Tiny structures resembling worm fossils were found in the meteorite’s interior, inside pockets indicative of the presence of ancient water. ALH84001 rose to an unprecedented level of infamy as paper after paper was released, with dozens of scientists arguing over whether this particular space rock had once harbored life. And in the process of this new, specific debate, an older and far more general debate was brought back into the public eye: the debate over the panspermia hypothesis.
Panspermia is the theory that microbes or organic molecules are found throughout space, moving from place to place on meteors, comets, or artificial vehicles; or even drifting naked through the void. This idea is a popular one in science fiction, found perhaps most notably in Michael Crichton’s 1969 novel The Andromeda Strain, but its numerous iterations appear in widely varying productions, from Star Trek to The X-Files to Doctor Who.
The most common versions of panspermia theory involve bacteria or other microbes traveling through space while dormant inside comets or meteors. According to this theory, sometimes, when a large meteor strikes a planet with enough force, native rocks from that planet are blasted upwards with sufficient velocity to escape the planet’s gravity, carrying biological materials with them. Aside from the biological aspect, this phenomenon has been observed and documented: Earth has many meteors that originated on the moon and Mars (including ALH84001), so it may not be a stretch to imagine that such a meteor might pick up organic material on its way out, assuming there was organic material to be picked up. Whether that material could survive a meteoric impact is another story although microbial life has proven surprisingly hardy in extreme conditions, including the vacuum of space.
However, the most plausible panspermia theory is probably pseudo-panspermia, which posits that planetary life can arise from space-born organic molecules (rather than fully formed cells hurtling through space). Organic molecules independent of life are indeed found in space: they are formed as part of the dust that gets ejected from a star during a supernova. Particles that resemble this organic dust have been found in ancient meteorites, leading some astronomers to believe that they may have been present during the formation of our solar system. Nascent planets were constantly pummeled with space debris over millions of years; it is certainly possible that some of these meteorites brought organic dust with them as they smashed into Earth. However, there is little evidence linking this to the origin of Earth life.
On the other end of the spectrum is directed panspermia: the dissemination of life or organic compounds by intelligent species. By far the most popular version of panspermia among the sci-fi crowd, the directed panspermia hypothesis also has some surprising support in the scientific community - notably by Francis Crick, co-discoverer of DNA’s role as genetic material. There is unsurprisingly almost no evidence that favors directed panspermia. Like all other panspermia theories, the problem is that there is little evidence either for or against it. It may seem a very implausible idea, but it is almost impossible to disprove.
- Emma Flickinger