Even these statistics belie the true importance of phosphorus, which scientists credit as the limiting factor for terrestrial life. This idea follows Justus von Liebig's law of the minimum, an early agricultural concept stating that a species responds only to the nutrient in shortest supply and that this is what limits the growth of a given population. Science popularizer Isaac Asimov, in his 1974 book Asimov on Chemistry
, put it most succinctly: "Life can multiply until all the phosphorus has gone and then there is an inexorable halt which nothing can prevent."
So where did Earth's phosphorus come from?
"Because phosphorus is much rarer in the environment than in life, understanding the behavior of phosphorus on the early Earth gives clues to life's orgin," said Matthew Pasek, a doctoral candidate at the University of Arizona's Lunar and Planetary Laboratory. Working with Dante Lauretta, assistant professor of planetary sciences at the university, Pasek argues that iron meteorites could have brought more phosphorus to Earth than occurs naturally. He presented his ideas at the 228th American Chemical Society national meeting in Philadelphia on Tuesday.
The most common terrestrial form of phosphorus is a mineral called apatite. When mixed with water, apatite releases only very small amounts of phosphate, the oxidized form in which phosphorus naturally is found. Scientists have tried heating apatite to high temperatures, combining it with various strange, super-energetic compounds.
"These experiments tended to use chemicals that were probably uncommon on the early Earth, so it's unclear how applicable they are to geochemical systems," Pasek told Astronomy
. "It's more intuitive to use simpler and more common compounds, such as those found in meteorites."
Lauretta conducted experiments showing that metal surfaces corroded in the early solar system in a way that concentrated phosphorus on them. "This natural mechanism of phosphorus concentration in the presence of . . . iron-based metal . . . made me think that . . . corrosion of meteoritic minerals could lead to the formation of important phosphorus-bearing biomolecules," he explained.
Inspired by these experiments, Pasek and Lauretta began looking at meteorites as a possible source of the element. Meteorites contain several different phosphorus-bearing minerals, but the most important, said Pasek, is iron-nickel phosphide, also known as schreibersite. This metallic compound is extremely rare on Earth, but iron meteorites are peppered with schreibersite grains or even pinkish-colored veins of the mineral. Iron meteorites became the focus of the study because schreibersite is between 10 and 100 times more common in iron meteorites than other types.
Last April, Pasek, Lauretta, and undergraduate student Virginia Smith, mixed schriebersite with de-ionized water at room temperature. They then analyzed the liquid mixture using nuclear magnetic resonance. "We saw a whole slew of different phosphorus compounds being formed," Pasek said. "One of the most interesting ones we found was P2
, one of the more biochemically useful forms of phosphate, similar to what's found in ATP." The analysis revealed numerous phosphate salts in different states of oxidation, Pasek told Astronomy
Previous experiments have formed P2
, or pyrophosphate, but at high temperature or under other extreme conditions. "This allows us to somewhat constrain where the origins of life may have occurred," Pasek said. "If you are going to have phosphate-based life, it likely would have had to occur near a freshwater region where a meteorite had recently fallen. We can go so far, maybe, as to say it was an iron meteorite."
Meteorites were critical for the evolution of life, argues Pasek, because of minerals like pyrophosphate, which is used in ATP, in photosynthesis, in forming new phosphate bonds with carbon-bearing compounds, and in a variety of other biochemical processes.
No one ever realized that such a critical stage in planetary evolution could be coupled to the origin of life.
"I think one of the most exciting aspects of this discovery is the fact that iron meteorites form by the process of planetesimal differentiation," Lauretta noted. The building blocks of planets, called planestesimals, form both a metallic core and a silicate mantle. Iron meteorites come from the metallic core, and other types of meteorites, called achondrites, represent the rocky mantle. Today's asteroids are what remains of our solar system's population of planetesimals.