The origin of life on Earth remains an unresolved question. Fossil evidence suggests the presence of life almost as soon as the planet cooled enough, following a period known as the Late Heavy Bombardment when inner solar system was a veritable shooting gallery of asteroid impacts. The short amount of time between planet-sterilizing collisions large enough to boil off the oceans and the emergence of life has suggested to some that the early Earth was seeded with life. This notion that life got its start elsewhere before coming to Earth is known as 'panspermia'. Despite seeming somewhat fanciful, several prominent scientists have written seriously about the idea -- including Lord Kelvin, Svante Arrhenius, and Francis Crick, one of the co-discoverers of the structure of DNA.
Panspermia comes in a few different varieties, depending on the mode of transportation. The most commonly considered being ballistic panspermia, in which microorganisms catch a ride inside a meteorite to another world. A large enough impact can blast small chunks off a planetary surface into space, where it might drift for several hundred thousand to millions of years before colliding with another planet. This process of exchange is well documented, as several meteorites have been identified as originating on Mars. The most famous of these being ALH84001, a Martian meteorite found in the Allan Hills region of Antarctica in 1984. This small rock rocketed into infamy in 1996 when President Clinton announced the discovery of "microfossils" inside, as seen with an electron microscope. The scientific community has gone back and forth over the ultimate nature of the shapes, whether biogenic or not, such that there is no consensus. One upshot of the debate, additional studies have made the case that ballistic panspermia is at least theoretically possible... exactly how likely is still unknown.
 |
| Supposed nano-fossils seen in the Martian meteorite ALH84001 |
Another flavor can be described as diffusive panspermia, in which small spores drifting in the wind reach the upper atmosphere, then continue into space, which drift along solar winds until settling on a new habitat and taking hold. Spores are reproductive structures well adapted to extended survival in unfavorable conditions, and are released in the billions upon billions of spores by plants, bacteria, fungi, algae, etc.. Given astronomically long odds as a means of interplanetary distribution, this seems almost ridiculously unlikely; but the possibility can't be completely ignored either.
Complicating both of these methods of interstellar distribution are the harsh conditions and vast distances of space. Ultraviolet light and other cosmic radiation is highly destructive to DNA and the complex molecules of life. Then there's surviving the ultracold temperature of 3 K (temperature in Kelvin, named for Lord Kelvin, mentioned above as a proponent of panspermia; also -270 ºC or -454 ºF), in the vacuum of space. Plus, the difficulty of remaining viable after the unimaginably long durations required to travel such vast distances. Directed panspermia gets around these complications by proposing an intelligent transfer of life between planets by extraterrestrials in spaceships. As with other theories that fall under the auspices of pseudo-science, this hypothesis provides an explanation without offering any realistic means of testing -- indeed, the lack of evidence is often hailed as definitive proof. That said... if humanity ever colonizes Mars, directed panspermia will become a reality; and thus it probably exists among any interplanetary species, if any exist.
(I have an irrational fear of scorpions. They're evil, and I want nothing to do with them. In my moments of levity, I like to rant about scorpions being on Earth as a result of unintentional panspermia. Scorpions are like the Norway rat of spacefaring civilizations. I mean look at them: scorpions are aliens, and they shouldn't exist here. They're an invasive species. The only possible explanation is that they were stowaways on a flying saucer, and a visiting intelligence accidentally left them here millions of years ago.)
As fascinating it might be to consider panspermia, it ultimately feels unsatisfying as a hypothesis regarding the origins of life, as it merely deflects the question. If life got here from elsewhere... how did it get started elsewhere? Prebiotic chemists study the conditions of early Earth, and try to cook up an analogue prebiotic soup to explain the origin of life. Out of this study has come a softer version, what might be described as contributive panspermia. Many of the building blocks of life are fairly common in the universe. Carbonaceous chondrites, meteorites containing organic compounds, probably contributed ingredients to the prebiotic soup, including amino acids and polycyclic aromatic hyrdocarbons. An experiment simulating cometary ice formation produced lipids, which can spontaneously form vesicle structures resembling a cell membrane when dissolved in water. The delivery to Earth of extra-terrestrially derived molecules could have been an essential step in the origin of life.
No comments:
Post a Comment