Unless you’re a farmer or a pharmacist, phosphorus likely rarely enters your consciousness. Yet its compounds form the structural backbone of our genetic code and drives the energy behind nearly all of life’s metabolism. But phosphorus’ bioavailability for prebiotic chemistry was hardly a fait accompli; its presence here on Earth may be serendipitous at best.
A recent paper appearing in The Journal of Geophysical Research: Space Physics, only emphasizes this point by outlining a scheme that describes how Earth’s phosphorus was actually delivered to our planet some 4.5 billion years ago.
The team led by researchers in the School of Chemistry at the University of Leeds in the U.K., note that when Earth formed any phosphorus that was present likely sank into our planet’s molten core. But they note that a portion of the interplanetary dust particles which orbit our Sun likely wound up in Earth’s atmosphere.
Such cosmic dust particles may deliver phosphorus to Earth’s atmosphere, the American geophysical union (AGU) reports. From there, the AGU notes that a series of chemical reactions repackage the element into biologically useful forms that eventually settle onto Earth’s surface.
Upon encroaching upon Earth’s atmosphere, however, this dust was affected by atmospheric friction that caused a portion of it to vaporize and melt. The paper’s authors describe a network of potential chemical reactions that outline the specific process by which this cosmic dust could produce biologically useful phosphorus molecules, the AGU notes. And it’s an ongoing process, happening even now.
The researchers were also able to predict which regions of the globe might annually receive the most phosphorus delivered by cosmic dust. Three mountain chains are thought to be the recipients of the most phosphorus delivered by such dust and include the northern Rocky Mountains, the Himalayas, and the southern Andes.
The Leeds team even posits that an atmospheric layer of the phosphorus-containing molecule OPO may encircle Earth some 90 km above our surface. The hope is that future atmospheric research will be able to detect this phosphorus layer’s existence.
But arguably, the main lesson in all of this is that even in our own Sun, phosphorus is hard to come by.
As a 2020 paper in The Astrophysical Journal Letters (APJL) points out, phosphorus is three orders of magnitude less abundant in the Sun than other important light elements. However, trying to determine its abundance around other stars is extraordinarily difficult, mainly because there are no spectral lines for detecting it at optical wavelengths. And as this paper points out, even using space telescopes to detect it in other stars will be no easy task, as its spectral lines in the near-ultraviolet are often blended and are difficult to discern.
Regardless, at present the Sun still seems to be phosphorus-rich when compared with other nearby stars, as the APJL authors note. But data on the abundance of phosphorus in other stars is only available for less than one percent of all stars. Thus, as the authors point out, this makes it difficult to make pronouncements about how prevalent phosphorus might be in other sunlike stars.
As the APJL authors note, there is a strong caveat. As they write, on rocky planets that form around host stars with substantially less phosphorus, the fact that much of it will be partitioned in a given planet’s core may rule out the possibility that a potential habitable extrasolar planet would have enough phosphorus for life to evolve.
The upshot is that in its bioavailable form, phosphorus here on Earth seems to be a once in a blue-moon proposition. The accretion of cosmic dust in our atmosphere appears to offer Earth an unexpected, continuous source of phosphorus replenishment. But it also gives astrobiologists yet another box that must be checked for life to evolve elsewhere.