Panspermia
Panspermia () is the hypothesis that life exists throughout the Universe, distributed by space dust, meteoroids, asteroids, comets, and planetoids, as well as by spacecraft carrying unintended contamination by microorganisms. Panspermia is a fringe theory with little support amongst mainstream scientists. Critics argue that it does not answer the question of the origin of life but merely places it on another celestial body. It is also criticized because it cannot be tested experimentally. Panspermia hypotheses propose (for example) that microscopic life-forms which can survive the effects of space (such as extremophiles) can become trapped in debris ejected into  space after collisions between planets and small Solar System bodies that harbor life. Panspermia studies concentrate not on how life began, but on methods that may distribute it in the Universe. 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—and continuously—distributed to planetary surfaces where life then emerged (abiogenesis). History. The first known mention of the term was in the writings of the fifth-century BC Greek philosopher Anaxagoras. Panspermia began to  assume a more scientific form through the proposals of Jöns Jacob Berzelius (1834), Hermann E. Richter (1865), Kelvin (1871), Hermann von Helmholtz (1879) and finally reaching the level of a detailed scientific hypothesis through the efforts of the Swedish chemist Svante Arrhenius (1903). Fred Hoyle (1915–2001) and Chandra Wickramasinghe (born 1939) were influential proponents of panspermia. In 1974 they proposed the hypothesis that some dust in interstellar space was largely organic (containing carbon), which Wickramasinghe later proved to be correct. Hoyle and Wickramasinghe further contended that life forms continue to enter the Earth's atmosphere, and may be responsible for  epidemic outbreaks, new diseases, and the genetic novelty necessary for macroevolution. 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 of space for about 1.5 years. Some organisms survived in an inactive state for considerable lengths of time, and those samples sheltered by simulated meteorite material provide experimental evidence for the likelihood of the hypothetical scenario of lithopanspermia. In October 2018, Harvard astronomers presented an analytical model that suggests matter—and potentially dormant  spores—can be exchanged across the vast distances between galaxies, a process termed 'galactic panspermia', and not be restricted to the limited scale of solar systems. The detection of an extra-solar object named ʻOumuamua crossing the inner Solar System in a hyperbolic orbit confirms the existence of a continuing material link with exoplanetary systems. In November 2019, scientists reported detecting, for the first time, sugar molecules, including ribose, in meteorites, suggesting that chemical processes on asteroids can produce some fundamentally essential bio-ingredients important to life, and supporting the notion of an RNA world prior to a DNA-based  origin of life on Earth, and possibly, as well, the notion of panspermia. Proposed mechanisms. Panspermia can be said to be either interstellar (between star systems) or interplanetary (between planets in the same star system); its transport mechanisms may include comets, radiation pressure and lithopanspermia (microorganisms embedded in rocks). Interplanetary transfer of nonliving material is well documented, as evidenced by meteorites of Martian origin found on Earth. Space probes may also be a viable transport mechanism for interplanetary cross-pollination in the Solar System or even beyond. However, space agencies have implemented planetary protection procedures to reduce the risk of  planetary contamination, although, as recently discovered, some microorganisms, such as Tersicoccus phoenicis, may be resistant to procedures used in spacecraft assembly clean room facilities. In 2012, mathematician Edward Belbruno and astronomers Amaya Moro-Martín and Renu Malhotra proposed that gravitational low-energy transfer of rocks among the young planets of stars in their birth cluster is commonplace, and not rare in the general galactic stellar population. Deliberate directed panspermia from space to seed Earth or sent from Earth to seed other planetary systems have also been proposed. 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's atmosphere with sufficient speed to cross interstellar space to other systems before the spores can be destroyed. In 2020, Paleobiologist Grzegorz Sadlok proposed the hypothesis that life can transit interstellar distances on nomadic exoplanets and/or its exomoons. In 2020, Avi Loeb and Amir Siraj wrote about the possible transfer of life by objects grazing the Earth's atmosphere and reaching exoplanetary systems. Radiopanspermia. In 1903, Svante Arrhenius published in his article "The Distribution of Life in Space", the hypothesis now called radiopanspermia, that microscopic  forms of life can be propagated in space, driven by the radiation pressure from stars. 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 particles only, such as single bacterial spores. The main criticism of radiopanspermia hypothesis came from Iosif Shklovsky and Carl Sagan, who pointed out the proofs of the lethal action of space radiation (UV and X-rays) in the cosmos. Regardless of the evidence, Wallis and  Wickramasinghe argued in 2004 that the transport of individual bacteria or clumps of bacteria, is overwhelmingly more important than lithopanspermia in terms of numbers of microbes transferred, even accounting for the death rate of unprotected bacteria in transit. 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 shielded against solar UV, the spores were capable of surviving in space for up to six years while embedded in clay or meteorite powder (artificial  meteorites). Minimal protection is required to shelter a spore against UV radiation: Exposure of unprotected DNA to solar UV and cosmic ionizing radiation break it up into its constituent bases. Also, exposing DNA to the ultrahigh vacuum of space alone is sufficient to cause DNA damage, so the transport of unprotected DNA or RNA during interplanetary flights powered solely by light pressure is extremely unlikely. 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, rocks that are greater than or equal to 1 meter in diameter are required to effectively shield resistant microorganisms, such as bacterial spores against galactic cosmic radiation. These results clearly negate the radiopanspermia hypothesis, which requires single spores accelerated by the radiation pressure of the Sun, requiring many years to travel between the planets, and support the likelihood of interplanetary transfer of microorganisms within asteroids or comets, the so-called lithopanspermia hypothesis. Lithopanspermia. 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 have become amenable to experimental testing. However, it has been argued, that even if organisms survive all 3 stages, the possibility of their immediate survival on a new world still remain relatively low due to evolutionary constraints in the form of a inverse proportionality between environments between worlds. Accidental panspermia. Thomas Gold, a professor of astronomy, suggested in 1960 the hypothesis of "Cosmic Garbage", that life on Earth might have originated accidentally from a pile of  waste products dumped on Earth long ago by extraterrestrial beings. Directed panspermia. Directed panspermia concerns the deliberate transport of microorganisms in space, sent to Earth to start life here, or sent from Earth to seed new planetary systems with life by introduced species of microorganisms on lifeless planets. The Nobel prize winner Francis Crick, along with Leslie Orgel proposed that life may have been purposely spread by an advanced extraterrestrial civilization, but considering an early "RNA world" Crick noted later that life may have originated on Earth. It has been suggested that 'directed' panspermia was proposed in order to counteract  various objections, including the argument that microbes would be inactivated by the space environment and cosmic radiation before they could make a chance encounter with Earth. Conversely, active directed panspermia has been proposed to secure and expand life in space. This may be motivated by biotic ethics that values, and seeks to propagate the basic patterns of our organic gene/protein life-form. A number of publications since 1979 have proposed the idea that directed panspermia could be demonstrated to be the origin of all life on Earth if a distinctive 'signature' message were found, deliberately implanted into either the  genome or the genetic code of the first microorganisms by our hypothetical progenitor. Pseudo-panspermia. Pseudo-panspermia (sometimes called soft panspermia, molecular panspermia or quasi-panspermia) proposes that the organic molecules used for life originated in space and were incorporated in the solar nebula, from which the planets condensed and were further—and continuously—distributed to planetary surfaces where life then emerged (abiogenesis). From the early 1970s it was becoming evident that interstellar dust consisted of a large component of organic molecules. The first suggestion came from Chandra Wickramasinghe, who proposed a polymeric composition based on the molecule formaldehyde (CHO ). 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 positively charged molecule then draws in a nearby reactant by electrostatic attraction of the neutral molecule's electrons. Molecules can also be generated by reactions between neutral atoms and molecules, although this process is generally slower. The dust plays a critical role of shielding the molecules from the ionizing effect of ultraviolet radiation emitted by stars. Mathematician Jason Guillory in his 2008 analysis  of C/C isotopic ratios of organic compounds found in the Murchison meteorite indicates a non-terrestrial origin for these molecules rather than terrestrial contamination. Biologically relevant molecules identified so far include uracil (an RNA nucleobase), and xanthine. These results demonstrate that many organic compounds which are components of life on Earth were already present in the early Solar System and may have played a key role in life's origin. In August 2009, NASA scientists identified one of the fundamental chemical building-blocks of life (the amino acid glycine) in a comet for the first time. In August 2011 , a report, based on NASA studies with meteorites found on Earth, was published suggesting building blocks of DNA (adenine, guanine and related organic molecules) may have been formed extraterrestrially in outer space. In October 2011, scientists reported that cosmic dust contains complex organic matter ("amorphous organic solids with a mixed aromatic-aliphatic structure") that could be created naturally, and rapidly, by stars. One of the scientists suggested that these complex organic compounds may have been related to the development of life on Earth and said that, "If this is the case, life on Earth may have had an easier time  getting started as these organics can serve as basic ingredients for life." 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 is located 400 light years from Earth. Glycolaldehyde is needed to form ribonucleic acid, or RNA, which is similar in function to DNA. This finding suggests that complex organic molecules may form in stellar systems prior to the formation of planets, eventually arriving on young planets early in their formation . 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 step along the path toward amino acids and nucleotides, the raw materials of proteins and DNA, respectively". Further, as a result of these transformations, the PAHs lose their spectroscopic signature which could be one of the reasons "for the lack of PAH detection in interstellar ice grains, particularly the outer regions of cold, dense clouds or the upper molecular layers of protoplanetary disks." In 2013, the Atacama Large Millimeter Array (ALMA Project ) confirmed that researchers have discovered an important pair of prebiotic molecules in the icy particles in interstellar space (ISM). The chemicals, found in a giant cloud of gas about 25,000 light-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 of an important amino acid. Researchers found a molecule called cyanomethanimine, which produces adenine, one of the four nucleobases that form the "rungs" in the ladder-like structure of DNA. The other molecule, called ethanamine, is thought to play a role in forming alanine , one of the twenty amino acids in the genetic code. 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. NASA ALMA scientist Anthony Remijan stated that finding these molecules in an interstellar gas cloud means that important building blocks for DNA and amino acids can 'seed' newly formed planets with the chemical precursors for life. 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. In February 2014, NASA announced a greatly upgraded database for tracking polycyclic aromatic hydrocarbons (PAHs) in the universe. According to scientists, more than 20% of the carbon in the universe may be associated with PAHs, possible starting materials for the formation of life. PAHs seem to have been formed shortly after the Big Bang, are widespread throughout the universe, and are associated with new stars and exoplanets. In March 2015, NASA scientists reported that, for the first time, complex DNA and RNA organic compounds of life , including uracil, cytosine and thymine, have been formed in the laboratory under outer space conditions, using starting chemicals, such as pyrimidine, found in meteorites. Pyrimidine, like polycyclic aromatic hydrocarbons (PAHs), the most carbon-rich chemical found in the Universe, may have been formed in red giants or in interstellar dust and gas clouds, according to the scientists. In May 2016, the Rosetta Mission team reported the presence of glycine, methylamine and ethylamine in the coma of 67P/Churyumov-Gerasimenko. This, plus the detection of phosphorus, is consistent with the hypothesis that comets played a crucial role in the emergence of  life on Earth. In 2019, the detection of extraterrestrial sugars in meteorites implied the possibility that extraterrestrial sugars may have contributed to forming functional biopolymers like RNA. In February 2022, peptides were experimentally proven to be formed under outer space conditions, using starting chemicals (CO, C, and NH) that are very abundant in the molecular clouds in the interstellar medium. In April 2022, scientists reported the detection of purine and pyrimidine nucleobases in several meteorites, including guanine, adenine, cytosine, uracil and thymine, and claimed that such meteoritic nucleobases could serve as "building blocks of DNA and RNA on the early  Earth". Protospermia. Betül Kaçar, the director of the NASA Astrobiology Consortium MUSE, calls sending the chemical capacity for life to emerge on another planetary body "protospermia". Reflecting the ethical implications of the possibility that humans are capable of instigating multiple origins of life under a broader array of circumstances than life currently exists, she wrote: "With protospermia, whatever arises after we provide a nudge toward biogenesis would be just as much a product of that environment as our life is of Earth. It would be unique and 'of' that destination body as much as its rocks on the ground and  the gasses in its atmosphere." Material-based panspermia. Kuhan Chandru (from the National University of Malaysia) and Tony Z. Jia (from the Earth-Life Science Institute, Japan) have suggested that a panspermia-seed need not be an organism. This is because an organism would struggle to facilitate any origins of life event on a new planet. Instead, they suggest a material-based seed (e.g., abiotically forming gels, polymers, ceramic, etc.) that may be capable of enduring spaceflight and, upon reaching a new planet, may kick-start the origins of life. They used a prebiotic polyester system as a material-based  Panspermia example, to hypothesize how such a system may be capable of doing panspermia-type origins of life. Extraterrestrial life. According to the panspermia hypothesis, microscopic life—distributed by meteoroids, asteroids and other small Solar System bodies—may exist throughout the universe. Of the bodies on which life is possible, living organisms could most easily enter the other bodies of the Solar System from Enceladus. It is generally agreed that the conditions required for the evolution of intelligent life as we know it are probably exceedingly rare in the universe, while simultaneously noting that simple single-celled microorganisms may be  more likely. Hypotheses on extraterrestrial sources of illnesses. Hoyle and Wickramasinghe have speculated that several outbreaks of illnesses on Earth are of extraterrestrial origins, including the 1918 flu pandemic, and certain outbreaks of polio and mad cow disease. For the 1918 flu pandemic they hypothesized that cometary dust brought the virus to Earth simultaneously at multiple locations—a view almost universally dismissed by experts on this pandemic. Hoyle also speculated that HIV came from outer space. After Hoyle's death, "The Lancet" published a letter to the editor from Wickramasinghe and two of his colleagues, in which they hypothesized that  the virus that causes severe acute respiratory syndrome (SARS) could be extraterrestrial in origin and not originated from chickens. "The Lancet" subsequently published three responses to this letter, showing that the hypothesis was not evidence-based, and casting doubts on the quality of the experiments cited by Wickramasinghe in his letter. A 2008 encyclopedia notes that "Like other claims linking terrestrial disease to extraterrestrial pathogens, this proposal was rejected by the greater research community." Hoaxes. A separate fragment of the Orgueil meteorite (kept in a sealed glass jar since its discovery) was found in 1965 to have a seed capsule  embedded in it, whilst the original glassy layer on the outside remained undisturbed. Despite great initial excitement, the seed was found to be that of a European Juncaceae or Rush plant that had been glued into the fragment and camouflaged using coal dust. The outer "fusion layer" was in fact glue. Whilst the perpetrator of this hoax is unknown, it is thought that they sought to influence the 19th century debate on spontaneous generation—rather than panspermia—by demonstrating the transformation of inorganic to biological matter. Extremophiles. Until the 1970s, life was thought to depend on its access to sunlight . Even life in the ocean depths, where sunlight cannot reach, was believed to obtain its nourishment either from consuming organic detritus rained down from the surface waters or from eating animals that did. It is now known that extremophiles, microorganisms with extraordinary capability to thrive in the harshest environments on Earth, can specialize to thrive in the deep-sea, ice, boiling water, acid, the water core of nuclear reactors, salt crystals, toxic waste and in a range of other extreme habitats that were previously thought to be inhospitable for life. Living bacteria found in ice core samples retrieved from  deep at Lake Vostok in Antarctica, have provided data for extrapolations to the likelihood of microorganisms surviving frozen in extraterrestrial habitats or during interplanetary transport. In order to test some of these organisms' potential resilience in outer space, plant seeds and spores of bacteria, fungi and ferns have been exposed to the harsh space environment. Research in outer space. The question of whether certain microorganisms can survive in the harsh environment of outer space has intrigued biologists since the beginning of spaceflight, and opportunities were provided to expose samples to space. The first American tests were made in 1966, during  the Gemini IX and XII missions, when samples of bacteriophage T1 and spores of "Penicillium roqueforti" were exposed to outer space for 16.8 h and 6.5 h, respectively. Other basic life sciences research in low Earth orbit started in 1966 with the Soviet biosatellite program Bion and the U.S. Biosatellite program. Thus, the plausibility of panspermia can be evaluated by examining life forms on Earth for their capacity to survive in space. The following experiments carried on low Earth orbit specifically tested some aspects of panspermia or lithopanspermia: ERA. The Exobiology Radiation Assembly (ERA) was a 1992 experiment on board  the European Retrievable Carrier (EURECA) on the biological effects of space radiation. EURECA was an uncrewed 4.5 tonne satellite with a payload of 15 experiments. It was an astrobiology mission developed by the European Space Agency (ESA). Spores of different strains of "Bacillus subtilis" and the "Escherichia coli" plasmid pUC19 were exposed to selected conditions of space (space vacuum and/or defined wavebands and intensities of solar ultraviolet radiation). After the approximately 11-month mission, their responses were studied 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 control experiment: BIOPAN. BIOPAN is a multi-user experimental facility installed on the external surface of the Russian Foton descent capsule. Experiments developed for BIOPAN are designed to investigate the effect of the space environment on biological material after exposure between 13 and 17 days. The experiments in BIOPAN are exposed to solar and cosmic radiation, the space vacuum and weightlessness, or a selection thereof. Of the 6 missions flown so far on BIOPAN between 1992 and 2007, dozens of  experiments were conducted, and some analyzed the likelihood of panspermia. Some bacteria, lichens ("Xanthoria elegans", "Rhizocarpon 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 cosmic radiation. EXOSTACK. The German EXOSTACK experiment was deployed on 7 April 1984 on board the Long Duration Exposure Facility satellite. 30% of "Bacillus subtilis" spores survived the nearly 6 years exposure when embedded in salt crystals, whereas 80% survived in the presence of glucose, which stabilize the structure of the cellular macromolecules , especially during vacuum-induced dehydration. If shielded against solar UV, spores of "B. subtilis" were capable of surviving in space for up to 6 years, especially if embedded in clay or meteorite powder (artificial meteorites). The data support the likelihood of interplanetary transfer of microorganisms within meteorites, the so-called lithopanspermia hypothesis. EXPOSE. EXPOSE is a multi-user facility mounted outside the International Space Station dedicated to astrobiology experiments. There have been three EXPOSE experiments flown between 2008 and 2015: EXPOSE-E, EXPOSE-R and EXPOSE-R2. Results from the orbital missions, especially the experiments "SEEDS" and "LiFE", concluded that  after an 18-month exposure, some seeds and lichens ("Stichococcus sp." and "Acarospora sp"., a lichenized fungal genus) may be capable to survive interplanetary travel if sheltered inside comets or rocks from cosmic radiation and UV radiation. The "LIFE", "SPORES", and "SEEDS" parts of the experiments provided information about the likelihood of lithopanspermia. These studies will provide experimental data to the lithopanspermia hypothesis, and they will provide basic data to planetary protection issues. Tanpopo. The Tanpopo mission is an orbital astrobiology experiment by Japan that is currently investigating the possible interplanetary transfer of life, organic compounds, and possible terrestrial particles  in low Earth orbit. The Tanpopo experiment took place at the Exposed Facility located on the exterior of Kibo module of the International Space Station. The mission collected cosmic dusts and other particles for three years by using an ultra-low density silica gel called aerogel. The purpose is to assess the panspermia hypothesis and the possibility of natural interplanetary transport of life and its precursors. Some of these aerogels were replaced every one or two years through 2018. Sample collection began in May 2015, and the first samples were returned to Earth in mid-2016. In August 2020, scientists reported  that bacteria from Earth, particularly "Deinococcus radiodurans" bacteria, which is highly resistant to environmental hazards, were found to survive for three years in outer space, based on studies conducted on the International Space Station. Hayabusa2. Hayabusa2 is an asteroid sample-return mission. In 2020, the spacecraft brought back a capsule containing a sample of carbon-rich asteroid dust from the asteroid 162173 Ryugu. Scientists believe this could provide clues about the ancient delivery of water and organic molecules to Earth. Seiichiro Watanabe from the Hayabusa project said: "There are a lot of samples and it seems they contain plenty of  organic matter, so I hope we can find out many things about how organic substances have developed on the parent body of Ryugu."