Word Encounters: Panspermia
How we communicate what we learn about science and sharing information are just as important as our experiments and research. Here's a regular series we'd like to begin on interesting science words and theories that have evolved to shed light on our understanding of life on Earth and what the future might hold. In this piece, staff writer Stewart Mittnacht explores an interesting but unproven scientific possibility.
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How life on Earth originated is a question that scientists and philosophers alike have pondered throughout the ages. Could life have originated elsewhere in the universe, such as Mars? The idea of panspermia continues to filter into mainstream science, despite opponents.
By Stewart Mittnacht
Panspermia: Greek for “all seed”
Panspermia refers to the potential for life to spread from one point of origin to many other points throughout the universe, either by natural means or artificial ones. Three criteria need to be met to support this idea: a mechanism of transport must exist; an organism must be able to survive a long journey through space; and an organism needs to be able to adapt to life in its new world.
Let’s start with rocks as an example. Rocks can, and have, traveled between planets. During rare but powerful events, such as the asteroid that drove dinosaurs to extinction, the resulting explosion caused small rocks to accelerate to high enough speeds to exit the gravity field of their home planet. Dubbed “ejecta,” these rocks sometimes have the good fortune to later get pulled toward another planet and captured.
If ejecta can be exchanged between planets in this fashion, perhaps a bacterium might hitch a ride between worlds, locked away in small but protected mineral layers—a concept known as lithopanspermia, literally “all seed by rock.” But could a prokaryote (single-celled organisms, including bacteria) possibly survive such a journey? Evidence indicates that the answer is a soft “yes,” depending on whether or not certain conditions are met.
In his book Worlds in the Making, published in 1908, Nobel prize-winning Swedish chemist and physicist Svante Arrhenius posited a theory of panspermism, noting that life on Earth may have originated from microorganisms present in outer space, an idea believed to have first originated with the Greek philosopher Anaxagoras. Francis Crick, one of the biologists who discovered the structure of DNA, examined the possibility in a paper in 1973. Today, the subject continues to spark discussion and debate in the academic and science communities.
For a bacterium to travel between worlds, first it must survive the substantial g-forces generated both by its launch into space as well as the descent to its new planet’s surface. Though no human could hope to survive such an ordeal, bacteria are hardy, and several experiments in which bacterial spores were intentionally allowed to re-enter earth from space (on the heat shield of an Orion rocket) demonstrated that in their desiccated state some Earth bacteria can indeed survive re-entry (provided that they are not on the side of their “lifeboat” directly exposed to the intense, friction-generated heat of re-entry).
Exit and entry, however, are only brief portions of a long journey. It is estimated to take tens of millions of years for ejecta to complete their interplanetary voyage, during which time our little seed of life must survive constant exposure to intense cosmic radiation and the cold vacuum of space. Here bacteria adapted for harsh, dry conditions have an advantage—the ability to enter a sessile state [fixed in one place] in which the cell becomes desiccated and all internal metabolism halts.
Under such conditions a single cell can “live” in stasis for thousands of years. Some salt-tolerant species of bacteria have been revived from halite crystals dating up to 65,000 years old. But over a longer time, a sessile bacterium’s genetic code would be destroyed by background radiation.
To survive for longer, the bacteria would need an opportunity to occasionally thaw out and undergo cellular division, during which time genetic material could be repaired by the cell. In young star systems, where cosmic collisions are more common, ejecta may occasionally collide with other pieces of interstellar rock and dust.
Such collisions, if spread out over a few thousand years, would heat the bacteria often enough for them to periodically thaw out, undergo a few cycles of cellular division and repair, then return to dormancy. This puts interplanetary panspermia in the realm of possibility, although for greater distances, such as travel between stars, a larger lifeboat is needed than a small rock.
Orphan planets—those that have been ejected from their parent star system—might sustain certain types of bacteria, such as extremophile archaea [a microorganism that can live in conditions of extreme temperature, acidity, alkalinity, or chemical concentration] for millions of years. These heat-loving organisms require only hot rocks and organic volatiles to survive, and they might be sustained deep within their planet by geothermal energy long after the surface has frozen over.
An alternative to natural panspermia is artificial panspermia, which can be accidental or intentional. Accidental panspermia might occur if bacteria were to stow away on a spacecraft. To avoid unintentionally introducing an invasive species to other planets with potential life, such as Mars, NASA and other space institutes follow strict decontamination protocols on all interplanetary vehicles. But no technique is truly foolproof, and the potential exists for some bacteria to survive the process. Intentional panspermia might occur in our near future, should humans begin colonizing other planets.
Biotic ethics argues that life is a rare and valuable state of matter, and that both as members of this group and as an intelligent entity it is our duty to strive to maintain its existence. By directed panspermia, we might sustain Earth-based life long after the Sun enters its red giant phase and engulfs our planet.