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Thursday, April 18, 2024

Early modern human

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Early_modern_human
100 to 80 thousand year old Skhul V from Israel

Early modern human (EMH), or anatomically modern human (AMH), are terms used to distinguish Homo sapiens (the only extant Hominina species) that are anatomically consistent with the range of phenotypes seen in contemporary humans, from extinct archaic human species. This distinction is useful especially for times and regions where anatomically modern and archaic humans co-existed, for example, in Paleolithic Europe. Among the oldest known remains of Homo sapiens are those found at the Omo-Kibish I archaeological site in south-western Ethiopia, dating to about 233,000 to 196,000 years ago, the Florisbad site in South Africa, dating to about 259,000 years ago, and the Jebel Irhoud site in Morocco, dated about 315,000 years ago.

Extinct species of the genus Homo include Homo erectus (extant from roughly 2 to 0.1 million years ago) and a number of other species (by some authors considered subspecies of either H. sapiens or H. erectus). The divergence of the lineage leading to H. sapiens out of ancestral H. erectus (or an intermediate species such as Homo antecessor) is estimated to have occurred in Africa roughly 500,000 years ago. The earliest fossil evidence of early modern humans appears in Africa around 300,000 years ago, with the earliest genetic splits among modern people, according to some evidence, dating to around the same time. Sustained archaic human admixture with modern humans is known to have taken place both in Africa and (following the recent Out-Of-Africa expansion) in Eurasia, between about 100,000 and 30,000 years ago.

Name and taxonomy

The binomial name Homo sapiens was coined by Linnaeus, 1758. The Latin noun homō (genitive hominis) means "human being", while the participle sapiēns means "discerning, wise, sensible".

The species was initially thought to have emerged from a predecessor within the genus Homo around 300,000 to 200,000 years ago. A problem with the morphological classification of "anatomically modern" was that it would not have included certain extant populations. For this reason, a lineage-based (cladistic) definition of H. sapiens has been suggested, in which H. sapiens would by definition refer to the modern human lineage following the split from the Neanderthal lineage. Such a cladistic definition would extend the age of H. sapiens to over 500,000 years.

Estimates for the split between the Homo sapiens line and combined Neanderthal/Denisovan line range from between 503,000 and 565,000 years ago; between 550,000 and 765,000 years ago; and (based on rates of dental evolution) possibly more than 800,000 years ago.

Extant human populations have historically been divided into subspecies, but since around the 1980s all extant groups have tended to be subsumed into a single species, H. sapiens, avoiding division into subspecies altogether.

Some sources show Neanderthals (H. neanderthalensis) as a subspecies (H. sapiens neanderthalensis). Similarly, the discovered specimens of the H. rhodesiensis species have been classified by some as a subspecies (H. sapiens rhodesiensis), although it remains more common to treat these last two as separate species within the genus Homo rather than as subspecies within H. sapiens.

All humans are considered to be a part of the subspecies H. sapiens sapiens, a designation which has been a matter of debate since a species is usually not given a subspecies category unless there is evidence of multiple distinct subspecies.

Age and speciation process

Derivation from H. erectus

The divergence of the lineage that would lead to H. sapiens out of archaic human varieties derived from H. erectus, is estimated as having taken place over 500,000 years ago (marking the split of the H. sapiens lineage from ancestors shared with other known archaic hominins). But the oldest split among modern human populations (such as the Khoisan split from other groups) has been recently dated to between 350,000 and 260,000 years ago, and the earliest known examples of H. sapiens fossils also date to about that period, including the Jebel Irhoud remains from Morocco (ca. 300,000 or 350–280,000 years ago), the Florisbad Skull from South Africa (ca. 259,000 years ago), and the Omo remains from Ethiopia (ca. 195,000, or, as more recently dated, ca. 233,000 years ago).

An mtDNA study in 2019 proposed an origin of modern humans in Botswana (and a Khoisan split) of around 200,000 years. However, this proposal has been widely criticized by scholars with the recent evidence overall (genetic, fossil, and archaeological) supporting an origin for H. sapiens approximately 100,000 years earlier and in a broader region of Africa than the study proposes.

In September 2019, scientists proposed that the earliest H. sapiens (and last common human ancestor to modern humans) arose between 350,000 and 260,000 years ago through a merging of populations in East and South Africa.

An alternative suggestion defines H. sapiens cladistically as including the lineage of modern humans since the split from the lineage of Neanderthals, roughly 500,000 to 800,000 years ago.

The time of divergence between archaic H. sapiens and ancestors of Neanderthals and Denisovans caused by a genetic bottleneck of the latter was dated at 744,000 years ago, combined with repeated early admixture events and Denisovans diverging from Neanderthals 300 generations after their split from H. sapiens, as calculated by Rogers et al. (2017).

The derivation of a comparatively homogeneous single species of H. sapiens from more diverse varieties of archaic humans (all of which were descended from the early dispersal of H. erectus some 1.8 million years ago) was debated in terms of two competing models during the 1980s: "recent African origin" postulated the emergence of H. sapiens from a single source population in Africa, which expanded and led to the extinction of all other human varieties, while the "multiregional evolution" model postulated the survival of regional forms of archaic humans, gradually converging into the modern human varieties by the mechanism of clinal variation, via genetic drift, gene flow and selection throughout the Pleistocene.

Since the 2000s, the availability of data from archaeogenetics and population genetics has led to the emergence of a much more detailed picture, intermediate between the two competing scenarios outlined above: The recent Out-of-Africa expansion accounts for the predominant part of modern human ancestry, while there were also significant admixture events with regional archaic humans.

Since the 1970s, the Omo remains, originally dated to some 195,000 years ago, have often been taken as the conventional cut-off point for the emergence of "anatomically modern humans". Since the 2000s, the discovery of older remains with comparable characteristics, and the discovery of ongoing hybridization between "modern" and "archaic" populations after the time of the Omo remains, have opened up a renewed debate on the age of H. sapiens in journalistic publications. H. s. idaltu, dated to 160,000 years ago, has been postulated as an extinct subspecies of H. sapiens in 2003. H. neanderthalensis, which became extinct about 40,000 years ago, was also at one point considered to be a subspecies, H. s. neanderthalensis.

H. heidelbergensis, dated 600,000 to 300,000 years ago, has long been thought to be a likely candidate for the last common ancestor of the Neanderthal and modern human lineages. However, genetic evidence from the Sima de los Huesos fossils published in 2016 seems to suggest that H. heidelbergensis in its entirety should be included in the Neanderthal lineage, as "pre-Neanderthal" or "early Neanderthal", while the divergence time between the Neanderthal and modern lineages has been pushed back to before the emergence of H. heidelbergensis, to close to 800,000 years ago, the approximate time of disappearance of H. antecessor.

Early Homo sapiens

Reconstruction of early Homo sapiens from Jebel Irhoud, Morocco c. 315 000 years BP

The term Middle Paleolithic is intended to cover the time between the first emergence of H. sapiens (roughly 300,000 years ago) and the period held by some to mark the emergence of full behavioral modernity (roughly by 50,000 years ago, corresponding to the start of the Upper Paleolithic).

Many of the early modern human finds, like those of Jebel Irhoud, Omo, Herto, Florisbad, Skhul, and Peștera cu Oase exhibit a mix of archaic and modern traits. Skhul V, for example, has prominent brow ridges and a projecting face. However, the brain case is quite rounded and distinct from that of the Neanderthals and is similar to the brain case of modern humans. It is uncertain whether the robust traits of some of the early modern humans like Skhul V reflects mixed ancestry or retention of older traits.

The "gracile" or lightly built skeleton of anatomically modern humans has been connected to a change in behavior, including increased cooperation and "resource transport".

There is evidence that the characteristic human brain development, especially the prefrontal cortex, was due to "an exceptional acceleration of metabolome evolution ... paralleled by a drastic reduction in muscle strength. The observed rapid metabolic changes in brain and muscle, together with the unique human cognitive skills and low muscle performance, might reflect parallel mechanisms in human evolution." The Schöningen spears and their correlation of finds are evidence that complex technological skills already existed 300,000 years ago, and are the first obvious proof of an active (big game) hunt. H. heidelbergensis already had intellectual and cognitive skills like anticipatory planning, thinking and acting that so far have only been attributed to modern man.

The ongoing admixture events within anatomically modern human populations make it difficult to estimate the age of the matrilinear and patrilinear most recent common ancestors of modern populations (Mitochondrial Eve and Y-chromosomal Adam). Estimates of the age of Y-chromosomal Adam have been pushed back significantly with the discovery of an ancient Y-chromosomal lineage in 2013, to likely beyond 300,000 years ago. There have, however, been no reports of the survival of Y-chromosomal or mitochondrial DNA clearly deriving from archaic humans (which would push back the age of the most recent patrilinear or matrilinear ancestor beyond 500,000 years).

Fossil teeth found at Qesem Cave (Israel) and dated to between 400,000 and 200,000 years ago have been compared to the dental material from the younger (120,000–80,000 years ago) Skhul and Qafzeh hominins.

Dispersal and archaic admixture

Overview map of the peopling of the world by anatomically modern humans (numbers indicate dates in thousands of years ago [ka])

Dispersal of early H. sapiens begins soon after its emergence, as evidenced by the North African Jebel Irhoud finds (dated to around 315,000 years ago). There is indirect evidence for H. sapiens presence in West Asia around 270,000 years ago.

The Florisbad Skull from Florisbad, South Africa, dated to about 259,000 years ago, has also been classified as representing early H. sapiens.

In September 2019, scientists proposed that the earliest H. sapiens (and last common human ancestor to modern humans) arose between 350,000 and 260,000 years ago through a merging of populations in East and South Africa.

Among extant populations, the Khoi-San (or "Capoid") hunters-gatherers of Southern Africa may represent the human population with the earliest possible divergence within the group Homo sapiens sapiens. Their separation time has been estimated in a 2017 study to be between 350 and 260,000 years ago, compatible with the estimated age of early H. sapiens. The study states that the deep split-time estimation of 350 to 260 thousand years ago is consistent with the archaeological estimate for the onset of the Middle Stone Age across sub-Saharan Africa and coincides with archaic H. sapiens in southern Africa represented by, for example, the Florisbad skull dating to 259 (± 35) thousand years ago.

H. s. idaltu, found at Middle Awash in Ethiopia, lived about 160,000 years ago, and H. sapiens lived at Omo Kibish in Ethiopia about 233,000-195,000 years ago. Two fossils from Guomde, Kenya, dated to at least (and likely more than) 180,000 years ago and (more precisely) to 300–270,000 years ago, have been tentatively assigned to H. sapiens and similarities have been noted between them and the Omo Kibbish remains. Fossil evidence for modern human presence in West Asia is ascertained for 177,000 years ago, and disputed fossil evidence suggests expansion as far as East Asia by 120,000 years ago.

In July 2019, anthropologists reported the discovery of 210,000 year old remains of a H. sapiens and 170,000 year old remains of a H. neanderthalensis in Apidima Cave, Peloponnese, Greece, more than 150,000 years older than previous H. sapiens finds in Europe.

A significant dispersal event, within Africa and to West Asia, is associated with the African megadroughts during MIS 5, beginning 130,000 years ago. A 2011 study located the origin of basal population of contemporary human populations at 130,000 years ago, with the Khoi-San representing an "ancestral population cluster" located in southwestern Africa (near the coastal border of Namibia and Angola).

Layer sequence at Ksar Akil in the Levantine corridor, and discovery of two fossils of Homo sapiens, dated to 40,800 to 39,200 years BP for "Egbert", and 42,400–41,700 BP for "Ethelruda".

While early modern human expansion in Sub-Saharan Africa before 130 kya persisted, early expansion to North Africa and Asia appears to have mostly disappeared by the end of MIS5 (75,000 years ago), and is known only from fossil evidence and from archaic admixture. Eurasia was re-populated by early modern humans in the so-called "recent out-of-Africa migration" post-dating MIS5, beginning around 70,000–50,000 years ago. In this expansion, bearers of mt-DNA haplogroup L3 left East Africa, likely reaching Arabia via the Bab-el-Mandeb, and in the Great Coastal Migration spread to South Asia, Maritime South Asia and Oceania between 65,000 and 50,000 years ago, while Europe, East and North Asia were reached by about 45,000 years ago. Some evidence suggests that an early wave of humans may have reached the Americas by about 40,000–25,000 years ago.

Evidence for the overwhelming contribution of this "recent" (L3-derived) expansion to all non-African populations was established based on mitochondrial DNA, combined with evidence based on physical anthropology of archaic specimens, during the 1990s and 2000s, and has also been supported by Y DNA and autosomal DNA. The assumption of complete replacement has been revised in the 2010s with the discovery of admixture events (introgression) of populations of H. sapiens with populations of archaic humans over the period of between roughly 100,000 and 30,000 years ago, both in Eurasia and in Sub-Saharan Africa. Neanderthal admixture, in the range of 1–4%, is found in all modern populations outside of Africa, including in Europeans, Asians, Papua New Guineans, Australian Aboriginals, Native Americans, and other non-Africans. This suggests that interbreeding between Neanderthals and anatomically modern humans took place after the recent "out of Africa" migration, likely between 60,000 and 40,000 years ago. Recent admixture analyses have added to the complexity, finding that Eastern Neanderthals derive up to 2% of their ancestry from anatomically modern humans who left Africa some 100 kya. The extent of Neanderthal admixture (and introgression of genes acquired by admixture) varies significantly between contemporary racial groups, being absent in Africans, intermediate in Europeans and highest in East Asians. Certain genes related to UV-light adaptation introgressed from Neanderthals have been found to have been selected for in East Asians specifically from 45,000 years ago until around 5,000 years ago. The extent of archaic admixture is of the order of about 1% to 4% in Europeans and East Asians, and highest among Melanesians (the last also having Denisova hominin admixture at 4% to 6% in addition to neanderthal admixture). Cumulatively, about 20% of the Neanderthal genome is estimated to remain present spread in contemporary populations.

In September 2019, scientists reported the computerized determination, based on 260 CT scans, of a virtual skull shape of the last common human ancestor to modern humans/H. sapiens, representative of the earliest modern humans, and suggested that modern humans arose between 350,000 and 260,000 years ago through a merging of populations in East and South Africa while North-African fossils may represent a population which introgressed into Neanderthals during the LMP.

Anatomy

Known archaeological remains of anatomically modern humans in Europe and Africa, directly dated, calibrated carbon dates as of 2013.

Generally, modern humans are more lightly built (or more "gracile") than the more "robust" archaic humans. Nevertheless, contemporary humans exhibit high variability in many physiological traits, and may exhibit remarkable "robustness". There are still a number of physiological details which can be taken as reliably differentiating the physiology of Neanderthals vs. anatomically modern humans.

Anatomical modernity

The term "anatomically modern humans" (AMH) is used with varying scope depending on context, to distinguish "anatomically modern" Homo sapiens from archaic humans such as Neanderthals and Middle and Lower Paleolithic hominins with transitional features intermediate between H. erectus, Neanderthals and early AMH called archaic Homo sapiens. In a convention popular in the 1990s, Neanderthals were classified as a subspecies of H. sapiens, as H. s. neanderthalensis, while AMH (or European early modern humans, EEMH) was taken to refer to "Cro-Magnon" or H. s. sapiens. Under this nomenclature (Neanderthals considered H. sapiens), the term "anatomically modern Homo sapiens" (AMHS) has also been used to refer to EEMH ("Cro-Magnons"). It has since become more common to designate Neanderthals as a separate species, H. neanderthalensis, so that AMH in the European context refers to H. sapiens, but the question is by no means resolved.

In this more narrow definition of H. sapiens, the subspecies Homo sapiens idaltu, discovered in 2003, also falls under the umbrella of "anatomically modern". The recognition of H. sapiens idaltu as a valid subspecies of the anatomically modern human lineage would justify the description of contemporary humans with the subspecies name Homo sapiens sapiens. However, biological anthropologist Chris Stringer does not consider idaltu distinct enough within H. sapiens to warrant its own subspecies designation.

A further division of AMH into "early" or "robust" vs. "post-glacial" or "gracile" subtypes has since been used for convenience. The emergence of "gracile AMH" is taken to reflect a process towards a smaller and more fine-boned skeleton beginning around 50,000–30,000 years ago.

Braincase anatomy

Anatomical comparison of skulls of H. sapiens (left) and H. neanderthalensis (right)
(in Cleveland Museum of Natural History)
Features compared are the braincase shape, forehead, browridge, nasal bone projection, cheek bone angulation, chin and occipital contour

The cranium lacks a pronounced occipital bun in the neck, a bulge that anchored considerable neck muscles in Neanderthals. Modern humans, even the earlier ones, generally have a larger fore-brain than the archaic people, so that the brain sits above rather than behind the eyes. This will usually (though not always) give a higher forehead, and reduced brow ridge. Early modern people and some living people do however have quite pronounced brow ridges, but they differ from those of archaic forms by having both a supraorbital foramen or notch, forming a groove through the ridge above each eye. This splits the ridge into a central part and two distal parts. In current humans, often only the central section of the ridge is preserved (if it is preserved at all). This contrasts with archaic humans, where the brow ridge is pronounced and unbroken.

Modern humans commonly have a steep, even vertical forehead whereas their predecessors had foreheads that sloped strongly backwards. According to Desmond Morris, the vertical forehead in humans plays an important role in human communication through eyebrow movements and forehead skin wrinkling.

Brain size in both Neanderthals and AMH is significantly larger on average (but overlapping in range) than brain size in H. erectus. Neanderthal and AMH brain sizes are in the same range, but there are differences in the relative sizes of individual brain areas, with significantly larger visual systems in Neanderthals than in AMH.

Jaw anatomy

Compared to archaic people, anatomically modern humans have smaller, differently shaped teeth. This results in a smaller, more receded dentary, making the rest of the jaw-line stand out, giving an often quite prominent chin. The central part of the mandible forming the chin carries a triangularly shaped area forming the apex of the chin called the mental trigon, not found in archaic humans. Particularly in living populations, the use of fire and tools requires fewer jaw muscles, giving slender, more gracile jaws. Compared to archaic people, modern humans have smaller, lower faces.

Body skeleton structure

The body skeletons of even the earliest and most robustly built modern humans were less robust than those of Neanderthals (and from what little we know from Denisovans), having essentially modern proportions. Particularly regarding the long bones of the limbs, the distal bones (the radius/ulna and tibia/fibula) are nearly the same size or slightly shorter than the proximal bones (the humerus and femur). In ancient people, particularly Neanderthals, the distal bones were shorter, usually thought to be an adaptation to cold climate. The same adaptation is found in some modern people living in the polar regions.

Height ranges overlap between Neanderthals and AMH, with Neanderthal averages cited as 164 to 168 cm (65 to 66 in) and 152 to 156 cm (60 to 61 in) for males and females, respectively, which is largely identical to pre-industrial average heights for AMH. Contemporary national averages range between 158 to 184 cm (62 to 72 in) in males and 147 to 172 cm (58 to 68 in) in females. Neanderthal ranges approximate the contemporary height distribution measured among Malay people, for one.

Recent evolution

Reconstruction of a modern man from southwestern Europe c. 30 000 years BP, London Natural History Museum.

Following the peopling of Africa some 130,000 years ago, and the recent Out-of-Africa expansion some 70,000 to 50,000 years ago, some sub-populations of H. sapiens had been essentially isolated for tens of thousands of years prior to the early modern Age of Discovery. Combined with archaic admixture this has resulted in significant genetic variation, which in some instances has been shown to be the result of directional selection taking place over the past 15,000 years, i.e., significantly later than possible archaic admixture events.

Some climatic adaptations, such as high-altitude adaptation in humans, are thought to have been acquired by archaic admixture. Introgression of genetic variants acquired by Neanderthal admixture have different distributions in European and East Asians, reflecting differences in recent selective pressures. A 2014 study reported that Neanderthal-derived variants found in East Asian populations showed clustering in functional groups related to immune and haematopoietic pathways, while European populations showed clustering in functional groups related to the lipid catabolic process. A 2017 study found correlation of Neanderthal admixture in phenotypic traits in modern European populations.

Physiological or phenotypical changes have been traced to Upper Paleolithic mutations, such as the East Asian variant of the EDAR gene, dated to c. 35,000 years ago.

Recent divergence of Eurasian lineages was sped up significantly during the Last Glacial Maximum (LGM), the Mesolithic and the Neolithic, due to increased selection pressures and due to founder effects associated with migration. Alleles predictive of light skin have been found in Neanderthals, but the alleles for light skin in Europeans and East Asians, associated with KITLG and ASIP, are (as of 2012) thought to have not been acquired by archaic admixture but recent mutations since the LGM. Phenotypes associated with the "white" or "Caucasian" populations of Western Eurasian stock emerge during the LGM, from about 19,000 years ago. Average cranial capacity in modern human populations varies in the range of 1,200 to 1,450 cm3 for adult males. Larger cranial volume is associated with climatic region, the largest averages being found in populations of Siberia and the Arctic. Both Neanderthal and EEMH had somewhat larger cranial volumes on average than modern Europeans, suggesting the relaxation of selection pressures for larger brain volume after the end of the LGM.

Examples for still later adaptations related to agriculture and animal domestication including East Asian types of ADH1B associated with rice domestication, or lactase persistence, are due to recent selection pressures.

An even more recent adaptation has been proposed for the Austronesian Sama-Bajau, developed under selection pressures associated with subsisting on freediving over the past thousand years or so.

Behavioral modernity

Lithic Industries of early Homo sapiens at Blombos Cave (M3 phase, MIS 5), Southern Cape, South Africa (c. 105,000 – 90,000 years old)

Behavioral modernity, involving the development of language, figurative art and early forms of religion (etc.) is taken to have arisen before 40,000 years ago, marking the beginning of the Upper Paleolithic (in African contexts also known as the Later Stone Age).

There is considerable debate regarding whether the earliest anatomically modern humans behaved similarly to recent or existing humans. Behavioral modernity is taken to include fully developed language (requiring the capacity for abstract thought), artistic expression, early forms of religious behavior, increased cooperation and the formation of early settlements, and the production of articulated tools from lithic cores, bone or antler. The term Upper Paleolithic is intended to cover the period since the rapid expansion of modern humans throughout Eurasia, which coincides with the first appearance of Paleolithic art such as cave paintings and the development of technological innovation such as the spear-thrower. The Upper Paleolithic begins around 50,000 to 40,000 years ago, and also coincides with the disappearance of archaic humans such as the Neanderthals.

Bifacial silcrete point of early Homo sapiens, from M1 phase (71,000 BCE) layer of Blombos Cave, South Africa

The term "behavioral modernity" is somewhat disputed. It is most often used for the set of characteristics marking the Upper Paleolithic, but some scholars use "behavioral modernity" for the emergence of H. sapiens around 200,000 years ago, while others use the term for the rapid developments occurring around 50,000 years ago. It has been proposed that the emergence of behavioral modernity was a gradual process.

Examples of behavioural modernity

Claimed "oldest known drawing by human hands", discovered in Blombos Cave in South Africa. Estimated to be a 73,000-year-old work of a Homo sapiens.

The equivalent of the Eurasian Upper Paleolithic in African archaeology is known as the Later Stone Age, also beginning roughly 40,000 years ago. While most clear evidence for behavioral modernity uncovered from the later 19th century was from Europe, such as the Venus figurines and other artefacts from the Aurignacian, more recent archaeological research has shown that all essential elements of the kind of material culture typical of contemporary San hunter-gatherers in Southern Africa was also present by at least 40,000 years ago, including digging sticks of similar materials used today, ostrich egg shell beads, bone arrow heads with individual maker's marks etched and embedded with red ochre, and poison applicators. There is also a suggestion that "pressure flaking best explains the morphology of lithic artifacts recovered from the c. 75-ka Middle Stone Age levels at Blombos Cave, South Africa. The technique was used during the final shaping of Still Bay bifacial points made on heat‐treated silcrete." Both pressure flaking and heat treatment of materials were previously thought to have occurred much later in prehistory, and both indicate a behaviourally modern sophistication in the use of natural materials. Further reports of research on cave sites along the southern African coast indicate that "the debate as to when cultural and cognitive characteristics typical of modern humans first appeared" may be coming to an end, as "advanced technologies with elaborate chains of production" which "often demand high-fidelity transmission and thus language" have been found at the South African Pinnacle Point Site 5–6. These have been dated to approximately 71,000 years ago. The researchers suggest that their research "shows that microlithic technology originated early in South Africa by 71 kya, evolved over a vast time span (c. 11,000 years), and was typically coupled to complex heat treatment that persisted for nearly 100,000 years. Advanced technologies in Africa were early and enduring; a small sample of excavated sites in Africa is the best explanation for any perceived 'flickering' pattern." Increases in behavioral complexity have been speculated to have been a linked to an earlier climatic change to much drier conditions between 135,000 and 75,000 years ago. This might have led to human groups who were seeking refuge from the inland droughts, expanded along the coastal marshes rich in shellfish and other resources. Since sea levels were low due to so much water tied up in glaciers, such marshlands would have occurred all along the southern coasts of Eurasia. The use of rafts and boats may well have facilitated exploration of offshore islands and travel along the coast, and eventually permitted expansion to New Guinea and then to Australia.

In addition, a variety of other evidence of abstract imagery, widened subsistence strategies, and other "modern" behaviors has been discovered in Africa, especially South, North, and East Africa, predating 50,000 years ago (with some predating 100,000 years ago). The Blombos Cave site in South Africa, for example, is famous for rectangular slabs of ochre engraved with geometric designs. Using multiple dating techniques, the site was confirmed to be around 77,000 and 100,000–75,000 years old. Ostrich egg shell containers engraved with geometric designs dating to 60,000 years ago were found at Diepkloof, South Africa. Beads and other personal ornamentation have been found from Morocco which might be as much as 130,000 years old; as well, the Cave of Hearths in South Africa has yielded a number of beads dating from significantly prior to 50,000 years ago, and shell beads dating to about 75,000 years ago have been found at Blombos Cave, South Africa. Specialized projectile weapons as well have been found at various sites in Middle Stone Age Africa, including bone and stone arrowheads at South African sites such as Sibudu Cave (along with an early bone needle also found at Sibudu) dating approximately 72,000–60,000 years ago some of which may have been tipped with poisons, and bone harpoons at the Central African site of Katanda dating ca. 90,000 years ago. Evidence also exists for the systematic heat treating of silcrete stone to increase its flake-ability for the purpose of toolmaking, beginning approximately 164,000 years ago at the South African site of Pinnacle Point and becoming common there for the creation of microlithic tools at about 72,000 years ago.

In 2008, an ochre processing workshop likely for the production of paints was uncovered dating to ca. 100,000 years ago at Blombos Cave, South Africa. Analysis shows that a liquefied pigment-rich mixture was produced and stored in the two abalone shells, and that ochre, bone, charcoal, grindstones and hammer-stones also formed a composite part of the toolkits. Evidence for the complexity of the task includes procuring and combining raw materials from various sources (implying they had a mental template of the process they would follow), possibly using pyrotechnology to facilitate fat extraction from bone, using a probable recipe to produce the compound, and the use of shell containers for mixing and storage for later use. Modern behaviors, such as the making of shell beads, bone tools and arrows, and the use of ochre pigment, are evident at a Kenyan site by 78,000-67,000 years ago. Evidence of early stone-tipped projectile weapons (a characteristic tool of Homo sapiens), the stone tips of javelins or throwing spears, were discovered in 2013 at the Ethiopian site of Gademotta, and date to around 279,000 years ago.

Expanding subsistence strategies beyond big-game hunting and the consequential diversity in tool types have been noted as signs of behavioral modernity. A number of South African sites have shown an early reliance on aquatic resources from fish to shellfish. Pinnacle Point, in particular, shows exploitation of marine resources as early as 120,000 years ago, perhaps in response to more arid conditions inland. Establishing a reliance on predictable shellfish deposits, for example, could reduce mobility and facilitate complex social systems and symbolic behavior. Blombos Cave and Site 440 in Sudan both show evidence of fishing as well. Taphonomic change in fish skeletons from Blombos Cave have been interpreted as capture of live fish, clearly an intentional human behavior.

Humans in North Africa (Nazlet Sabaha, Egypt) are known to have dabbled in chert mining, as early as ≈100,000 years ago, for the construction of stone tools.

Evidence was found in 2018, dating to about 320,000 years ago at the site of Olorgesailie in Kenya, of the early emergence of modern behaviors including: the trade and long-distance transportation of resources (such as obsidian), the use of pigments, and the possible making of projectile points. The authors of three 2018 studies on the site observe that the evidence of these behaviors is roughly contemporary with the earliest known Homo sapiens fossil remains from Africa (such as at Jebel Irhoud and Florisbad), and they suggest that complex and modern behaviors began in Africa around the time of the emergence of Homo sapiens.

In 2019, further evidence of Middle Stone Age complex projectile weapons in Africa was found at Aduma, Ethiopia, dated 100,000–80,000 years ago, in the form of points considered likely to belong to darts delivered by spear throwers.

Pace of progress during Homo sapiens history

Homo sapiens technological and cultural progress appears to have been very much faster in recent millennia than in Homo sapiens early periods. The pace of development may indeed have accelerated, due to massively larger population (so more humans extant to think of innovations), more communication and sharing of ideas among human populations, and the accumulation of thinking tools. However it may also be that the pace of advancements always looks relatively faster to humans in the time they live, because previous advances are unrecognised.

Archaic humans

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Archaic_humans
Homo rhodesiensis ("Broken Hill Cranium"): dated to 324,000 to 274,000 years ago.

Archaic humans is a broad category denoting all species of the genus Homo that are not Homo sapiens (which are known as modern humans). Among the earliest modern human remains are those from Jebel Irhoud in Morocco (about 315 ka), Florisbad in South Africa (259 ka), and Omo-Kibish I (Omo I) in southern Ethiopia (c. 233 or 195 ka). Some examples of archaic humans include H. antecessor (1200–770 ka), H. bodoensis (1200–300 ka), H. heidelbergensis (600–200 ka), Neanderthals (H. neanderthalensis; 430–40 ka), H. rhodesiensis (300–125 ka) and Denisovans (H. denisova; 285–52 ka),

Most archaic humans had a brain size averaging 1,200 to 1,400 cubic centimeters, which overlaps with the range of modern humans. Notable exceptions include Homo naledi and Homo floresiensis, having cranial capacities of 465-610 and 380 cubic centimeters, respectively.

Archaic humans are distinguished from anatomically modern humans by having a thick skull, prominent supraorbital ridges (brow ridges) and the lack of a prominent chin.

Anatomically modern humans appeared around 300,000 years ago in Africa, and 70,000 years ago gradually supplanted the "archaic" human varieties. Non-modern varieties of Homo are certain to have survived until after 30,000 years ago, and perhaps until as recently as 12,000 years ago. According to recent genetic studies, modern humans may have bred with two or more groups of archaic humans, including Neanderthals and Denisovans. Other studies have cast doubt on admixture being the source of the shared genetic markers between archaic and modern humans, pointing to an ancestral origin of the traits which originated 500,000–800,000 years ago. In August 2023, scientists reported the discovery of an unknown ancient human hominin that may have lived 300,000 years ago in China.

Terminology and definition

The category archaic human lacks a single, agreed definition. According to one definition, Homo sapiens is a single species comprising several subspecies that include the archaics and modern humans. Under this definition, modern humans are referred to as Homo sapiens sapiens and archaics are also designated with the prefix "Homo sapiens". For example, the Neanderthals are Homo sapiens neanderthalensis, and Homo heidelbergensis is Homo sapiens heidelbergensis. Other taxonomists prefer not to consider archaics and modern humans as a single species but as several different species. In this case the standard taxonomy is used, i.e. Homo rhodesiensis, or Homo neanderthalensis.

The evolutionary dividing lines that separate modern humans from archaic humans and archaic humans from Homo erectus are unclear. The earliest known fossils of anatomically modern humans such as the Omo remains from 233,000-195,000 years ago, Homo sapiens idaltu from 160,000 years ago, and Qafzeh remains from 90,000 years ago are recognizably modern humans. These early modern humans possess a number of archaic traits, such as moderate, but not prominent, brow ridges.

Brain size expansion

Anatomical comparison of the skulls of anatomically modern humans (left) and Homo neanderthalensis (right)

The emergence of archaic humans is sometimes used as an example of punctuated equilibrium. This occurs when a species undergoes significant biological evolution within a relatively short period. Subsequently, the species undergoes very little change for long periods until the next punctuation. The brain size of archaic humans expanded significantly from 900 cm3 (55 cu in) in erectus to 1,300 cm3 (79 cu in). Since the peak of human brain size during the archaics, it has begun to decline.

Origin of language

Robin Dunbar has argued that archaic humans were the first to use language. Based on his analysis of the relationship between brain size and hominin group size, he concluded that because archaic humans had large brains, they must have lived in groups of over 120 individuals. Dunbar argues that it was not possible for hominins to live in such large groups without using language, otherwise there could be no group cohesion and the group would disintegrate. By comparison, chimpanzees live in smaller groups of up to 50 individuals.

Fossils

Wednesday, April 17, 2024

Volcanism on Io

From Wikipedia, the free encyclopedia
Io, with two plumes erupting from its surface

Volcanism on Io, a moon of Jupiter, is represented by the presence of volcanoes, volcanic pits and lava flows on the surface. Io's volcanic activity was discovered in 1979 by Linda Morabito, an imaging scientist working on Voyager 1. Observations of Io by passing spacecraft and Earth-based astronomers have revealed more than 150 active volcanoes. As of 2004, up to 400 such volcanoes are predicted to exist based on these observations. Io's volcanism makes the satellite one of only four known currently volcanically or cryovolcanically active worlds in the Solar System (the others being Earth, Saturn's moon Enceladus, and Neptune's moon Triton.)

First predicted shortly before the Voyager 1 flyby, the heat source for Io's volcanism comes from tidal heating produced by its forced orbital eccentricity. This differs from Earth's internal heating, which is derived primarily from radioactive isotope decay and primordial heat of accretion. Io's eccentric orbit leads to a slight difference in Jupiter's gravitational pull on the satellite between its closest and farthest points on its orbit, causing a varying tidal bulge. This variation in the shape of Io causes frictional heating in its interior. Without this tidal heating, Io might have been similar to the Moon, a world of similar size and mass, geologically dead and covered with numerous impact craters.

Io's volcanism has led to the formation of hundreds of volcanic centres and extensive lava formations, making it the most volcanically active body in the Solar System. Three different types of volcanic eruptions have been identified, differing in duration, intensity, lava effusion rate, and whether the eruption occurs within a volcanic pit (known as a patera). Lava flows on Io, tens or hundreds of kilometres long, have primarily basaltic composition, similar to lavas seen on Earth at shield volcanoes such as Kīlauea in Hawaii. Although most of the lava on Io is made of basalt, a few lava flows consisting of sulfur and sulfur dioxide have been seen. In addition, eruption temperatures as high as 1,600 K (1,300 °C; 2,400 °F) were detected, which can be explained by the eruption of high-temperature ultramafic silicate lavas.

As a result of the presence of significant quantities of sulfurous materials in Io's crust and on its surface, some eruptions propel sulfur, sulfur dioxide gas, and pyroclastic material up to 500 kilometres (310 mi) into space, producing large, umbrella-shaped volcanic plumes. This material paints the surrounding terrain in red, black, and/or white, and provides material for Io's patchy atmosphere and Jupiter's extensive magnetosphere. Spacecraft that have flown by Io since 1979 have observed numerous surface changes as a result of Io's volcanic activity.

Further observations by the Juno orbiter of volcanism and volcanic plumes on Io were made during a 3 February 2024 flyby.

Discovery

Discovery image of active volcanism on Io. The plumes of Pele and Loki are visible above the limb and at the terminator, respectively.

Before the Voyager 1 encounter with Io on March 5, 1979, Io was thought to be a dead world much like the Moon. The discovery of a cloud of sodium surrounding Io led to theories that the satellite would be covered in evaporites.

Hints of discoveries to come arose from Earth-based infrared observations taken in the 1970s. An anomalously high thermal flux, compared to the other Galilean satellites, was discovered during measurements taken at an infrared wavelength of 10 μm while Io was in Jupiter's shadow. At the time, this heat flux was attributed to the surface having a much higher thermal inertia than Europa and Ganymede. These results were considerably different from measurements taken at wavelengths of 20 μm, which suggested that Io had similar surface properties to the other Galilean satellites. Robert Nelson and Bruce Hapke attempted to explain these features in Io's spectrum by suggesting fumarolic activity as a mechanism for producing short-chain sulfur allotropes on Io's surface. It has since been determined that the greater flux at shorter wavelengths was due to the combined flux from Io's volcanoes and solar heating, whereas solar heating provides a much greater fraction of the flux at longer wavelengths. A sharp increase in Io's thermal emission at 5 μm was observed on February 20, 1978 by Witteborn, et al. The group considered volcanic activity at the time, in which case the data was fit into a region on Io 8,000 square kilometres (3,100 sq mi) in size at 600 K (300 °C; 600 °F). However, the authors considered that hypothesis unlikely, and instead focused on emission from Io's interaction with Jupiter's magnetosphere.

Shortly before the Voyager 1 encounter, Stan Peale, Patrick Cassen, and R. T. Reynolds published a paper in the journal Science predicting a volcanically modified surface and a differentiated interior, with distinct rock types rather than a homogeneous blend. They based this prediction on models of Io's interior that took into account the massive amount of heat produced by the varying tidal pull of Jupiter on Io caused by its slightly eccentric orbit. Their calculations suggested that the amount of heat generated for an Io with a homogeneous interior would be three times greater than the amount of heat generated by radioactive isotope decay alone. This effect would be even greater with a differentiated Io.

Voyager 1 observation of Loki Patera and nearby lava flows and volcanic pits

Voyager 1's first images of Io revealed a lack of impact craters, suggesting a very young surface. Craters are used by geologists to estimate the age of a planetary surface; the number of impact structures increase with the age of the planetary surface. Instead, Voyager 1 observed a multi-coloured surface, pockmarked with irregular-shaped depressions, which lacked the raised rims characteristic of impact craters. Voyager 1 also observed flow features formed by low-viscosity fluid and tall, isolated mountains that did not resemble terrestrial volcanoes. The surface observed suggested that, just as Peale and colleagues had theorized, Io was heavily modified by volcanism.

On March 8, 1979, three days after passing Jupiter, Voyager 1 took images of Jupiter's moons to help mission controllers determine the spacecraft's exact location, a process called optical navigation. By processing images of Io to enhance the visibility of background stars, navigation engineer Linda Morabito found a 300-kilometre (190 mi) tall cloud along its limb. At first, she suspected the cloud to be a moon behind Io, but no suitably sized body would have been in that location. The feature was determined to be a plume generated by active volcanism at a dark depression later named Pele. Following this discovery, eight other plumes were located in Voyager images of Io. These plumes were later named after mythological deities associated with fire, volcanoes, or mayhem: Loki (two separate plumes), Prometheus, Volund, Amirani, Maui, Marduk, and Masubi. Thermal emission from multiple sources, indicative of cooling lava, were also found. Surface changes were observed when images that had been acquired by Voyager 2 were compared to those taken four months previously by Voyager 1, including new plume deposits at Aten Patera and Surt.

Heat source

Io's main source of internal heat comes from the tidal forces generated by Jupiter's gravitational pull. This external heating differs from the internal heat source for volcanism on Earth, which is a result of radioactive isotope decay and residual heat from accretion. In the Earth, these internal heat sources drive mantle convection, which in turn causes volcanism through plate tectonics.

The tidal heating of Io is dependent on its distance from Jupiter, its orbital eccentricity, the composition of its interior, and its physical state. Its Laplace orbital resonance with Europa and Ganymede maintains Io's eccentricity and prevents tidal dissipation within Io from circularizing its orbit. The eccentricity leads to vertical differences in Io's tidal bulge of as much as 100 metres (330 ft) as Jupiter's gravitational pull varies between the periapsis and apoapsis points in Io's orbit. This varying tidal pull also produces friction in Io's interior, enough to cause significant tidal heating and melting. Unlike Earth, where most of its internal heat is released by conduction through the crust, on Io internal heat is released via volcanic activity and generates the satellite's high heat flow (global total: 0.6–1.6 × 1014 W). Models of its orbit suggest that the amount of tidal heating within Io changes with time, and that the current heat flow is not representative of the long-term average. The observed release of heat from Io's interior is greater than estimates for the amount presently generated from tidal heating, suggesting that Io is cooling after a period of greater flexing.

Composition

Jupiter moon Io volcanic activity (12/14/2022/left and 03/01/2023)
Voyager 1 image of volcanic pits and lava flows near Ra Patera

Analysis of Voyager images led scientists to believe that the lava flows on Io were composed mostly of various forms of molten elemental sulfur. The colouration of the flows was found to be similar to its various allotropes. Differences in the lava colour and brightness are a function of the temperature of polyatomic sulfur and the packing and bonding of its atoms. An analysis of the flows that radiate out from Ra Patera revealed differently colored materials, all associated with liquid sulfur, at different distances from the vent: dark albedo material close to the vent at 525 K (252 °C; 485 °F), red material in the central part of each flow at 450 K (177 °C; 350 °F), and orange material at the farthest ends of each flow at 425 K (152 °C; 305 °F). This colour pattern corresponds to flows radiating out from a central vent, cooling as the lava travels away from it. In addition, temperature measurements of thermal emission at Loki Patera taken by Voyager 1's Infrared Interferometer Spectrometer and Radiometer (IRIS) instrument were consistent with sulfur volcanism. However, the IRIS instrument was not capable of detecting wavelengths that are indicative of higher temperatures. This meant that temperatures consistent with silicate volcanism were not discovered by Voyager. Despite this, Voyager scientists deduced that silicates must play a role in Io's youthful appearance, from its high density and the need for silicates to support the steep slopes along patera walls. The contradiction between the structural evidence and the spectral and temperature data following the Voyager flybys led to a debate in the planetary science community regarding the composition of Io's lava flows, whether they were composed of silicate or sulfurous materials.

Earth-based infrared studies in the 1980s and 1990s shifted the paradigm from one of primarily sulfur volcanism to one where silicate volcanism dominates, and sulfur acts in a secondary role. In 1986, measurements of a bright eruption on Io's leading hemisphere revealed temperatures of at least 900 K (600 °C; 1,200 °F). This is higher than the boiling point of sulfur (715 K or 442 °C or 827 °F), indicating a silicate composition for at least some of Io's lava flows. Similar temperatures were also observed at the Surt eruption in 1979 between the two Voyager encounters, and at the eruption observed by Witteborn and colleagues in 1978. In addition, modeling of silicate lava flows on Io suggested that they cooled rapidly, causing their thermal emission to be dominated by lower temperature components, such as solidified flows, as opposed to the small areas covered by still molten lava near the actual eruption temperature.

Thermal emission map of Io by Galileo

Silicate volcanism, involving basaltic lava with mafic to ultramafic (magnesium-rich) compositions, was confirmed by the Galileo spacecraft in the 1990s and 2000s from temperature measurements of Io's numerous hot spots, locations where thermal emission is detected, and from spectral measurements of Io's dark material. Temperature measurements from Galileo's Solid-State Imager (SSI) and Near-Infrared Mapping Spectrometer (NIMS) revealed numerous hot spots with high-temperature components ranging from at least 1,200 K (900 °C; 1,700 °F) to a maximum of 1,600 K (1,300 °C; 2,400 °F), like at the Pillan Patera eruption in 1997. Initial estimates during the course of the Galileo mission suggesting eruption temperatures approaching 2,000 K (1,700 °C; 3,100 °F) have since proven to be overestimates because the wrong thermal models were used to calculate the temperatures. Spectral observations of Io's dark material suggested the presence of orthopyroxenes, such as enstatite, which are magnesium-rich silicate minerals common in mafic and ultramafic basalt. This dark material is seen in volcanic pits, fresh lava flows, and pyroclastic deposits surrounding recent, explosive volcanic eruptions. Based on the measured temperature of the lava and the spectral measurements, some of the lava may be analogous to terrestrial komatiites. Compressional superheating, which could increase the temperature of magma during ascent to the surface during an eruption, may also be a factor in some of the higher temperature eruptions.

Although temperature measurements of Io's volcanoes settled the sulfur-versus-silicates debate that persisted between the Voyager and Galileo missions at Jupiter, sulfur and sulfur dioxide still play a significant role in the phenomena observed on Io. Both materials have been detected in the plumes generated at Io's volcanoes, with sulfur being a primary constituent of Pele-type plumes. Bright flows have been identified on Io, at Tsũi Goab Fluctus, Emakong Patera, and Balder Patera for example, that are suggestive of effusive sulfur or sulfur dioxide volcanism.

Eruption styles

Observations of Io by spacecraft and Earth-based astronomers have led to the identification of differences in the types of eruptions seen on the satellite. The three main types identified include intra-patera, flow-dominated, and explosion-dominated eruptions. They differ in terms of duration, energy released, brightness temperature (determined from infrared imaging), type of lava flow, and whether it is confined within volcanic pits.

Intra-patera eruptions

Tupan Patera, an example of a volcanic depression

Intra-patera eruptions occur within volcanic depressions known as paterae, which generally have flat floors bounded by steep walls. Paterae resemble terrestrial calderas, but it is unknown whether they form when an empty magma chamber collapses, like their terrestrial cousins. One hypothesis suggests that they are produced through the exhumation of volcanic sills, with the overlying material either being blasted out or integrated into the sill. Some paterae display evidence for multiple collapses, similar to the calderas atop Olympus Mons on Mars or Kīlauea on Earth, suggesting that they may occasionally form like volcanic calderas. Because the formation mechanism is still uncertain, the general term for these features uses the Latin descriptor term employed by the International Astronomical Union in naming them, paterae. Unlike similar features on Earth and Mars, these depressions generally do not lie at the peak of shield volcanoes and are larger, with an average diameter of 41 kilometres (25 mi). Patera depths have been measured for only a few paterae and typically exceed 1 km. The largest volcanic depression on Io is Loki Patera at 202 kilometres (126 mi) across. Whatever the formation mechanism, the morphology and distribution of many paterae suggest that they are structurally controlled, with at least half bounded by faults or mountains.

Infrared image showing night-time thermal emission from the lava lake Pele

This eruption style can take the form of either lava flows, spreading across the floor of the paterae, or lava lakes. Except for observations by Galileo during its seven close flybys, it can be difficult to tell the difference between a lava lake and a lava flow eruption on a patera floor, due to inadequate resolution and similar thermal emission characteristics. Intra-patera lava flow eruptions, such as the Gish Bar Patera eruption in 2001, can be just as voluminous as those seen spreading out across the Ionian plains. Flow-like features have also been observed within a number of paterae, like Camaxtli Patera, suggesting that lava flows periodically resurface their floors.

Ionian lava lakes are depressions partially filled with molten lava covered by a thin solidified crust. These lava lakes are directly connected to a magma reservoir lying below. Observations of thermal emission at several Ionian lava lakes reveal glowing molten rock along the patera margin, caused by the lake's crust breaking up along the edge of the patera. Over time, because the solidified lava is denser than the still-molten magma below, this crust can founder, triggering an increase in thermal emission at the volcano. For some lava lakes, like the one at Pele, this occurs continuously, making Pele one of the brightest emitters of heat in the near-infrared spectrum on Io. At other sites, such as at Loki Patera, this can occur episodically. During an overturning episode at these more quiescent lava lakes, a wave of foundering crust spreads out across the patera at the rate of about 1 kilometre (0.6 mi) per day, with new crust forming behind it until the entire lake has been resurfaced. Another eruption would only begin once the new crust has cooled and thickened enough for it to no longer be buoyant over the molten lava. During an overturning episode, Loki can emit up to ten times more heat than when its crust is stable.

Flow-dominated eruptions (Promethean Volcanism)

Culann Patera, an example of a flow-dominated eruption

Flow-dominated eruptions are long-lived events that build up extensive, compound lava flows. The extent of these flows makes them a major terrain type on Io. In this style of eruption, magma emerges onto the surface from vents on the floor of paterae, vents surrounding paterae, or from fissures on the plains, producing inflated, compound lava flows similar to those seen at Kīlauea in Hawaii. Images from the Galileo spacecraft revealed that many of Io's major flows, like those at Prometheus and Amirani, are produced by the build-up of small breakouts of lava on top of older flows. Flow-dominated eruptions differ from explosion-dominated eruptions by their longevity and their lower energy output per unit of time. Lava erupts at a generally steady rate, and flow-dominated eruptions can last for years or decades.

Active flow fields more than 300 kilometres (190 mi) long have been observed on Io at Amirani and Masubi. A relatively inactive flow field named Lei-Kung Fluctus covers more than 125,000 square kilometres (48,000 sq mi), an area slightly larger than Nicaragua. The thickness of flow fields was not determined by Galileo, but the individual breakouts on their surface are likely to be 1 m (3 ft) thick. In many cases, active lava breakouts flow out onto the surface at locations tens to hundreds of kilometres from the source vent, with low amounts of thermal emission observed between it and the breakout. This suggests that lava flows through lava tubes from the source vent to the breakout.

Although these eruptions generally have a steady eruption rate, larger outbreaks of lava have been observed at many flow-dominated eruption sites. For example, the leading edge of the Prometheus flow field moved 75 to 95 kilometres (47 to 59 mi) between observations by Voyager in 1979 and Galileo in 1996. Although generally dwarfed by explosion-dominated eruptions, the average flow rate at these compound flow fields is much greater than what is observed at similar contemporary lava flows on Earth. Average surface coverage rates of 35–60 square metres (380–650 sq ft) per second were observed at Prometheus and Amirani during the Galileo mission, compared to 0.6 square metres (6.5 sq ft) per second at Kīlauea.

Explosion-dominated eruptions (Pillanian Volcanism)

Galileo images of active lava flows and fountains at Tvashtar Paterae in 1999

Explosion-dominated eruptions are the most pronounced of Io's eruption styles. These eruptions, sometimes called "outburst" eruptions from their Earth-based detections, are characterized by their short duration (lasting only weeks or months), rapid onset, large volumetric flow rates, and high thermal emission. They lead to a short-lived, significant increase in Io's overall brightness in the near-infrared. The most powerful volcanic eruption observed on Io was an "outburst" eruption at Surt, observed by Earth-based astronomers on February 22, 2001.

Explosion-dominated eruptions occur when a body of magma (called a dike) from deep within Io's partially molten mantle reaches the surface at a fissure. This results in a spectacular display of lava fountains. During the beginning of the outburst eruption, thermal emission is dominated by strong, 1–3 μm infrared radiation. It is produced by a large amount of exposed, fresh lava within the fountains at the eruption source vent. Outburst eruptions at Tvashtar in November 1999 and February 2007 centred on a 25-kilometre (16 mi) long, 1-kilometre (0.62 mi) tall lava "curtain" produced at a small patera nested within the larger Tvashtar Paterae complex.

The large amount of exposed molten lava at these lava fountains has provided researchers with their best opportunity to measure the actual temperatures of Ionian lavas. Temperatures suggestive of an ultramafic lava composition similar to Pre-Cambrian komatiites (about 1,600 K or 1,300 °C or 2,400 °F) are dominant at such eruptions, though superheating of the magma during ascent to the surface cannot be ruled out as a factor in the high eruption temperatures.

Two Galileo images, taken 168 days apart, showing the effects of an explosion-dominated eruption at Pillan Patera in 1997

Although the more explosive, lava-fountaining stage may last only a few days to a week, explosion-dominated eruptions can continue for weeks to months, producing large, voluminous silicate lava flows. A major eruption in 1997 from a fissure north-west of Pillan Patera produced more than 31 cubic kilometres (7.4 cu mi) of fresh lava over a 2+12- to 5+12-month period, and later flooded the floor of Pillan Patera. Observations by Galileo suggest lava coverage rates at Pillan between 1,000 and 3,000 square metres (11,000 and 32,000 sq ft) per second during the 1997 eruption. The Pillan flow was found to be 10 m (33 ft) thick, compared to the 1 m (3 ft) thick flows observed at the inflated fields at Prometheus and Amirani. Similar, rapidly emplaced lava flows were observed by Galileo at Thor in 2001. Such flow rates are similar to those seen at Iceland's Laki eruption in 1783 and in terrestrial flood basalt eruptions.

Explosion-dominated eruptions can produce dramatic (but often short-lived) surface changes around the eruption site, such as large pyroclastic and plume deposits produced as gas exsolves from lava fountains. The 1997 Pillan eruption produced a 400 km (250 mi) wide deposit of dark, silicate material and bright sulfur dioxide. The Tvashtar eruptions of 2000 and 2007 generated a 330 km (210 mi) tall plume that deposited a ring of red sulfur and sulfur dioxide 1,200 km (750 mi) wide. Despite the dramatic appearance of these features, without continuous resupply of material, the vent surroundings often revert to their pre-eruption appearance over a period of months (in the case of Grian Patera) or years (as at Pillan Patera).

Plumes

Sequence of five New Horizons images, taken over eight minutes, showing Io's volcano Tvashtar erupting material 330 kilometres (210 mi) above its surface

The discovery of volcanic plumes at Pele and Loki in 1979 provided conclusive evidence that Io was geologically active. Generally, plumes form when volatiles like sulfur and sulfur dioxide are ejected skyward from Io's volcanoes at speeds reaching 1 kilometre per second (0.62 mi/s), creating umbrella-shaped clouds of gas and dust. Additional materials that might be found in the volcanic plumes include sodium, potassium, and chlorine. Although striking in appearance, volcanic plumes are relatively uncommon. Of the 150 or so active volcanoes observed on Io, plumes have only been observed at a couple of dozen of them. The limited area of Io's lava flows suggests that much of the resurfacing needed to erase Io's cratering record must come from plume deposits.

A plume, about 100 km high, erupting from the Masubi region of Io in July 1999

The most common type of volcanic plume on Io are dust plumes, or Prometheus-type plumes, produced when encroaching lava flows vaporize underlying sulfur dioxide frost, sending the material skyward. Examples of Prometheus-type plumes include Prometheus, Amirani, Zamama, and Masubi. These plumes are usually less than 100 kilometres (62 mi) tall with eruption velocities around 0.5 kilometres per second (0.31 mi/s). Prometheus-type plumes are dust-rich, with a dense inner core and upper canopy shock zone, giving them an umbrella-like appearance. These plumes often form bright circular deposits, with a radius ranging between 100 and 250 kilometres (62 and 155 mi) and consisting primarily of sulfur dioxide frost. Prometheus-type plumes are frequently seen at flow-dominated eruptions, helping make this plume type quite long-lived. Four out of the six Prometheus-type plumes observed by Voyager 1 in 1979 were also observed throughout the Galileo mission and by New Horizons in 2007. Although the dust plume can be clearly seen in sunlit visible-light images of Io acquired by passing spacecraft, many Prometheus-type plumes have an outer halo of fainter, more gas-rich material reaching heights approaching that of the larger, Pele-type plumes.

Io's largest plumes, Pele-type plumes, are created when sulfur and sulfur dioxide gas exsolve from erupting magma at volcanic vents or lava lakes, carrying silicate pyroclastic material with them. The few Pele-type plumes that have been observed are usually associated with explosion-dominated eruptions, and are short-lived. The exception to this is Pele, which is associated with a long-lived active lava lake eruption, though the plume is thought to be intermittent. The higher vent temperatures and pressures associated with these plumes generate eruption speeds of up to 1 kilometre per second (0.62 mi/s), allowing them to reach heights of between 300 and 500 kilometres (190 and 310 mi). Pele-type plumes form red (from short-chain sulfur) and black (from silicate pyroclastics) surface deposits, including large 1,000 kilometres (620 mi)-wide red rings, as seen at Pele. The erupted sulfurous components of Pele-type plumes are thought to be the result of an excess amount of sulfur in Io's crust and a decrease in sulfur solubility at greater depths in Io's lithosphere. They are generally fainter than Prometheus-type plumes as a result of the low dust content, causing some to be called stealth plumes. These plumes are sometimes only seen in images acquired while Io is in the shadow of Jupiter or those taken in ultraviolet. The little dust that is visible in sunlit images is generated when sulfur and sulfur dioxide condense as the gases reach the top of their ballistic trajectories. That is why these plumes lack the dense central column seen in Prometheus-type plumes, in which dust is generated at the plume source. Examples of Pele-type plumes have been observed at Pele, Tvashtar, and Grian.

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