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Can comets or meteorites in outer space contain extraterrestrial microscopic lifeforms?

Q: Can comets or meteorites in outer space contain extraterrestrial microscopic lifeforms?

Krishna: Panspermia (1) [from Ancient Greek (sperma) 'seed'] 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.

Panspermia comes in many forms, such as radiopanspermia, lithopanspermia, and directed panspermia. Regardless of its form, the theories generally propose that microbes able to survive in Space (such as certain types of bacteria or plant spores can become trapped in debris ejected into space after collisions between planets and small Solar System bodies that harbor life. This debris containing the lifeforms is then transported by meteors between bodies in a solar system, or even across solar systems within a galaxy. In this way, panspermia studies concentrate not on how life began but on methods that may distribute it within the Universe. This point is often used as a criticism of the theory.

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. Historically, disputes over the merit of this theory centered on whether life is ubiquitous or emergent throughout the Universe. Due to its long history, the theory maintains support today, with some work being done to develop mathematical treatments of how life might migrate naturally throughout the Universe. Its long history also lends itself to extensive speculation and hoaxes that have arisen from meteoritic events.

The idea that life can be distributed throughout the universe, from planet to planet, is called panspermia. Some lifeforms, particularly the extremophiles, are hardy enough to survive the extreme conditions of space. Meteorites and comets are the most discussed methods for transportation, and the story is fascinating.

Lithopanspermia is the name given to the transfer of organisms across space using rocks. Transporting life-containing rocks from the ground to another planet might seem like a conundrum at first, but the mechanism is simple enough. During the impact of an asteroid or a comet with a planet's surface, the pressure is so great that the material forming the impact crater is ejected at a speed reaching escape velocity, allowing chunks of rock to escape the confining threshold of the atmosphere. Although a great portion of the impacted area experiences high enough temperatures to easily destroy most life, a significant fraction of the rock may experience the lower temperatures and pressures required to permit the survival of amino acids and microscopic organisms.

Once in space, these microbes face the task of surviving the ''vacuum, weightlessness, temperature extremes, cosmic rays, and, for those close to the surface of the rock, ultraviolet radiation from the sun'' (Lunine). We know that some organisms do possess the ability to survive these conditions. For example, tardigrades, also known as "water bears," have been shown to survive exposure to solar radiation while in space. However, organisms well beneath the surface of the rock fare the best chance of survival because they are shielded from ultraviolet radiation.

Bacteria that form spores, which are extremely tolerant of low temperatures and normally dangerous conditions, could survive within rock for millions of years. Once our rock reaches the end of its journey and plunges into the atmosphere of another planet, the organisms within may survive, provided the rock is large enough to offer the necessary heat shielding. Without it, any chances of life inside it would be burned up and destroyed. It's in this way that interplanetary travel of microorganisms is thought to occur over billions of years. They're a bit like geological spaceships.

Does this mean that panspermia could have seeded life across the galaxy? Not really. There's the enormous distance between stars, and a timescale in the tens of millions of years for a trip. The longer the period of time, the less likely life is to survive a trip to another world.

In the mind of mainstream science, this makes interstellar panspermia an unlikely occurrence. Fortunately, we can still wonder about it within our own solar system. If cosmic dispersal has been occurring for so long, one wonders if natural selection still occurs in space. If so, then might the descendants of space-faring life would be the most hardy, radiation-resistant, long-lasting, and efficient? If these microbes are capable of surviving in space, then why haven't we noticed any yet? One might expect to find the descendants of planetary impact survivors across the solar system. Scientists are posing these relevant questions (2).

However, it seems highly unlikely that life in the form of biological cells began in comets. However, there is now direct evidence that some of the so-called chemical ‘building blocks’ of life – organic molecules – can be found in comets (3).

So the answer to your question is chemical building blocks can be found in comets and meteorites. But we haven’t found any living organisms in them so far. If living organisms were brought by these space vehicles to Earth, they would have special radiation resistant and long-lasting features.

Though both planet seeds and bacterial spores have been proposed as potentially viable vehicles, their ability to not only survive in space for the required time, but also survive atmospheric entry is debated.

Exposing DNA to the ultra high vacuum of space alone is sufficient to cause DNA damage, so the transport of unprotected life during interplanetary flights is extremely unlikely.

Footnotes:

1. https://en.wikipedia.org/wiki/Panspermia#:~:text=Panspermia%20propo...

2. https://www.nature.com/scitable/blog/postcards-from-the-universe/li...

3. https://www.esa.int/Science_Exploration/Space_Science/Rosetta/Where....

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