PanspermiaPanspermia (from Ancient Greek πᾶν (pan), meaning 'all', andσπέρμα (sperma), meaning 'seed') is the hypothesis that life existsthroughout the Universe, distributed by space dust,[1]

meteoroids,[2] asteroids, comets,[3] planetoids,[4] and also byspacecraft carrying unintended contamination bymicroorganisms.[5][6][7] Distribution may have occurredspanning galaxies, and so may not be restricted to the limitedscale of solar systems.[8][9]

Panspermia hypotheses propose (for example) that microscopiclife-forms that can survive the effects of space (such asextremophiles) can become trapped in debris ejected into spaceafter collisions between planets and small Solar System bodiesthat harbor life.[10][11] Some organisms may travel dormant foran extended amount of time before colliding randomly with other planets or intermingling with protoplanetary disks. Undercertain ideal impact circumstances (into a body of water, for example), and ideal conditions on a new planet's surfaces, it ispossible that the surviving organisms could become active and begin to colonize their new environment. At least one report findsthat endospores from a type of Bacillus bacteria found in Morocco can survive being heated to 420 °C (788 °F), making theargument for Panspermia even stronger.[12] Panspermia studies concentrate not on how life began, but on the methods that maycause its distribution in the Universe.[13][14][15]

Pseudo-panspermia (sometimes called "soft panspermia" or "molecular panspermia") argues that the pre-biotic organic building-blocks of life originated in space, became incorporated in the solar nebula from which planets condensed, and were further—andcontinuously—distributed to planetary surfaces where life then emerged (abiogenesis).[16][17] From the early 1970s, it started tobecome evident that interstellar dust included a large component of organic molecules. Interstellar molecules are formed bychemical reactions within very sparse interstellar or circumstellar clouds of dust and gas.[18] The dust plays a critical role inshielding the molecules from the ionizing effect of ultraviolet radiation emitted by stars.[19]

The chemistry leading to life may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch whenthe Universe was only 10 to 17 million years old. Though the presence of life is confirmed only on the Earth, some scientiststhink that extraterrestrial life is not only plausible, but probable or inevitable. Probes and instruments have started examiningother planets and moons in the Solar System and in other planetary systems for evidence of having once supported simple life,and projects such as SETI attempt to detect radio transmissions from possible extraterrestrial civilizations.

History

Proposed mechanismsRadiopanspermiaLithopanspermiaAccidental panspermiaDirected panspermiaPseudo-panspermia

Panspermia proposes that bodies such as cometstransported life forms such as bacteria—completewith their DNA—through space to the Earth

Contents

Extraterrestrial lifeHypotheses on extraterrestrial sources of illnessesCase studiesHoaxes

ExtremophilesResearch in outer space

ERABIOPANEXOSTACKEXPOSETanpopo

Criticism

See also

References

Further reading

External links

The first known mention of the term was in the writings of the 5th-century BC Greek philosopher Anaxagoras.[20] Panspermiabegan to assume a more scientific form through the proposals of Jöns Jacob Berzelius (1834),[21] Hermann E. Richter (1865),[22]

Kelvin (1871),[23] Hermann von Helmholtz (1879)[24][25] and finally reaching the level of a detailed scientific hypothesisthrough the efforts of the Swedish chemist Svante Arrhenius (1903).[26]

Fred Hoyle (1915–2001) and Chandra Wickramasinghe (born 1939) were influential proponents of panspermia.[27][28] In 1974they proposed the hypothesis that some dust in interstellar space was largely organic (containing carbon), which Wickramasinghelater proved to be correct.[29][30][31] Hoyle and Wickramasinghe further contended that life forms continue to enter the Earth'satmosphere, and may be responsible for epidemic outbreaks, new diseases, and the genetic novelty necessary formacroevolution.[32]

In an Origins Symposium presentation on April 7, 2009, physicist Stephen Hawking stated his opinion about what humans mayfind when venturing into space, such as the possibility of alien life through the theory of panspermia: "Life could spread fromplanet to planet or from stellar system to stellar system, carried on meteors."[33]

Three series of astrobiology experiments have been conducted outside the International Space Station between 2008 and 2015(EXPOSE) where a wide variety of biomolecules, microorganisms, and their spores were exposed to the solar flux and vacuum ofspace for about 1.5 years. Some organisms survived in an inactive state for considerable lengths of time,[34][35] and those samplessheltered by simulated meteorite material provide experimental evidence for the likelihood of the hypothetical scenario oflithopanspermia.[36]

Several simulations in laboratories and in low Earth orbit suggest that ejection, entry and impact is survivable for some simpleorganisms. In 2015, remains of biotic material were found in 4.1 billion-year-old rocks in Western Australia, when the youngEarth was about 400 million years old.[37][38] According to one researcher, "If life arose relatively quickly on Earth … then itcould be common in the universe."[37]

In April 2018, a Russian team published a paper which disclosed that they found DNA on the exterior of the ISS from land andmarine bacteria similar to those previously observed in superficial micro layers at the Barents and Kara seas' coastal zones. Theyconclude "The presence of the wild land and marine bacteria DNA on the ISS suggests their possible transfer from the

History

stratosphere into the ionosphere with the ascending branch of the global atmospheric electrical circuit. Alternatively, the wild landand marine bacteria as well as the ISS bacteria may all have an ultimate space origin."[39]

In October 2018, Harvard astronomers presented an analytical model that suggests matter—and potentially dormant spores—canbe exchanged across the vast distances between galaxies, a process termed 'galactic panspermia', and not be restricted to thelimited scale of solar systems.[8][9] The detection of an extra-solar object named ʻOumuamua crossing the inner Solar System in ahyperbolic orbit confirms the existence of a continuing material link with exoplanetary systems.[40]

Panspermia can be said to be either interstellar (between star systems) or interplanetary (between planets in the same starsystem);[41][42] its transport mechanisms may include comets,[43][44] radiation pressure and lithopanspermia (microorganismsembedded in rocks).[45][46][47] Interplanetary transfer of nonliving material is well documented, as evidenced by meteorites ofMartian origin found on Earth.[47] Space probes may also be a viable transport mechanism for interplanetary cross-pollination inthe Solar System or even beyond. However, space agencies have implemented planetary protection procedures to reduce the riskof planetary contamination,[48][49] although, as recently discovered, some microorganisms, such as Tersicoccus phoenicis, maybe resistant to procedures used in spacecraft assembly clean room facilities.[5][6]

In 2012, mathematician Edward Belbruno and astronomers Amaya Moro-Martín and Renu Malhotra proposed that gravitationallow-energy transfer of rocks among the young planets of stars in their birth cluster is commonplace, and not rare in the generalgalactic stellar population.[50][51] Deliberate directed panspermia from space to seed Earth[52] or sent from Earth to seed otherplanetary systems have also been proposed.[53][54][55][56] One twist to the hypothesis by engineer Thomas Dehel (2006),proposes that plasmoid magnetic fields ejected from the magnetosphere may move the few spores lifted from the Earth'satmosphere with sufficient speed to cross interstellar space to other systems before the spores can be destroyed.[57][58]

In 1903, Svante Arrhenius published in his article The Distribution of Life in Space,[59] the hypothesis now calledradiopanspermia, that microscopic forms of life can be propagated in space, driven by the radiation pressure from stars.[60]

Arrhenius argued that particles at a critical size below 1.5 μm would be propagated at high speed by radiation pressure of the Sun.However, because its effectiveness decreases with increasing size of the particle, this mechanism holds for very tiny particlesonly, such as single bacterial spores.[61]

The main criticism of radiopanspermia hypothesis came from Iosif Shklovsky and Carl Sagan, who pointed out the proofs of thelethal action of space radiations (UV and X-rays) in the cosmos.[62] Regardless of the evidence, Wallis and Wickramasingheargued in 2004 that the transport of individual bacteria or clumps of bacteria, is overwhelmingly more important thanlithopanspermia in terms of numbers of microbes transferred, even accounting for the death rate of unprotected bacteria intransit.[63]

Then, data gathered by the orbital experiments ERA, BIOPAN, EXOSTACK and EXPOSE, determined that isolated spores,including those of B. subtilis, were killed if exposed to the full space environment for merely a few seconds, but if shieldedagainst solar UV, the spores were capable of surviving in space for up to six years while embedded in clay or meteorite powder(artificial meteorites).[61][64]

Minimal protection is required to shelter a spore against UV radiation: Exposure of unprotected DNA to solar UV and cosmicionizing radiation break it up into its constituent bases.[65][66][67] Also, exposing DNA to the ultrahigh vacuum of space alone issufficient to cause DNA damage, so the transport of unprotected DNA or RNA during interplanetary flights powered solely bylight pressure is extremely unlikely.[67]

Proposed mechanisms

Radiopanspermia

The feasibility of other means of transport for the more massive shielded spores into the outer Solar System – for example,through gravitational capture by comets – is at this time unknown.

Based on experimental data on radiation effects and DNA stability, it has been concluded that for such long travel times, boulder-sized rocks which are greater than or equal to 1 meter in diameter are required to effectively shield resistant microorganisms, suchas bacterial spores against galactic cosmic radiation.[68][69] These results clearly negate the radiopanspermia hypothesis, whichrequires single spores accelerated by the radiation pressure of the Sun, requiring many years to travel between the planets, andsupport the likelihood of interplanetary transfer of microorganisms within asteroids or comets, the so-called lithopanspermiahypothesis.[61][64]

Lithopanspermia, the transfer of organisms in rocks from one planet to another either through interplanetary or interstellar space,remains speculative. Although there is no evidence that lithopanspermia has occurred in the Solar System, the various stages havebecome amenable to experimental testing.[70]

Planetary ejection — For lithopanspermia to occur, researchers have suggested that microorganisms mustsurvive ejection from a planetary surface which involves extreme forces of acceleration and shock withassociated temperature excursions. Hypothetical values of shock pressures experienced by ejected rocks areobtained with Martian meteorites, which suggest the shock pressures of approximately 5 to 55 GPa, accelerationof 3 Mm/s2 and jerk of 6 Gm/s3 and post-shock temperature increases of about 1 K to 1000 K.[71][72] Todetermine the effect of acceleration during ejection on microorganisms, rifle and ultracentrifuge methods weresuccessfully used under simulated outer space conditions.[70]

Survival in transit — The survival of microorganisms has been studied extensively using both simulatedfacilities and in low Earth orbit. A large number of microorganisms have been selected for exposure experiments.It is possible to separate these microorganisms into two groups, the human-borne, and the extremophiles.Studying the human-borne microorganisms is significant for human welfare and future manned missions; whilstthe extremophiles are vital for studying the physiological requirements of survival in space.[70]

Atmospheric entry — An important aspect of the lithopanspermia hypothesis to test is that microbes situated onor within rocks could survive hypervelocity entry from space through Earth's atmosphere (Cockell, 2008). As withplanetary ejection, this is experimentally tractable, with sounding rockets and orbital vehicles being used formicrobiological experiments.[70][71] B. subtilis spores inoculated onto granite domes were subjected tohypervelocity atmospheric transit (twice) by launch to a ∼120 km altitude on an Orion two-stage rocket. Thespores were shown to have survived on the sides of the rock, but they did not survive on the forward-facingsurface that was subjected to a maximum temperature of 145 °C.[73] The exogenous arrival of photosyntheticmicroorganisms could have quite profound consequences for the course of biological evolution on the inoculatedplanet. As photosynthetic organisms must be close to the surface of a rock to obtain sufficient light energy,atmospheric transit might act as a filter against them by ablating the surface layers of the rock. Althoughcyanobacteria have been shown to survive the desiccating, freezing conditions of space in orbital experiments,this would be of no benefit as the STONE experiment showed that they cannot survive atmospheric entry.[74]

Thus, non-photosynthetic organisms deep within rocks have a chance to survive the exit and entry process. (Seealso: Impact survival.) Research presented at the European Planetary Science Congress in 2015 suggests thatejection, entry and impact is survivable for some simple organisms.[75]

Thomas Gold, a professor of astronomy, suggested in 1960 the hypothesis of "Cosmic Garbage", that life on Earth might haveoriginated accidentally from a pile of waste products dumped on Earth long ago by extraterrestrial beings.[76]

Directed panspermia concerns the deliberate transport of microorganisms in space, sent to Earth to start life here, or sent fromEarth to seed new planetary systems with life by introduced species of microorganisms on lifeless planets. The Nobel prizewinner Francis Crick, along with Leslie Orgel proposed that life may have been purposely spread by an advanced extraterrestrial

Lithopanspermia

Accidental panspermia

Directed panspermia

civilization,[52] but considering an early "RNA world" Crick noted later that life may have originated on Earth.[77] It has beensuggested that 'directed' panspermia was proposed in order to counteract various objections, including the argument that microbeswould be inactivated by the space environment and cosmic radiation before they could make a chance encounter with Earth.[68]

Conversely, active directed panspermia has been proposed to secure and expand life in space.[55][78] This may be motivated bybiotic ethics that values, and seeks to propagate, the basic patterns of our organic gene/protein life-form.[79] The panbioticprogram would seed new planetary systems nearby, and clusters of new stars in interstellar clouds. These young targets, wherelocal life would not have formed yet, avoid any interference with local life.

For example, microbial payloads launched by solar sails at speeds up to 0.0001 c (30,000 m/s) would reach targets at 10 to 100light-years in 0.1 million to 1 million years. Fleets of microbial capsules can be aimed at clusters of new stars in star-formingclouds, where they may land on planets or be captured by asteroids and comets and later delivered to planets. Payloads maycontain extremophiles for diverse environments and cyanobacteria similar to early microorganisms. Hardy multicellularorganisms (rotifer cysts) may be included to induce higher evolution.[80]

The probability of hitting the target zone can be calculated from where A(target)

is the cross-section of the target area, dy is the positional uncertainty at arrival; a – constant (depending on units), r(target) is theradius of the target area; v the velocity of the probe; (tp) the targeting precision (arcsec/yr); and d the distance to the target, guidedby high-resolution astrometry of 1×10−5 arcsec/yr (all units in SIU). These calculations show that relatively near targetstars(Alpha PsA, Beta Pictoris) can be seeded by milligrams of launched microbes; while seeding the Rho Ophiochus star-forming cloud requires hundreds of kilograms of dispersed capsules.[55]

Directed panspermia to secure and expand life in space is becoming possible because of developments in solar sails, preciseastrometry, extrasolar planets, extremophiles and microbial genetic engineering.[81][82] After determining the composition ofchosen meteorites, astroecologists performed laboratory experiments that suggest that many colonizing microorganisms and someplants could obtain many of their chemical nutrients from asteroid and cometary materials.[83] However, the scientists noted thatphosphate (PO4) and nitrate (NO3–N) critically limit nutrition to many terrestrial lifeforms.[83] With such materials, and energyfrom long-lived stars, microscopic life planted by directed panspermia could find an immense future in the galaxy.[84]

A number of publications since 1979 have proposed the idea that directed panspermia could be demonstrated to be the origin ofall life on Earth if a distinctive 'signature' message were found, deliberately implanted into either the genome or the genetic codeof the first microorganisms by our hypothetical progenitor.[85][86][87][88]

In 2013 a team of physicists claimed that they had found mathematical and semiotic patterns in the genetic code which they thinkis evidence for such a signature.[89][90][91] This claim has been challenged by biologist PZ Myers who said, writing inPharyngula:

Unfortunately, what they’ve so honestly described is good old honest garbage ... Their methods failed torecognize a well-known functional association in the genetic code; they did not rule out the operation of naturallaw before rushing to falsely infer design ... We certainly don’t need to invoke panspermia. Nothing in the geneticcode requires design. and the authors haven’t demonstrated otherwise.[92]

In a later peer-reviewed article, the authors address the operation of natural law in an extensive statistical test, and draw the sameconclusion as in the previous article.[93] In special sections they also discuss methodological concerns raised by PZ Myers andsome others.

Pseudo-panspermia

Pseudo-panspermia (sometimes called soft panspermia, molecular panspermia or quasi-panspermia) proposes that the organicmolecules used for life originated in space and were incorporated in the solar nebula, from which the planets condensed and werefurther —and continuously— distributed to planetary surfaces where life then emerged (abiogenesis).[16][17] From the early1970s it was becoming evident that interstellar dust consisted of a large component of organic molecules. The first suggestioncame from Chandra Wickramasinghe, who proposed a polymeric composition based on the molecule formaldehyde (CH2O).[94]

Interstellar molecules are formed by chemical reactions within very sparse interstellar or circumstellar clouds of dust and gas.Usually this occurs when a molecule becomes ionized, often as the result of an interaction with cosmic rays. This positivelycharged molecule then draws in a nearby reactant by electrostatic attraction of the neutral molecule's electrons. Molecules canalso be generated by reactions between neutral atoms and molecules, although this process is generally slower.[18] The dust playsa critical role of shielding the molecules from the ionizing effect of ultraviolet radiation emitted by stars.[19]

A 2008 analysis of 12C/13C isotopic ratios of organic compounds found in the Murchison meteorite indicates a non-terrestrialorigin for these molecules rather than terrestrial contamination. Biologically relevant molecules identified so far include uracil, anRNA nucleobase, and xanthine.[95][96] These results demonstrate that many organic compounds which are components of life onEarth were already present in the early Solar System and may have played a key role in life's origin.[97]

In August 2009, NASA scientists identified one of the fundamental chemical building-blocks of life (the amino acid glycine) in acomet for the first time.[98]

In August 2011, a report, based on NASA studies with meteorites found on Earth, was published suggesting building blocks ofDNA (adenine, guanine and related organic molecules) may have been formed extraterrestrially in outer space.[99][100][101] InOctober 2011, scientists reported that cosmic dust contains complex organic matter ("amorphous organic solids with a mixedaromatic-aliphatic structure") that could be created naturally, and rapidly, by stars.[102][103][104] One of the scientists suggestedthat these complex organic compounds may have been related to the development of life on Earth and said that, "If this is thecase, life on Earth may have had an easier time getting started as these organics can serve as basic ingredients for life."[102]

In August 2012, and in a world first, astronomers at Copenhagen University reported the detection of a specific sugar molecule,glycolaldehyde, in a distant star system. The molecule was found around the protostellar binary IRAS 16293-2422, which islocated 400 light years from Earth.[105][106] Glycolaldehyde is needed to form ribonucleic acid, or RNA, which is similar infunction to DNA. This finding suggests that complex organic molecules may form in stellar systems prior to the formation ofplanets, eventually arriving on young planets early in their formation.[107]

In September 2012, NASA scientists reported that polycyclic aromatic hydrocarbons (PAHs), subjected to interstellar medium(ISM) conditions, are transformed, through hydrogenation, oxygenation and hydroxylation, to more complex organics – "a stepalong the path toward amino acids and nucleotides, the raw materials of proteins and DNA, respectively".[108][109] Further, as aresult of these transformations, the PAHs lose their spectroscopic signature which could be one of the reasons "for the lack ofPAH detection in interstellar ice grains, particularly the outer regions of cold, dense clouds or the upper molecular layers ofprotoplanetary disks."[108][109]

In 2013, the Atacama Large Millimeter Array (ALMA Project) confirmed that researchers have discovered an important pair ofprebiotic molecules in the icy particles in interstellar space (ISM). The chemicals, found in a giant cloud of gas about 25,000light-years from Earth in ISM, may be a precursor to a key component of DNA and the other may have a role in the formation ofan important amino acid. Researchers found a molecule called cyanomethanimine, which produces adenine, one of the fournucleobases that form the "rungs" in the ladder-like structure of DNA.[110]

The other molecule, called ethanamine, is thought to play a role in forming alanine, one of the twenty amino acids in the geneticcode. Previously, scientists thought such processes took place in the very tenuous gas between the stars. The new discoveries,however, suggest that the chemical formation sequences for these molecules occurred not in gas, but on the surfaces of ice grains

in interstellar space.[110] NASA ALMA scientist Anthony Remijan stated that finding these molecules in an interstellar gas cloudmeans that important building blocks for DNA and amino acids can 'seed' newly formed planets with the chemical precursors forlife.[111]

In March 2013, a simulation experiment indicate that dipeptides (pairs of amino acids) that can be building blocks of proteins,can be created in interstellar dust.[112]

In February 2014, NASA announced a greatly upgraded database (http://www.astrochem.org/pahdb/) for tracking polycyclicaromatic hydrocarbons (PAHs) in the universe. According to scientists, more than 20% of the carbon in the universe may beassociated with PAHs, possible starting materials for the formation of life. PAHs seem to have been formed shortly after the BigBang, are widespread throughout the universe, and are associated with new stars and exoplanets.[113]

In March 2015, NASA scientists reported that, for the first time, complex DNA and RNA organic compounds of life, includinguracil, cytosine and thymine, have been formed in the laboratory under outer space conditions, using starting chemicals, such aspyrimidine, found in meteorites. Pyrimidine, like polycyclic aromatic hydrocarbons (PAHs), the most carbon-rich chemical foundin the Universe, may have been formed in red giants or in interstellar dust and gas clouds, according to the scientists.[114]

In May 2016, the Rosetta Mission team reported the presence of glycine, methylamine and ethylamine in the coma of67P/Churyumov-Gerasimenko.[115] This, plus the detection of phosphorus, is consistent with the hypothesis that comets played acrucial role in the emergence of life on Earth.

The chemistry of life may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch when theUniverse was only 10–17 million years old.[116][117][118] According to the panspermia hypothesis, microscopic life—distributedby meteoroids, asteroids and other small Solar System bodies—may exist throughout the universe.[119] Nonetheless, Earth is theonly place in the universe known by humans to harbor life.[120][121] The sheer number of planets in the Milky Way galaxy,however, may make it probable that life has arisen somewhere else in the galaxy and the universe. It is generally agreed that theconditions required for the evolution of intelligent life as we know it are probably exceedingly rare in the universe, whilesimultaneously noting that simple single-celled microorganisms may be more likely.[122]

The extrasolar planet results from the Kepler mission estimate 100–400 billion exoplanets, with over 3,500 as candidates orconfirmed exoplanets.[123] On 4 November 2013, astronomers reported, based on Kepler space mission data, that there could beas many as 40 billion Earth-sized planets orbiting in the habitable zones of sun-like stars and red dwarf stars within the MilkyWay Galaxy.[124][125] 11 billion of these estimated planets may be orbiting sun-like stars.[126] The nearest such planet may be 12light-years away, according to the scientists.[124][125]

It is estimated that space travel over cosmic distances would take an incredibly long time to an outside observer, and with vastamounts of energy required. However, some scientists hypothesize that faster-than-light interstellar space travel might be feasible.This has been explored by NASA scientists since at least 1995.[127]

Hoyle and Wickramasinghe have speculated that several outbreaks of illnesses on Earth are of extraterrestrial origins, includingthe 1918 flu pandemic, and certain outbreaks of polio and mad cow disease. For the 1918 flu pandemic they hypothesized thatcometary dust brought the virus to Earth simultaneously at multiple locations—a view almost universally dismissed by experts onthis pandemic. Hoyle also speculated that HIV came from outer space.[128]

Extraterrestrial life

Hypotheses on extraterrestrial sources of illnesses

After Hoyle's death, The Lancet published a letter to the editor from Wickramasinghe and two of his colleagues,[129] in whichthey hypothesized that the virus that causes severe acute respiratory syndrome (SARS) could be extraterrestrial in origin and notoriginated from chickens. The Lancet subsequently published three responses to this letter, showing that the hypothesis was notevidence-based, and casting doubts on the quality of the experiments referenced by Wickramasinghe in his letter.[130][131][132] A2008 encyclopedia notes that "Like other claims linking terrestrial disease to extraterrestrial pathogens, this proposal was rejectedby the greater research community."[128]

In April 2016, Jiangwen Qu of the Department of Infectious Disease Control in China presented a statistical study suggesting that"extremes of sunspot activity to within plus or minus 1 year may precipitate influenza pandemics." He discussed possiblemechanisms of epidemic initiation and early spread, including speculation on primary causation by externally derived viralvariants from space via cometary dust.[133]

A meteorite originating from Mars known as ALH84001 was shown in 1996 to contain microscopic structuresresembling small terrestrial nanobacteria. When the discovery was announced, many immediately conjecturedthat these were fossils and were the first evidence of extraterrestrial life — making headlines around the world.Public interest soon started to dwindle as most experts started to agree that these structures were not indicativeof life, but could instead be formed abiotically from organic molecules. However, in November 2009, a team ofscientists at Johnson Space Center, including David McKay, reasserted that there was "strong evidence that lifemay have existed on ancient Mars", after having reexamined the meteorite and finding magnetitecrystals.[134][135]

On May 11, 2001, two researchers from the University of Naples claimed to have found viable extraterrestrialbacteria inside a meteorite. Geologist Bruno D'Argenio and molecular biologist Giuseppe Geraci claim thebacteria were wedged inside the crystal structure of minerals, but were resurrected when a sample of the rockwas placed in a culture medium.[136][137][138]

An Indian and British team of researchers led by Chandra Wickramasinghe reported on 2001 that air samplesover Hyderabad, India, gathered from the stratosphere by the Indian Space Research Organisation (ISRO) onJanuary 21, 2001, contained clumps of living cells.[139] Wickramasinghe calls this "unambiguous evidence for thepresence of clumps of living cells in air samples from as high as 41 km, above which no air from lower downwould normally be transported".[140][141] Two bacterial and one fungal species were later independently isolatedfrom these filters which were identified as Bacillus simplex, Staphylococcus pasteuri and Engyodontium albumrespectively.[142][143] Pushkar Ganesh Vaidya from the Indian Astrobiology Research Centre reported in 2009that "the three microorganisms captured during the balloon experiment do not exhibit any distinct adaptationsexpected to be seen in microorganisms occupying a cometary niche".[144][145]

In 2005 an improved experiment was conducted by ISRO. On April 20, 2005, air samples were collected from theupper atmosphere at altitudes ranging from 20 km to more than 40 km.[146] The samples were tested at two labsin India. The labs found 12 bacterial and 6 different fungal species in these samples. The fungi were Penicilliumdecumbens, Cladosporium cladosporioides, Alternaria sp. and Tilletiopsis albescens. Out of the 12 bacterialsamples, three were identified as new species and named Janibacter hoylei (after Fred Hoyle), Bacillusisronensis (named after ISRO) and Bacillus aryabhattai (named after the ancient Indian mathematician,Aryabhata). These three new species showed that they were more resistant to UV radiation than similarbacteria.[147][148]

Some other researchers have retrieved bacteria from the stratosphere since the1970s.[149] Atmospheric sampling by NASA in 2010 before and after hurricanes, collected314 different types of bacteria; the study suggests that large-scale convection duringtropical storms and hurricanes can then carry this material from the surface higher up intothe atmosphere.[150][151]

Another proposed mechanism of spores in the stratosphere is lifting by weather and Earth magnetism up to theionosphere into low Earth orbit, where Russian astronauts retrieved DNA from a known sterile exterior surface ofthe International Space Station.[39] The Russian scientists then also speculated the possibility "that commonterrestrial bacteria are constantly being resupplied from space."[39]

In 2013, Dale Warren Griffin, a microbiologist working at the United States Geological Survey noted that virusesare the most numerous entities on Earth. Griffin speculates that viruses evolved in comets and on other planets

Case studies

and moons may be pathogenic to humans, so he proposed to also look for viruses on moons and planets of theSolar System.[152]

A separate fragment of the Orgueil meteorite (kept in a sealed glass jar since its discovery) was found in 1965 to have a seedcapsule embedded in it, whilst the original glassy layer on the outside remained undisturbed. Despite great initial excitement, theseed was found to be that of a European Juncaceae or Rush plant that had been glued into the fragment and camouflaged usingcoal dust. The outer "fusion layer" was in fact glue. Whilst the perpetrator of this hoax is unknown, it is thought that they soughtto influence the 19th century debate on spontaneous generation — rather than panspermia — by demonstrating the transformationof inorganic to biological matter.[153]

Until the 1970s, life was thought to depend on its access to sunlight. Even life inthe ocean depths, where sunlight cannot reach, was believed to obtain itsnourishment either from consuming organic detritus rained down from thesurface waters or from eating animals that did.[154] However, in 1977, during anexploratory dive to the Galapagos Rift in the deep-sea exploration submersibleAlvin, scientists discovered colonies of assorted creatures clustered aroundundersea volcanic features known as black smokers.[154]

It was soon determined that the basis for this food chain is a form of bacteriumthat derives its energy from oxidation of reactive chemicals, such as hydrogen orhydrogen sulfide, that bubble up from the Earth's interior. This chemosynthesisrevolutionized the study of biology by revealing that terrestrial life need not beSun-dependent; it only requires water and an energy gradient in order to exist.

It is now known that extremophiles, microorganisms with extraordinarycapability to thrive in the harshest environments on Earth, can specialize tothrive in the deep-sea,[155][156][157] ice, boiling water, acid, the water core ofnuclear reactors, salt crystals, toxic waste and in a range of other extremehabitats that were previously thought to be inhospitable forlife.[158][159][160][161] Living bacteria found in ice core samples retrieved from3,700 metres (12,100 ft) deep at Lake Vostok in Antarctica, have provided datafor extrapolations to the likelihood of microorganisms surviving frozen inextraterrestrial habitats or during interplanetary transport.[162] Also, bacteria have been discovered living within warm rock deepin the Earth's crust.[163]

In order to test some of these organisms' potential resilience in outer space, plant seeds and spores of bacteria, fungi and fernshave been exposed to the harsh space environment.[160][161][164] Spores are produced as part of the normal life cycle of manyplants, algae, fungi and some protozoans, and some bacteria produce endospores or cysts during times of stress. These structuresmay be highly resilient to ultraviolet and gamma radiation, desiccation, lysozyme, temperature, starvation and chemicaldisinfectants, while metabolically inactive. Spores germinate when favourable conditions are restored after exposure to conditionsfatal to the parent organism.

Although computer models suggest that a captured meteoroid would typically take some tens of millions of years before collisionwith a planet,[50] there are documented viable Earthly bacterial spores that are 40 million years old that are very resistant toradiation,[50][56] and others able to resume life after being dormant for 25 million years,[165] suggesting that lithopanspermia life-

Hoaxes

Extremophiles

Hydrothermal vents are able tosupport extremophile bacteria onEarth and may also support life inother parts of the cosmos.

transfers are possible via meteorites exceeding 1 m in size.[50]

The discovery of deep-sea ecosystems, along with advancements in the fields of astrobiology, observational astronomy anddiscovery of large varieties of extremophiles, opened up a new avenue in astrobiology by massively expanding the number ofpossible extraterrestrial habitats and possible transport of hardy microbial life through vast distances.[70]

The question of whether certain microorganisms can survive in the harsh environment of outer space has intrigued biologistssince the beginning of spaceflight, and opportunities were provided to expose samples to space. The first American tests weremade in 1966, during the Gemini IX and XII missions, when samples of bacteriophage T1 and spores of Penicillium roquefortiwere exposed to outer space for 16.8 h and 6.5 h, respectively.[61][70] Other basic life sciences research in low Earth orbit startedin 1966 with the Soviet biosatellite program Bion and the U.S. Biosatellite program. Thus, the plausibility of panspermia can beevaluated by examining life forms on Earth for their capacity to survive in space.[166] The following experiments carried on lowEarth orbit specifically tested some aspects of panspermia or lithopanspermia:

The Exobiology Radiation Assembly (ERA) was a 1992 experiment on boardthe European Retrievable Carrier (EURECA) on the biological effects of spaceradiation. EURECA was an unmanned 4.5 tonne satellite with a payload of 15experiments.[167] It was an astrobiology mission developed by the EuropeanSpace Agency (ESA). Spores of different strains of Bacillus subtilis and theEscherichia coli plasmid pUC19 were exposed to selected conditions of space(space vacuum and/or defined wavebands and intensities of solar ultravioletradiation). After the approximately 11-month mission, their responses werestudied in terms of survival, mutagenesis in the his (B. subtilis) or lac locus(pUC19), induction of DNA strand breaks, efficiency of DNA repair systems,and the role of external protective agents. The data were compared with those of a simultaneously running ground controlexperiment:[168][169]

The survival of spores treated with the vacuum of space, however shielded against solar radiation, issubstantially increased, if they are exposed in multilayers and/or in the presence of glucose as protective.All spores in "artificial meteorites", i.e. embedded in clays or simulated Martian soil, are killed.Vacuum treatment leads to an increase of mutation frequency in spores, but not in plasmid DNA.Extraterrestrial solar ultraviolet radiation is mutagenic, induces strand breaks in the DNA and reduces survivalsubstantially.Action spectroscopy confirms results of previous space experiments of a synergistic action of space vacuum andsolar UV radiation with DNA being the critical target.The decrease in viability of the microorganisms could be correlated with the increase in DNA damage.The purple membranes, amino acids and urea were not measurably affected by the dehydrating condition ofopen space, if sheltered from solar radiation. Plasmid DNA, however, suffered a significant amount of strandbreaks under these conditions.[168]

BIOPAN is a multi-user experimental facility installed on the external surface of the Russian Foton descent capsule. Experimentsdeveloped for BIOPAN are designed to investigate the effect of the space environment on biological material after exposurebetween 13 and 17 days.[170] The experiments in BIOPAN are exposed to solar and cosmic radiation, the space vacuum andweightlessness, or a selection thereof. Of the 6 missions flown so far on BIOPAN between 1992 and 2007, dozens of experimentswere conducted, and some analyzed the likelihood of panspermia. Some bacteria, lichens (Xanthoria elegans, Rhizocarpon

Research in outer space

ERA

EURECA facility deployment in 1992

BIOPAN

geographicum and their mycobiont cultures, the black Antarctic microfungi Cryomyces minteri and Cryomyces antarcticus),spores, and even one animal (tardigrades) were found to have survived the harsh outer space environment and cosmicradiation.[171][172][173][174]

The German EXOSTACK experiment was deployed on 7 April 1984 on boardthe Long Duration Exposure Facility statellite. 30% of Bacillus subtilis sporessurvived the nearly 6 years exposure when embedded in salt crystals, whereas80% survived in the presence of glucose, which stabilize the structure of thecellular macromolecules, especially during vacuum-induceddehydration.[61][175]

If shielded against solar UV, spores of B. subtilis were capable of surviving inspace for up to 6 years, especially if embedded in clay or meteorite powder(artificial meteorites). The data support the likelihood of interplanetary transferof microorganisms within meteorites, the so-called lithopanspermiahypothesis.[61]

EXPOSE is a multi-user facility mounted outside the International Space Stationdedicated to astrobiology experiments.[164] There have been three EXPOSEexperiments flown between 2008 and 2015: EXPOSE-E, EXPOSE-R andEXPOSE-R2. Results from the orbital missions, especially the experiments SEEDS[176] andLiFE,[177] concluded that after an 18-month exposure, some seeds and lichens(Stichococcus sp. and Acarospora sp., a lichenized fungal genus) may be capableto survive interplanetary travel if sheltered inside comets or rocks from cosmicradiation and UV radiation.[164][178] The LIFE, SPORES, and SEEDS parts ofthe experiments provided information about the likelihood oflithopanspermia.[179][180][181] These studies will provide experimental data tothe lithopanspermia hypothesis,[180] and they will provide basic data toplanetary protection issues.

The Tanpopo mission is an orbital astrobiology experiment by Japan that is currently investigating the possible interplanetarytransfer of life, organic compounds, and possible terrestrial particles in low Earth orbit. The Tanpopo experiment took place at theExposed Facility located on the exterior of Kibo module of the International Space Station. The mission collected cosmic dustsand other particles for three years by using an ultra-low density silica gel called aerogel. The purpose is to assess the panspermiahypothesis and the possibility of natural interplanetary transport of life and its precursors.[182][183] Some of these aerogels werereplaced every one or two years through 2018.[184] Sample collection began in May 2015, and the first samples were returned toEarth in mid-2016.[185] Analyses are ongoing.

Panspermia is often criticized because it does not answer the question of the origin of life but merely places it on another celestialbody. It was also criticized because it was thought it could not be tested experimentally.[70]

EXOSTACK

EXOSTACK on the Long DurationExposure Facility satellite.

EXPOSE

Location of the astrobiologyEXPOSE-E and EXPOSE-R facilitieson the International Space Station

Tanpopo

Criticism

Wallis and Wickramasinghe argued in 2004 that the transport of individualbacteria or clumps of bacteria, is overwhelmingly more important thanlithopanspermia in terms of numbers of microbes transferred, even accountingfor the death rate of unprotected bacteria in transit.[186] Then it was found thatisolated spores of B. subtilis were killed by several orders of magnitude ifexposed to the full space environment for a mere few seconds. Though theseresults may seem to negate the original panspermia hypothesis, the type ofmicroorganism making the long journey is inherently unknown and also itsfeatures unknown. It could then be impossible to dismiss the hypothesis basedon the hardiness of a few earth-evolved microorganism. Also, if shielded againstsolar UV, spores of Bacillus subtilis were capable of surviving in space for up to6 years, especially if embedded in clay or meteorite powder (artificialmeteorites). The data support the likelihood of interplanetary transfer ofmicroorganisms within meteorites, the so-called lithopanspermia hypothesis.[61]

Abiogenesis – The natural process by which life arises from non-living matterAnthropic principle – Philosophical consideration that observations of the universe must be compatible with theobserversAstrobiology – Science concerned with life in the universeAstrobiology journalAstrobiology MagazineCryptobiosisDrake equation – Probabilistic argument to estimate the number of alien civilizations in the galaxyEarliest known life forms – Putative fossilized microorganisms found near hydrothermal ventsFermi paradox – The apparent contradiction between the lack of evidence and high probability estimates for theexistence of extraterrestrial civilizationsFine-tuned UniverseInterplanetary contamination – Biological contamination of a planetary body by a space probe or spacecraftList of microorganisms tested in outer spacePlanetary protection – A guiding principle in the design of an interplanetary mission, aiming to prevent biologicalcontamination of both the target celestial body and the EarthRare Earth hypothesis – Hypothesis that complex extraterrestrial life is improbable and extremely rareRed rain in KeralaTanpopo (mission) – An orbital astrobiology experiment investigating the potential interplanetary transfer of life,organic compounds, and possible terrestrial particles in the low Earth orbitTholin – Class of molecules formed by ultraviolet irradiation of organic compounds

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Dust collector with aerogel blocks

See also

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