Explaining Earth’s Carbon: Enter the ‘Soot Line’

Let’s take a look at how Earth’s carbon came to be here, through the medium of two new papers. This is a process most scientists have assumed involved molecules in the original solar nebula that wound up on our world through accretion as the gases cooled and the carbon molecules precipitated. But the first of the papers (both by the same team, though with different lead authors) points out that gas molecules carrying carbon won’t do the trick. When carbon vaporizes, it does not condense back into a solid, and that calls for some explanation.

University of Michigan scientist Jie Li is lead author of the first paper, which appears in Science Advances. The analysis here says that carbon in the form of organic molecules produces much more volatile species when it is vaporized, and demands low temperatures to form solids. Moreover, says Li, it does not condense back into organic form.

“The condensation model has been widely used for decades. It assumes that during the formation of the sun, all of the planet’s elements got vaporized, and as the disk cooled, some of these gases condensed and supplied chemical ingredients to solid bodies. But that doesn’t work for carbon.”

Most of Earth’s carbon, the researchers believe, accumulated directly from the interstellar medium well after the protoplanetary disk had formed and warmed; it was never vaporized in the way the condensation model suggests. Interesting concepts come into play here, among them the cleverly titled ‘soot line,’ in analogy to the ‘snow line’ in planetary systems. This marker has a lot to do with how carbon behaves. The authors use astronomical observations and modeling to explore the concept. From the paper — watch what happens as the disk warms:

Astronomical observations show that approximately half of the cosmically available carbon entered the protoplanetary disk as volatile ices and the other half as carbonaceous organic solids. As the disk warms up from 20 K, all the volatile carbon carriers sublimate by 120 K, followed by the conversion of major refractory carbon carriers into CO and other gases near a characteristic temperature of ~500 K… The sublimation sequence of carbon exhibits a “cliff” where dust grains in an accreting disk lose most of their carbon to gas within a narrow temperature range near 500 K.

The ‘cliff’ is another good analogy. It’s the edge of the soot line:

The division between the stability fields of solid and gas carbon carriers corresponds to the “soot line,” a term coined to describe the location where the irreversible destruction of presolar polycyclic hydrocarbons via thermally driven reactions in the planet-forming region of disks occurred.

Modeling the sublimation process and loss of carbon in the solar nebula, the authors chart the soot line as it migrates with time as the system matures and the pressure and temperature of the disk evolve. Shortly after the birth of the Sun, the soot line might have extended out 10s of AU, but as the accretion rate diminished, it would have migrated inward. A carbon poor early Earth, then, would be the result of formation during the period when the soot line was well beyond Earth’s orbit, during the first million years, when accretion rates were high.

Image: This is Figure 2 from the paper. Caption: Fig. 2 Schematic illustration of the soot line in a protoplanetary disk: The soot line (red parabola) delineates the phase boundary between solid and gaseous carbon carriers. In the accretion-dominated disk phase, it is located far from the proto-Sun and divides carbon-poor dust and pebbles (green dots) from carbon-rich ones (dark blue dots). Within 1 Ma, as a result of the transition to a radiation-dominated, or passive, disk phase, the soot line migrates inside Earth’s current orbit. Note that the Si-rich and C-rich solids do not represent distinct reservoirs because carbonaceous material is likely associated with silicates. They are provided for ease in illustration. Credit: Li et al.

Drawing again from the paper:

If the bulk carbon content of Earth is low, then most of its source materials must have lost carbon through sublimation early in the nebula’s history or by additional processes such as planetesimal differentiation. Constraining the fraction of carbon-depleted source material accreted by Earth requires us to constrain the maximum amount of carbon in the bulk Earth.

Which the authors do by determining the maximum amount of carbon the Earth’s core could contain — after all, they mention planetary differentiation — a figure that turns out to be less than half a percent of Earth’s mass. Says Li’s colleague Edwin Bergin (University of Michigan):

“We asked how much carbon could you stuff in the Earth’s core and still be consistent with all the constraints. There’s uncertainty here. Let’s embrace the uncertainty to ask what are the true upper bounds for how much carbon is very deep in the Earth, and that will tell us the true landscape we’re within.”

The paper points to a severe carbon deficit in the newly formed Earth, and suggests still more about the environment producing it. Centimeter-to-meter sized pebbles delivering mass as they drift inward from the outer Solar System would carry both water and carbon. Simulations of their movement show that a giant planet core in the disk would create a pressure bump where drifting pebbles would pile up, diminishing the supply of carbon for the emerging inner system. The carbon-poor composition of iron meteorites is cited as evidence of this early carbon depletion.

In the second paper, the same group of researchers examined these iron meteorites, which represent the metallic cores of planetesimals, looking at how they retained carbon in their early formation. Here melting and loss of carbon is apparent. Marc Hirschmann (University of Minnesota) led the second study, which included the same co-authors along with Li:

“Most models have the carbon and other life-essential materials such as water and nitrogen going from the nebula into primitive rocky bodies, and these are then delivered to growing planets such as Earth or Mars.. But this skips a key step, in which the planetesimals lose much of their carbon before they accrete to the planets.”

Thus we see two different aspects of carbon loss, highlighting the delicate nature of carbon, so necessary for climate regulation but capable of creating Venus-like conditions when found in excess. The loss of carbon in the early Earth may play an essential role in our planet’s habitability. How carbon loss occurs in other planetary systems is a topic that will require a multidisciplinary approach involving both astronomy and geochemistry. As the second paper suggests, it’s a topic that could be vital to life’s chances elsewhere:

…the volatile-depleted character of parent body cores reflects processes that affected whole planetesimals. As the parent bodies of iron meteorites formed early in solar system history and likely represent survivors of a planetesimal population that was mostly consumed during planet formation, they are potentially good analogs for the compositions of planetesimals and embryos accreted to terrestrial planets. Less depleted chondritic bodies, which formed later and did not experience such significant devolatilization, are possibly less apt models for the building blocks of terrestrial planets. More globally, the process of terrestrial planet formation appears to be dominated by volatile carbon loss at all stages, making the journey of carbon-dominated interstellar precursors (C/Si > 1) to carbon-poor worlds inevitable.

Thus sublimation, not condensation, tells the tale of carbon abundance on Earth, with presumably the same processes at work elsewhere in the galaxy.

The Li paper is “Earth’s carbon deficit caused by early loss through irreversible sublimation,” Science Advances Vol. 7, No. 14 (2 April 2021). Full text. The Hirschmann paper is “Early volatile depletion on planetesimals inferred from C–S systematics of iron meteorite parent bodies,” Proceedings of the National Academy of Sciences Vol. 118 (30 March 2021). Full text.

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