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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Panspermia | 4/5 | https://en.wikipedia.org/wiki/Panspermia | reference | science, encyclopedia | 2026-05-05T03:37:50.050920+00:00 | kb-cron |
Planetary ejection – For lithopanspermia to occur, microorganisms must first survive ejection from a planetary surface (assuming they do not form on meteorites, as suggested in), which involves extreme forces of acceleration and shock with associated temperature rises. Hypothetical values of shock pressures experienced by ejected rocks are obtained from Martian meteorites, which suggest pressures of approximately 5 to 55 GPa, acceleration of 3 Mm/s2, jerk of 6 million km/s3 and post-shock temperature increases of about 1 K to 1000 K. Though these conditions are extreme, some organisms appear able to survive them. Survival in transit – Once in space, the microorganisms have to make it to their next destination for lithopanspermia to be successful. 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, both human-borne microbes (significant for future crewed missions) and extremophiles (significant for determining the physiological requirements of survival in space). Bacteria in particular can exhibit a survival mechanism whereby a colony generates a biofilm that enhances its protection against UV radiation. Atmospheric entry – The final stage of lithopanspermia, is re-entry onto a viable planet via its atmosphere. This requires that the organisms are able to further survive potential atmospheric ablation. Tests of this stage could use sounding rockets and orbital vehicles. B. subtilis spores inoculated onto granite domes were twice subjected to hypervelocity atmospheric transit by launch to a ~120 km altitude on an Orion two-stage rocket. The spores survived on the sides of the rock, but not on the forward-facing surface that reached 145 °C. 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 can survive the desiccating, freezing conditions of space, the STONE experiment showed that they cannot survive atmospheric entry. Small non-photosynthetic organisms deep within rocks might survive the exit and entry process, including impact survival. Lithopanspermia, described by the mechanism above, can be either interplanetary or interstellar. It is possible to quantify panspermia models and treat them as viable mathematical theories. For example, a recent study of planets of the Trappist-1 planetary system presents a model for estimating the probability of interplanetary panspermia, similar to studies in the past done about Earth-Mars panspermia. This study found that lithopanspermia is 'orders of magnitude more likely to occur' in the Trappist-1 system as opposed to the Earth-to-Mars scenario. According to their analysis, the increase in probability of lithopanspermia is linked to an increased probability of abiogenesis amongst the Trappist-1 planets. In a way, these modern treatments attempt to keep panspermia as a contributing factor to abiogenesis, as opposed to a theory that directly opposes it. In line with this, it is suggested that if biosignatures could be detected on two (or more) adjacent planets, that would provide evidence that panspermia is a potentially required mechanism for abiogenesis. As of yet, no such discovery has been made. Lithopanspermia has also been hypothesized to operate between stellar systems. One mathematical analysis, estimating the total number of rocky or icy objects that could potentially be captured by planetary systems within the Milky Way, has concluded that lithopanspermia is not necessarily bound to a single stellar system. This not only requires these objects have life in the first place, but also that it survives the journey. Thus intragalactic lithopanspermia is heavily dependent on the survival lifetime of organisms, as well as the velocity of the transporter. Again, there is no evidence that such a process has, or can occur.
==== Counterarguments ==== The complex nature of the requirements for lithopanspermia, as well as evidence against the longevity of bacteria being able to survive under these conditions, makes lithopanspermia a difficult theory to support. That being said, impact events did occur often in the early solar system and still occur today, such as within the asteroid belt.
=== Directed panspermia ===
First proposed in 1972 by Nobel prize winner Francis Crick along with Leslie Orgel, directed panspermia is the theory that life was deliberately brought to Earth by a higher intelligent being from another planet. In light of the evidence at the time that it seems unlikely for an organism to have been delivered to Earth via radiopanspermia or lithopanspermia, Crick and Orgel proposed this as an alternative theory, though it is worth noting that Orgel was less serious about the claim. They do acknowledge that the scientific evidence is lacking, but discuss what kinds of evidence would be needed to support the theory. In a similar vein, Thomas Gold suggested that life on Earth might have originated accidentally from a pile of 'Cosmic Garbage' dumped on Earth long ago by extraterrestrial beings. These theories are often considered more science fiction, however, Crick and Orgel use the principle of cosmic reversibility to argue for it. This principle is based on the fact that if our species is capable of infecting a sterile planet, then what is preventing another technological society from having done that to Earth in the past? They concluded that it would be possible to deliberately infect another planet in the foreseeable future. As far as evidence goes, Crick and Orgel argued that given the universality of the genetic code, it follows that an infective theory for life is viable. Directed panspermia could, in theory, be demonstrated by finding a distinctive 'signature' message had been deliberately implanted into either the genome or the genetic code of the first microorganisms by our hypothetical progenitor, some 4 billion years ago. However, there is no known mechanism that could prevent mutation and natural selection from removing such a message over long periods of time.