Plant perception (physiology)

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https://en.wikipedia.org/wiki/Plant_perception_(physiology) 

The leaf closing after touch in Mimosa pudica depends on electrical signals

Plant perception is the ability of plants to sense and respond to the environment by adjusting their morphology and physiology. Botanical research has revealed that plants are capable of reacting to a broad range of stimuli, including chemicals, gravity, light, moisture, infections, temperature, oxygen and carbon dioxide concentrations, parasite infestation, disease, physical disruption, sound, and touch. The scientific study of plant perception is informed by numerous disciplines, such as plant physiology, ecology, and molecular biology.

Aspects of perception

Light

Main articles: Phototropism, photomorphogenesis, photoperiodism, and shade avoidance

 

The sunflower, a common heliotropic plant which perceives and reacts to sunlight by slow turning movement

Many plant organs contain photoreceptors (phototropins, cryptochromes, and phytochromes), each of which reacts very specifically to certain wavelengths of light. These light sensors tell the plant if it is day or night, how long the day is, how much light is available, and where the light is coming from. Shoots generally grow towards light, while roots grow away from it, responses known as phototropism and skototropism, respectively. They are brought about by light-sensitive pigments like phototropins and phytochromes and the plant hormone auxin.

Many plants exhibit certain behaviors at specific times of the day; for example, flowers that open only in the mornings. Plants keep track of the time of day with a circadian clock. This internal clock is synchronized with solar time every day using sunlight, temperature, and other cues, similar to the biological clocks present in other organisms. The internal clock coupled with the ability to perceive light also allows plants to measure the time of the day and so determine the season of the year. This is how many plants know when to flower (see photoperiodism). The seeds of many plants sprout only after they are exposed to light. This response is carried out by phytochrome signalling. Plants are also able to sense the quality of light and respond appropriately. For example, in low light conditions, plants produce more photosynthetic pigments. If the light is very bright or if the levels of harmful ultraviolet radiation increase, plants produce more of their protective pigments that act as sunscreens.

Gravity

Main article: Gravitropism

To orient themselves correctly, plants must be able to sense the direction of gravity. The subsequent response is known as gravitropism.

In roots, gravity is sensed and translated in the root tip, which then grows by elongating in the direction of gravity. In shoots, growth occurs in the opposite direction, a phenomenon known as negative gravitropism. Poplar stems can detect reorientation and inclination (equilibrioception) through gravitropism.

Vine (Vitis) tendril. Note how the plant reaches for and wraps around the galvanised wire provided for the purpose. This is a very tough twig and appears to have no other purpose than support for the plant. Nothing else grows from it. It must reach out softly, then wrap around and then dry and toughen. See more at thigmotropism.

At the root tip, amyloplasts containing starch granules fall in the direction of gravity. This weight activates secondary receptors, which signal to the plant the direction of the gravitational pull. After this occurs, auxin is redistributed through polar auxin transport and differential growth towards gravity begins. In the shoots, auxin redistribution occurs in a way to produce differential growth away from gravity.

For perception to occur, the plant often must be able to sense, perceive, and translate the direction of gravity. Without gravity, proper orientation will not occur and the plant will not effectively grow. The root will not be able to uptake nutrients or water, and the shoot will not grow towards the sky to maximize photosynthesis.

Touch

Main articles: Thigmotropism and thigmomorphogenesis

All plants are able to sense touch. Thigmotropism is directional movement that occurs in plants responding to physical touch. Climbing plants, such as tomatoes, exhibit thigmotropism, allowing them to curl around objects. These responses are generally slow (on the order of multiple hours), and can best be observed with time-lapse cinematography, but rapid movements can occur as well. For example, the so-called "sensitive plant" (Mimosa pudica) responds to even the slightest physical touch by quickly folding its thin pinnate leaves such that they point downwards, and carnivorous plants such as the Venus flytrap (Dionaea muscipula) produce specialized leaf structures that snap shut when touched or landed upon by insects. In the Venus flytrap, touch is detected by cilia lining the inside of the specialized leaves, which generate an action potential that stimulates motor cells and causes movement to occur.

Smell

Wounded or infected plants produce distinctive volatile odors, (e.g. methyl jasmonate, methyl salicylate, green leaf volatiles), which can in turn be perceived by neighboring plants. Plants detecting these sorts of volatile signals often respond by increasing their chemical defenses or and prepare for attack by producing chemicals which defend against insects or attract insect predators.

Vibration

Plants upregulate chemical defenses such as glucosinolate and anthocyanin in response to vibrations created during herbivory.

Signal transduction

Plant hormones and chemical signals

Further information: Plant hormone

Plants systematically use hormonal signalling pathways to coordinate their development and morphology.

Plants produce several signal molecules usually associated with animal nervous systems, such as glutamate, GABA, acetylcholine, melatonin, and serotonin. They may also use ATP, NO, and ROS for signaling in similar ways as animals do.

Electrophysiology

Plants have a variety of methods of delivering electrical signals. The four commonly recognized propagation methods include action potentials (APs), variation potentials (VPs), local electric potentials (LEPs), and systemic potentials (SPs)

Although plant cells are not neurons, they can be electrically excitable and can display rapid electrical responses in the form of APs to environmental stimuli. APs allow for the movement of signaling ions and molecules from the pre-potential cell to the post-potential cell(s). These electrophysiological signals are constituted by gradient fluxes of ions such as H⁺, K⁺, Cl⁻, Na⁺, and Ca²⁺ but it is also thought that other electrically charge ions such as Fe³⁺, Al³⁺, Mg²⁺, Zn²⁺, Mn²⁺, and Hg²⁺ may also play a role in downstream outputs. The maintenance of each ions electrochemical gradient is vital in the health of the cell in that if the cell would ever reach equilibrium with its environment, it is dead. This dead state can be due to a variety of reasons such as ion channel blocking or membrane puncturing.

These electrophysiological ions bind to receptors on the receiving cell causing downstream effects result from one or a combination of molecules present. This means of transferring information and activating physiological responses via a signaling molecule system has been found to be faster and more frequent in the presence of APs.

These action potentials can influence processes such as actin-based cytoplasmic streaming, plant organ movements, wound responses, respiration, photosynthesis, and flowering. These electrical responses can cause the synthesis of numerous organic molecules, including ones that act as neuroactive substances in other organisms such as calcium ions.

The ion flux across cells also influence the movement of other molecules and solutes. This changes the osmotic gradient of the cell, resulting in changes to turgor pressure in plant cells by water and solute flux across cell membranes. These variations are vital for nutrient uptake, growth, many types of movements (tropisms and nastic movements) among other basic plant physiology and behavior.(Higinbotham 1973; Scott 2008; Segal 2016).

Thus, plants achieve behavioural responses in environmental, communicative, and ecological contexts.

Signal perception

Plant behavior is mediated by phytochromes, kinins, hormones, antibiotic or other chemical release, changes of water and chemical transport, and other means.

Plants have many strategies to fight off pests. For example, they can produce a slew of different chemical toxins against predators and parasites or they can induce rapid cell death to prevent the spread of infectious agents. Plants can also respond to volatile signals produced by other plants. Jasmonate levels also increase rapidly in response to mechanical perturbations such as tendril coiling.

In plants, the mechanism responsible for adaptation is signal transduction. Adaptive responses include:

• Active foraging for light and nutrients. They do this by changing their architecture, e.g. branch growth and direction, physiology, and phenotype.
• Leaves and branches being positioned and oriented in response to a light source.
• Detecting soil volume and adapting growth accordingly, independently of nutrient availability.
• Defending against herbivores.

Plant intelligence

Further information: Plant cognition

Plants do not have brains or neuronal networks like animals do; however, reactions within signalling pathways may provide a biochemical basis for learning and memory in addition to computation and basic problem solving. Controversially, the brain is used as a metaphor by some plant perception researchers to provide an integrated view of signalling.

Plants respond to environmental stimuli by movement and changes in morphology. They communicate while actively competing for resources. In addition, plants accurately compute their circumstances, use sophisticated cost–benefit analysis, and take tightly controlled actions to mitigate and control diverse environmental stressors. Plants are also capable of discriminating between positive and negative experiences and of learning by registering memories from their past experiences. Plants use this information to adapt their behaviour in order to survive present and future challenges of their environments.

Plant physiology studies the role of signalling to integrate data obtained at the genetic, biochemical, cellular, and physiological levels, in order to understand plant development and behaviour. The neurobiological view sees plants as information-processing organisms with rather complex processes of communication occurring throughout the individual plant. It studies how environmental information is gathered, processed, integrated, and shared (sensory plant biology) to enable these adaptive and coordinated responses (plant behaviour); and how sensory perceptions and behavioural events are 'remembered' in order to allow predictions of future activities upon the basis of past experiences. Plants, it is claimed by some plant physiologists, are as sophisticated in behaviour as animals, but this sophistication has been masked by the time scales of plants' responses to stimuli, which are typically many orders of magnitude slower than those of animals.

It has been argued that although plants are capable of adaptation, it should not be called intelligence per se, as plant neurobiologists rely primarily on metaphors and analogies to argue that complex responses in plants can only be produced by intelligence. "A bacterium can monitor its environment and instigate developmental processes appropriate to the prevailing circumstances, but is that intelligence? Such simple adaptation behaviour might be bacterial intelligence but is clearly not animal intelligence." However, plant intelligence fits a definition of intelligence proposed by David Stenhouse in a book about evolution and animal intelligence, in which he describes it as "adaptively variable behaviour during the lifetime of the individual". Critics of the concept have also argued that a plant cannot have goals once it is past the developmental stage of seedling because, as a modular organism, each module seeks its own survival goals and the resulting organism-level behavior is not centrally controlled. This view, however, necessarily accommodates the possibility that a tree is a collection of individually intelligent modules cooperating, competing, and influencing each other to determine behavior in a bottom-up fashion. The development into a larger organism whose modules must deal with different environmental conditions and challenges is not universal across plant species, however, as smaller organisms might be subject to the same conditions across their bodies, at least, when the below and aboveground parts are considered separately. Moreover, the claim that central control of development is completely absent from plants is readily falsified by apical dominance.

The Italian botanist Federico Delpino wrote on the idea of plant intelligence in 1867. Charles Darwin studied movement in plants and in 1880 published a book, The Power of Movement in Plants. Darwin concludes:

It is hardly an exaggeration to say that the tip of the radicle thus endowed [..] acts like the brain of one of the lower animals; the brain being situated within the anterior end of the body, receiving impressions from the sense-organs, and directing the several movements.

In 2020, Paco Calvo studied the dynamic of plant movements and investigated whether French beans deliberately aim for supporting structures. Calvo said: “We see these signatures of complex behaviour, the one and only difference being is that it’s not neural-based, as it is in humans. This isn’t just adaptive behaviour, it’s anticipatory, goal-directed, flexible behaviour.”

In philosophy, there are few studies of the implications of plant perception. Michael Marder put forth a phenomenology of plant life based on the physiology of plant perception. Paco Calvo Garzon offers a philosophical take on plant perception based on the cognitive sciences and the computational modeling of consciousness.

Comparison with neurobiology

Plant sensory and response systems have been compared to the neurobiological processes of animals. Plant neurobiology concerns mostly the sensory adaptive behaviour of plants and plant electrophysiology. Indian scientist J. C. Bose is credited as the first person to research and talk about the neurobiology of plants. Many plant scientists and neuroscientists, however, view the term "plant neurobiology" as a misnomer, because plants do not have neurons.

The ideas behind plant neurobiology were criticised in a 2007 article published in Trends in Plant Science by Amedeo Alpi and 35 other scientists, including such eminent plant biologists as Gerd Jürgens, Ben Scheres, and Chris Sommerville. The breadth of fields of plant science represented by these researchers reflects the fact that the vast majority of the plant science research community rejects plant neurobiology as a legitimate notion. Their main arguments are that:

• "Plant neurobiology does not add to our understanding of plant physiology, plant cell biology or signaling".
• "There is no evidence for structures such as neurons, synapses or a brain in plants".
• The common occurrence of plasmodesmata in plants "poses a problem for signaling from an electrophysiological point of view", since extensive electrical coupling would preclude the need for any cell-to-cell transport of ‘neurotransmitter-like' compounds.

The authors call for an end to "superficial analogies and questionable extrapolations" if the concept of "plant neurobiology" is to benefit the research community. Several responses to this criticism have attempted to clarify that the term "plant neurobiology" is a metaphor and that metaphors have proved useful on previous occasions. Plant ecophysiology describes this phenomenon.

Parallels in other taxa

The concepts of plant perception, communication, and intelligence have parallels in other biological organisms for which such phenomena appear foreign to or incompatible with traditional understandings of biology, or have otherwise proven difficult to study or interpret. Similar mechanisms exist in bacterial cells, choanoflagellates, fungal hyphae, and sponges, among many other examples. All of these organisms, despite being devoid of a brain or nervous system, are capable of sensing their immediate and momentary environment and responding accordingly. In the case of unicellular life, the sensory pathways are even more primitive in the sense that they take place on the surface of a single cell, as opposed to within a network of many related cells.

Thermal runaway

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Thermal_runaway 

 

Diagram of thermal runaway

Thermal runaway describes a process that is accelerated by increased temperature, in turn releasing energy that further increases temperature. Thermal runaway occurs in situations where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result. It is a kind of uncontrolled positive feedback.

In chemistry (and chemical engineering), thermal runaway is associated with strongly exothermic reactions that are accelerated by temperature rise. In electrical engineering, thermal runaway is typically associated with increased current flow and power dissipation. Thermal runaway can occur in civil engineering, notably when the heat released by large amounts of curing concrete is not controlled. In astrophysics, runaway nuclear fusion reactions in stars can lead to nova and several types of supernova explosions, and also occur as a less dramatic event in the normal evolution of solar-mass stars, the "helium flash".

Some climate researchers have postulated that a global average temperature increase of 3–4 degrees Celsius above the preindustrial baseline could lead to a further unchecked increase in surface temperatures. For example, releases of methane, a greenhouse gas more potent than CO₂, from wetlands, melting permafrost and continental margin seabed clathrate deposits could be subject to positive feedback.

Chemical engineering

Chemical reactions involving thermal runaway are also called thermal explosions in chemical engineering, or runaway reactions in organic chemistry. It is a process by which an exothermic reaction goes out of control: the reaction rate increases due to an increase in temperature, causing a further increase in temperature and hence a further rapid increase in the reaction rate. This has contributed to industrial chemical accidents, most notably the 1947 Texas City disaster from overheated ammonium nitrate in a ship's hold, and the 1976 explosion of zoalene, in a drier, at King's Lynn. Frank-Kamenetskii theory provides a simplified analytical model for thermal explosion. Chain branching is an additional positive feedback mechanism which may also cause temperature to skyrocket because of rapidly increasing reaction rate.

Chemical reactions are either endothermic or exothermic, as expressed by their change in enthalpy. Many reactions are highly exothermic, so many industrial-scale and oil refinery processes have some level of risk of thermal runaway. These include hydrocracking, hydrogenation, alkylation (S_N2), oxidation, metalation and nucleophilic aromatic substitution. For example, oxidation of cyclohexane into cyclohexanol and cyclohexanone and ortho-xylene into phthalic anhydride have led to catastrophic explosions when reaction control failed.

Thermal runaway may result from unwanted exothermic side reaction(s) that begin at higher temperatures, following an initial accidental overheating of the reaction mixture. This scenario was behind the Seveso disaster, where thermal runaway heated a reaction to temperatures such that in addition to the intended 2,4,5-trichlorophenol, poisonous 2,3,7,8-tetrachlorodibenzo-p-dioxin was also produced, and was vented into the environment after the reactor's rupture disk burst.

Thermal runaway is most often caused by failure of the reactor vessel's cooling system. Failure of the mixer can result in localized heating, which initiates thermal runaway. Similarly, in flow reactors, localized insufficient mixing causes hotspots to form, wherein thermal runaway conditions occur, which causes violent blowouts of reactor contents and catalysts. Incorrect equipment component installation is also a common cause. Many chemical production facilities are designed with high-volume emergency venting, a measure to limit the extent of injury and property damage when such accidents occur.

At large scale, it is unsafe to "charge all reagents and mix", as is done in laboratory scale. This is because the amount of reaction scales with the cube of the size of the vessel (V ∝ r³), but the heat transfer area scales with the square of the size (A ∝ r²), so that the heat production-to-area ratio scales with the size (V/A ∝ r). Consequently, reactions that easily cool fast enough in the laboratory can dangerously self-heat at ton scale. In 2007, this kind of erroneous procedure caused an explosion of a 2,400 U.S. gallons (9,100 L)-reactor used to metalate methylcyclopentadiene with metallic sodium, causing the loss of four lives and parts of the reactor being flung 400 feet (120 m) away. Thus, industrial scale reactions prone to thermal runaway are preferably controlled by the addition of one reagent at a rate corresponding to the available cooling capacity.

Some laboratory reactions must be run under extreme cooling, because they are very prone to hazardous thermal runaway. For example, in Swern oxidation, the formation of sulfonium chloride must be performed in a cooled system (−30 °C), because at room temperature the reaction undergoes explosive thermal runaway.

Microwave heating

Microwaves are used for heating of various materials in cooking and various industrial processes. The rate of heating of the material depends on the energy absorption, which depends on the dielectric constant of the material. The dependence of dielectric constant on temperature varies for different materials; some materials display significant increase with increasing temperature. This behavior, when the material gets exposed to microwaves, leads to selective local overheating, as the warmer areas are better able to accept further energy than the colder areas—potentially dangerous especially for thermal insulators, where the heat exchange between the hot spots and the rest of the material is slow. These materials are called thermal runaway materials. This phenomenon occurs in some ceramics.

Electrical engineering

Some electronic components develop lower resistances or lower triggering voltages (for nonlinear resistances) as their internal temperature increases. If circuit conditions cause markedly increased current flow in these situations, increased power dissipation may raise the temperature further by Joule heating. A vicious circle or positive feedback effect of thermal runaway can cause failure, sometimes in a spectacular fashion (e.g. electrical explosion or fire). To prevent these hazards, well-designed electronic systems typically incorporate current limiting protection, such as thermal fuses, circuit breakers, or PTC current limiters.

To handle larger currents, circuit designers may connect multiple lower-capacity devices (e.g. transistors, diodes, or MOVs) in parallel. This technique can work well, but is susceptible to a phenomenon called current hogging, in which the current is not shared equally across all devices. Typically, one device may have a slightly lower resistance, and thus draws more current, heating it more than its sibling devices, causing its resistance to drop further. The electrical load ends up funneling into a single device, which then rapidly fails. Thus, an array of devices may end up no more robust than its weakest component.

The current-hogging effect can be reduced by carefully matching the characteristics of each paralleled device, or by using other design techniques to balance the electrical load. However, maintaining load balance under extreme conditions may not be straightforward. Devices with an intrinsic positive temperature coefficient (PTC) of electrical resistance are less prone to current hogging, but thermal runaway can still occur because of poor heat sinking or other problems.

Many electronic circuits contain special provisions to prevent thermal runaway. This is most often seen in transistor biasing arrangements for high-power output stages. However, when equipment is used above its designed ambient temperature, thermal runaway can still occur in some cases. This occasionally causes equipment failures in hot environments, or when air cooling vents are blocked.

Semiconductors

Silicon shows a peculiar profile, in that its electrical resistance increases with temperature up to about 160 °C, then starts decreasing, and drops further when the melting point is reached. This can lead to thermal runaway phenomena within internal regions of the semiconductor junction; the resistance decreases in the regions which become heated above this threshold, allowing more current to flow through the overheated regions, in turn causing yet more heating in comparison with the surrounding regions, which leads to further temperature increase and resistance decrease. This leads to the phenomenon of current crowding and formation of current filaments (similar to current hogging, but within a single device), and is one of the underlying causes of many semiconductor junction failures.

Bipolar junction transistors (BJTs)

Leakage current increases significantly in bipolar transistors (especially germanium-based bipolar transistors) as they increase in temperature. Depending on the design of the circuit, this increase in leakage current can increase the current flowing through a transistor and thus the power dissipation, causing a further increase in collector-to-emitter leakage current. This is frequently seen in a push–pull stage of a class AB amplifier. If the pull-up and pull-down transistors are biased to have minimal crossover distortion at room temperature, and the biasing is not temperature-compensated, then as the temperature rises both transistors will be increasingly biased on, causing current and power to further increase, and eventually destroying one or both devices.

One rule of thumb to avoid thermal runaway is to keep the operating point of a BJT so that V_ce ≤ 1/2V_cc

Another practice is to mount a thermal feedback sensing transistor or other device on the heat sink, to control the crossover bias voltage. As the output transistors heat up, so does the thermal feedback transistor. This in turn causes the thermal feedback transistor to turn on at a slightly lower voltage, reducing the crossover bias voltage, and so reducing the heat dissipated by the output transistors.

If multiple BJT transistors are connected in parallel (which is typical in high current applications), a current hogging problem can occur. Special measures must be taken to control this characteristic vulnerability of BJTs.

In power transistors (which effectively consist of many small transistors in parallel), current hogging can occur between different parts of the transistor itself, with one part of the transistor becoming more hot than the others. This is called second breakdown, and can result in destruction of the transistor even when the average junction temperature seems to be at a safe level.

Power MOSFETs

Power MOSFETs typically increase their on-resistance with temperature. Under some circumstances, power dissipated in this resistance causes more heating of the junction, which further increases the junction temperature, in a positive feedback loop. As a consequence, power MOSFETs have stable and unstable regions of operation. However, the increase of on-resistance with temperature helps balance current across multiple MOSFETs connected in parallel, so current hogging does not occur. If a MOSFET transistor produces more heat than the heatsink can dissipate, then thermal runaway can still destroy the transistors. This problem can be alleviated to a degree by lowering the thermal resistance between the transistor die and the heatsink. See also Thermal Design Power.

Metal oxide varistors (MOVs)

Metal oxide varistors typically develop lower resistance as they heat up. If connected directly across an AC or DC power bus (a common usage for protection against electrical transients), a MOV which has developed a lowered trigger voltage can slide into catastrophic thermal runaway, possibly culminating in a small explosion or fire. To prevent this possibility, fault current is typically limited by a thermal fuse, circuit breaker, or other current limiting device.

Tantalum capacitors

Tantalum capacitors are, under some conditions, prone to self-destruction by thermal runaway. The capacitor typically consists of a sintered tantalum sponge acting as the anode, a manganese dioxide cathode, and a dielectric layer of tantalum pentoxide created on the tantalum sponge surface by anodizing. It may happen that the tantalum oxide layer has weak spots that undergo dielectric breakdown during a voltage spike. The tantalum sponge then comes into direct contact with the manganese dioxide, and increased leakage current causes localized heating; usually, this drives an endothermic chemical reaction that produces manganese(III) oxide and regenerates (self-heals) the tantalum oxide dielectric layer.

However, if the energy dissipated at the failure point is high enough, a self-sustaining exothermic reaction can start, similar to the thermite reaction, with metallic tantalum as fuel and manganese dioxide as oxidizer. This undesirable reaction will destroy the capacitor, producing smoke and possibly flame.

Therefore, tantalum capacitors can be freely deployed in small-signal circuits, but application in high-power circuits must be carefully designed to avoid thermal runaway failures.

Digital logic

The leakage current of logic switching transistors increases with temperature. In rare instances, this may lead to thermal runaway in digital circuits. This is not a common problem, since leakage currents usually make up a small portion of overall power consumption, so the increase in power is fairly modest — for an Athlon 64, the power dissipation increases by about 10% for every 30 degrees Celsius. For a device with a TDP of 100 W, for thermal runaway to occur, the heat sink would have to have a thermal resistivity of over 3 K/W (kelvins per watt), which is about 6 times worse than a stock Athlon 64 heat sink. (A stock Athlon 64 heat sink is rated at 0.34 K/W, although the actual thermal resistance to the environment is somewhat higher, due to the thermal boundary between processor and heatsink, rising temperatures in the case, and other thermal resistances.) Regardless, an inadequate heat sink with a thermal resistance of over 0.5 to 1 K/W would result in the destruction of a 100 W device even without thermal runaway effects.

Batteries

When handled improperly, or if manufactured defectively, some rechargeable batteries can experience thermal runaway resulting in overheating. Sealed cells will sometimes explode violently if safety vents are overwhelmed or nonfunctional. Especially prone to thermal runaway are lithium-ion batteries, most markedly in the form of the lithium polymer battery. Reports of exploding cellphones occasionally appear in newspapers. In 2006, batteries from Apple, HP, Toshiba, Lenovo, Dell and other notebook manufacturers were recalled because of fire and explosions. The Pipeline and Hazardous Materials Safety Administration (PHMSA) of the U.S. Department of Transportation has established regulations regarding the carrying of certain types of batteries on airplanes because of their instability in certain situations. This action was partially inspired by a cargo bay fire on a UPS airplane. One of the possible solutions is in using safer and less reactive anode (lithium titanates) and cathode (lithium iron phosphate) materials — thereby avoiding the cobalt electrodes in many lithium rechargeable cells — together with non-flammable electrolytes based on ionic liquids.

Astrophysics

Runaway thermonuclear reactions can occur in stars when nuclear fusion is ignited in conditions under which the gravitational pressure exerted by overlying layers of the star greatly exceeds thermal pressure, a situation that makes possible rapid increases in temperature through gravitational compression. Such a scenario may arise in stars containing degenerate matter, in which electron degeneracy pressure rather than normal thermal pressure does most of the work of supporting the star against gravity, and in stars undergoing implosion. In all cases, the imbalance arises prior to fusion ignition; otherwise, the fusion reactions would be naturally regulated to counteract temperature changes and stabilize the star. When thermal pressure is in equilibrium with overlying pressure, a star will respond to the increase in temperature and thermal pressure due to initiation of a new exothermic reaction by expanding and cooling. A runaway reaction is only possible when this response is inhibited.

Helium flashes in red giant stars

When stars in the 0.8–2.0 solar mass range exhaust the hydrogen in their cores and become red giants, the helium accumulating in their cores reaches degeneracy before it ignites. When the degenerate core reaches a critical mass of about 0.45 solar masses, helium fusion is ignited and takes off in a runaway fashion, called the helium flash, briefly increasing the star's energy production to a rate 100 billion times normal. About 6% of the core is quickly converted into carbon. While the release is sufficient to convert the core back into normal plasma after a few seconds, it does not disrupt the star, nor immediately change its luminosity. The star then contracts, leaving the red giant phase and continuing its evolution into a stable helium-burning phase.

Novae

A nova results from runaway hydrogen fusion (via the CNO cycle) in the outer layer of a carbon-oxygen white dwarf star. If a white dwarf has a companion star from which it can accrete gas, the material will accumulate in a surface layer made degenerate by the dwarf's intense gravity. Under the right conditions, a sufficiently thick layer of hydrogen is eventually heated to a temperature of 20 million K, igniting runaway fusion. The surface layer is blasted off the white dwarf, increasing luminosity by a factor on the order of 50,000. The white dwarf and companion remain intact, however, so the process can repeat. A much rarer type of nova may occur when the outer layer that ignites is composed of helium.

X-ray bursts

Analogous to the process leading to novae, degenerate matter can also accumulate on the surface of a neutron star that is accreting gas from a close companion. If a sufficiently thick layer of hydrogen accumulates, ignition of runaway hydrogen fusion can then lead to an X-ray burst. As with novae, such bursts tend to repeat and may also be triggered by helium or even carbon fusion. It has been proposed that in the case of "superbursts", runaway breakup of accumulated heavy nuclei into iron group nuclei via photodissociation rather than nuclear fusion could contribute the majority of the energy of the burst.

Type Ia supernovae

A type Ia supernova results from runaway carbon fusion in the core of a carbon-oxygen white dwarf star. If a white dwarf, which is composed almost entirely of degenerate matter, can gain mass from a companion, the increasing temperature and density of material in its core will ignite carbon fusion if the star's mass approaches the Chandrasekhar limit. This leads to an explosion that completely disrupts the star. Luminosity increases by a factor of greater than 5 billion. One way to gain the additional mass would be by accreting gas from a giant star (or even main sequence) companion. A second and apparently more common mechanism to generate the same type of explosion is the merger of two white dwarfs.

Pair-instability supernovae

A pair-instability supernova is believed to result from runaway oxygen fusion in the core of a massive, 130–250 solar mass, low to moderate metallicity star. According to theory, in such a star, a large but relatively low density core of nonfusing oxygen builds up, with its weight supported by the pressure of gamma rays produced by the extreme temperature. As the core heats further, the gamma rays eventually begin to pass the energy threshold needed for collision-induced decay into electron-positron pairs, a process called pair production. This causes a drop in the pressure within the core, leading it to contract and heat further, causing more pair production, a further pressure drop, and so on. The core starts to undergo gravitational collapse. At some point this ignites runaway oxygen fusion, releasing enough energy to obliterate the star. These explosions are rare, perhaps about one per 100,000 supernovae.

Comparison to nonrunaway supernovae

Not all supernovae are triggered by runaway nuclear fusion. Type Ib, Ic and type II supernovae also undergo core collapse, but because they have exhausted their supply of atomic nuclei capable of undergoing exothermic fusion reactions, they collapse all the way into neutron stars, or in the higher-mass cases, stellar black holes, powering explosions by the release of gravitational potential energy (largely via release of neutrinos). It is the absence of runaway fusion reactions that allows such supernovae to leave behind compact stellar remnants.

Upper Paleolithic

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https://en.wikipedia.org/wiki/Upper_Paleolithic 

 

Expansion of early modern humans from Africa.

The Upper Paleolithic (or Upper Palaeolithic) is the third and last subdivision of the Paleolithic or Old Stone Age. Very broadly, it dates to between 50,000 and 12,000 years ago (the beginning of the Holocene), according to some theories coinciding with the appearance of behavioral modernity in early modern humans, until the advent of the Neolithic Revolution and agriculture.

Anatomically modern humans (i.e. Homo sapiens) are believed to have emerged in Africa around 300,000 years ago, it has been argued by some that their ways of life changed relatively little from that of archaic humans of the Middle Paleolithic, until about 50,000 years ago, when there was a marked increase in the diversity of artefacts found associated with modern human remains. This period coincides with the most common date assigned to expansion of modern humans from Africa throughout Asia and Eurasia, which contributed to the extinction of the Neanderthals.

The Upper Paleolithic has the earliest known evidence of organized settlements, in the form of campsites, some with storage pits. Artistic work blossomed, with cave painting, petroglyphs, carvings and engravings on bone or ivory. The first evidence of human fishing is also found, from artefacts in places such as Blombos cave in South Africa. More complex social groupings emerged, supported by more varied and reliable food sources and specialized tool types. This probably contributed to increasing group identification or ethnicity.

The peopling of Australia most likely took place before c. 60 ka. Europe was peopled after c. 45 ka. Anatomically modern humans are known to have expanded northward into Siberia as far as the 58th parallel by about 45 ka (Ust'-Ishim man). The Upper Paleolithic is divided by the Last Glacial Maximum (LGM), from about 25 to 15 ka. The peopling of the Americas occurred during this time, with East and Central Asia populations reaching the Bering land bridge after about 35 ka, and expanding into the Americas by about 15 ka. In Western Eurasia, the Paleolithic eases into the so-called Epipaleolithic or Mesolithic from the end of the LGM, beginning 15 ka. The Holocene glacial retreat begins 11.7 ka (10th millennium BC), falling well into the Old World Epipaleolithic, and marking the beginning of the earliest forms of farming in the Fertile Crescent.

Lifestyle and technology

See also: Hunter-gatherer, Aurignacian, and Behavioral modernity

Both Homo erectus and Neanderthals used the same crude stone tools. Archaeologist Richard G. Klein, who has worked extensively on ancient stone tools, describes the stone tool kit of archaic hominids as impossible to categorize. He argues that almost everywhere, whether Asia, Africa or Europe, before 50,000 years ago all the stone tools are much alike and unsophisticated.

Flint Knives, Ahmarian Culture, Nahal Boqer, Israel, 47,000-40,000 BP. Israel Museum.

Firstly among the artefacts of Africa, archeologists found they could differentiate and classify those of less than 50,000 years into many different categories, such as projectile points, engraving tools, knife blades, and drilling and piercing tools. These new stone-tool types have been described as being distinctly differentiated from each other; each tool had a specific purpose. The early modern humans who expanded into Europe, commonly referred to as the Cro-Magnons, left many sophisticated stone tools, carved and engraved pieces on bone, ivory and antler, cave paintings and Venus figurines.

The Neanderthals continued to use Mousterian stone tool technology and possibly Châtelperronian technology. These tools disappeared from the archeological record at around the same time the Neanderthals themselves disappeared from the fossil record, about 40,000 cal BP.

Stone core for making fine blades, Boqer Tachtit, Negev, Israel, circa 40,000 BP.

Settlements were often located in narrow valley bottoms, possibly associated with hunting of passing herds of animals. Some of them may have been occupied year round, though more commonly they appear to have been used seasonally; people moved between the sites to exploit different food sources at different times of the year. Hunting was important, and caribou/wild reindeer "may well be the species of single greatest importance in the entire anthropological literature on hunting."

Technological advances included significant developments in flint tool manufacturing, with industries based on fine blades rather than simpler and shorter flakes. Burins and racloirs were used to work bone, antler and hides. Advanced darts and harpoons also appear in this period, along with the fish hook, the oil lamp, rope, and the eyed needle. Fishing of pelagic fish species and navigating the open ocean is evidenced by sites from Timor and Buka (Solomon Islands).

The changes in human behavior have been attributed to changes in climate, encompassing a number of global temperature drops. These led to a worsening of the already bitter cold of the last glacial period (popularly but incorrectly called the last ice age). Such changes may have reduced the supply of usable timber and forced people to look at other materials. In addition, flint becomes brittle at low temperatures and may not have functioned as a tool.

Changes in climate and geography

The Upper Paleolithic covered the second half of the Last glacial period from 50,000 to 10,000 before present, until the warming of the Holocene. Ice core data from Antarctica and Greenland.

The climate of the period in Europe saw dramatic changes, and included the Last Glacial Maximum, the coldest phase of the last glacial period, which lasted from about 26.5 to 19 kya, being coldest at the end, before relatively rapid warming (all dates vary somewhat for different areas, and in different studies). During the Maximum, most of Northern Europe was covered by an ice-sheet, forcing human populations into the areas known as Last Glacial Maximum refugia, including modern Italy and the Balkans, parts of the Iberian Peninsula and areas around the Black Sea.

This period saw cultures such as the Solutrean in France and Spain. Human life may have continued on top of the ice sheet, but we know next to nothing about it, and very little about the human life that preceded the European glaciers. In the early part of the period, up to about 30 kya, the Mousterian Pluvial made northern Africa, including the Sahara, well-watered and with lower temperatures than today; after the end of the Pluvial the Sahara became arid.

European Last Glacial Maximum refuges, 20,000 BP.

  Solutrean and Proto Solutrean Cultures

  Epigravettian Culture

The Last Glacial Maximum was followed by the Allerød oscillation, a warm and moist global interstadial that occurred around 13.5 to 13.8 kya. Then there was a very rapid onset, perhaps within as little as a decade, of the cold and dry Younger Dryas climate period, giving sub-arctic conditions to much of northern Europe. The Preboreal rise in temperatures also began sharply around 10.3 kya, and by its end around 9.0 kya had brought temperatures nearly to present day levels, although the climate was wetter. This period saw the Upper Paleolithic give way to the start of the following Mesolithic cultural period.

As the glaciers receded sea levels rose; the English Channel, Irish Sea and North Sea were land at this time, and the Black Sea a fresh-water lake. In particular the Atlantic coastline was initially far out to sea in modern terms in most areas, though the Mediterranean coastline has retreated far less, except in the north of the Adriatic and the Aegean. The rise in sea levels continued until at least 7.5 kya (5500 BC), so evidence of human activity along Europe's coasts in the Upper Paleolithic is mostly lost, though some traces have been recovered by fishing boats and marine archaeology, especially from Doggerland, the lost area beneath the North Sea.

Timeline

50,000–40,000 BP

Anatomically Modern Humans known archaeological remains in Europe and Africa, directly dated, calibrated carbon dates as of 2013.

 

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".

50,000 BP

• Numerous Aboriginal stone tools were found in gravel sediments in Castlereagh, Sydney, Australia. At first when these results were new they were controversial; more recently dating of the same strata has revised and corroborated these dates.
• Start of the Mousterian Pluvial in North Africa.
• Occupants of the Fa-Hien Lena cave, Sri Lanka had developed bow and arrow technology 48,000 BP (though the earliest known bow and arrow technology dates to about 65,000 BP from Sibudu Cave, South Africa).

45,000–43,000 BP

• Earliest evidence of modern humans found in Europe, in Southern Italy. These are indirectly dated.
• Earliest mathematical artifact, the notched Lebombo bone, a possible tally stick or lunar calendar, dated to 44,000–43,000 BP in Eswatini (Swaziland), southern Africa
• Oldest-known mining in archaeological record, the Ngwenya Mine in Swaziland, at about 43,000 years ago, where humans mined hematite to make the red pigment ochre
• Earliest directly dated figurative cave art of mankind at Leang Bulu' Sipong on Sulawesi, Indonesia.

43,000–41,000 BP

• Ornaments and skeletal remains of modern humans, at Ksar Akil in Lebanon. These are directly dated.
• Denisova hominins live in the Altai Mountains (Russia, China, Mongolia, and Kazakhstan)

40,000–30,000 BP

40,000–35,000 BP

• First human inhabitants in Perth, Australia, as evidenced by archaeological findings on the Upper Swan River.
• During this time period, Melbourne, Australia was occupied by hunter-gatherers.
• Early cultural centre in the Swabian Alps, oldest depiction of a human being (Venus of Hohle Fels), beginning of the Aurignacian.
• Löwenmensch figure created in Hohlenstein-Stadel, one of the earliest figurative art. It is now in Ulmer Museum, Ulm, Germany.
• The first flutes appear in Germany.
• Most of the giant vertebrates and megafauna in Australia became extinct.
• Fishing of pelagic fish species at Jerimalai shelter, Timor.

Venus of Laussel, an Upper Paleolithic (Gravettian) carving.

• Examples of cave art in Spain are dated from around 40,000 BP, making them the oldest examples of cave art yet discovered in Europe (see: Caves of Nerja). Scientists theorise that the paintings may have been made by Neanderthals, rather than by homo sapiens. (BBC) (Science)
• Wall painting with horses, rhinoceroses and aurochs is made at Chauvet Cave, Vallon-Pont-d'Arc, Ardéche gorge, France. Discovered in December 1994.
• Evidence for continued Neanderthal presence in the Iberian Peninsula at 37,000 years ago was published in 2017.
• Archaeological studies support human presence in the Chek Lap Kok area (now Hong Kong International Airport) from 35,000 to 39,000 years ago.
• Zar, Yataghyeri, Damjili and Taghlar caves in Azerbaijan.
• First evidence of people inhabiting Japan.

35,000 BP

• Kostenki XVII, a layer of the Kostenki (Kostyonki) site, on the middle Don River, was occupied by the early upper paleolithic Spitsyn culture.

30,000 BP

• First ground stone tools appear in Japan.
• End of the Mousterian Pluvial in North Africa.
• The area of Sydney was occupied by Aboriginal Australians during this time period, as evidenced by radiocarbon dating. In an archaeological dig in Parramatta, Western Sydney, it was found that the Aboriginals used charcoal, stone tools and possible ancient campfires.
• First human settlement in Alice Springs, Northern Territory, Australia.
• Kilu Cave at Buka in the Solomons is evidence for the first human settlement of an oceanic island and for navigating the open ocean.

The Venus of Brassempouy is preserved in the Musée d'Archéologie Nationale at Saint-Germain-en-Laye, near Paris.

30,000–20,000 BP

29,000–25,000 BP

• Last eruption of the Ciomadul volcano in Romania.
• Venus of Dolní Věstonice (Czech Republic). It is the oldest known ceramic in the world.
• Venus of Willendorf, Austria, created. It is now at the Natural History Museum, Vienna.
• The Red Lady of Paviland lived around 29,000–26,000 years ago. Recent evidence has come to light that he was a tribal chief.
• Human settlement in Beijing, China dates from about 27,000 to 10,000 years ago.

24,000 BP

• Start of the second Mousterian Pluvial in North Africa.

23,000 BP

• Venus of Petřkovice is created at Petřkovice in Ostrava, Czech Republic. It is now in Archeological Institute, Brno.

22,000 BP

• Last Glacial Maximum: Venus of Brassempouy, Grotte du Pape, Brassempouy, Landes, France, created. It is now at Musée des Antiquités Nationales, Saint-Germain-en-Laye.

21,000 BP

• Artifacts suggests early human activity occurred at some point in Canberra, Australia. Archaeological evidence of settlement in the region includes inhabited rock shelters, rock art, burial places, camps and quarry sites, and stone tools and arrangements.
• End of the second Mousterian Pluvial in North Africa.

20,000–10,000 BP

Main article: Epipaleolithic

• Last Glacial Maximum. Mean sea levels are believed to be 110 to 120 metres (360 to 390 ft) lower than present, with the direct implication that many coastal and lower riverine valley archaeological sites of interest are today under water.

18,000 BP

• Spotted Horses, Pech Merle cave, Dordogne, France are painted. Discovered in December, 1994.
• Ibex-headed spear-thrower, from Le Mas-d'Azil, Ariège, France, is made. It is now at Musée de la Préhistoire, Le Mas d'Azil.
• Mammoth-bone village in Mezhyrich, Ukraine is inhabited.

17,000 BP

Lascaux, a UNESCO World Heritage Site.

• Spotted human hands are painted at Pech Merle cave, Dordogne, France. Discovered in December 1994.
• Oldest Dryas stadial.
• Hall of Bulls at Lascaux in France is painted. Discovered in 1940. Closed to the public in 1963.
• Bird-Headed man with bison and Rhinoceros, Lascaux, is painted.
• Lamp with ibex design, from La Mouthe cave, Dordogne, France, is made. It is now at Musée des Antiquités Nationales, Saint-Germain-en-Laye.
• Paintings in Cosquer Cave are made, where the cave mouth is now under water at Cap Margiou, France.

15,000 BP

• Bølling interstadial.
• Bison, Le Tuc d'Audoubert, Ariège, France.
• Paleo-Indians move across North America, then southward through Central America.
• Pregnant woman and deer (?), from Laugerie-Basse, France was made. It is now at Musée des Antiquités Nationales, St.-Germain-en-Laye.

14,000 BP

Reindeer Age articles

• Older Dryas stadial, Allerød interstadial.
• Paleo-Indians searched for big game near what is now the Hovenweep National Monument.
• Bison, on the ceiling of a cave at Altamira, Spain, is painted. Discovered in 1879. Accepted as authentic in 1902.
• Younger Dryas stadial.
• Beginning of the Holocene extinction.

12,000 BP

• Wooden buildings in South America (Chile).
• First pottery vessels (Japan).

11,000 BP

• First evidence of human settlement in Argentina.
• The Arlington Springs Man dies on the island of Santa Rosa, off the coast of California, United States.
• Human remains deposited in caves which are now located off the coast of Yucatán, Mexico.
• Creswellian culture settlement on Hengistbury Head, England, dates from around this year.

10,000 BP

• Evidence of a massacre near Lake Turkana, Kenya indicates upper paleolithic warfare.

Cultures

The Upper Paleolithic in the Franco-Cantabrian region:

• The Châtelperronian culture was located around central and south western France, and northern Spain. It appears to be derived from the Mousterian culture, and represents the period of overlap between Neanderthals and Homo sapiens. This culture lasted from approximately 45,000 BP to 40,000 BP.
• The Aurignacian culture was located in Europe and south west Asia, and flourished between 43,000 and 26,000 BP. It may have been contemporary with the Périgordian (a contested grouping of the earlier Châtelperronian and later Gravettian cultures).
• The Gravettian culture was located across Europe. Gravettian sites generally date between 33,000 and 20,000 BP.
• The Solutrean culture was located in eastern France, Spain, and England. Solutrean artifacts have been dated c. 22,000 to 17,000 BP.
• The Magdalenian culture left evidence from Portugal to Poland during the period from 17,000 to 12,000 BP.

• Central and east Europe:
  • 33,000 BP, Gravettian culture in southern Ukraine.
  • 30,000 BP, Szeletian culture
  • 22,000 BP, Pavlovian, Aurignacian cultures
  • 13,000 BP, Ahrensburg culture (Western Germany, Netherlands, England)
  • 12,000 BP, Epigravettian
• North and west Africa, and Sahara:
  • 32,000 BP, Aterian culture (Algeria, Libya)
  • 12,000 BP, Ibero-Maurusian (a.k.a. Oranian, Ouchtatian), and Sebilian cultures
  • 10,000 BP, Capsian culture (Tunisia, Algeria)
• Central, south, and east Africa:
  • 50,000 BP, Fauresmith culture
  • 30,000 BP, Stillbayan culture
  • 12,000 BP, Lupembian culture
  • 11,000 BP, Magosian culture (Zambia, Tanzania)
  • 9,000 BP, Wiltonian culture
• West Asia (including Middle East):
  • 50,000 BP, Jabroudian culture (Levant)
  • 40,000 BP, Amoudian culture
  • 30,000 BP, Emireh culture
  • 20,000 BP, Aurignacian culture
  • 12,000 BP, Kebarian, Athlitian cultures
• South, central and northern Asia:
  • 30,000 BP, Angara culture
  • 11,000 BP, Khandivili culture
• East and southeast Asia:
  • 50,000 BP, Ngandong culture
  • 30,000 BP, Sen-Doki culture
  • 16,000 BP, Jōmon period starts in Ancient Japan
  • 12,000 BP, pre-Jōmon ceramic culture (Japan)
  • 10,000 BP, Hoabinhian culture (Northern Vietnam)
  • 9,000 BP, Jōmon culture (Japan)
• Oceania
  • 40,000 BP, Whadjuk and Noongar culture (Perth, Australia)
  • 35,000 BP, Wurundjeri, Boonwurrung and Wathaurong culture (Melbourne, Australia)
  • 30,000 BP, Eora and Darug culture (Sydney, Australia)
  • 30,000 BP, Arrernte culture (Alice Springs, Central Australia)