asteroid or comet impacting the Moon. 'Secondary impact craters' form from the impact of ejecta expelled during the impact. With a impact, a ejecta curtain at first forms immediately. During a impact, the energy transferred to the ground goes into melting and vapourising the impactor and parts of the surface and excavating and ejecting vast amounts of material from the ground. Crater central peaks are formed, should a crater impact be large enough, as the Moon's crust rebounds after the tremendous stress of an impact is released. Some craters lack a central peak as they feature a peak ring instead, a ring circling around the crater's center. Such a ring originates from uplifted crustal rocks within minutes of impact. The energies of impacts are so high that rocks no longer behave as brittle solids, but rather as deformable plastic. As the crater forms, the bottom of the crater is first pressed down, then it rebounds. For craters above 12 miles (20 km) in diameter, the rebound is so strong that material from depth is actually brought up and forms a central peak. Impact melt is thrown straight up during the impact and then comes back down, creating mountains almost instantaneously. In not so large craters, over time micrometeorites and other small impactors will grind and erode these steep slopes into smooth mountains. Surface material is scattered farthest, while the deepest material, which usually comes up in big chunks, remains closest to the crater rim. A abundance of large rocks near a crater hint to the crater's age. A crater depth of excavation is roughly one tenth its diameter. violently. In low-energy impacts, a simple bowl-shaped crater results. Some impact craters may come bouldery, like the result of bolides impacting solid material with more cohesive materials producing larger boulders when they are impacted. Impacts at most angles produce circular craters, impacts with incidence angles under 15 degree from the horizontal may create elliptical craters. When regolith only is excavated that means that a impactor could not reach deep under the regolith, into the the underlying mare basalt. Usually, ejecta material on the rim comes from the deepest part of the crater, and ejecta farther away from the crater comes from shallower depths. When an impact ejecta blanket is not uniform, the ejecta is defined as asymmetric. Craters with asymmetric ejecta are either caused by pre-impact differences in composition, unusual topography, or a oblique or low angle of impact. Asteroids hit the Moon at fantastically high speeds, greater than 35,000 miles per hour (53,000 km/h), and most of the craters left by these impacts are circular. However, the shape of the crater (or the distribution of the ejecta blanket) changes when the the angle between the asteroid path and the surface becomes small, 15° or less. In such a low angle impact, the ejecta has more momentum in the direction of travel of the impactor, which causes the asymmetric ray patterns as butterfly or dragonfly-shaped ejecta is very common for craters formed at those low impact angles. In a case for example when the extent of ejecta extends to the North and South and a lack of ejecta to the East, that probably indicates that the impactor probably came from the east. Another example of an oblique impact is Messier Crater, where the shape of the crater is elliptical or oblong due to the angle of impact. A 'butterfly' pattern in the ejecta may also indicate a oblique impact. The ejecta rays generally are high reflectance relative to the surrounding terrain. In most cases, a well-defined, high reflectance ray pattern suggests the relative youth of an impact crater. However, these rays may be compositional if the crater excavated material from beneath the surface layers. Large impacts are catastrophic events for the local area. Besides the huge craters they leave behind, impacts may heat portions of the crust to such high temperatures that rocks melt and flow like lava. These melts run downhill, cool and solidify, leaving behind flow features. When material is excavated during impact, much of that material is deposited very close to the crater - generally within 1 crater diameter. This material is called the continuous ejecta blanket. However, most impacts are so energetic that ejecta is deposited also much farther away. This far-reaching ejecta is part of the discontinuous ejecta blanket and is responsible for forming secondary crater chains, clusters, and rays. The erosive energy of secondary ejecta is quite large with wall and rim of existing craters deep grooved for example. Post-impact modification occurs at a crater once formed, like slumping, fracturing or downslope movement of material from the rim, increasing the size of a crater. Recent studies have shown that the older highland impactor population can be clearly distinguished from the younger population in the lunar 'maria' with the highlands having a greater density of large impactor. That implies that the earlier population of impactors had a proportionally greater number of large fragments than the population that characterized later lunar history. The transition occurred about the time of the Orientale impact basin, about 3.8 billion years ago, which is, on a other hand, the period too of the end of the Heavy Bombardment Period. After a impact melt during impact the floor of a crater may cool differentially across the crater floor such that some areas appear smooth while others are hummocky. Ejecta rays of some lunar craters are extensive, high-reflectance ejecta rays of some lunar craters, like Copernicus or Tycho that extend across nearby mare and overlap ejecta from other craters are a other occurrence of some large craters at the Moon. Polar craters of the Moon have been found recently with slopes of 36 degrees over several thousands of yards, which could cause landslides. Younger craters typically hold lots of boulders spreading out of the cavity as older ones have smoother surface or overall subdued or degraded shape. Huge numbers of small impacts over time are believed to be the main factor breaking boulders into small pieces and smoothing the surface. The process, over a duration of a billion of years or two eventually leads to the disparition of small-sized craters. On older Lunar surface, each new small-size crater appearing, erases on average, one older crater of comparable size, a state known as 'equilibrium'

Landslides are a common form of mass wasting on both the Moon and on Earth. This process exposes fresh material with, generally a higher albedo. Straight gullies on a other hand, are seen along craters' walls and are flows of dry material

click to a illustration of a typical lunar landscape as seen from the surface. This view was taken in the Plain of Descartes, a region in lunar highlands during the Apollo 16 mission and formed of 27 separate frames NASA. picture site 'Amateur Astronomy' based on a picture NASA

The geological eras of the Moon are the following, as established after the Apollo program.

Pre-Nectarian		4.55-3.92 billion years ago
Nectarian		3.92-3.85 billion years ago
Early Imbrian		3.85-3.80 billion years ago
Late Imbrian		3.80-3.20 billion years ago
Eratosthenian		3.20-11.0 billion years ago
Copernician		11.0 billion years ago-now

Before the Apollo program results, the geology of the Moon comprised 4 geological eras only with the Pre-Imbrian (4.55-3.85 billion years ago), the Imbrian (3.85-3.20 billion years ago), the Eratosthenian (3.20-11.0 billion years ago) and the Copernician (11.0 billion years ago-now)

samples of linear and sinuous lunar rilles. A small part of Rupes Recta is featured center. picture site 'Amateur Astronomy'

Linear rilles are surface manifestations of structural faulting, or tectonic stress, that formed when the lunar crust was pulled apart. The widths of these linear rilles range from as little as a few yards (meters) to kilometers (miles or less) across; Rupes Recta for example is between 0.6-1.9 mile (1-3 km) wide across its length. In addition, Rupes Recta is composed of several echelon segments -the linear rille is not a single, uninterrupted 62 miles (100 km) length fault! There are at least 5 large fault segments that range from about 5 to 30 miles (8 km to 50 km) in length. Sinuous rilles on the Moon like Vallis Schröteri, Rimae Posidonius, or Rimae Prinz, as far as they are concerned are believed to primarily result from lunar volcanism. Thrust faults, or lobate scarps similar to those at Mercury, and which are known on the Moon since the Apollo program era, are a hint to that cooling of Moon's interior is causing a contraction of the lunar sphere. Such a process is pushing such fault structures, obliquely from the lunar underground upwards as the recent NASA LRO orbiter saw them globally distributed. Those scarps are no more than 6.2-mile (10-km) wide and only tens of yards or meters high. They are likely very young, geologically speaking as Moon shrinked by some 328 ft (100 m) over the course of 1 billion years, which should be the age of them. Earth’s gravity added to the process and influenced the pattern of orientations of fault scarps. Moon, in terms of those scarps even could still be geologically active today with Moon enduring moonquakes and still shrinking. Fault shrinking also is seen with narrow trenches typically much longer than they are wide, or linear valleys, known as graben. Such graben are showing that the contractional forces which shrank the Moon globally because of the cooling of a still hot interior, likely were not large as such small graben might have never formed. That shrinking of Moon with time causes Moonshakes and fault areas. The Moon's tectonic action is especially visible in Mare Frigoris. A weak contraction suggests that the Moon, unlike the terrestrial planets, did not completely melt in the very early stages of its evolution. Rather, observations support an alternative view that only the Moon's exterior initially melted forming an ocean of molten rock. By 1998 the Lunar Prospector spacecraft spotted a strong peak in the gamma-ray spectrum at a terrain between the craters Compton and Belkovich, a thorium hot spot as much of Moon’s thorium lies mostly on the nearside. Most detailed images in 2011 have since revealed numerous volcanic features, some large, and some small, examples of silicic (rich in silica relative to basalt) volcanism. Hence such volcanoes that litter the Moon's far side are of the rare silicate types and not of the basaltic volcanoes seen on the near side. Moon’s volcanic activity slowed gradually instead of stopping abruptly a billion years ago, with rock deposits less than 100 million years old, and called 'mare patches,' a result of late volcanism at Moon. Such patches are less than a third of a mile (500 meters) across. The lunar mantle thus had to remain hot enough to provide magma for those small-volume eruptions, which might be a important contributor to the recent geological history of our Moon

Like a example of the Moon's geology complexity, many fractures on the Moon are seen in the floors of ancient, flat-floored highlands craters. Such fracture networks often encircle all or part of the crater floor, and in some areas they show accumulated deposits of dark volcanic material. The wall of such a fracture, in the northeastern floor of Alphonsus crater, for example, has been seen mantled by a dark, fine-grained pyroclastic deposit that appears to have moved down the wall of the fracture toward the floor as the wall of the fracture is composed of light-colored rocks that are typical of the lunar highlands (mostly composed of anorthosite). Rocks and boulders of this bright material have also moved down the fracture wall altogether. It is likely that the dark volcanic material came from a nearby volcanic vent located along this fracture network. Pyroclastic deposits such as those observed in Alphonsus crater are formed by violently explosive eruptions of basaltic magma and may have formed in conjunction with massive outpourings of surface lava flows to the west in nearby Mare Nubium. The mare deposits in Mare Nubium are ancient, about 3.2 to 3.5 billion years old. If Alphonsus pyroclastic deposits and Mare Nubium were indeed related, then it is likely that the Alphonsus pyroclastic deposits are about the same age. The Alphonsus pyroclastic deposits are sometimes associated with low cones that have symmetric dark 'haloes'; these cones resemble cinder cones or small volcanoes on Earth. hinting to the presence of volcanoes, thus, in the floor of Alphonsus. In part because of these fascinating volcanoes in the floor of Alphonsus, this area was considered as a possible landing site for the Apollo 16 and Apollo 17 missions. The Ranger 9 spacecraft impacted in Alphonsus to the northeast of the central peak. Scientific interest in this crater remains high, and so Alphonsus is a high-priority target for future expeditions to the Moon. Lunar swirls are odd markings of light and dark on the surface of the Moon as they can be tens of miles across and appear in groups or just as an isolated feature. The bright areas in the swirls appear to be less weathered than their surroundings. They appear where ancient bits of magnetic field are embedded in the lunar crust (although not every 'fossil' magnetic field on the Moon has a lunar swirl). Several phenomena can cause material exposed to space to change both physically and chemically as three prominent theories exist about their formation: swirls and magnetic fields formed from plumes of material ejected by comet impacts; Moon’s fine dust particles lofted by micrometeoroid bombardment are sorted by a existing magnetic field over the swirls, forming light and dark patterns; or particles in solar wind respond to magnetic forces and magnetic field shields the surface from weathering by the latter

The floor of Alphonsus Crater (left) and a detailed view of the fracture (right; the fracture rim is to the upper right as the bottom towards the lower left. picture NASA/GSFC/Arizona State University

Some recent studies of rock samples collected by the Apollo missions through a advanced spectrography method allowing to create a image of the minerals beyond determining its composition, have revealed that carbon is extant at the Moon as until now it was thought the result from the solar wind. Organic elements have been found in small quantities by 2015 inside lunar samples. Carbon in fact survived from around 3.8 billion years ago, when the moon was heavily bombarded by meteorites. Carbon either came from the object that impacted or it condensed from the carbon-rich gas that was released during impact. Between 1969 and 1972 six Apollo missions brought back 842 pounds (382 kg) of lunar rocks, core samples, pebbles, sand and dust from the lunar surface. The six space flights returned 2,200 separate samples from six different exploration sites on the Moon. NASA is keeping its collection of rocks at Johnson Space Center in Houston and a facility in New Mexico. 140,000 subsamples subsequently were created from the original rock samples as black market originating from rocks given to nations worldwide at the time of the Apollo program or from owners which had been gifted by NASA is a problem nowadays. NASA also made the decision to keep some lunar samples completely untouched as an investment in the future, allowing them to be analyzed with more advanced technologies as they are developed. As far as colors on the Moon are concerned, they are dominantly controlled by variations in iron and titanium content. The mare regions have low reflectance because they contain relatively high amounts of iron oxide (FeO). Some mare basalts contain unusually high amounts of titanium oxide (TiO2) in addition to iron oxide, making for even lower reflectance. TiO2 also shifts the color of the mare from red to blue. That can even yields difference in colors between two mare, like, for example, Mare Tranquillitatis being bluer compared to a more brownish Mare Serenitatis. Grain size and physical state of the surface material also effect the reflectance (or albedo). The LCROSS impact plume in 2009 revealed that there were several volatile species detected in the Cabeus crater cold-trapping region near the southern pole, including mercury atoms and hydrogen (H2) molecules. Mercury atoms would generally be hopping across the surface and eventually migrating toward the colder polar regions. Volatile materials generally can get transported from warm to cold areas on the Moon. During the 2013 GRAIL craft's impacts, mercury and enhancements of atomic hydrogen were seen in the plume. It was a surprise to find mercury in a lit area as it is likely a product yielded during the Moon's formation, which is reasonably concentrated near the surface despite exposed to the space environement like micro-meteorids or the solar wind

Billions of years ago, the Moon might have had a atmosphere, a one thicker than the atmosphere of Mars today and was likely capable of weathering rocks and producing windstorms, and maybe the source of some, if not all, of the water detected on the Moon. Such a atmosphere was released by lava eruptions releasing volatiles. That atmosphere was short-lived, at about 70 million years and made primarily of carbon monoxide, sulfur and water. Lava eruptions also released most of water still found at Moon nowadays. Moon today has no atmosphere, but a thin exosphere instead with about none in terms of gas atoms or molecules. Argon is released by daytime from the lunar surface. Lunar vacuum is voider still than that typically used for experiments in laboratories on Earth. Physical processes such as meteoroid stream impacts, the bombardment of helium and hydrogen particles from the Sun, thermal absorption, and space weathering constantly modify Moon’s surface as they work within the tenuous, one 25-trillionth the density of Earth’s, lunar exosphere. Increase in exospheric gases occurs when the rain of meteoroid impacts increases during a stream. Interplanetary grains can hit the lunar surface at speeds exceeding 21 miles (34 kilometers) per second, releasing immense heat, and vaporizing part of the soil and meteoroids themselves. NASA's Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft in 2015 has confirmed the existence of the gas neon in the lunar exosphere. Noble gas Argon-40 is also found in the lunar exosphere since a altitude of around 62 miles, and a essential constituent of it. Helium seen into Moon's 'atmosphere' since the Apollo program experiments might originate from radioactive decay inside our satellite or from an exterior source, such as the solar wind. Helium from the inside could be released during lunar quakes. Helium abundances have been seen increasing when night progresses which might be due to atmospheric cooling, which concentrates atoms at lower altitude. Argon is also present in the lunar atmosphere. Static electricity is known to play an important role on the airless, dusty Moon as that could also on many different bodies, including asteroids and comets, with surface continuously bombarded by ultraviolet light from the Sun and exposed to a rain of charged particles as some of those bodies are found within the bubble of a magnetosphere. On small objects with low gravity, dust grains might even be able to overcome the force of gravity and escape into space. Moon is engulfed in a permanent, but lopsided, dust cloud that increases in density when annual events like the Geminids meteor shower spew shooting stars. The cloud however is primarily made up of tiny dust grains kicked up from the surface by the impact of high-speed, interplanetary dust particles. The first hints of a cloud of dust around the Moon came in the late 1960s when NASA cameras aboard unmanned moon landers captured a bright glow during lunar sunsets. Several years later, Apollo astronauts orbiting the moon reported a significant glow above the lunar surface when approaching sunrise. A pending question about the lunar atmosphere is the one of the LHG, faint rays appearing at up to 62 miles (100 km) or low (barely a yard -1m) above the Moon's terminator. Such scattered light might be due to heavy (low) or small (high) particles of Moon's dust kicked upwards due a positive electrical charge acquired when solar ultraviolet radiation hit those and eject negative-charged electrons. As dust is following a ballistic path and eventually fall back to the surface. Only a handful of LHG observations have been made however and little is known about them. On the other hand, the solar-wind particles are producting varied interactions with the Moon's minerals as they strike the surface. 10 percent of the solar-wind particles striking the Moon escape to space as ENAs which amounts to roughly 150 tons of recycled hydrogen atoms per year. Several transient gases in the atmosphere also could account for the phenomenon as, should soil particles be kicked off they likely would not fall back to the surface as many flat rocky surfaces, as seen by astronauts of the Apollo program, should, at the contrary, receive some Moon dust, which is not the case. Recent studies in 2013 have summarized that dust at Moon may escape to space or bounce long distances due to varied interactions with micro-meteorids, for example, rough surfaces or the solar wind. Leaping lunar dust, on a other hand, is also seen near shadowed craters as it is lofted above the surface in a sunlit area and jump over the shadowed region, bouncing back and forth between sunlit areas on opposite sides, or locally trapped over a shaded region of 1 to 10 yards (meters) in size. This effect should be especially prominent during dusk and dawn at the terminator when light constrat is maximum and with long shadows. Lunar dust usually is a indicator of unusual surface electric fields with shaded regions negatively charged compared to sunlit ones yielding a locally complex, electric field, or a dipole field, over the shaded region. Those fields likely account for views of dust swarm or canopies or 'twilight' by Surveyor spacecraft. Perturbations due to local conditions like rim height, roughness, or interference by the solar wind, cause the dust to eventually either fall into the crater or be launched away. Each 29 days around Full Moon, our Moon is crossing the Earth magnetic tail. As far as water on the Moon is concerned, as its importance might appear like a refueling source for space missions, large amounts of it have been found conserved in the darker areas of large polar craters and likely brought there by comets and asteroids slamming into the Moon at the time of the Great Bombardment Period, 4 billion years ago. Intense asteroid bombardments in its youth, coupled with its weak gravity and the Sun's powerful radiation left the Moon first with almost no atmosphere hence no water. However scientists had come to theorize that deep craters at the lunar poles would be in permanent shadow and thus extremely cold, and able to trap volatile material like water or ice if such material were somehow transported there, perhaps by comet impacts or chemical reactions with hydrogen, a major component of the solar wind. Other studies have shown that water exists too, in a quantity global of over the amount of water in the Great Lakes, USA, all over the Moon in the dirt ou subsurface rocks. That water likely was produced when the lunar magma ocean that is thought to have formed at some point during the compacting process of the rocks which agglomerated to form Moon, began to cool. During this cooling, water either escaped into space or was preserved as hydroxyl molecules in the crystallizing minerals. Beyond water ice, further studies are showing that metallic elements too were released by the impact, like mercury, magnesium, calcium, or sodium and even a bit of silver. About 2.2 to 4.4 pounds (1-2 kilograms) of sodium were released or one to two percent of the amount of water released as Earth's oceans have a comparable sodium to water ratio. Sodium might have been brought to Moon by comets and sodium atoms bouncing across the lunar surface until cold trapped in the polar crater or the solar wind carries small amounts of sodium, which could become embedded in the lunar surface, as the solar wind also might also liberate sodium from lunar rocks, which are about 0.4 percent sodium. Sodium is also liberated from lunar rocks by meteoroid impacts. Two percent sodium to water is consistent with the amount of sodium in comets

As far as water on Moon is concerned, recent observations by India's Chandrayaan-1, NASA's Cassini mission or the EPOXI-extended Deep Impact, have clearly detected the spectral signal of water or, the hydroxyl group (a bond of oxygen and hydrogen) on the Moon! The signal was found stronger near the poles, or in the morning, and lowest by noon, or low in the lunar maria. Those traces amounts of water or water-like elements on the Moon, likely are chemicals trapped in the materials at the surface and they likely are originating from the interaction of the solar wind with moon soil. As that is about 45 percent oxygen combined with silicate minerals, the protons of the solar wind, which are positively charged hydrogen atoms are breaking these oxygen bounds, bringing to that, with free oxygen, and hydrogen roaming in Moon's soil, that combines back eventually into water or hydroxil. A strongest amount of water traces at the poles are due to a migration allowed by the daily temperature changes at Moon, as the water traces eventually get to the poles where they accumulate in the permanently shadowed ares of the large craters. Since 2009 the LRO-LCROSS mission impact really brought the evidence that there is water ice, in large amounts, in the southern polar regions of the Moon! check more from our page dedicated to the LCROSS discovery. These deposits may be slightly more abundant on crater slopes in the southern hemisphere that face the lunar South Pole. The hydrogen-bearing material is volatile and may be in the form of water molecules or hydroxyl molecules (a oxygen atom bound to a hydrogen one) that are loosely bound to the lunar surface. Such ice water would be available for manned return to the Moon! Should such an explanation be reliable, one would have to find there too remains of the water which had been brought unto the Moon by comets, by the beginnings of the solar system, so that one could deal with quantities -instead of amounts- of water! Lunar sample 74220, the famous high-titanium 'orange glass soil' of volcanic origin collected during the Apollo 17 mission, by 2011 has provided the evidence for water content of the inclusions, which were formed during explosive eruptions on the Moon approximately 3.7 billion years ago. In contrast to most volcanic deposits, the lunar melt inclusions are encased in crystals that prevent the escape of water and other volatiles during eruption. Such a process not only hint to amount of water contained in lunar magma -and 100 times higher than previously thought- as it also might hint to that water found a lunar poles might also come from lunar magma (a other view is that locations of polar ice at both lunar poles are antipodal hinting to that poles were there in the past as the 3.5-billion-year-old hot spot that created Oceanus Procellarum might have also caused the poles' shift). The results might also raise questions about aspects of the impact theory of how the Moon was created as it predicted very low water content of lunar rock due to catastrophic degassing during the collision. Shackleton crater, on a other hand, one of the polar craters were water is extant, has its walls bright a evidence for ice on both the crater's floor and walls as the water at Moon, generally, is about 3 feet (one meter) below the surface. The icy deposits on our Moon likely are mixed in with the regolith, yielding a surface frost. Water ice can persist for millions or billions of years. Any meteoroids which can penetrate as deep as 3 inches (8 centimeters) into the lunar regolith can vaporize water out of it, which can participate in turn to the feeble lunar atmosphere -- in a proportion of two thirds. Such shocks might also be at the origine of water at Moon's poles. South Pole's frozen water may date back billions of years and has been untainted by the Sun's radiation or the geological processes that otherwise constantly churn and renew planetary surfaces. Lunar water is either H20 or OH (hydroxyl, a more reactive relative of it)

the Moon's apparent diameter today and in 1 to 2 billion years from now (left); a total Sun eclipse today and a eclipse in 1 to 2 billion years (right)

On the other hand, at the same time, both Earth and Moon were much closer, and were each rotating more rapidly than today. Moon was certainly about between 12,000-29,000 miles (19,000-47,000 km) away from Earth only, yielding a much swifter orbit than today (about 4.19 hours), as Earth was rotating a few hours less than today, yielding a length of day at the early Earth shorter by the same amount. Dinosaurs, 70 million years ago were still having a day shorter than today by 2 hours. Along the eons, both bodies continued to interact. As Moon is orbiting Earth, it makes appear a tidal bulge at Earth -the tides as well as a real crustal bulge. As the Earth is rotating, this bulge in turn exerts a gravitational effect on Moon, accelerating it on its orbit as it is very slowly acting on the length of the day. Actually this translates into the Moon getting further from Earth, hence slowing its orbital period as the Earth's day is slowly getting longer. Moon is getting distant by 1.49" (3.8 cm) per year as Earth's day is slowing by 0.002 seconds per century. This will eventually lead to that Earth and Moon will get tidally-locked at 47 days. Moon will orbit in 47 days, as Earth's day will be of 47 days too! This should not take place before several tens of billions of years however. At the present Earth's rotation rate, it takes 3 million years to get the day 1-minute longer. At the end of the evolution, Moon will be 348,000 miles (560,000 km) away from the Earth, instead of 239,000 (385,000 km) today. A by-side effect of this evolution is that solar eclipses, at one point, will cease to exist. This should occur definitively about 1 to 2 billion years from now. The process will occur gradually as total eclipses will slowly turn into hybrid, then annular only. The last total solar eclipse on Earth will be seen for the last time 600 millions years from now. At the end of the evolution, there will be annular eclipses only, with the eclipsed Sun visible by 4.8' each side of the occulting Moon, and partial eclipses. The decrease rate of total eclipse is rated this way: an incremental decrease of the apparent diameter of the Moon of 1.8" brings an average of one eclipse per century to cease being total to become hybrid. An additional view is that Moon will eventually be torn apart as the Sun, becoming a red giant since 5 billion years from now, will push our moon back toward Earth to a destructive, eventual close encounter. In terms of gravity, Moon holds 'mascons' (short for 'mass concentrations'), which are regions of higher mass and gravity, found with Moon's large impact basins, as they might be linked with dense, solidified basaltic lavas. Such gravity anomalies make that the low-lunar orbits are unstable. A mission in the 2010's confirmed that lunar mascons were generated when large asteroids or comets impacted the ancient Moon, when its interior was much hotter than it is now, as Moon's light crust and dense mantle combined with the shock of a large impact to create the distinctive pattern of density anomalies. The origin of lunar mascons had been a mystery in planetary science since their discovery in 1968 by a team at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif. But it was not clear until now how much of the unseen excess mass resulted from lava filling the crater or iron-rich mantle upwelling to the crust. With a target pattern a mascon on a lunar map displays a bulls-eye with a gravity surplus, surrounded with a gravity deficit. A ring with a gravity surplus surrounds the bulls-eye and the inner ring. This pattern arises as a natural consequence of crater excavation, collapse and cooling following an impact. The increase in density and gravitational pull at a mascon's bulls-eye is caused by lunar material melted from the heat of a long-ago asteroid impact. Over millions of years Moon's subtle adjustment to any large impact might eventually create a gravitational 'high spot' like in great basins. As far as the gravitational relation between the Moon and Earth (as the Sun also intervenes), generally, the mutual pulling of the two bodies is powerful enough to stretch them both, so they wind up shaped a little like two eggs with their ends pointing toward one another. On Earth, the tension has an especially strong effect on the oceans, because water moves so freely, and is the driving force behind tides with differences in altitudes up to 2 feet. Earth’s distorting effect on the moon, called the lunar body tide, or a solid tide, is more difficult to detect, because the Moon is solid except for its small core. Even so, there is enough force to raise a bulge about 20 inches (51 centimeters) high on the near side of the moon and similar one on the far side