For all the handwringing over the amount of carbon wreaking havoc on our global climate, Earth is remarkably carbon poor. Carbon is only a trace element in Earth and a minor element in the Sun, write the authors of The Sixth Element: How Carbon Shapes Our World, out next month from Princeton University Press.
Despite problems with humanity’s use of carbon-based fossil fuels, our whole existence is based on this element’s ability to create rich chemistry, co-authors Theodore P. Snow, professor emeritus at the University of Colorado in Boulder and Don Brownlee, professor emeritus at the University of Washington in Seattle, note.
What is amazing is how rare carbon is in the whole Earth; the total carbon abundance is only a few hundred parts per million, Brownlee told me via email. Yet on Earth, carbon was surely the critical elixir that led to the evolution of the complex molecules and chemical pathways that made life possible, he says.
Ironically, the most carbon-rich bodies in the solar system are not the Sun or the planets, but smaller bodies such as comets and asteroids, leftover planetary building blocks that have escaped collisions with planets or ejection from solar orbits for more than 4 billion years, Snow and Brownlee write.
Even so, Earth has a layered structure, and carbon —- the sixth element on the periodic table —- is present at all levels from the top of the atmosphere to the our planet’s core.
As To Why Earth Is So Carbon Poor?
Earth formed in the Sun’s habitable zone where carbon did not efficiently form solids, says Brownlee. Earth is very carbon poor compared to the typical asteroids and comets that formed further form the Sun are often considered to be preserved building blocks of solid planets, he says.
But carbon can cause trouble.
Carbon is the only chemical element with its own tax; we are spending unknown billions of dollars learning about how to cope with it; and we repeatedly hear that our use of carbon is going to destroy the Earth, says Brownlee. The production of fossil fuels was nature’s gift to us but the global warming it is producing has numerous serious effects, he says.
Challenges Ahead
Rising temperatures due to accumulating Carbon dioxide will lead to sea level rise and cause changes in crop growing regions and global weather extremes, but it is impossible that anything that humans could currently do could ever destroy our planet, he says.
And despite its relative rarity here on Earth, carbon’s ability to bond with elements to form a nearly infinite number of compounds is likely why we are here to talk about it. But could life in our solar system have gone a different way and be based on an element like silicon, instead of carbon?
Silicon is not a cosmically rare element (it is the seventh most abundant in the Galaxy)—but carbon is about four times more abundant, Snow and Brownlee note. On Earth, silicon is much more abundant (26 percent by mass) than carbon, they write.
As for finding silicon-based here in our solar system?
Warm, wet primitive meteorites loaded with silicon were heated in the first few million years of solar system history, says Brownlee. We have looked at thousands of lunar samples, thousands of meteorites and even samples of a comet but have not seen any evidence that silicon in these billion-year-old materials was ever involved in any process that might be considered life, he says.
What about silicon-based life beyond your solar system?
Even if we had thousands of excellent spectra of exoplanets we would still probably could not know of Silicon-based life because there wouldn’t be any Silicon-bearing gas in their atmospheres, says Brownlee. Life on Earth is trivially easy to detect by extraterrestrials because it created a strange atmosphere (nitrogen, oxygen and carbon dioxide) that could not exist by normal chemical processes, he says.
How is carbon is distributed within our own galaxy?
There is probably an optimum amount of carbon for a planet to have life, says Brownlee, but who knows what this is? Too much can lead to bad atmospheres (like Venus) and too little might be just too little for life to start, he says.
Fundamental Questions Remain
One such question is how carbon actually finds its way into Earthlike planets.
The Sun and early solar system had vast amounts of carbon (the 4th most abundant element after hydrogen, helium and oxygen) but Earth formed from solids and most of the carbon atoms were in the form of gaseous carbon monoxide, says Brownlee.
This is just the sort of conundrum that The Sixth Element highlights. Meticulously researched and comprehensive in scope, the book will be an asset to science libraries for decades to come.
