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Cometary Origins

We’ve learned a great deal about comet 67P Churyumov-Gerasimenko thanks to the European Space Agency’s Rosetta mission. What stands out to me is the fact that 40 percent of the comet, in terms of mass, is made up of organic compounds — combinations of hydrogen, carbon, oxygen and nitrogen. These ‘building blocks’ of life on Earth are readily available and could have been delivered over and over again through our planet’s long impact history.

But where did the organic compounds themselves come from? Jean-Loup Bertaux (CNRS / UPMC / Université de Versailles Saint-Quentin-en-Yvelines), working with Rosine Lallement (Observatoire de Paris / CNRS / Université Paris Diderot), has put forth the idea that the organics formed in interstellar space long before the formation of the Solar System.

The idea no longer seems as startling as it once might have, thanks to our continuing study of what are called diffuse interstellar bands, or DIBs. Revealed by spectroscopic studies, these are absorptions at specific wavelengths that turn up in the spectra of various astronomical objects, caused by the absorption of light in the interstellar medium. DIBs began to be recorded in the early 1900s, with over 100 of them being identified from the ultraviolet through the near infrared.

Image: Relative strengths of known diffuse interstellar bands. Credit: NASA/ Peter Jenniskens.

Diffuse interstellar bands appear to show organic material, found in most cases in roughly the same proportions wherever they are observed. In dense clouds of matter like those that will spawn stars, however, DIB absorption drops.

The mechanism? Bertaux and Lallement believe the organic molecules that produce DIBs clump together in such dense environments, and the clumped matter absorbs less radiation than the free-floating matter. “The decrease of DIB during line–of-sight crossings of the dense cores of interstellar clouds,” the authors write, “and the simultaneous steepening of the Far UV part of the reddening curve suggest that DIB carriers coagulate and are constituent of the very small grains eventually preserved in comets.”

In other words, organic molecules from the interstellar medium would have been conserved as the early nebula around growing young stars like the Sun coalesced, and would gradually have been incorporated into cometary nuclei, as comets like 67P/CG formed through the gradual accretion of grains of growing size. The authors also note that the ratio of organics could be increased by other organic molecules in the interstellar medium that do not show up in DIBs.

Image: The nucleus of comet 67P Churyumov-Gerasimenko (“Chury”) as seen by the European Rosetta space probe. Credit: ESA / Rosetta / MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA.

We’re dealing with pristine material, for the organics would have remained in the comets ever since their formation 4.6 billion years ago. The prospect for future missions is that we should be able to fetch a sample of these materials that could help us understand their true identity. If they are interstellar in origin, then the same materials should have been delivered to many systems. Thus the dust of comet 67P Churyumov-Gerasimenko gives us clues to an interstellar absorption feature that could mark the spread of organic compounds throughout the galaxy.

The paper is Bertaux & Lallement, “Diffuse Interstellar Bands carriers and cometary organic material,” published by Monthly Notices of the Royal Astronomical Society 31 August 2017 (abstract).


{ 15 comments… add one }
  • John walker September 8, 2017, 17:04

    40% total mass organics. Do you have a source for that value? I’d like to learn more.

    • Paul Gilster September 8, 2017, 20:24

      CNRS put this figure out in press materials on the Bertaux paper:


      Seems high to me as well.

      • John walker September 11, 2017, 17:55

        I requested clarification from the cnrs press office. Dr. Bertaux sent me this:

        The results from the Rosetta mission are.
        1. The dust to ice mass ratio is between 4 and 6.
        2. the mass ratio of organic to mineral +sulfide is around 1, see the reference to a conference in Montevideo in April 2017, a results from the Cosima team:
        Baklouti D. et al. (2017), Asteroids, Comets, Meteors – ACM2017 – Montevideo, April

        Taking the lowest number 4 for the dust to ice mass ratio, it makes the bulk composition
        40% organic, 40% mineral +sulfide, 20% water ice.
        Taking the highest number 6 for the dust to ice mass ratio, it makes the bulk composition
        42% organic, 42% mineral +sulfide, 16% water ice.

        Surprising? yes, surprising! but in fact it is in line with results obtained on comet Halley in 1986, though less accurate.

        JL Bertaux

        • John walker September 11, 2017, 18:19

          Dr. Bertaux subsequently kindly sent me a copy of his paper. From the paper:
          “During the two years at the comet, the COSIMA instrument on board Rosetta collected at low velocity more than 31,000 particles, released from 67P/CG before and after the comet’s perihelion. About 250 particles, ranging from ~50 to ~1000 µm in size, were analysed with the TOF-SIMS technique (Baklouti et al. 2017). The dust particles are void of ice, which had ample time to sublimate before collection and analysis by COSIMA, if there were any when leaving the nucleus. As reported by Baklouti et al. (2017), the collected dust is composed of ∼54 % of minerals, and ∼46 % of organic matter in mass. Therefore the cometary mass ratio of organic/minerals is RC=46/54=0.85 ± 0.4 (with a rather large error bar of ∼50% according to the authors) in the non-volatile fraction of the comet.”

          • Paul Gilster September 11, 2017, 21:50

            Well done, John. Many thanks for following this up!

            • John walker September 12, 2017, 11:16

              I just want to add on average the bodies generally referred to as “carbonaceous” chondrites are comprised of only around 5% organics by mass. When one looks for primary sources of primordial organics in the solar system it seems comets may be key. Estimated total mass of all comets is around 1000 times more than that of all main belt and Kuiper belt objects combined. Although the vast majority of those comets drift in the nether regions of the Oort Cloud and most of that mass will be in the form of volatiles that’s still a lot of organics.

  • Alex Tolley September 8, 2017, 17:19

    40 percent of the comet, in terms of mass, is made up of organic compounds

    That seems really high. Other data I have seen suggest that comets are more like 1% organic matter, most of which is CH4. (e.g. Organic comp ounds on comet 67P/Churyumo v-Gerasimenko revealed
    by COSAC mass spectrometry

    Possibly the surface might be highly enriched, but the bulk of nucleus?

  • Spaceman September 9, 2017, 13:30

    Sounds like a very high percentage of organics. Exciting, however, to think that the raw materials of life may have been delivered here from comets and/or asteroids. It also makes me think about the possibility of extrasolar planets whose systems may have comets and asteroids with significantly more organic compounds compared to the comets and asteroids in our solar system…would such a system have a higher probability of abiogenesis (assuming it is broadly similar in other respects to our own)?

  • Harold Daughety September 9, 2017, 23:05

    The low chemical bond energy in the CHON system and the reversibility of reactions, the branched chain forming ability of of the system, and the vast but not unlimited 3 dimensional structures seems to suggest that simple living forms are probably common throughout the galaxy. When conditions permit, more complex systems will arise, the ultimate form of which will be determined largely by the environment. Consumption of one life form by another is the most efficient means of acquiring living tissue, by building from the complex molecular structures of the prey. Thus the ultimate apex life form will be determined in part on which single cell species triumphs most often. Whether or not there are even remote parallels to human evolution is an unanswerable question, but it seem to me to be possible, even probable, that there are. Whether or not we can recognize the aliens as intelligent is debatable.

  • Hop David September 10, 2017, 10:25

    According to Wikipedia, CHON rank among the top 5 most common chemically active elements in the universe (iron is almost tied with nitrogen)

    Hydrogen 73.9%
    Helium 24%
    Oxygen 1.04%
    Carbon .46%
    Neon .134%
    Iron .109%
    Nitrogen .096%

    Given this list I would expect most of the solid material in the Oort objects to be water. Some carbon dioxide ice. Rust, ammonia.

    Robert Walker has speculated Oort planets (if they exist) might have oceans of liquid hydrogen and neon.
    My conversation with Walker:
    Walker’s Space Stack Exchange question:

    The objects in the deep cold of interstellar space are likely very strange and interesting places.

    I seem to recall a science fiction writer (Baxter?) of a race that adapted themselves to deeply cryogenic temperatures so their race would endure as the universe cools. The beings were more or less globular and their surface was very reflective and mirror like. The volume between stars is certainly much greater than the volumes we’re accustomed to thinking of.

    • Harold Daughety September 17, 2017, 21:15

      It seems to be a misstatement, the the most common chemically active . . this is a list of elements by abundance. Helium is not chemically active.

  • Silent Hunter September 10, 2017, 17:04

    Is it just me or does 67P Churyumov-Gerasimenko look like a bit like Serenity.

  • Andrew Palfreyman September 11, 2017, 0:19

    Comets form one rather flakey model of why Tabby’s Star behaves as it does. In case you’re unaware, she’s at it again in spades, as of September 10th 2017. Probably worth revisiting some time soon.

  • ljk September 19, 2017, 12:45

    If this model is correct, it could drastically change how astrobiology searches for life on alien worlds!

    Rocks, not bacteria, triggered Earth’s oxidation

    Changes in the planet’s crust paved the way for an oxygen atmosphere and the evolution of complex life, writes Andrew Masterson.


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