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Wednesday, June 12, 2024

Moai

From Wikipedia, the free encyclopedia
Moai facing inland at Ahu Tongariki, restored by Chilean archaeologist Claudio Cristino in the 1990s

Moai or moʻai (/ˈm./ MOH-eye; Spanish: moái; Rapa Nui: moʻai, lit.'statue') are monolithic human figures carved by the Rapa Nui people on Rapa Nui (Easter Island) in eastern Polynesia between the years 1250 and 1500. Nearly half are still at Rano Raraku, the main moai quarry, but hundreds were transported from there and set on stone platforms called ahu around the island's perimeter. Almost all moai have overly large heads, which account for three-eighths of the size of the whole statue. They also have no legs. The moai are chiefly the living faces (aringa ora) of deified ancestors (aringa ora ata tepuna).

The statues still gazed inland across their clan lands when Europeans first visited the island in 1722, but all of them had fallen by the latter part of the 19th century. The moai were toppled in the late 18th and early 19th centuries, possibly as a result of European contact or internecine tribal wars.

The production and transportation of the more than 900 statues is considered a remarkable creative and physical feat. The tallest moai erected, called Paro, was almost 10 metres (33 ft) high and weighed 82 tonnes (81 long tons; 90 short tons). The heaviest moai erected was a shorter but squatter moai at Ahu Tongariki, weighing 86 tonnes (85 long tons; 95 short tons). One unfinished sculpture, if completed, would be approximately 21 m (69 ft) tall, with a weight of about 145–165 tonnes (143–162 long tons; 160–182 short tons). Statues are still being discovered as of 2023.

Description

Moai set in the hillside at Rano Raraku

The moai are monolithic statues, and their minimalist style reflects forms found throughout Polynesia. Moai are carved from volcanic tuff (solidified ash). The human figures would be outlined in the rock wall first, then chipped away until only the image was left. The over-large heads (a three-to-five ratio between the head and the trunk, a sculptural trait consistent with the Polynesian belief in the sanctity of the chiefly head) have heavy brows and elongated noses with a distinctive fish-hook-shaped curl of the nostrils. The lips protrude in a thin pout. Like the nose, the ears are elongated and oblong in form. The jaw lines stand out against the truncated neck. The torsos are heavy, sometimes, the clavicles are subtly outlined in stone too. The arms are carved in bas relief and rest against the body in various positions, hands and long slender fingers resting along the crests of the hips, meeting at the hami (loincloth), with the thumbs sometimes pointing towards the navel. Generally, the anatomical features of the backs are not detailed, but sometimes bear a ring and girdle motif on the buttocks and lower back. Except for one kneeling moai, the statues do not have clearly visible legs.

Moʻai quarry at Rano Raraku

Though moai are whole-body statues, they are often referred to as "Easter Island heads" in some popular literature. This is partly because of the disproportionate size of most moai heads, and partly because many of the images for the island showing upright moai are of the statues on the slopes of Rano Raraku, many of which are buried to their shoulders. Some of the "heads" at Rano Raraku have been excavated and their bodies seen, and observed to have markings that had been protected from erosion by their burial. 

The average height of the moai is about 4 m (13 ft), with the average width at the base around 1.6 m (5.2 ft). These massive creations usually weigh around 12.5 tonnes (13.8 tons) each.

All but 53 of the more than 900 moai known to date were carved from tuff (a compressed volcanic ash) from Rano Raraku, where 394 moai in varying states of completion are still visible today. There are also 13 moai carved from basalt, 22 from trachyte and 17 from fragile red scoria. At the end of carving, the builders would rub the statue with pumice.

Characteristics

Re-erected tuff moai at Ahu Tahai with restored pukao and replica eyes

Easter Island statues are known for their large, broad noses and big chins, along with rectangle-shaped ears and deep eye slits. Their bodies are normally squatting, with their arms resting in different positions and are without legs. The majority of the ahu are found along the coast and face inland towards the community. There are some inland ahu such as Ahu Akivi. These moai face the community but given the small size of the island, also appear to face the coast.

Eyes

In 1979, Sergio Rapu Haoa and a team of archaeologists discovered that the hemispherical or deep elliptical eye sockets were designed to hold coral eyes with either black obsidian or red scoria pupils. The discovery was made by collecting and reassembling broken fragments of white coral that were found at the various sites. Subsequently, previously uncategorized finds in the Easter Island museum were re-examined and recategorized as eye fragments. It is thought that the moai with carved eye sockets were probably allocated to the ahu and ceremonial sites, suggesting that a selective Rapa Nui hierarchy was attributed to the moai design until its demise with the advent of the religion revolving around the tangata manu.

Symbolism

Many archaeologists suggest that "[the] statues were thus symbols of authority and power, both religious and political. But they were not only symbols. To the people who erected and used them, they were actual repositories of sacred spirit. Carved stone and wooden objects in ancient Polynesian religions, when properly fashioned and ritually prepared, were believed to be charged by a magical spiritual essence called mana."

Archaeologists believe that the statues were a representation of the ancient Polynesians' ancestors. The moai statues face away from the ocean and towards the villages as if to watch over the people. The exception is the seven Ahu Akivi which face out to sea to help travelers find the island. There is a legend that says there were seven men who waited for their king to arrive. A study in 2019 concluded that ancient people believed that quarrying of the moai might be related to improving soil fertility and thereby critical food supplies.

Pukao topknots and headdresses

The more recent moai had pukao on their heads, which represent the topknot of the chieftains. According to local tradition, the mana was preserved in the hair. The pukao were carved out of red scoria, a very light rock from a quarry at Puna Pau. Red itself is considered a sacred color in Polynesia. The added pukao suggest a further status to the moai.

Markings

Petroglyphs on the back of an excavated moai.

When first carved, the surface of the moai was polished smooth by rubbing with pumice. However, the easily worked tuff from which most moai were carved is easily eroded, such that the best place to see the surface detail is on the few moai carved from basalt or in photographs and other archaeological records of moai surfaces protected by burials.

Those moai that are less eroded typically have designs carved on their backs and posteriors. The Routledge expedition of 1914 established a cultural link between these designs and the island's traditional tattooing, which had been repressed by missionaries a half-century earlier. Until modern DNA analysis of the islanders and their ancestors, this was key scientific evidence that the moai had been carved by the Rapa Nui and not by a separate group from South America.

At least some of the moai were painted. One moai in the collection of the Metropolitan Museum of Art was decorated with a reddish pigment. Hoa Hakananai'a was decorated with maroon and white paint until 1868, when it was removed from the island. It is now housed in the British Museum, London, but demands have been made for its return to Rapa Nui.

History

Map of Easter Island using moai to show locations of various ahu

The statues were carved by the Polynesian colonizers of the island, mostly between 1250 and 1500. In addition to representing deceased ancestors, the moai, once they were erected on ahu, may also have been regarded as the embodiment of powerful living or former chiefs and important lineage status symbols. Each moai presented a status: "The larger the statue placed upon an ahu, the more mana the chief who commissioned it had." The competition for grandest statue was ever prevalent in the culture of the Easter Islanders. The proof stems from the varying sizes of moai.

Completed statues were moved to ahu mostly on the coast, then erected, sometimes with pukao, red stone cylinders, on their heads. Moai must have been very time-consuming to craft and transport; not only would the actual carving of each statue require effort and resources, but the finished product was then hauled to its final location and erected.

An incomplete moai in the quarry at Rano Raraku

The quarries in Rano Raraku appear to have been abandoned abruptly, with a litter of stone tools and many completed moai outside the quarry awaiting transport and almost as many incomplete statues still in situ as were installed on ahu. In the nineteenth century, this led to conjecture that the island was the remnant of a sunken continent and that most completed moai were under the sea. That idea has long been debunked, and now it is generally believed that:

  • Some statues were rock carvings and never intended to be completed.
  • Some were incomplete because, when inclusions were encountered, the carvers would abandon a partial statue and start a new one. Tuff is a soft rock with occasional lumps of much harder rock included in it.
  • Some completed statues at Rano Raraku were placed there permanently and not parked temporarily awaiting removal.
  • Some were indeed incomplete when the statue-building era came to an end.

Craftsmen

It is not known exactly which group in the communities were responsible for carving statues. Oral traditions suggest that the moai were carved either by a distinguished class of professional carvers who were comparable in status to high-ranking members of other Polynesian craft guilds, or, alternatively, by members of each clan. The oral histories show that the Rano Raraku quarry was subdivided into different territories for each clan.

Transportation

Since the island was largely treeless by the time the Europeans first visited, the movement of the statues was a mystery for a long time; pollen analysis has now established that the island was almost totally forested until 1200 CE. It is well known that around the years 800 and 1200, tree felling was done en masse, this being discovered thanks to the study of pollen fossil and carbon footprints.

Ahu Akivi, the furthest inland of all the ahu

It is not exactly known how the moai were moved across the island, however, there are numerous studies and theories discussing the topic. Earlier researchers assumed that the process required human energy, ropes, and possibly wooden sledges (sleds) or rollers, as well as leveled tracks across the island (the "Easter Island roads"). Another theory suggests that the moai were placed on top of logs and were rolled to their destinations. If that theory is correct, it would take 50–150 people to move the moai. The most recent study demonstrates from the evidence in the archaeological record that the statues were harnessed with ropes from two sides and made to "walk" by tilting them from side to side while pulling forward, in a vertical way.

Oral histories recount how various natives used divine power to command the statues to walk. The earliest accounts say a king named Tuu Ku Ihu moved them with the help of the god Makemake, while later stories tell of a woman who lived alone on the mountain ordering them about at her will. Scholars currently support the theory that the main method was that the moai were "walked" upright (some assume by a rocking process), as laying it prone on a sledge (the method used by the Easter Islanders to move stone in the 1860s) would have required an estimated 1500 people to move the largest moai that had been successfully erected. In 1998, Jo Anne Van Tilburg suggested fewer than half that number could do it by placing the sledge on lubricated rollers. In 1999, she supervised an experiment to move a nine-tonne moai. A replica was loaded on a sledge built in the shape of an A frame that was placed on rollers and 60 people pulled on several ropes in two attempts to tow the moai. The first attempt failed when the rollers jammed up. The second attempt succeeded when tracks were embedded in the ground. This was on flat ground and used eucalyptus wood rather than the native palm trees.

Sign indicating the protected status of the moai

In 1986, Pavel Pavel, Thor Heyerdahl and the Kon-Tiki Museum experimented with a five-tonne moai and a nine-tonne moai. With a rope around the head of the statue and another around the base, using eight workers for the smaller statue and 16 for the larger, they "walked" the moai forward by swiveling and rocking it from side to side; however, the experiment was ended early due to damage to the statue bases from chipping. Despite the early end to the experiment, Thor Heyerdahl estimated that this method for a 20-tonne statue over Easter Island terrain would allow 320 feet (100 m) per day. Other scholars concluded that it was probably not the way the moai were moved due to the reported damage to the base caused by the "shuffling" motion.

Around the same time, archaeologist Charles Love experimented with a 10-tonne replica. His first experiment found rocking the statue to walk it was too unstable over more than a few hundred yards. He then found that placing the statue upright on two sled runners atop log rollers, 25 men were able to move the statue 150 feet (46 m) in two minutes. In 2003, further research indicated this method could explain supposedly regularly spaced post holes (his research on this claim has not yet been published) where the statues were moved over rough ground. He suggested the holes contained upright posts on either side of the path so that as the statue passed between them, they were used as cantilevers for poles to help push the statue up a slope without the requirement of extra people pulling on the ropes and similarly to slow it on the downward slope. The poles could also act as a brake when needed.

Based on detailed studies of the statues found along prehistoric roads, archaeologists Terry Hunt and Carl Lipo have shown that the pattern of breakage, form and position of statues is consistent with an upright hypothesis for transportation. Hunt and Lipo argue that when the statues were carved at a quarry, the sculptors left their bases wide and curved along the front edge. They showed that statues along the road have a center of mass that causes the statue to lean forward. As the statue tilts forward, it rocks sideways along its curved front edge and takes a step. Large flakes are seen broken off the sides of the bases. They argue that once the statue was walked down the road and installed in the landscape, the wide and curved base was carved down.

Recent experimental recreations have proven that it is fully possible that the moai were literally walked from their quarries to their final positions by ingenious use of ropes. Teams of workers would have worked to rock the moai back and forth, creating the walking motion and holding the moai upright. If correct, it can be inferred that the fallen road moai were the result of the teams of balancers being unable to keep the statue upright, and it was presumably not possible to lift the statues again once knocked over. However, the debate continues.

Birdman cult

Originally, Easter Islanders had a paramount chief or single leader. Through the years the power levels veered from sole chiefs to a warrior class known as matatoʻa. The therianthropic figure of a half bird and half-man was the symbol of the matatoʻa; the distinct character connected the sacred site of Orongo. The new cult prompted battles of tribes over worship of ancestry. Creating the moai was one way the islanders would honor their ancestors; during the height of the birdman cult there is evidence which suggests that the construction of moai stopped.

Petroglyph of a birdman with an egg in hand.

"One of the most fascinating sights at Orongo are the hundreds of petroglyphs carved with birdman and Makemake images. Carved into solid basalt, they have resisted ages of harsh weather. It has been suggested that the images represent birdman competition winners. Over 480 birdman petroglyphs have been found on the island, mostly around Orongo." Orongo, the site of the cult's festivities, was a dangerous landscape which consisted of a "narrow ridge between a 1,000-foot (300 m) drop into the ocean on one side and a deep crater on the other". Considered the sacred spot of Orongo, Mata Ngarau was the location where birdman priests prayed and chanted for a successful egg hunt. "The purpose of the birdman contest was to obtain the first egg of the season from the offshore islet Motu Nui. Contestants descended the sheer cliffs of Orongo and swam to Motu Nui where they awaited the coming of the birds. Having procured an egg, the contestant swam back and presented it to his sponsor, who then was declared birdman for that year, an important status position."

Moai Kavakava

These figures are much smaller than the better-known stone moai. They are made of wood and have a small, slender aspect, giving them a sad appearance. These figures are believed to have been made after the civilization on Rapa Nui began to collapse, which is why they seem to have a more emaciated appearance to them.

1722–1868 toppling of the moai

Toppled moai

At some point after the 1722 Jacob Roggeveen arrival, all of the moai that had been erected on ahu were toppled, with the last standing statues reported in 1838 by Abel Aubert du Petit-Thouars, and no upright statues by 1868, apart from the partially buried ones on the outer slopes of Rano Raraku.

Oral histories include one account of a clan pushing down a single moai in the night, but others refer to the "earth shaking", and there are indications that at least some of them fell down due to earthquakes. Some of the moai toppled forward such that their faces were hidden, and often fell in such a way that their necks broke; others fell off the back of their platforms. Today, about 50 moai have been re-erected on their ahus or at museums elsewhere.

The Rapa Nui people were then devastated by the slave trade that began at the island in 1862. Within a year, the individuals that remained on the island were sick, injured, and lacking leadership. The survivors of the slave raids had new company from landing missionaries. Over time, the remaining populace converted to Christianity. Slowly, Native Easter Islanders began to be assimilated, as their tattoos and body paint were banned by the new Christian proscriptions, after which they were then subjected to removal from a portion of their native lands and made to reside on a much smaller portion of the island, while the rest was used for farming by the Peruvians.

Removal of moai from Easter Island

Original moai at the Louvre Museum, in Paris

Ten or more moai have been removed from Easter Island and transported to locations around the world, including the ones today displayed at the Louvre Museum in Paris and the British Museum in London.

Replicas and casts

Several other locations displays replicas (casts) of moai, including the Natural History Museum of Los Angeles County; the Auckland Museum; the American Museum of Natural History; and the campus of the American University.

Preservation and restoration

Early European drawing of moai, in the lower half of a 1770 Spanish map of Easter Island; the original manuscript maps of the Spanish expedition are in Naval Museum of Madrid and in The Jack Daulton Collection, US.

From 1955 to 1978, an American archaeologist, William Mulloy, undertook extensive investigation of the production, transportation and erection of Easter Island's monumental statuary. Mulloy's Rapa Nui projects include the investigation of the Akivi-Vaiteka Complex and the physical restoration of Ahu Akivi (1960); the investigation and restoration of Ahu Ko Te Riku and Ahu Vai Uri and the Tahai Ceremonial Complex (1970); the investigation and restoration of two ahu at Hanga Kio'e (1972); the investigation and restoration of the ceremonial village at Orongo (1974) and numerous other archaeological surveys throughout the island.

The Rapa Nui National Park and the moai were included in the 1972 UN convention concerning the protection of the world's cultural and natural heritage and consequently on the 1995 list of UNESCO World Heritage Sites.

The statues have been mapped by a number of groups over the years, including efforts by Father Sebastian Englert and Chilean researchers. The EISP (Easter Island Statue Project) conducted research and documentation on many of the moai on Rapa Nui and the artifacts held in museums overseas. The purpose of the project is to understand the figures original use, context, and meaning, with the results being provided to the Rapa Nui families and the island's public agencies that are responsible for conservation and preservation of the moai. Other studies include work by Britton Shepardson[51],Terry L. Hunt and Carl P. Lipo.

In 2008, a Finnish tourist chipped a piece off the ear of one moai. The tourist was fined $17,000 in damages and was banned from the island for three years.

In 2020, an unoccupied truck rolled into a moai, destroying the statue and causing 'incalculable damage'.

In 2022, an unknown number of moai in Rano Raraku were damaged by a wildfire that covered an area of around 150 to 250 acres. The Mayor of Rapa Nui, Pedro Edmunds Paoa, stated the fire was started intentionally. Other authorities believe the damage to some of the affected statues is "irreparable".

Unicode character

In 2010, moai was included as a "moyai" emoji (🗿) in Unicode version 6.0 under the code point U+1F5FF as "Japanese stone statue like Moai on Easter Island".

The official Unicode name for the emoji is spelt "moyai" as the emoji actually depicts the Moyai Statue near Shibuya Station in Tokyo. The Moyai Statue was a gift from the people of Nii-jima (an island 163 kilometres (101 mi) from Tokyo but administratively part of the city) inspired by Easter Island moai. The name of the statue was derived by combining moai and a word from the Nii-jima Japanese dialect moyai (催合い) 'helping each other'.

As the Unicode adopted proprietary emoji initially used by Japanese mobile carriers in the 1990s, inconsistent drawings were adopted for this emoji by various companies with proprietary emoji images, depicting either a moai or the Moyai Statue. The Google and Microsoft emoji initially resembled the Moyai Statue in Tokyo; however, the emoji were later revised to resemble moai.

Notwithstanding its intended purpose, the emoji is commonly used in Internet culture to represent a deadpan expression or used to convey that something is being said in a particularly dry, ironic, or sarcastic fashion.

Flyby (spaceflight)

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Flyby_(spaceflight)
Imagery collected by Voyager 2 of Ganymede during its flyby of the Jovian system
Galileo spacecraft encounters asteroid 243 Ida

A flyby (/ˈflb/) is a spaceflight operation in which a spacecraft passes in proximity to another body, usually a target of its space exploration mission and/or a source of a gravity assist (also called swing-by) to impel it towards another target. Spacecraft which are specifically designed for this purpose are known as flyby spacecraft, although the term has also been used in regard to asteroid flybys of Earth for example. Important parameters are the time and distance of closest approach.

Spacecraft flyby

Flyby maneuvers can be conducted with a planet, a natural satellite or a non-planetary object such as a small Solar System body.

Planetary flybys have occurred with Mars or Earth for example:

An example of a comet flyby is when International Cometary Explorer (formerly ISEE-3) passed about 4,800 miles (7,700 km) from the nucleus of Comet Giacobini-Zinner in September 1985.

Another application of the flyby is of Earth's Moon, usually called a lunar flyby. The Apollo 13 spacecraft had an exploded oxygen tank, and therefore had to flyby around the Moon. The Artemis 2 will include a lunar flyby.

Mars

Illustration of the MarCO 6U cubesat relay flyby probes and technology demonstrators for the Mars InSight lander; the flybys provided bent pipe communication support during the landing in 2018

In regards to Mars flybys, a related concept is a Mars flyby rendezvous, where a spacecraft does not enter orbit but rendezvous before or after a flyby of the planet with another spacecraft. Mars flyby rendezvous was evaluated at NASA's Manned Spacecraft Center in the 1960s. At that time NASA developed designs for a combination of a Mars lander, short-stay surface habitat, and ascent vehicle called a Mars Excursion Module (MEM); the ascent stage performed the rendezvous with a different spacecraft that did a flyby of Mars without entering orbit or landing. Compared to MOR, a flyby rendezvous means one spacecraft does not have to orbit Mars, so the resources needed on a return journey to Earth are not taken in and out of Mars orbit for example. (See also Mars cycler)

Mariner IV flyby of Mars in July 1965 returned more accurate atmospheric data about Mars and much closer views of its surface then previously.

Mariner 6 and Mariner 7 flyby of Mars in 1969 caused another breakthrough in knowledge about the planet. The Mariner 6 & 7 infrared radiometer results from he flyby showed that the atmosphere of Mars was composed mostly of carbon dioxide (CO2), and they were also able to detect trace amounts water on the surface of Mars.

Rosetta, swung by around Mars at a distance of 250 km and performed a gravity assist. This is the closest flyby of Mars.

In 2018, the twin Mars Cube One performed a flyby to relay communication for InSight lander EDL (they were launched towards Mars with the cruise stage carrying the InSight lander). Both MarCOs reached Mars and successfully relayed data during the Entry, Descent, and Landing phase of Insight on November 26, 2018.

Meanwhile, Tianwen-1 Deployable Camera, imaged Tianwen-1 in on its transit to Mars, in September 2020 and made a flyby of Mars around 10 February, 2021 according to its trajectory thought for Mars, before entering the deep space or a solar orbit.

Kuiper belt

The New Horizons spacecraft was planning to fly by the Kuiper belt object 486958 Arrokoth on New Year's Day 2019, after its successful flyby of the dwarf planet Pluto in 2015.

On the night of December 31, 2018 to the morning of January 1, 2019 New Horizons performed the most distant flyby to date, of the Kuiper belt object Arrokoth. New Horizons previously did a flyby of Pluto in July 2015, and that was at about 32.9 AU (astronomical units) from the Sun, while the New Year's Day 2019 flyby of the Kuiper object Arrokoth was at 43.6 AU.

Diagram of the trajectory of New Horizons during its flyby of Pluto

Cassini

Animation of Cassini's trajectory around Saturn from 1 May 2004 to 15 September 2017
   Cassini ·   Saturn ·   Enceladus ·    Titan ·    Iapetus

Cassini-Huygens (launched 1997), which orbited Saturn (from 2004–2017) performed flybys of many of Saturn's moons including Titan. Cassini-Huygen's had its first flyby of Titan in October 2004. For further examples of Cassini flybys of Saturn's moons see Timeline of Cassini-Huygens.

Cassini conducted many flybys at various distances of the moons of Saturn. It achieved 126 flybys of Titan, and its final close flyby was on April 22, 2017 prior to its retirement.

An animation of the Cassini spacecraft trajectory around Saturn over 10 years, during which it passed closely by many moons of Saturn, is at right.

Comets

Flyby of comet Hartley 2 on Nov. 4, 2010 (EPOXI mission)

International Cometary Explorer (ISEE-3) passed through the plasma tail of comet Giacobini-Zinner doing a flyby of the distance of 7,800 km (4,800 mi) of the nucleus on September 11, 1985.

In 2010, the Deep Impact spacecraft, on the EPOXI mission did a flyby of comet Hartley 2.

Natural flyby

During an asteroid flyby of Earth, sometimes they are imaged by radar. Animation of 2014 JO25, which had an Earth flyby in 2017

Flyby is also sometimes loosely used to describe when, for example, an asteroid approaches and coasts by the Earth.

This was also the term for when a comet did a flyby of Mars in 2014.

P/2016 BA14 was radar imaged at distance of 2.2 million miles (3,500,000 km) from Earth in 2016, during its flyby. This enabled the size of the nucleus to be calculated to about 3,300 feet (1 km) in diameter.

On December 16, 2018 the short period comet 46P/Wirtanen had its closest approach of Earth, coming within 7.1 million miles or 11.4 million kilometres (one of its closest approaches to Earth).

Prebiotic atmosphere

From Wikipedia, the free encyclopedia
The pale orange dot, an artist's impression of the early Earth which is believed to have appeared orange through its hazy methane rich prebiotic second atmosphere, being somewhat comparable to Titan's atmosphere

The prebiotic atmosphere is the second atmosphere present on Earth before today's biotic, oxygen-rich third atmosphere, and after the first atmosphere (which was mainly water vapor and simple hydrides) of Earth's formation. The formation of the Earth, roughly 4.5 billion years ago, involved multiple collisions and coalescence of planetary embryos. This was followed by a <100 million year period on Earth where a magma ocean was present, the atmosphere was mainly steam, and surface temperatures reached up to 8,000 K (14,000 °F). Earth's surface then cooled and the atmosphere stabilized, establishing the prebiotic atmosphere. The environmental conditions during this time period were quite different from today: the Sun was ~30% dimmer overall yet brighter at ultraviolet and x-ray wavelengths, there was a liquid ocean, it is unknown if there were continents but oceanic islands were likely, Earth's interior chemistry (and thus, volcanic activity) was different, and there was a larger flux of impactors (e.g. comets and asteroids) hitting Earth's surface.

Studies have attempted to constrain the composition and nature of the prebiotic atmosphere by analyzing geochemical data and using theoretical models that include our knowledge of the early Earth environment. These studies indicate that the prebiotic atmosphere likely contained more CO2 than the modern Earth, had N2 within a factor of 2 of the modern levels, and had vanishingly low amounts of O2. The atmospheric chemistry is believed to have been "weakly reducing", where reduced gases like CH4, NH3, and H2 were present in small quantities. The composition of the prebiotic atmosphere was likely periodically altered by impactors, which may have temporarily caused the atmosphere to have been "strongly reduced".

Constraining the composition of the prebiotic atmosphere is key to understanding the origin of life, as it may facilitate or inhibit certain chemical reactions on Earth's surface believed to be important for the formation of the first living organism. Life on Earth originated and began modifying the atmosphere at least 3.5 billion years ago and possibly much earlier, which marks the end of the prebiotic atmosphere.

Environmental context

Establishment of the prebiotic atmosphere

Earth is believed to have formed over 4.5 billion years ago by accreting material from the solar nebula. Earth's Moon formed in a collision, the Moon-forming impact, believed to have occurred 30-50 million years after the Earth formed. In this collision, a Mars-sized object named Theia collided with the primitive Earth and the remnants of the collision formed the Moon. The collision likely supplied enough energy to melt most of Earth's mantle and vaporize roughly 20% of it, heating Earth's surface to as high as 8,000 K (~14,000 °F). Earth's surface in the aftermath of the Moon-forming impact was characterized by high temperatures (~2,500 K), an atmosphere made of rock vapor and steam, and a magma ocean. As the Earth cooled by radiating away the excess energy from the impact, the magma ocean solidified and volatiles were partitioned between the mantle and atmosphere until a stable state was reached. It is estimated that Earth transitioned from the hot, post-impact environment into a potentially habitable environment with crustal recycling, albeit different from modern plate tectonics, roughy 10-20 million years after the Moon-forming impact, around 4.4 billion years ago. The atmosphere present from this point in Earth's history until the origin of life is referred to as the prebiotic atmosphere.

It is unknown when exactly life originated. The oldest direct evidence for life on Earth is around 3.5 billion years old, such as fossil stromatolites from North Pole, Western Australia. Putative evidence of life on Earth from older times (e.g. 3.8 and 4.1 billion years ago) lacks additional context necessary to claim it is truly of biotic origin, so it is still debated. Thus, the prebiotic atmosphere concluded 3.5 billion years ago or earlier, placing it in the early Archean Eon or mid-to-late Hadean Eon.

Environmental factors

Knowledge of the environmental factors at play on early Earth is required to investigate the prebiotic atmosphere. Much of what we know about the prebiotic environment comes from zircons - crystals of zirconium silicate (ZrSiO4). Zircons are useful because they record the physical and chemical processes occurring on the prebiotic Earth during their formation and they are especially durable. Most zircons that are dated to the prebiotic time period are found at the Jack Hills formation of Western Australia, but they also occur elsewhere. Geochemical data from several prebiotic zircons show isotopic evidence for chemical change induced by liquid water, indicating that the prebiotic environment had a liquid ocean and a surface temperature that did not cause it to freeze or boil. It is unknown when exactly the continents emerged above this liquid ocean. This adds uncertainty to the interaction between Earth's prebiotic surface and atmosphere, as the presence of exposed land determines the rate of weathering processes and provides local environments that may be necessary for life to form. However, oceanic islands were likely. Additionally, the oxidation state of Earth's mantle was likely different at early times, which changes the fluxes of chemical species delivered to the atmosphere from volcanic outgassing.

Environmental factors from elsewhere in the solar system also affected prebiotic Earth. The Sun was ~30% dimmer overall around the time the Earth formed. This means greenhouse gases may have been required in higher levels than present day to keep Earth from freezing over. Despite the overall reduction in energy coming from the Sun, the early Sun emitted more radiation in the ultraviolet and x-ray regimes than it currently does. This indicates that different photochemical reactions may have dominated early Earth's atmosphere, which has implications for global atmospheric chemistry and the formation of important compounds that could lead to the origin of life. Finally, there was a significantly higher flux of objects that impacted Earth - such as comets and asteroids - in the early solar system. These impactors may have been important in the prebiotic atmosphere because they can deliver material to the atmosphere, eject material from the atmosphere, and change the chemical nature of the atmosphere after their arrival.

Atmospheric composition

The exact composition of the prebiotic atmosphere is unknown due to the lack of geochemical data from the time period. Current studies generally indicate that the prebiotic atmosphere was "weakly reduced", with elevated levels of CO2, N2 within a factor of 2 of the modern level, negligible amounts of O2, and more hydrogen-bearing gases than the modern Earth (see below). Noble gases and photochemical products of the dominant species were also present in small quantities.

Carbon dioxide

Carbon dioxide (CO2) is an important component of the prebiotic atmosphere because, as a greenhouse gas, it strongly affects the surface temperature; also, it dissolves in water and can change the ocean pH. The abundance of carbon dioxide in the prebiotic atmosphere is not directly constrained by geochemical data and must be inferred.

Evidence suggests that the carbonate-silicate cycle regulates Earth's atmospheric carbon dioxide abundance on timescales of about 1 million years. The carbonate-silicate cycle is a negative feedback loop that modulates Earth's surface temperature by partitioning carbon between the atmosphere and the mantle via several surface processes. It has been proposed that the processes of the carbonate-silicate cycle would result in high CO2 levels in the prebiotic atmosphere to offset the lower energy input from the faint young Sun. This mechanism can be used to estimate the prebiotic CO2 abundance, but it is debated and uncertain. Uncertainty is primarily driven by a lack of knowledge about the area of exposed land, early Earth's interior chemistry and structure, the rate of reverse weathering and seafloor weathering, and the increased impactor flux. One extensive modeling study suggests that CO2 was roughly 20 times higher in the prebiotic atmosphere than the preindustrial modern value (280 ppm), which would result in a global average surface temperature around 259 K (6.5 °F) and an ocean pH around 7.9. This is in agreement with other studies, which generally conclude that the prebiotic atmospheric CO2 abundance was higher than the modern one, although the global surface temperature may still be significantly colder due to the faint young Sun.

Nitrogen

Nitrogen in the form of N2 is 78% of Earth's modern atmosphere by volume, making it the most abundant gas. N2 is generally considered a background gas in the Earth's atmosphere because it is relatively unreactive due to the strength of its triple bond. Despite this, atmospheric N2 was at least moderately important to the prebiotic environment because it impacts the climate via Rayleigh scattering and it may have been more photochemically active under the enhanced x-ray and ultraviolet radiation from the young Sun. N2 was also likely important for the synthesis of compounds believed to be critical for the origin of life, such as hydrogen cyanide (HCN) and amino acids derived from HCN. Studies have attempted to constrain the prebiotic atmosphere N2 abundance with theoretical estimates, models, and geologic data. These studies have resulted in a range of possible constraints on the prebiotic N2 abundance. For example, a recent modeling study that incorporates atmospheric escape, magma ocean chemistry, and the evolution of Earth's interior chemistry suggests that the atmospheric N2 abundance was probably less than half of the present day value. However, this study fits into a larger body of work that generally constrains the prebiotic N2 abundance to be between half and double the present level.

Oxygen

Oxygen in the form of O2 makes up 21% of Earth's modern atmosphere by volume. Earth's modern atmospheric O2 is due almost entirely to biology (e.g. it is produced during oxygenic photosynthesis), so it was not nearly as abundant in the prebiotic atmosphere. This is favorable for the origin of life, as O2 would oxidize organic compounds needed in the origin of life. The prebiotic atmosphere O2 abundance can be theoretically calculated with models of atmospheric chemistry. The primary source of O2 in these models is the breakdown and subsequent chemical reactions of other oxygen containing compounds. Incoming solar photons or lightning can break up CO2 and H2O molecules, freeing oxygen atoms and other radicals (i.e. highly reactive gases in the atmosphere). The free oxygen can then combine into O2 molecules via several chemical pathways. The rate at which O2 is created in this process is determined by the incoming solar flux, the rate of lightning, and the abundances of the other atmospheric gases that take part in the chemical reactions (e.g. CO2, H2O, OH), as well as their vertical distributions. O2 is removed from the atmosphere via photochemical reactions that mainly involve H2 and CO near the surface. The most important of these reactions starts when H2 is split into two H atoms by incoming solar photons. The free H then reacts with O2 and eventually forms H2O, resulting in a net removal of O2 and a net increase in H2O. Models that simulate all of these chemical reactions in a potential prebiotic atmosphere show that an extremely small atmospheric O2 abundance is likely. In one such model that assumed values for CO2 and H2 abundances and sources, the O2 volume mixing ratio is calculated to be between 10−18 and 10−11 near the surface and up to 10−4 in the upper atmosphere.

Hydrogen and reduced gases

The hydrogen abundance in the prebiotic atmosphere can be viewed from the perspective of reduction-oxidation (redox) chemistry. The modern atmosphere is oxidizing, due to the large volume of atmospheric O2. In an oxidizing atmosphere, the majority of atoms that form atmospheric compounds (e.g. C) will be in an oxidized form (e.g. CO2) instead of a reduced form (e.g. CH4). In a reducing atmosphere, more species will be in their reduced, generally hydrogen-bearing forms. Because there was very little O2 in the prebiotic atmosphere, it is generally believed that the prebiotic atmosphere was "weakly reduced" - although some argue that the atmosphere was "strongly reduced". In a weakly reduced atmosphere, reduced gases (e.g. CH4 and NH3) and oxidized gases (e.g CO2) are both present. The actual H2 abundance in the prebiotic atmosphere has been estimated by doing a calculation that takes into account the rate at which H2 is volcanically outgassed to the surface and the rate at which it escapes to space. One of these recent calculations indicates that the prebiotic atmosphere H2 abundance was around 400 parts per million, but could have been significantly higher if the source from volcanic outgassing was enhanced or atmospheric escape was less efficient than expected. The abundances of other reduced species in the atmosphere can then be calculated with models of atmospheric chemistry.

Post-impact atmospheres

It has been proposed that the large flux of impactors in the early solar system may have significantly changed the nature of the prebiotic atmosphere. During the time period of the prebiotic atmosphere, it is expected that a few asteroid impacts large enough to vaporize the oceans and melt Earth's surface could have occurred, with smaller impacts expected in even larger numbers. These impacts would have significantly changed the chemistry of the prebiotic atmosphere by heating it up, ejecting some of it to space, and delivering new chemical material. Studies of post-impact atmospheres indicate that they would have caused the prebiotic atmosphere to be strongly reduced for a period of time after a large impact. On average, impactors in the early solar system contained highly reduced minerals (e.g. metallic iron) and were enriched with reduced compounds that readily enter the atmosphere as a gas. In these strongly reduced post-impact atmospheres, there would be significantly higher abundances of reduced gases like CH4, HCN, and perhaps NH3. Reduced, post-impact atmospheres after the ocean condensed are predicted to last up to tens of millions of years before returning to the background state.

Relationship to the origin of life

The prebiotic atmosphere can supply chemical ingredients and facilitate environmental conditions that contribute to the synthesis of organic compounds involved in the origin of life. For example, potential compounds involved in the origin of life were synthesized in the Miller-Urey experiment. In this experiment, assumptions must be made about what gases were present in the prebiotic atmosphere. Proposed important ingredients for the origin of life include (but are not limited to) methane (CH4), ammonia (NH3), phosphate, hydrogen cyanide (HCN), various organics, and various photochemical byproducts. The atmospheric composition will impact the stability and production of these compounds at Earth's surface. For example, the "weakly reduced" prebiotic atmosphere may produce some, but not all, of these ingredients via reactions with lightning. On the other hand, the production and stability of origin of life ingredients in a strongly reduced atmosphere are greatly enhanced, making post-impact atmospheres particularly relevant. It is also proposed that the conditions required for the origin of life could have emerged locally, in a system that is isolated from the atmosphere (e.g. a hydrothermal vent). However, compounds such as cyanides used to make nucleobases of RNA would be too dilute in the ocean, unlike lakes on land. Once life originated and started interacting with the atmosphere, the prebiotic atmosphere transitioned into the post-biotic atmosphere, by definition.

Sumerian literature

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Sumerian_literature
Sumerian inscription on a ceramic stone plaque.

Sumerian literature constitutes the earliest known corpus of recorded literature, including the religious writings and other traditional stories maintained by the Sumerian civilization and largely preserved by the later Akkadian and Babylonian empires. These records were written in the Sumerian language in the 3rd and 2nd millennia BC during the Middle Bronze Age.

The Sumerians invented one of the first writing systems, developing Sumerian cuneiform writing out of earlier proto-writing systems by about the 30th century BC. The Sumerian language remained in official and literary use in the Akkadian and Babylonian empires, even after the spoken language disappeared from the population; literacy was widespread, and the Sumerian texts that students copied heavily influenced later Babylonian literature. The basic genres of Sumerian literature were literary catalogues, narrative/mythological compositions, historical compositions, letters and legal documents, disputation poems, proverbs, and other texts which do not belong to these prior categories.

Poetry

Most Sumerian literature is written in left-justified lines, and could contain line-based organization such as the couplet or the stanza, but the Sumerian definition of poetry is unknown. It is not rhymed, although “comparable effects were sometimes exploited.” Though rhymeless, the intricate patterns of similar and alternating sounds of vowels and consonants and the similar and alternating verb and noun endings give the language a musical resonance. It did not use syllabo-tonic versification, and the writing system precludes detection of rhythm, metre, rhyme, or alliteration. Quantitative analysis of other possible poetic features seems to be lacking, or has been intentionally hidden by the scribes who recorded the writing.

Literary genres and topics

Genre is often the first judgement made of ancient literature; types of literature were not clearly defined, and all Sumerian literature incorporated poetic aspects. Sumerian poems demonstrate basic elements of poetry, including lines, imagery, and metaphor. Humans, gods, talking animals, and inanimate objects were all incorporated as characters. Suspense and humor were both incorporated into Sumerian stories. These stories were primarily shared orally, though they were also recorded by scribes. Some works were associated with specific musical instruments or contexts and may have been performed in specific settings. Sumerian literature did not use titles, instead being referred to by the work's first line.

Based on the categorization work of Miguel Civil, Modern assyriologists have divided the extant corpus of Sumerian literature into broad categories including "Literary Catalogs", "Narratives and Mythological Compositions", "Historical Compositions and Praise Poetry", "Letters, Letter Prayers and Laws", "Hymns and Songs", "Heterogenous Compositions" (including Wisdom literature), and "Proverbs".

Literary catalogs

Narrative and mythological compositions

Historical compositions

Letters and laws

Hymns

Disputation poems

Proverbs

Heterogeneous compositions

Variational method (quantum mechanics)

From Wikipedia, the free encyclopedia https://en.wikipedia.org/wiki/Variational_method_(quantum_mechanics) ...