Pseudo-panspermia (sometimes called soft panspermia, molecular panspermia or quasi-panspermia) is a well-supported hypothesis for a stage in the origin of life. The theory first asserts that many of the small organic molecules used for life originated in space (for example, being incorporated in the solar nebula, from which the planets condensed). It continues that these organic molecules were distributed to planetary surfaces, where life then emerged on Earth and perhaps on other planets. Pseudo-panspermia differs from the fringe theory of panspermia, which asserts that life arrived on Earth from distant planets. [1]
Theories of the origin of life have been current since the 5th century BC, when the Greek philosopher Anaxagoras proposed an initial version of panspermia: life arrived on earth from the heavens. [3] In modern times, full panspermia has little support amongst mainstream scientists. [1] Pseudo-panspermia, in which molecules are formed and transported through space is however well-supported. [2]
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 ionised, 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. [4] The dust plays a critical role of shielding the molecules from the ionizing effect of ultraviolet radiation emitted by stars. [5] The Murchison meteorite contains the organic molecules uracil and xanthine, [6] [7] which must therefore already have been present in the early Solar System, where they could have played a role in the origin of life. [8]
Nitriles, key molecular precursors of the RNA World scenario, are among the most abundant chemical families in the universe and have been found in molecular clouds in the center of the Milky Way, protostars of different masses, meteorites and comets, and also in the atmosphere of Titan, the largest moon of Saturn. [9] [10]
Evidence for the extraterrestrial creation of organic molecules includes both their discovery in various contexts in space, and their laboratory synthesis under extraterrestrial conditions:
| Molecule | Class | Body | Notes |
|---|---|---|---|
| Glycine | Amino acid | Comet | NASA, 2009 [11] |
| mixed aromatic-aliphatic compounds | Cosmic dust | 2011 [12] [13] | |
| Glycolaldehyde | Sugar-related | Around a protostar | Copenhagen University, 2012 [14] [15] Precursor of RNA [16] |
| Cyanomethanimine, Ethanimine | Imines | Icy particles in interstellar space | Precursors of nucleobase adenine, and of amino acid alanine [17] |
| polycyclic aromatic hydrocarbons (PAHs) | widespread, 20% of carbon in universe | NASA, 2014 [18] | |
| Glycine, Methylamine, Ethylamine | Amino acid, amines | Coma of comet 67P/Churyumov-Gerasimenko | Rosetta Mission, 2016 [19] |
| Uracil, Niacin | Nucleobase, vitamer | 162173 Ryugu | Hayabusa2 , 2023 [20] [21] |
| Molecule | Class | Conditions | Notes |
|---|---|---|---|
| Precursors of amino acids and nucleotides | Interstellar medium | NASA, 2012, starting from polycyclic aromatic hydrocarbons (PAHs) [22] [23] | |
| Uracil, Cytosine, Thymine | Nucleobases | Pyrimidine, outer space | NASA, 2015 [24] |
| Peptides | outer space, using CO, C, NH3 | Materials common in molecular clouds of interstellar medium [25] |
Organic molecules can then be distributed to planets including Earth both when the planets formed and later. If the materials from which planets formed contained organic molecules, and were not destroyed by heat or other processes, then these would be available for abiogenesis on those planets.
Later distribution is by means of bodies such as comets and asteroids. These may fall to the planetary surface as meteorites, releasing any molecules they are carrying as they vaporise on impact or later as they erode. Findings of organic molecules in meteorites include:
| Molecule | Class | Notes |
|---|---|---|
| Adenine, Guanine | Nucleobase | NASA, 2011 [26] [27] |
| Sugars | In "primitive meteorites" [28] | |
| Guanine, Adenine, Cytosine, Uracil, Thymine | Nucleobases | 2022 [29] |
| Asteroid | Location | Notes |
|---|---|---|
| 24 Themis | Asteroid Belt | NASA, Jet Propulsion Laboratory, Near Earth Objects, life on Earth [30] |
| 269 Justitia | Asteroid Belt | NASA, JPL Small-Body Database [31] |
Astrobiology is a scientific field within the life and environmental sciences that studies the origins, early evolution, distribution, and future of life in the universe by investigating its deterministic conditions and contingent events. As a discipline, astrobiology is founded on the premise that life may exist beyond Earth.
Extraterrestrial life, alien life, or colloquially simply aliens is life which does not originate from Earth. No extraterrestrial life has yet been conclusively detected. Such life might range from simple forms such as prokaryotes to intelligent beings, possibly bringing forth civilizations that might be far more advanced than humanity. The Drake equation speculates about the existence of sapient life elsewhere in the universe. The science of extraterrestrial life is known as astrobiology.
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, known as directed panspermia. The theory argues that life did not originate on Earth, but instead evolved somewhere else and seeded life as we know it.
Astrochemistry is the study of the abundance and reactions of molecules in the universe, and their interaction with radiation. The discipline is an overlap of astronomy and chemistry. The word "astrochemistry" may be applied to both the Solar System and the interstellar medium. The study of the abundance of elements and isotope ratios in Solar System objects, such as meteorites, is also called cosmochemistry, while the study of interstellar atoms and molecules and their interaction with radiation is sometimes called molecular astrophysics. The formation, atomic and chemical composition, evolution and fate of molecular gas clouds is of special interest, because it is from these clouds that solar systems form.
Cosmochemistry or chemical cosmology is the study of the chemical composition of matter in the universe and the processes that led to those compositions. This is done primarily through the study of the chemical composition of meteorites and other physical samples. Given that the asteroid parent bodies of meteorites were some of the first solid material to condense from the early solar nebula, cosmochemists are generally, but not exclusively, concerned with the objects contained within the Solar System.
A biosignature is any substance – such as an element, isotope, molecule, or phenomenon – that provides scientific evidence of past or present life on a planet. Measurable attributes of life include its complex physical or chemical structures, its use of free energy, and the production of biomass and wastes.
Nalin Chandra Wickramasinghe is a Sri Lankan-born British mathematician, astronomer and astrobiologist of Sinhalese ethnicity. His research interests include the interstellar medium, infrared astronomy, light scattering theory, applications of solid-state physics to astronomy, the early Solar System, comets, astrochemistry, the origin of life and astrobiology. A student and collaborator of Fred Hoyle, the pair worked jointly for over 40 years as influential proponents of panspermia. In 1974 they proposed the hypothesis that some dust in interstellar space was largely organic, later proven to be correct.
Carbonaceous chondrites or C chondrites are a class of chondritic meteorites comprising at least 8 known groups and many ungrouped meteorites. They include some of the most primitive known meteorites. The C chondrites represent only a small proportion (4.6%) of meteorite falls.
The Murchison meteorite is a meteorite that fell in Australia in 1969 near Murchison, Victoria. It belongs to the carbonaceous chondrite class, a group of meteorites rich in organic compounds. Due to its mass and the fact that it was an observed fall, the Murchison meteorite is one of the most studied of all meteorites.
Cosmic dust – also called extraterrestrial dust, space dust, or star dust – is dust that occurs in outer space or has fallen onto Earth. Most cosmic dust particles measure between a few molecules and 0.1 mm (100 μm), such as micrometeoroids. Larger particles are called meteoroids. Cosmic dust can be further distinguished by its astronomical location: intergalactic dust, interstellar dust, interplanetary dust, and circumplanetary dust. There are several methods to obtain space dust measurement.
The PAH world hypothesis is a speculative hypothesis that proposes that polycyclic aromatic hydrocarbons (PAHs), known to be abundant in the universe, including in comets, and assumed to be abundant in the primordial soup of the early Earth, played a major role in the origin of life by mediating the synthesis of RNA molecules, leading into the RNA world. However, as yet, the hypothesis is untested.
Active asteroids are small Solar System bodies that have asteroid-like orbits but show comet-like visual characteristics. That is, they show a coma, tail, or other visual evidence of mass-loss, but their orbits remain within Jupiter's orbit. These bodies were originally designated main-belt comets (MBCs) in 2006 by astronomers David Jewitt and Henry Hsieh, but this name implies they are necessarily icy in composition like a comet and that they only exist within the main-belt, whereas the growing population of active asteroids shows that this is not always the case.
Extraterrestrial material refers to natural objects now on Earth that originated in outer space. Such materials include cosmic dust and meteorites, as well as samples brought to Earth by sample return missions from the Moon, asteroids and comets, as well as solar wind particles.
In biology, abiogenesis or the origin of life is the natural process by which life has arisen from non-living matter, such as simple organic compounds. The prevailing scientific hypothesis is that the transition from non-living to living entities on Earth was not a single event, but a process of increasing complexity involving the formation of a habitable planet, the prebiotic synthesis of organic molecules, molecular self-replication, self-assembly, autocatalysis, and the emergence of cell membranes. The transition from non-life to life has never been observed experimentally, but many proposals have been made for different stages of the process.
An interstellar object is an astronomical object in interstellar space that is not gravitationally bound to a star. This term can also be applied to an object that is on an interstellar trajectory but is temporarily passing close to a star, such as certain asteroids and comets. In the latter case, the object may be called an interstellar interloper.
Interstellar ice consists of grains of volatiles in the ice phase that form in the interstellar medium. Ice and dust grains form the primary material out of which the Solar System was formed. Grains of ice are found in the dense regions of molecular clouds, where new stars are formed. Temperatures in these regions can be as low as 10 K, allowing molecules that collide with grains to form an icy mantle. Thereafter, atoms undergo thermal motion across the surface, eventually forming bonds with other atoms. This results in the formation of water and methanol. Indeed, the ices are dominated by water and methanol, as well as ammonia, carbon monoxide and carbon dioxide. Frozen formaldehyde and molecular hydrogen may also be present. Found in lower abundances are nitriles, ketones, esters and carbonyl sulfide. The mantles of interstellar ice grains are generally amorphous, becoming crystalline only in the presence of a star.
Zita Carla Torrão Pinto Martins, OSE, is a Portuguese astrobiologist, and an associate professor at Instituto Superior Técnico. She was a Royal Society University Research Fellow (URF) at Imperial College London. Her research explores how life may have begun on Earth by looking for organic compounds in meteorite samples.
Asteroidal water is water or water precursor deposits such as hydroxide (OH−) that exist in asteroids. The "snow line" of the Solar System lies outside of the main asteroid belt, and the majority of water is expected in minor planets. Nevertheless, a significant amount of water is also found inside the snow line, including in near-earth objects (NEOs).
Hemolithin is a proposed protein containing iron and lithium, of extraterrestrial origin, according to an unpublished preprint. The result has not been published in any peer-reviewed scientific journal. The protein was purportedly found inside two CV3 meteorites, Allende and Acfer-086, by a team of scientists led by Harvard University biochemist Julie McGeoch. The report of the discovery was met with some skepticism and suggestions that the researchers had extrapolated too far from incomplete data.
Although they were part of the scientific establishment – Hoyle at Cambridge and Wickramasinghe at the University of Wales – their views on the topic were far from mainstream, and panspermia remains a fringe theory
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