What is the common thread among the following items: pacemakers, spark plugs, fountain pens and compass bearings? Give up? All of them currently use (or used in early versions) the two densest elements, osmium and iridium. These two elements play important roles in technological advancements. However, if certain special events hadn’t occurred early in Earth’s history, no osmium or iridium would exist near the planet’s surface.
Not only are iridium and osmium dense, they also exhibit good electrical conductivity, have high melting points, and resist corrosion in extreme environments. Consequently, components made from these two elements retain their shape well and don’t degrade with repeated use. The utility of such materials in pacemakers and spark plugs are obvious. Other important scientific roles required such qualities also. Starting in 1889, the International Prototype Meter and kilogram standard were made from an iridium alloy. The standard meter was replaced by another reference more convenient to reproduce, but the standard iridium alloy kilogram remains in use today.
Many space probes employ an electrical generator that uses the decay of radioactive isotopes for power. The temperatures of such devices reach up to 2,000° Celsius, thus, requiring an iridium-based alloy to contain the radioactive fuel. The Voyager, Viking, Pioneer, Cassini, Galileo, and New Horizons programs all employed such technology. The stylus used in early phonographs also used an osmium alloy to improve durability until the development of synthetic diamonds. Perhaps less important from a scientific perspective, the osmium stylus certainly benefitted the now-multibillion dollar music industry in its early days.
Yet if early Earth had not developed in a just-right fashion the crust might not have contained any useful amounts of osmium or iridium. Both elements love iron (scientists use the term siderophile for this property). Given a chance, siderophillic elements will bond with iron—especially under high temperatures. When Earth first formed, its interior exhibited such high temperatures that all the iron melted and sank to form Earth’s core. Consequently, any osmium, iridium, or other iron-loving elements were also deposited in the core, rather than in the crust.
A recent scientific study by a Canadian and an American confirms that the processes that occurred during Earth’s formation cannot account for the amount of osmium and iridium in the crust today. Instead, some thing or event must have supplied additional quantities of these elements after the planet formed.
Astronomers also know that a period of heavy meteor showers occurred over 3.9 billion years ago, culminating in the Late Heavy Bombardment (LHB). These meteors, ranging in size from dust particles up to miles wide, contained a significant fraction of siderophillic elements, like osmium and iridium. Although some of these impactors probably sterilized Earth’s surface, they ensured that Earth’s crust contains usable quantities of scientifically and technologically important elements.
These results highlight the critical role of the LHB in preparing Earth for technologically advanced civilizations. Along with reducing the number of impact events experienced by more advanced life, the fact that the LHB seeded Earth’s surface with scientifically important elements seems to suggest that an intelligent Being used this meteor shower to prepare the planet for humanity’s arrival.