Interstellar Rocks Miss the Mark

Interstellar Rocks Miss the Mark

by Roger Wiens

Where, when, and how did life originate? Answers to these questions prove more elusive than ever to the science community—specifically to those who demand a naturalistic answer. Because of the short timescales required for the complicated leap from nonlife to life on planet Earth, some nontheistic scientists propose that life originated elsewhere in the galaxy. This thinking belongs to a theory called “panspermia” (pan-SPUR-mee-uh), which says that life on Earth got its start in dormant life arriving from space.

This theory’s popularity grows despite intractable problems, including this one: How did life survive in space for any reasonable length of time without special shielding from deadly and ubiquitous cosmic rays? Meteorites have been suggested as a possible safe carrier from one planet (or planetary system) to another.

At the 32d annual Lunar and Planetary Science Conference, Dr. H. Jay Melosh, from the University of Arizona, reported on a study that tested the probability of just one aspect of the meteor-transport hypothesis. That study looked at Earth’s likelihood of capturing interstellar wandering rocks. Capture of rocks (meteors) does occur, of course, because of gravity. Earth, for example, “captures” about fifteen rocks per year from our near neighbor, Mars. (Planets also eject rocks as a result of comet, meteor, and asteroid collisions.)

Size and mass determine a planet’s odds for success in capturing interstellar rocks. Capture is most efficient when it involves the giant planets. Melosh has estimated that Jupiter captures about 1×10-8 objects per year. In other words, on average, Jupiter captures roughly one interstellar meteorite from another stellar system every 100 million years. How does Earth compare? The study reveals that Earth captures only about 3×10-16 per year (one every three quadrillion years) on its own, and about 1×10-12 per year (one every trillion years) in a process by which the interstellar meteor is first captured into the solar system by Jupiter and then eventually lands on Earth. Translation: Odds are less than one in 200 that Earth would capture even one such meteor in its entire history

Such odds are very low. In truth, they are lower than the study suggests, for the assumptions underlying the calculations were admittedly “liberal.” For example, a Jupiter-sized object and a terrestrial planet were assumed to exist around each and every star in the solar vicinity, a scenario astronomers know to be false.

The odds against panspermia grow worse as one considers the narrow window of time between Earth’s formation and the first appearance (and subsequent reappearances) of life on Earth. Melosh did not address the issue of whether capture by Earth might have been more efficient in the primitive solar system when gas drag (the gravity and other physical effects of a dense gas cloud) may have played a role in meteorite capture. Melosh’s work is the first detailed study of cross-fertilization by meteorites.

In all likelihood, more such studies will follow. An attempt may be made to salvaget the hypothesis, but the outcome seems predictable. People who had expected panspermia to solve the problem of the rapid origin of life will have to look elsewhere – beyond the heavens.