Life in Interstellar Space

I shall discuss the interstellar space hypothesis a little, because it displays a common symptom of a scientific disease: the event in question is difficult to explain, so it must have happened "somewhere else".

Interstellar Space: the "Somewhere Else" Theory

Many times in the history of science, professionals and amateurs alike have relied on a "somewhere else" theory, sometimes with good reason and sometimes with no good reason. For example, early explorers expected to find a massive southern continent to balance the large northern land masses, and that continent was always placed in parts of the world that had not yet been explored by Westerners. Finally, there was no more unexplored space for it to occupy, and the theory was abandoned. Maybe we should distrust "somewhere else" theories for the origin of life.

Interstellar material consists of gases and dust particles ejected from exploding stars. Interstellar space is bathed in intense radiation that includes UV light and highly energetic charged particles (cosmic rays). Occasionally, molecules may react in interesting ways as their energy levels are raised by this radiation. Astronomers have now identified about fifty kinds of interstellar molecules, including ice, ammonia, cyanide, acetylene, formic acid, methane, methanol, ethanol, and even more complex hydrocarbons. There is no concentrated witches brew of these components, because they are scattered in a near vacuum. But organic molecules could be attracted to interstellar dust grains and accumulate in concentrated mixtures on their surfaces, probably in ices.

Some meteorites have large quantities of organic carbon compounds trapped within them; some carbon-rich meteorites have as much as 6% organic content. The planetoid Sedna is distinctly red, probably because it is coated with organic tars. New Scientist, 15 April 2005.

Some comets have strongly elliptical orbits that take them quickly inward through planetary systems, then outward again back into interstellar space. Such comets could slowly sweep up dust, ices, and organic molecules from interstellar space, then evaporate or explode them off again in the warmer planetary regions of their orbits. Comets may be 30% water ice, 40% various organic ices, and as much as 20% organic sludge. If this is true, then there is more organic material in the comets of the solar system than there is on all the planets put together.

Organic substances rich in H, C, O, and N were detected on Halley's comet on its 1987 visit, including CO2 and water ices, ammonia, cyanide, nucleic-acid bases (purines and pyrimidines), amino acids (adenine), and formaldehyde. In theory, adding warm water could make this into a life-evolving mixture.

One might occasionally find water within a comet, as it passes close to the sun: the implication is that water-based chemistry might then occur, possibly leading to life. However, at best, a comet would have a little water for a fraction of 1% of its lifeŅa week or two every few decades. Comets that orbit close enough to the Sun to melt some of their ice have a short life expectancy, too. So I come back to the conclusion that comets may be great places to generate organic molecules, but hopeless places to expect the origin of life.

Overall, I think there is no question of living cells evolving in interstellar space or on comets or meteorites without liquid water. At best, comets and meteorites provide an important source of organic compounds that might eventually find their way onto a planet. And if organic compounds from comets or meteorites survived the shock of delivery and also survived on the planetary surface, many other energy sources would surely have produced similar organic compounds on the planet itself. Earth could have received as much as 15% of the organic matter in its early oceans from comets and meteorites, with most of the rest generated in the atmosphere and the oceans by UV light.

A few scientists are excited about the possibility of some sort of life evolving in space. The idea has not been helped by claims that viral epidemics, such as influenza, occur on Earth today because viruses are still falling on from outer space (the Cosmic Sneeze Hypothesis?). [Hoyle, F., et al. 1986. Viruses from Space. Cardiff, Wales: University College Cardiff Press.]

Last revised September 30, 2005

Links checked September 30, 2005

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