Sciency Words A to Z: Panspermia

Welcome to a special A to Z Challenge edition of Sciency Words!  Sciency Words is an ongoing series here on Planet Pailly about the definitions and etymologies of science or science-related terms.  In today’s post, P is for:


Pictured above is a tardigrade, also known as a water bear.  Tardigrades are pudgy-looking, almost cuddly-looking (depending on the photograph) microorganisms that reside here on Earth.  But due to some alleged confusion about their genetic lineage, it was at one time speculated that tardigrades might have come from somewhere else.  Perhaps they migrated to Earth through a process called panspermia.

The word panspermia goes all the way back to ancient Greece, and it can be translated to mean “seeds everywhere,” as in the seeds of life have spread all over the universe.  According to this article by Chandra Wickramasinghe, one of the leading proponents of the panspermia hypothesis, serious scientific discussion of the idea began in the late 1800’s.  In Wickramasinghe’s article, Lord Kelvin is quoted thusly:

… Hence, and because we all confidently believe that there are at present, and have been from time immemorial, many worlds of life besides our own, we must regard it as probable in the highest degree that there are countless seed-bearing meteoritic stones moving through space.  If at the present instant no life existed upon the Earth, one such stone falling upon it might, by what we blindly call natural causes, lead to its becoming covered with vegetation.

While Lord Kelvin and other late 19th and early 20th Century scientists may have taken panspermia seriously, the idea soon fell out of vogue.  Yes, from time to time, perhaps a “meteoritic” impact or a volcanic explosion (or maybe even a really strong gust of wind) might loft a few bacterial spores into a planet’s upper atmosphere.  Perhaps those spores could then escape into space.  But surely those spores would not survive for long after that.

It was British astronomer Fred Hoyle and Sri Lankan-born British astronomer Chandra Wickramasinghe who repopularized panspermia in the 1970’s and 80’s.  As explained in this paper, titled “Progress Towards the Vindication of Panspermia,” the Hoyle-Wickramasinghe interpretation of panspermia is founded on two basic premises: that microorganisms “have an almost indefinite persistence and viability,” and that once they find themselves in the right environment, “microbes can replicate exponentially.”

I’d say both of those premises make sense.  We all know how rapidly microbes can replicate given the chance.  As for their “almost indefinite persistence and vitality”… tardigrades famously survived in the vacuum of space.  No air, no water, extreme temperatures, extreme radiation… the tardigrades handled space quite well. Other microorganisms also survived similar experiments.

But could these microbes survive the thousands or perhaps millions of years it would take to travel from one planet to another?  Hoyle and Wickramasinghe clearly thought so, and they made some pretty outrageous claims about how much biological material we should expect to find drifting through open space.

[…] by 1983 we inferred confidently that some 30 percent of the carbon in interstellar dust clouds had to be tied up in the form of organic dust that matched the properties of degraded or desiccated bacteria.

Panspermia is now one of the most important concepts in the field of astrobiology.  It’s gained a lot of credence, especially since the discovery of those bacteria shaped objects in the ALH84001 meteorite.  However, although it is an important concept, it also remains highly speculative.  As I’ve said several times now in these A to Z posts, astrobiologists must hold themselves to the same standards as a court of law: proof beyond a reasonable doubt.  And there are still plenty of reasonable doubts about panspermia.

Next time on Sciency Words A to Z, call me old fashioned, but I prefer it when planets have names.