Panspermia (from Ancient Greek πᾶν (pan)  'all ', and σπέρμα (sperma)  'seed') is the hypothesis, first proposed in the 5th century BC by the Greek philosopher Anaxagoras, that life exists throughout the Universe, distributed by space dust,^[1] meteoroids,^[2] asteroids, comets,^[3] and planetoids,^[4] as well as by spacecraft carrying unintended contamination by microorganisms.^[5][6][7] Panspermia is a fringe theory with little support amongst mainstream scientists.^[8] 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.^[9]

Panspermia proposes (for example) that microscopic lifeforms 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.^[10] Panspermia studies concentrate not on how life began, but on methods that may distribute it in the Universe.^[11][12][13]

Pseudo-panspermia (sometimes called soft panspermia or molecular panspermia) is the well-attested hypothesis that many of 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).^[14][15]

History

The first known mention of the term was in the writings of the fifth-century BC Greek philosopher Anaxagoras.^{[16][page needed]} Panspermia began to assume a more scientific form through the proposals of Jöns Jacob Berzelius (1834),^[17] Hermann E. Richter (1865),^[18] Kelvin (1871),^[19] Hermann von Helmholtz (1879)^[20][21] and finally reaching the level of a detailed scientific hypothesis through the efforts of the Swedish chemist Svante Arrhenius (1903).^[22]

Fred Hoyle (1915–2001) and Chandra Wickramasinghe (born 1939) were influential proponents of panspermia.^[23][24] In 1974 they proposed the hypothesis that some dust in interstellar space was largely organic (containing carbon), which Wickramasinghe later proved to be correct.^[25][26][27] 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.^[28]

Three series of astrobiology experiment (EXPOSE)s were conducted outside the International Space Station between 2008 and 2015 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,^[29][30] and those samples sheltered by simulated meteorite material provide experimental evidence for the hypothetical scenario of lithopanspermia.^[31]

In 2018, Harvard astronomers presented a model that suggests that 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.^[32][33] 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.^[34]

Overview

Panspermia requires

1) that organic molecules originated in space (perhaps to be distributed to Earth)

2) that life originated from these molecules, extraterrestrially

3) that this extraterrestrial life was transported Earth.

The creation and distribution of organic molecules from space is now uncontroversial; it is known as pseudo-panspermia.^[14] The existence of extraterrestrial life is unconfirmed but scientifically possible.^[35] The transport of such life to Earth is considered pseudo-science.^[8]

Origination and distribution of organic molecules: Pseudo-panspermia

Pseudo-panspermia is the well-supported hypothesis that many of the small organic molecules used for life originated in space, and were distributed to planetary surfaces. Life then emerged on Earth, and perhaps on other planets, by the processes of abiogenesis.^[14][15] Evidence for pseudo-panspermia includes the discovery of organic compounds such as sugars, amino acids, and nucleobases in meteorites and other extraterrestrial bodies,^{[36][37][38][39]} and the formation of similar compounds in the laboratory under outer space conditions.^[40][41]

Proposed mechanisms for transport of life to Earth

Panspermia can be said to be either interstellar (between star systems) or interplanetary (between planets in the same star system);^[42][43] its transport mechanisms may include comets,^[44][45] radiation pressure and lithopanspermia (microorganisms embedded in rocks).^[46][47][48] Interplanetary transfer of nonliving material is well documented, as evidenced by meteorites of Martian origin found on Earth.^[48] 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,^[49][50] although, as recently discovered, some microorganisms, such as Tersicoccus phoenicis, may be 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 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.^[51][52] Deliberate directed panspermia from space to seed Earth^[53] or sent from Earth to seed other planetary systems have also been proposed.^{[54][55][56][57]} 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.^[58][59] In 2020, Paleobiologist Grzegorz Sadlok proposed the hypothesis that life can transit interstellar distances on nomadic exoplanets and/or its exomoons.^[60] 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.^[61]

Radiopanspermia

In 1903, Svante Arrhenius published in his article The Distribution of Life in Space,^[62] the hypothesis now called radiopanspermia, that microscopic forms of life can be propagated in space, driven by the radiation pressure from stars.^[63] 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.^[64]

The main criticism of radiopanspermia came from Iosif Shklovsky and Carl Sagan, who pointed out the evidence for the lethal action of space radiation (UV and X-rays) in the cosmos.^[65] 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.^[66]

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).^[64][67]

Spores would need to be heavily protected against UV radiation: exposure of unprotected DNA to solar UV and cosmic ionizing radiation would break it up into its constituent bases.^[68][69][70] 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.^[70]

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, for such long travel times, rocks at least 1 meter in diameter are required to effectively shield resistant microorganisms, such as bacterial spores against galactic cosmic radiation.^[71][72] 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.^[64][67]

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.^[9]

• Planetary ejection – For lithopanspermia to occur, microorganisms must survive ejection from a planetary surface, which involves extreme forces of acceleration and shock with associated temperature excursions. Hypothetical values of shock pressures experienced by ejected rocks are obtained with Martian meteorites, which suggest the shock pressures of approximately 5 to 55 GPa, acceleration of 3 Mm/s² and jerk of 6 Gm/s³ and post-shock temperature increases of about 1 K to 1000 K.^[73][74] To determine the effect of acceleration during ejection on microorganisms, rifle and ultracentrifuge methods were successfully used under simulated outer space conditions.^[9]
• Survival in transit – The survival of microorganisms has been studied extensively using both simulated facilities 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 crewed missions; whilst the extremophiles are vital for studying the physiological requirements of survival in space.^[9]
• Atmospheric entry – An important aspect of the lithopanspermia hypothesis to test is that microbes situated on or within rocks could survive hypervelocity entry from space through Earth's atmosphere (Cockell, 2008). As with planetary ejection, this is experimentally tractable, with sounding rockets and orbital vehicles being used for microbiological experiments.^[9][73] B. subtilis spores inoculated onto granite domes were subjected to hypervelocity atmospheric transit (twice) by launch to a ∼120 km altitude on an Orion two-stage rocket. The spores were shown to have survived on the sides of the rock, but they did not survive on the forward-facing surface that was subjected to a maximum temperature of 145 °C.^[75] The exogenous arrival of photosynthetic microorganisms could have quite profound consequences for the course of biological evolution on the inoculated planet. 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. Although cyanobacteria 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.^[76] Thus, non-photosynthetic organisms deep within rocks have a chance to survive the exit and entry process. (See also: Impact survival.) Research presented at the European Planetary Science Congress in 2015 suggests that ejection, entry and impact is survivable for some simple organisms.^[77][78]

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.^[79]

Accidental panspermia

The astronomer Thomas Gold 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.^[80]

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,^[53] but considering an early "RNA world" Crick noted later that life may have originated on Earth.^[81] 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.^[71]

Conversely, active directed panspermia has been proposed to secure and expand life in space.^[56] This may be motivated by biotic ethics that values, and seeks to propagate the basic patterns of our organic gene/protein life-form.^[82]

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.^{[83][84][85][86]}

Protospermia

The astrobiologist Betül Kaçar 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."^[87]

Material-based panspermia

Kuhan Chandru and Tony Z. Jia have suggested that a panspermia-seed need not be an organism. 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.^[88] 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.^[89][90]

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.^[91]