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What is the timing and relative importance of each of the processes responsible for the delivery of volatiles to the terrestrial planets, and what conditions are required for those volatiles to make both organics and liquid water?

  1. Site Admin


    What is the timing and relative importance of each of the processes responsible for the delivery of volatiles to the terrestrial planets, and what conditions are required for those volatiles to make both organics and liquid water?

  2. Sean Raymond


    Below is a cut and paste of a comment I made on the more general discussion board. This is just to re-introduce what I believe are the central processes that determine a planet’s water content. (Sorry it’s so long — number 4 is probably the most important, although there is some debate)

    Based on current thinking in planet formation here are what I consider the key steps in determining a planet’s water content (comments welcome):

    1. The composition of the molecular cloud from which the star and planets form. I’m no chemist but certain compositions favor the formation of different molecules. I believe that the C/O ratio is one of these (Gaidos 2000) but someone else can chime in and correct me if needed.

    2. The properties of the gaseous protoplanetary disk. More massive or more viscous disks are strongly heated by viscous heating, thereby moving the snow line farther from the star. Low-mass or low-viscosity disks can have very close-in snow lines (within 1 AU for a Sun-like star; e.g., Sasselov & Lecar 2000) and thus seed terrestrial embryos with water.

    3. The presence of short-lived radionuclides (“SLRs”), especially Al-26 and Fe-60. The amount of heat released at early times by these SLRs is huge, enough to desiccate large planetesimals (Grimm & McSween 1993). The origin of SLRs in the Solar Nebula is still debated: they may have been injected by a nearby supernova or winds from a massive star, or potentially produced locally by spallation in the innermost parts of the disk. It is thus unclear what the abundance of SLRs is for a typical protoplanetary disk. In general, it is thought that the more SLRs, the more heating of planetesimals, the more distant the snow line, and the less water delivery to Earth-like planets (Desch & Leshin 2004, Gaidos et al 2007, Morris & Desch 2009).

    4. The dynamics of planetary accretion. If a terrestrial planet forms with a narrow feeding zone then its composition is determined solely by the local composition — for the habitable zone this means the planet is probably dry (modulo other effects listed above). However, if the orbital dynamics causes mixing between different radial zones then the planetary composition is an average that can include a contribution from more volatile elements like water that condensed farther out. Numerous dynamical factors affect water delivery during accretion: the surface density of solids in the disk (Raymond et al 2005, 2007), the presence, mass and eccentricity of any giant planets in the system (Chambers & Cassen 2002, Raymond et al 2004, 2007a, 2007b, 2009, O’Brien et al 2006), and the dynamics of the giant planets, i.e. whether they migrated (Raymond et al 2006, Mandell et al 2007). In the context of the Solar System a new model called the “Grand Tack” proposes that Jupiter migrated inward then outward, and in that context water delivery occurred during the outward migration (Walsh et al 2011).

    5. The long term water delivery by impacts of small bodies (asteroids, comets). In the Solar System this contribution is small (less than a couple % of the total water budget) but in other systems it could be considerable.

    6. Long term water erosion or retention. Water can be lost from the upper atmosphere by photo-dissociation followed by H loss, so this depends on the stellar UV flux and its long-term evolution.

  3. Francis McCubbin


    Based on estimates we presently have for terrestrial bodies in the Solar System, The Earth, Moon, and Mars may all have within an order of magnitude, the same water content (100’s-1000’s ppm in the bulk silicate). What does this mean regarding the governing processes of water delivery and the timing of water delivery to those bodies?

  4. Sean Raymond


    Francis, I suspect this is a signature of water delivery being dominated by small bodies — this makes the process much less stochastic than if it’s dominated by a few large water-rich embryos. (I actually wrote a whole paper on just this topic in 2007).

  5. Sean Raymond


    A question: what do you think is the ideal water content for habitability? Are water worlds habitable? And, during planet formation, are there processes that regulate the amount of water a planet can accrete? One that I can think of is the mechanism of Genda & Abe 2005 who showed that water-rich planets lose more water in giant impacts than rock-dominated planets because the impedance of water is less than for rocks.

  6. Francis McCubbin


    Sean, I think water is very important, but there is a fine interplay between the stability of liquid water, which requires somewhat oxidizing conditions and the stability of reduced carbon molecules and organic compounds, which require somewhat reducing conditions. Habitability likely lies within the region where water stability overlaps the stability of many organic molecules.

  7. Francis McCubbin


    Question: Are organic molecules delivered late after condensation or are they formed during planetary differentiation processes? Which processes is likely more important for abiogenesis?

  8. Sean Raymond


    I’ll throw another thought experiment out there. Using only objects in the Solar System, what is the ideal orbital configuration to maximize the number of habitable planets? For instance, I have heard it suggested that if Venus and Mars formed on each other’s orbits, both could be habitable and we might have 3 habitable planets in the Solar System instead of just one.

  9. Sanjay Dosaj


    NO COMPLEX SCIENCE BUT ONLY COMMON SENSE COUPLED WITH A LITTLE ACQUIRED KNOWLEDGE IS REQUIRED TO UNDERSTAND THE CREATION OF THE ALMIGHTY. This is a statement from my yet to be published book on positive science. To understand the creation we should first of all believe that a solution always lies very close to any problem. Considering the question of life on extra terrestrial bodies I will say that there is a trigger that if understood and activated any planet can become as alive as earth. Intensive study and long and deep thinking has enabled me to understand this trigger for Mars, my paper ‘TRIGGER LIFE ON MARS ‘ is due to be published shortly and it will end the debate on how to trigger life on any extra terrestrial body. Any comments or criticizm is welcome, not only on this site but also to my personal email sanjaydosaj@me.com

  10. Sanjay Dosaj


    Sean can you please provide the link to your paper.

  11. Sean Raymond


    For instance, would Europa or Titan be habitable if they were on the Moon’s orbit? And would Earth be habitable if it were tidally heated like Europa (I assume Io’s level of tidal heating would be too much)?

  12. Sean Raymond


    The paper I mentioned before on how stochastic water delivery is can be found here: http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2005astro.ph.10285R&db_key=PRE (click on “arXiv e-print” for a free version)

  13. Francis McCubbin


    Sean, your questions have inspired another question. Does habitability have anything to do with abiogenesis? It seems that we could add life to a habitable world and it could be sustained, for example, martian meteorites and even chondrites are often covered in terrestrial endolithic bacteria munching away at the scrumptious mineralogy, but what conditions in a planet should we look for that would favor abiogenesis and would they necessarily be the same as the conditions for habitability?

  14. Murthy S Gudipati


    Hi All, I think one of key issues that we need to understand is the delivery of volatiles to Earth through early heavy bombardment involving comets (and perhaps asteroids). Though there are hints that this could have triggered prebiotic processes and origin(s) of life on Earth, we have a long way to go before we accept or reject this pathway.

    Comets being key conveyers of materials from interstellar medium to inner solar system, their composition is well studies, but still poorly understood. Comet impacts are almost not studied. Whether the organic material would survive, or new organics are synthesized or whether the water that was brought on to Earth by comets was critical? These are questions that need NAI focus. We need to understand how volatiles evolve on Comets until they impact a planetary body.

    We also have to understand, if such volatile transport by comets to Earth were to be true, did or didn’t Mars go through similar processes and if yes, what happened to these organic volatiles on Mars?

    Regards Murthy Gudipati

  15. Sean Raymond


    Murthy good point. Cometary/asteroidal bombardments may be common in young systems, although it doesn’t seem likely that they are too common (at least not uniquitous) in older systems (Booth et al 2009). More study of how those impacts affect the planetary surface are definitely needed. Another interesting connection is that these bombardments are directly observable around other stars by the hot dust that is created. There are a few old stars with lots of hot dust that are thought to be undergoing bombardments right now (e.g., Tau Ceti, Eta Corvi, Vega). The origin of the hot dust is probably in-scattered cometary bodies, although the dynamical mechanism at work is not yet known. (see recent papers by Amy Bonsor et al if you are interested)

  16. Ali Mallakin


    Refer to Murthy, obviously he has good knowledge of the many aspects but they are ongoing study on comets and asteroids and they are known to be affluent in ice and organic molecules and are important sources of biogenic elements. In comparison asteroids with less organic materials may be more important as their lower impact speeds allow unaffected material to reach the surfaces of the terrestrial planets. Future study can examine the conditions inside asteroidal bodies that produced, changed and shattered organic compounds.

    Also the topic is very much related to planetary science and when plate tectonics begun the planetary evolution. The oxidation condition of the early Earth mantle possibly ruled the distribution of reducing gases. Earth formation could be through cold accumulation with early reduced stage before complete differentiation of the mantle, and a later on oxidized stage after differentiation.

    Looking at the early atmosphere in relation to synthesis of organic materials, one may determine the redox balance of the crust–mantle–ocean–atmosphere system. Endogenous organic synthesis seems to depend as primordial atmosphere was mainly reducing with low O2 levels. There is still an argue on the subject of the composition of the early atmosphere. Other than that it is known to have little O2. Oxygen could be produced by photodissociation of water.

  17. Sean Raymond


    Francis, your comment on abiogenesis could be inherently linked to the bombardment issue. If life develops on one planet in a system it can be spread to other planets via orbital spreading of impact ejecta. That is, if the life in question is tough enough to survive two big impacts and a while in free space!

  18. Ali Mallakin


    In addition more evidence for oxygen production from oxygenic photosynthesis and an addition to the atmosphere comes from the Banded iron formations (BIFs). In today’s Earth, high O2 levels allow for the photochemical formation of a significant amount of ozone. Ozone served as the major shield of highly energetic UV light. Although UV can be an important source of energy to synthesize organics, it is also a way of destroying them. That is why the early atmosphere was possibly not oxidizing, even it does not prove it was very reducing.

  19. Francis McCubbin


    Ali, I completely agree that the evolution of a planets redox state is an important consideration for planetary habitability and abiogenesis, and we have evidence in the geologic record that the Earth has had quite a dynamic past when it comes to the redox chemistry in the hydrosphere. Perhaps it was this dynamic history that allowed for the coexistence of organic molecules and liquid water. When we look at the limited rock record we have for Mars, the very surface seems to be quite oxidized, but the interior seems to be orders of magnitude more reducing than the presnt day terrestrial upper mantle. Could it be possible that a mixing mechanism like Plate tectonics is required to fuel the changes in redox state at a planet’s surface, and perhaps this is the key to abiogenesis on Earth (a bit of a stretch, but perhaps an interesting thought?)

  20. Ali Mallakin


    Just be careful about some of the terminologies, abiogenesis is a natural process by which life arises from simple organic compounds, it may/can not be correlated to comets and asteroids bombardment issue. Meaning and definition can be very important. Just on the side of the current discussion, I remember yesterday someone referred to meaningless things like “life” or “consciousness”, which everyone knows what it means, but no one knows how to define it. Meaning is known to be “what it is” and a definition is “the terms we use to explain it’. Something can have a meaning without a concrete recognizable definition such as love or hate, and likewise something can have a definition but no valuable meaning. Sometimes the dilemma is when someone gives more value to the definition than the meaning. A definition is only good to the point that it matches the meaning. When the meaning is unclear, it is pointless to rely on definitions. Declaring that a definition is wrong in these circumstances is also worthless, since the definition is not as significant as the meaning. Only after the meaning is understood is it the right time to focus on definitions again.

  21. Ali Mallakin


    Yes, Francis plate tectonics can be very important in abiogenesis. Noting that my previous comment was refering to Sean’s point.

  22. Murthy S Gudipati


    Dear All,

    Sorry for my intermittent responses as I’m at the AAS meeting – sneeking out for a few minutes now and then. One interesting question Sean brought up is that there are several such “cometary like” bombardment going on in other stars. In fact, this could be an excellent exoplanet research – to detect comet impacts on exoplanets (odds are very low though) – but that is not the focus of NAI. Comets crashing into another star may not give us more insight than observing comets now and then falling into our own Sun.

    Irrespective of the terminology we use the key question is: how and when(where) atoms and molecules in comets transformed into molecules (which may be different from the commonly accepted amino acids etc) that was necessary to trigger the biological evolution.

    In another posting I discussed that these two epochs do not necessarily be in the same “space and time” regime, but like comets, the first part could occur somewhere in the presolar or evolved star bodies – the formation of prebiotic molecules and then the second part is when they met another body (like Earth) with other necessary conditions ready – through comet impacts, then life could have evolved.

    Evidence that early life has evolved very soon after the formation of our solar system – perhaps soon after late heavy bombardment – is NOT in contradiction to the above thinking.

    We all are on the same page! Murthy

  23. David Eric Smith


    Francis, hi, In response to your question

    “Could it be possible that a mixing mechanism like Plate tectonics is required to fuel the changes in redox state at a planet’s surface, and perhaps this is the key to abiogenesis on Earth (a bit of a stretch, but perhaps an interesting thought?)”

    I know it may be a bit off the center of this topic of deposited volatiles, but I hope we will be able to pursue it and a suite of related questions in one of the fora. Redox equilibrium is a killer, and in planets that are not driven to some significant mantle disequilibrium, it is hard to understand how mantle carbon and particularly Fe don’t just evolve in quasi-equilibrium phase relations. What it would be nice to drill down on is distinctions between tectonics sensu stricto involving subduction and long-range transport, in which the weak force of gravity and the diffuse energy source of heat have a chance to accumulate disequilibria over long-range convection, and shorter range processes, including any kind of rifting, fractional melt separations leading to continental cratons, and volcanism either within the craton footprint or at margins. How much disequilibrium do we need, and what is the relation to convection-cell scales.

    The one place where this could be related to the long-term fate of volatiles concerns entrainment and trapped fluid as well as hydrates.

    I am not an expert in this field, and grateful to be corrected for things I have misunderstood, but these are questions that have bothered me for some time.

  24. Pauli Erik Laine


    Hi all!

    I have not yet read all your comments, but this sounds much like question of habitability. And we define habitability as limits where current extremophiles can survive today (temperature, radiation, pH, etc.). And also what we know about life: elements we are build of (NHCOPS), some energy source and liquid water.

    I think that the temperature sets the limit. It also defines when there can be liquid water. Of course there has to be elements to build chemical bonds, and an energy gradient to keep things going.

  25. Alan P Boss


    On the question of the survival of water and other volatiles, there is an excellent story posted today on space.com based on a paper given at the AAS meeting by Sean’s former colleague Rory Barnes. You can read the story at this web site:

    http://www.space.com/21437-alien-life-white-dwarfs-failed-stars.html

    Rory makes the point that certain seemingly habitable stellar environments (e.g., white dwarfs, brown dwarfs) might not be so habitable.