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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.

{ 18 comments… add one }
  • Geoffrey Hillend April 6, 2021, 17:19

    The carbon in the early solar system was changed to soot and graphite by the heat of the Sun. It seems to me condensation is not mutually exclusive with condensation which is what happened beyond the snow line. It does not seem like this paper shows anything new. Furthermore, carbon has come from comets and hydrocarbons are pretty much equally distributed throughout the solar system supporting the accretion disk theory and condensation.

  • Geoffrey Hillend April 6, 2021, 17:20

    Excuse me, I meant condensation is not mutually exclusive from sublimation.

  • Michael Fidler April 7, 2021, 0:11

    Interesting, but the lower end of the Hertzsprung–Russell diagram where the M dwarfs are at may have a different story to tell. The carbon molecules that exist in their stellar atmosphere seem to show that the planets around these stars should have carbon. The lower density of the Trappist 1 planets point toward water worlds with carbon as a main ingredient. This fact may indicate a very different geologic ecosystem with carbon being the main compound in the underlying plate tectonics lithosphere. Mountains made of diamonds and earthquake faults from diamond’s lattice shear strain. How water and carbon mix in these worlds may make for a very large variety of life forms.


    Can I blow your mind for a minute?
    There are bacteria that do photosynthesis from the infrared glow of hydrothermal vents at the bottom of the ocean.


    Not only is this good for life on Europa and Enceladus but also planets around low end K dwarfs and M dwarfs stars since that is their peak in blackbody radiation at around 750 nm.


    The possibility of panspermia from the low end K dwarfs and M dwarfs that make up 85 percent of the stars in our galaxy seems obvious…

    • Alex Tolley April 7, 2021, 10:15

      Bacteria that MUST photosynthesize using IR? That is indeed very interesting, if true. I will read the paper and comment further, as this would change things (a little, as the energy input rate remains very poor compared to the solar radiation at 1AU and chemotrophy still seems like the better bet for energy harvesting).

      • Alex Tolley April 7, 2021, 15:12

        a href=”https://mmbr.asm.org/content/mmbr/75/2/361.full.pdf”>Microbial Ecology of the Dark Ocean above, at, and
        below the Seafloor

        Somewhat surprisingly, there is also mounting evidence for
        nonsolar phototrophic metabolism occurring at hydrothermal
        vents. The first evidence for this process came from the observation
        of novel photoreceptors in vent macrofauna that are
        never exposed to sunlight (574). Further efforts confirmed that
        infrared and even visible light is generated from black body
        radiation from the high-temperature venting fluids at the
        seafloor (573). More recently, an anaerobic phototrophic sulfur-
        oxidizing bacterium designated GSB1 was isolated from
        vent fluids at the basalt-hosted East Pacific Rise vents (40).
        This bacterium uses bacteriochlorophyll c and the carotenoid
        chlorobactene to harness light energy, and the species is phylogenetically
        related to the green sulfur bacteria Chlorobium
        and Prosthecochloris. Additionally, genetic surveys of the inside
        an active vent for genes that encode the carbon fixation protein
        RubisCO recovered sequences most closely related to cyanobacteria
        and green sulfur bacteria, both of which are phototrophic
        groups, further indicating the presence of microorganisms
        utilizing photoradiation for energy (585). Future work
        is necessary to identify how prevalent phototrophic forms of
        metabolism may be at hydrothermal vents.

        (Emphasis mine)

        From this, I think that the idea that these bacteria are photosynthesizing with IR light alone is incorrect and that visible light, even if very low intensity, is still present to transfer the needed energy. So perhaps the idea that these bacteria can use IR light from hot rocks is not the case.

        Regarding the logic for life in subsurface oceans being able to use heat radiation to fix carbon (in addition to chemotrophy), there is the issue of how these photosynthetic pigments evolved. Are they evolved from chemotrophs, or are they evolved from an organism that lives in sunlight, and teh horizontal gene transfer was the source that may subsequently have evolved? If the latter, this would imply that subsurface ocean organisms would have to remain chemotrophic.

        Lastly, oceanic vents represent a very small fraction of the ocean floor. The amount of heat transfer, even if collected from the rest of the world and funneled through these vents, would represent a fraction of the energy of the incident solar radiation that Earth receives. This suggests to me that any life that exists around these vents, would be very sparse, even if using both photosynthesis and chemosynthesis to fix carbon.

        But I am open to surprises…

        • Michael Fidler April 8, 2021, 19:55

          “Lastly, oceanic vents represent a very small fraction of the ocean floor.”

          We know more about Mars surface then the earths deep oceans.

          Hundreds Of Hydrothermal Chimneys Discovered On The Seafloor Off The Pacific Northwest.

          “MBARI researchers searched through the data from the AUV to identify all chimneys taller than three meters (the minimum height that could be reliably separated from background topography). They were amazed to find 572 chimneys. Many of the newly discovered chimneys were close to vent sites that had been studied for decades.”


          Take a close look at the pictures in the original paper – it covers an extremely small area with 572 hydrothermal chimneys and there are a huge number of areas just like it.

          Hydrothermal Chimney Distribution on the Endeavour Segment, Juan de Fuca Ridge.


          • Alex Tolley April 9, 2021, 8:31

            Hydrothermal vents occur where there are hotspots in teh ocean floor, typically at ocean ridges. Look at any map of ocean floor topography and you can see that their combined area is a small fraction of the ocean floor. Logic should tell you that if this was not the case, the deep ocean would not be able to maintain a 4C temperature as the vents would heat the deep ocean.

            Your argument seems (at least in my interpretation) analogous to viewing a city and maintaining that the planet is mostly covered in buildings when we know that this is not the case.

            • Michael Fidler April 9, 2021, 9:49

              Well, your logic means Europa and Enceladus are dead worlds. My logic says Europa Hydrothermal Chimneys are 1000 feet high and Enceladus are 2 miles high and covered with life… You need to change your perspective, get yourself out of Davy Jones’ Locker and open your mind (out-of-the-box thinking).

              • Alex Tolley April 9, 2021, 12:38

                I didn’t say that subsurface oceans would be a priori sterile, but rather that life would be very sparse overall, as the energy available would be very limited. While photosynthesis may occur at these vents (in addition to chemotrophy), the relative energy available compared to Earth’s sunlight would be so low that the support for carbon fixation would be very low, implying low biomass too.

              • Michael April 10, 2021, 15:34

                Lol, I had to laugh to myself Michael.

    • Alex Tolley April 7, 2021, 13:54

      Despite the source of the paper (PNAS) I am not convinced of their discovery. The GSB1 culture apparently survives in the absence of any light and in teh presence of supposedly toxic O2. No experiment was shown that they grow in teh presence of IR light (AFAICS) but only in the presence of weak incandescent and fluorescent light – ie light in the shorter wavelengths needed. The authors argument is that these bugs are found in a smoker plume where light is only in the IR and therefore, based on teh assumption they are obligate photosynthesizers, they must be using the IR energy of the plume. To my mind, this was not proven. Are the authors sure they are obligate photosynthesizers? Their survival in the absence of light suggests perhaps not.

      I am not a microbiologist, but I would certainly like to read responses by microbiologists. I will attempt to see if any are available via citations of this paper. However, call me skeptical.

    • Dave Moore April 7, 2021, 15:54

      After reading this post carefully, I think these papers comport with our knowledge of Carbon Stars.

      At a high temperature in a nebula, the chemically stable equilibrium between Carbon, Hydrogen and Oxygen is a mixture of CO/C and H2 depending on the Carbon to Oxygen ratios. Below a certain temperature, this thermodynamic equilibria switches to H2O, CH4/C and H2. The sun’s formation nebula was very oxygen rich, so even after much Oxygen got incorporated in silicates there was sufficient left over to convert all the inner nebula carbon into CO, which been highly volatile was pushed to the outer nebula. In effect, the Oxygen in our nebula burnt all the carbon out of our inner system.

      In a Carbon rich nebula, the carbon will combine with all the oxygen left over from silicate formation, removing it from the inner system, and the left over will condense to form graphite grains (as it does in Carbon Stars), which will trigger planetesimal formation.

      This mechanism may account for the paucity of planets close to the sun in our system.

  • ljk April 7, 2021, 9:43

    This just in…

    Origin of Life Possibly Begins With XNA Before RNA, DNA

    Mark Bustos

    Apr 07, 2021 02:05 AM EDT

    A new study explains how DNA-like molecules came from XNAs – xeno nucleic acids – which might be the precursor to the origin of life as we know it.

    Researchers from Nagoya University in Japan published the study titled “Nonenzymatic polymerase-like template-directed synthesis of acyclic L-threoninol nucleic acid” in the journal Nature Communications, suggesting a new possibility about how life started. Furthermore, the new model suggested by Japanese researchers has applications in future plans to develop artificial life and biotechnology applications.

    Full article here:


  • Alex Tolley April 7, 2021, 10:11

    This seems to imply that worlds dominated by carbon rather than silicon may not exist. I do see Michael Fidler’s comment contesting that regarding M_dwarfs. I haven’t read the paper, but for someone who has, do the authors indicate that rocky worlds should be mainly Si, or are there circumstances where C could still dominate?

  • David Walker April 7, 2021, 10:31

    Paul Gilster has captured well the points about carbon delivery and state in the early solar system and made them accessible. Geoffrey Hillend however seems to not accept a central point of the Li et al. paper, that the condensation of carbon is not a reversible process of the sublimation because of the state in which the carbon is delivered. Once evaporated it will not readily recondense as soot. Before dismissing this paper as not new, Geoffrey Hillend should have explained why he believes the sublimation is reversible.

  • Geoffrey Hillend April 7, 2021, 19:14

    Maybe I misread the paper. I thought it says there is not any carbon in the out solar system. Also the idea that Venus is the result of excess carbon implies a dubious origin for carbon. Venus did not have any more carbon than Earth. It was the fact that carbon is an atmophile like hydrogen and nitrogen and the increase in brightness of our Sun and increased surface temperature moving it out of the life belt and causing a loss of H2O splitting into H and O through photolysis and atmospheric jeans escape and solar wind stripping.

    The carbonaceous chondrites also contain volatiles which were formed inside the snow line and have the original composition of the primordial accretion disk where they were formed first. The carbon cycle shows that the sublimation process is not irreversible, otherwise we couldn’t have a balanced, self regulated climate. Earth did not loose any carbon since it is a heavier element which is why Venus has an excess of CO2 and no little water, hydrogen and oxygen. Carbon can change forms and combine with other elements and that is what exactly happens with the limestone or calcium carbonate build up on the bottom of our oceans thanks to the carbon cycle which regulates are climate.

  • John walker April 8, 2021, 7:42

    Very interesting papers. Always fascinating to witness the upending of remarkable decades old false assumptions. Assumptions that more thorough consideration could exposed much earlier.

    The papers contains a few typos which may make it’s reading unnecessarily misleading for some as seen in a couple of the comments above. But, today there were also some very astute comments made here. Thanks to everyone for their contributions. Once again I regret the lack of a like button.

  • Geoffrey Hillend April 8, 2021, 16:18

    Quote from the paper “Explaining Earth’s Carbon: Enter the ‘Soot Line’ “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.” I can’t agree with this. For one thing the sublimation temperature of carbon is a whopping 6622 F, 3550 C. The boiling point of carbon is 7952 F or 4200 C. The temperature of lava is 2000 F. The whole idea of gases carbon and a soot line in the solar system is just wrong non existent and the accretion disk theory does not imply that. One should check the facts before one makes a theory.

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