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Planetary Interiors a Key to Habitability

Interdisciplinary approaches to new data offer a robust way to see past the conventions of a specialized field, noting connections that provide perspective and deepen understanding. That idea is sound across many disciplines, but it is getting new emphasis with an essay in Science asking whether we have not been too blinkered in our approach to astrobiology. After all, reams have been written about studying exoplanet atmospheres for biomarkers, but shouldn’t we be studying how atmospheres couple to planetary interiors?

“We need a better understanding of how a planet’s composition and interior influence its habitability, starting with Earth,” says Anat Shahar (Carnegie Institution for Science), one of the paper’s four authors. “This can be used to guide the search for exoplanets and star systems where life could thrive, signatures of which could be detected by telescopes.”

Thus the paper’s call for merging data from astronomical observations, mathematical modeling and simulations, and laboratory experiments on planetary interiors. We can assume key building blocks of rocky planets like those similar to Earth, knowing to expect silicon, magnesium, hydrogen, iron, oxygen and carbon. But each planet will have its own specific abundances, its own history shaped by its position in its stellar system and its interior chemistry, all of which will help to determine whether or not it has oceans, their size, and the nature of its atmosphere.

Shahar, along with Carnegie’s Peter Driscoll, Alycia Weinberger, and George Cody, proceed to explain the significance of understanding these factors if we want to make the call on habitability, citing the range of outcomes possible from different compositions:

Composition determines the internal material properties associated with heat and mass transport, like melting temperature, thermal and electrical conductivity, viscosity, and the abundance and partitioning of radiogenic isotopes. These properties control the heat budget and thermal evolution of a planet. The amount of water accreted during formation will affect the ocean volume at the surface, which in turn is influenced by water cycling between the surface and the deep Earth. The composition and subsequent partitioning of elements in the interior will determine the oxidation state of the mantle and therefore whether the species that are outgassed to the atmosphere are enriched or reduced (11). The physical parameters of high-pressure phases of rock that might exist in deep exoplanetary mantles control their water capacity, rate of heat transfer, likelihood of global convection, and rate of core cooling.

This figure from the paper illustrates the significance of plate tectonics:

Image Credit: N. Desai/Science.

The contingent nature of planetary evolution is clear as we study what can happen to a world over billions of years in the evolution from protoplanet through differentiation of the interior, impact history and the emergence of plate tectonics and development of a magnetic field. What the authors are arguing is that coherent research on these matters is not the work of a single discipline. Indeed:

Observations of stellar, disk, and planetesimal compositions must be combined with experimental studies of mineral physics and melting behavior to serve as inputs to planet formation and geodynamic models. In turn, the results of those modeling efforts will provide feedbacks into the observations and experiments by making predictions and identifying the compositions and material properties that are most important for habitability.

So as we learn about exoplanetary atmospheres, and we are on the edge of great strides in this area with the next generation of large ground- and space-based telescopes, we’ll need to put what we learn in the context of planetary interiors and their role in evolving a life-sustaining atmosphere. The idea that habitability is hugely influenced by planetary interiors is sensible, even obvious — think of the Earth without plate tectonics — but our approach to these habitability questions will surely be enriched by crossover studies of the kind the authors describe.

After all, as opposed to straight characterization of an atmosphere, learning about the interior planetary processes needed for life will be difficult. We can make the first call based on our evaluation of planet densities, available through combined transit and radial velocity studies. But density gives us only a crude insight into planetary composition. Our best recourse, then, is the combination of modeling, experimentation, and observations that will help us learn whether planets unlike our own may still have internal processes that can support and sustain life.

The paper is Shahar et al., “What makes a planet habitable?” Science Vol. 364, Issue 6439 (03 May 2019), pp. 434-435 (full text).

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Comments on this entry are closed.

  • Abelard Lindsey May 8, 2019, 13:38

    The other problem with having no plate tectonics is the “global resurfacing events” that occur every few hundred million years that render the planet uninhabitable.

  • Alex Tolley May 8, 2019, 13:39

    What we want to know is not habitability, but inhabited or not. At the moment, all we have for exoplanets is spectrographic data of the atmosphere to look for biomarkers as a proxy for life. At some point we will have direct imaging of the surface that may detect multicellular life, like forests. Much later we will have probes that can do planetary surveys.

    I think there has been plenty of discussion on Nasa’s avoidance of directly searching for life on Mars since the Viking probes. Searching for water, or previous evidence of surface water, is a proxy for habitability, but it avoids looking for signs of past life – e.g. fossil stromatolites, or subsurface microbial life itself. Maybe I am biased, but I am primarily interested in the life forms, not whether a planet could have life.

    Planetary interiors are hard to study, event on Earth. Determining them for exoplanets is going to be very hard. I suspect so hard, that we will detect life on these worlds before we can determine their interiors. More problematic is that we have historically assumed Earth as the gold standard for habitability. Life may be more varied and adaptive than we think. Just suppose we find life on Mars despite its apparent current lack of habitability today based on the criteria shown in the figure. Then what? Even more speculative, what if we find life in the icy moons? That would greatly extend the criteria for habitability.

    Where this habitability will be important is when we discover worlds that should be inhabited. but are not. A prebiotic Earth analog. These may be the perfect candidates for “terraforming” fn our future civilization has a long, even altruistic, outlook.

    • Alex T. May 9, 2019, 2:57

      Alex it seams that you bolded all problematic points in this approach, I 100% agree that life (if exists) will be detected much earlier than a planet internal structure could be minimally explored.
      This hypothesis cannot help in ET life searches, opposite.

      • Bruce D. Mayfield May 9, 2019, 10:34

        Nevertheless it looks like a realistic assessment. Science should progress by investigation of what actually exists rather than by merely seeking to prove what one hopes to be true.

    • Ron S. May 9, 2019, 10:01

      “when we discover worlds that should be inhabited. but are not”

      This is a problem. An inhabited exo-planet can exist for well over a billion years before there are biosignatures detectable at a great distance. That is, life exists under the prevailing primitive conditions until it is there long enough to markedly modify the surface, oceans and atmosphere. For example, assuming life arose on Mars would it have been detectable from afar before going extinct? Possibly (probably?) not.

  • Gary Wilson May 8, 2019, 14:04

    I think we’re all hoping that there is an abundance of habitable planets out there. Our only current knowledge, based on our own planet suggests that many, many factors contribute to habitability. Starting with the nature of the core and surrounding layers and extending to how much water is initially present and whether a stabilizing moon is present. As we extend our reach outward we will gradually accumulate enough information to make some sort of informed guess as to how often life will occur out there. That will bring us to the next question of intelligent life. The quietness we detect in the EM spectrum allows us to come up with a few plausible explanations ranging from rarity of intelligent life to dark forest hypotheses to Great Filters. We’re at the very beginning of beginning to add supportive or contradictory data to these ideas. The future couldn’t be more exciting!

  • Gary Wilson May 8, 2019, 14:14

    Amazingly, Earth was only prebiotic for a few hundred million years Alex. I think if you have a primordial soup it doesn’t take long to make something living with it. The same will be the case out there. I would think it will be rare to find planets in habitable zones at that stage. Nature seeks living complexity and will always find a way to generate it. The great battle against entropy continues.

    • Alex Tolley May 8, 2019, 18:49

      If that is the situation, then I would expect almost all habitable planets to be inhabited. If so, then biomarkers will be present. We will not need to understand the interior. For most worlds, habitable will simply mean inhabited, by microbes at least.

      The one potentially interesting scenario is finding habitable worlds that appear biologically lifeless, yet show evidence of technology, either past and/or present. Past technology on a habitable but lifeless world might indicate that the civilization failed some “Great Filter” event, either natural or self-inflicted. Current technology on such a world may indicate a machine civilization.

      • Robin Datta May 8, 2019, 20:44

        Not to forget the “chela” of Robert Forward’s “Dragon’s Egg”. Even the surface of a neutron star itself might be habitable.

      • Robin Datta May 8, 2019, 21:06

        A related discussion on Cool Worlds:
        Why we might be alone in the Universe

  • Gary Wilson May 8, 2019, 14:15

    Great comment by the way Alex. You are always thought provoking.

    • Alex Tolley May 8, 2019, 18:49

      Thank you. I appreciate the compliment.

  • Thomas Goodey May 8, 2019, 14:37

    The brute fact is that we don’t really know what is deep inside our own Earth, let alone other solar system planets, let alone exoplanets. All our knowledge is indirect and inferential – hypothetical. That’s why the upcoming mission to asteroid 16 Psyche – the “mission to the all-metal world” – is so important. It’ll get there in 2026, we hope, and we think that’ll be the first time we will actually see what’s in the middle of a planet. Psyche seems to be the core of a proto-planet that was stripped by a huge collision; at least, that is the current theory.

  • Gary Wilson May 8, 2019, 16:09

    That’s an absolutely brilliant mission Thomas. What a chance to explore such an intriguing body. I wonder if it actually ever was a planetary core or was busily accreting matter in a pre-planetary disc and got forced out of its early orbit before achieving adequate size? Lots of very interesting questions to answer.

  • wdk May 8, 2019, 19:13

    The illustration above accompanying the Science article shows two distinct hemispheres: one in which there is core to near surface convection and the other where it is stymied. The illustration needs to be a bifurcation to make its point, but I question whether the alternatives are so starkly outlined.

    In some earlier discussions about potential exoplanet magnetic fields a month or two ago, I submitted that the internal temperatures for ferrous rich interiors should be below iron’s Curie temperature, assuming that that was where the action lies or begins on the terrestrial globe.

    In the previous entry article to the current one, it is suggested that the sun and its surrounding disk coalesced near “merging neutron stars” which released a large number of high atomic number radio isotopes, the upshot being that the planetary interiors would have a higher internal heat content ( or source) than objects formed near the more widely assumed supernova event. And more frequent. Well, there would go another argument for a unique Earth. Or nearly so.

    But if the radio isotopes were absent, then the heat transfer requirement would be reduced and the Curie temperature boundary could be obtained by other means, probably later in history. Fracturing would
    be a more important geological element vs. flow. And then the diameter of a world with a magnetic field might be different due to these factors.

    And we still have to match the planet interior with the star, based on radiation distribution and gravitational grip.

    The modeling of exoplanets within systems and over time should continue to be interesting and informative.

    Well, I would submit that it depends on what variables it takes to get an interior temperature that is below the Curie temperature when you need it.

  • Geoffrey Hillend May 8, 2019, 19:34

    I think a planet has to have a magnetic field to be habitable. Consequently, the type of planetary interior is crucial to habitability.

    • Alex T. May 9, 2019, 3:04

      This hypothesis cannot be proven, never.

      • Bruce D. Mayfield May 9, 2019, 10:41

        Perhaps not, but that still doesn’t disprove Geoffrey’s statements, which I think are reasonable assumptions unless and until life is found on a world lacking a protective mag field.

        • Alex Tolley May 9, 2019, 12:02

          We need to be a bit careful with our terms. Habitable means in the HZ with liquid water on the surface, like Earth. A living world may not have those conditions.

          A magnetic field is simply a proxy for some other condition that makes the planet habitable – retaining an atmosphere, or ensuring a molten core is present to drive plate tectonics via mantle stirring and heat flow, etc.

          Life, however, may be more adaptable than our planet’s example suggests. It may be present in planets both inside and outside the HZ.
          IMO, until we find out, it is premature to to make this claim.

          As a practical matter, I would like to know how we can even test this hypothesis. Suppose we do detect life on a number of exoplanets through some unambiguous biomarkers [Those worlds that are “Earth-like” only]. How are we going to also determine the presence or absence of a magnetic field on those worlds to test the hypothesis? Is there some way to do that?

          • Bruce D. Mayfield May 9, 2019, 17:13

            On your last paragraph Alex Tolley we have already been able to detect aurora on exoplanets. See Scientific American article from Jan. 22, 2013: “Alien Auroras May Light Up Exoplanet Night Skies – Scientists have found evidence of polar lights on exoplanets and hope the discovery will add insight as to the strength of a planet’s magnetic field”

            Any planet that shows any evidence of possible life is going to garner as much attention as we can possibly give it, so if has auroral activity we’re quite likely to find it.

            • Alex Tolley May 10, 2019, 13:57

              Good point about aurora being sufficiently visible to show the presence of a magnetic field. I retract my comment about magnetic field detection. So now we do theoretically have a means to test whether biomarkers are present only within the superset of worlds with magnetic fields. While that will not include life that we cannot detect, it would be an interesting statistic.

  • Michael C. Fidler May 9, 2019, 7:18

    The example that is being used is the heaven and hell scenario, funny we are still stuck at that level. Heaven the Earth and the Hell Venus, we do have two other type of examples in our own solar system, Io and Titan. Between the two extremes being used is a lot of gray that we have no idea what actual exist and it could be full of psychedelic color instead of grey. As I have mentioned before the variety in planetary geology and chemical mixing taking place in planets may be way beyond anything we have seen in astrophysics, because of so many factors effecting the evolution of said planet.

    • Michael C. Fidler May 10, 2019, 23:31

      From Twitter, Lisa Kaltenegger: Interesting read. How life on Earth affected its inner workings.

      “It is well known that life on Earth and the geology of the planet are intertwined, but a new study provides fresh evidence for just how deep—literally—that connection goes. Geoscientists at Caltech and UC Berkeley have identified a chemical signature in igneous rocks recording the onset of oxygenation of Earth’s deep oceans—a signal that managed to survive the furnace of the mantle. This oxygenation is of great interest, as it ushered in the modern era of high atmospheric and oceanic oxygen levels, and is believed to have allowed the diversification of life in the sea.

      Their findings, which were published in Proceedings of the National Academy of Science on April 11, support a leading theory about the geochemistry of island arc magmas and offer a rare example of biological processes on the planet’s surface affecting the inner Earth.”

      https://phys.org/news/2019-05-life-earth-affected.html

  • Harry R Ray May 9, 2019, 9:59

    The most common form of water in the universe is superionic ice, mostly locked up in the interiors of ice giant planets. Well, guess what! According to Quantum Magazine, suoerionic ice has JUST BEEN CREATED IN THE LAB(www.quantummagazine,com), and given the designation Ice VIII. As you remember, Ice VII was created in the lab two years ago, and found in nature just last year! Next up would be you know what! GETTING CLOSERRRRRRRRRR!!!!!!!!!!!!!!!!!!

  • Harry R Ray May 9, 2019, 10:05

    OOPS! I meant QuantaMagazine(www.quantamagazine.org). Check it out before welllllllllll…………..lol(I hope).

    • ljk May 9, 2019, 11:21
      • Harry R Ray May 16, 2019, 11:39

        The interiors of TRAPPIST-1d and f may be magma rich. “Ground based follow-up observations of TRAPPIST-1 in the infrared. ” by A.Y. Burdanov, S. M. Lederer, M. Gillon, L. Pelrez, E. Ducrot, J. De Witt, E. Jenin, A. H. M. J. Triaud. C. L. Lidmani, L. Spitler, B-O Demory, D. Queloz, V. Van Grotel. The variability(if proven) in the transit depths of the above mentioned planets could be due to extreme volcanism, dust storms, or a thin opaque(in the ir)atmosphere that expands and contracts due to some as yet unknown mechanism.

  • ljk May 9, 2019, 10:11

    Unfathomably deep oceans on alien water worlds?

    Posted by Paul Scott Anderson in Space | May 9, 2019

    Distant water exoplanets might have oceans thousands of miles deep. That’s in contrast to Earth’s ocean, which is about 6.8 miles (about 11 km) deep at its deepest point.

    https://earthsky.org/space/exoplanet-water-worlds-deep-oceans-2019-study

    • Alex Tolley May 9, 2019, 14:06

      I haven’t read the paper, just the linked article. A mini-Neptune with radius 2-4x Earths (13-26km) and 25% water by mass, potentially thousands of km deep is interesting. The high pressures creating a dense ice at depth suggest that the ocean would be isolated from the rocky core, leaving it very short of heavier, trace elements, terrestrial life requires, like magnesium in chlorophyll. While life might have difficulty starting there, it may possibly be a refuge for migrating life. Such deep oceans would be more than enough protection for local cosmic events to sterilize a planet.

      For planetary engineers, a veritable mine to transport water to dry planets or space habitats. Also a fantastically large supply of deuterium and tritium for energy.

      If such worlds are as prevalent as the authors suggest, they should be easy to detect. Maybe those mini-Neptunes that seem almost unclassifiable based on size and density are existing examples?

      • ljk May 10, 2019, 9:31

        Ocean worlds may indeed be some of the best places in the Universe to develop and evolve life, but intelligent life that builds and uses technology is another matter.

        Cetaceans have been around far longer than humans, yet they have not built starships (that we know of).

        Of course it is time to bring up my favorite Douglas Adams quote again:

        “For instance, on the planet Earth, man had always assumed that he was more intelligent than dolphins because he had achieved so much—the wheel, New York, wars and so on—whilst all the dolphins had ever done was muck about in the water having a good time. But conversely, the dolphins had always believed that they were far more intelligent than man—for precisely the same reasons.”

        ― Douglas Adams, The Hitchhiker’s Guide to the Galaxy

        https://www.goodreads.com/quotes/811-for-instance-on-the-planet-earth-man-had-always-assumed

      • Harry R Ray May 12, 2019, 13:41

        This definitely puts planets like Kepler 22b back in the “primitive life” conversation, Not because of the mega-oceans, but because of “thin”(i.e. 1 to 100 bar)atmospheres whereupon future astronauts floating on the planets’ surfaces in some kind of naval craft can still see the planet’s sun(i.e. NOT pitch black sky as seen from the surface). Even more intreguing is the possibility of primative life migrating from the mega-ocean onto ice sheets. Should said ice sheets over deep time receive enough life sustaining minerals to make life sustainable via meteoric bombardment, could such primitive life EVOLVE into more COMPLEX forms, and if so, how complex?

        • Alex Tolley May 12, 2019, 23:45

          I am not aware of life evolving on icy surfaces. Certainly we see no Earth life that can solely live on Antarctica (penguins feed in the surrounding ocean) or permanent glaciers on mountain tops. However, that isn’t necessary for large animals and plants to evolve. One could have large floating mats of macro algae that might provide some large surface environment, a super-large equivalent of the floating mats of kelp that exist on Earth. Fish have evolved solely in an aqueous environment. But whether all this is possible on a water world IDK. We only have Earth as a model, with relatively shallow seas, and interaction of land areas to maintain a carbon cycle and micro-nutrient cycling. Life in a water world might take a very different course, assuming abiogenesis is possible there at all.

          • Alex Tolley May 12, 2019, 23:48

            Hopefully biomarkers may indicate whether life exists on such worlds. The age of interstellar exploration would be an exciting time if it starts with the knowledge that life exists on a range of different planet types, not just Earth analogs.

  • ljk May 9, 2019, 10:12

    Was moon created from proto-Earth magma?

    Posted by Eleanor Imster in Space | May 9, 2019

    A new study suggests that our moon formed from a splash of magma when a large object crashed into a proto-Earth covered in a fiery ocean.

    https://earthsky.org/space/moon-created-impact-proto-earth-magma

  • Geoffrey Hillend May 9, 2019, 17:39

    I like your optimism, Alex Tolley about the hardiness of life, but I limit myself to the idea that life had to have stable, safe and secure foothold first and then it adapted to the extreme conditions on Earth like the thermophiles, etc. I’ll be surprised if we find bio sign gases like CH4, O3, O2, N2 on Earth size exoplanets in the life belt. I don’t think they will stay there for a geological time scale.

    We might get lucky though. If we do, I will have to revise my ultra conservatism and limitation of life to an Earth twin with a Moon which is necessary for a magnetic field which I think is contingent fast rotation and angular momentum from a collision with another planet like Theia.

    • ljk May 10, 2019, 9:37

      We know of creatures on Earth that live in pools of boiling sulfuric acid. There is a microorganism that was found swimming in the reactor pool of a nuclear power plant that could take over four thousand times the amount of radiation that would turn most other terran life into crispy bacon in seconds. And do not forget those tube worms and giant clams and crabs that thrive around boiling hot geothermal vents in otherwise total darkness and bitter cold of the deep ocean.

      There are creatures that live at the bottom of deepest oceans seven miles down and miles under our planet’s crust. We also discovered that certain bacteria and other similar types can survive being directly exposed to space.

      They may not be the kind of intelligence we are looking for, but they are real examples that life can survive in conditions once thought impossible just a few decades ago. This holds out a lot of hope for life elsewhere. We already know the Universe is full of complex organic molecules, the building blocks of life.

      There may also be forms of life we have only guessed at or have no idea about, as we are only really beginning on this scientific journey of discovery.

  • Alex T. May 10, 2019, 3:13

    By the way the hypothesis that strong magnetic field of a planet can be created ONLY by hot, liquid, rotating planet core is also under big question and have to proved before we can apply such pattern to alien worlds.
    I’m sure everyone have/own lot of examples of rigid bodies that has very strong magnetic fields, why this variant cannot be applied to “frozen” planet core – is big surprise to me.
    If hot ferromagnetic material cooled in strong magnetic field – it will form constant magnet. How we can apply this scenario to the whole planet is big question, but how someone can suppose that it is impossible in our huge Universe?
    So we are discussing here hypothesis, that is based on the other hypothesis, it is even cannot be called theory right now.
    And finally to add some arguments to my point of view :
    We have perfect example in our Solar system – the Venus, that seams to be volcanic active , as sequence – it seams to have a liquid core inside and in same time it has very week magnetic field.

  • Geoffrey Hillend May 10, 2019, 18:10

    Magnets are solid, but molten iron is different. A magnet has most of it’s atoms magnetic moments all lined up north and south which causes the magnetic field, but non magnetic metals don’t have that and molten iron won’t emit magnetic field unless it moves in circles; Charged particles move in circles create a magnetic field which happens by the fast rotation of our Earth which creates the magnetic field.

    Venus’s magnetic field is too weak to block the solar wind due to a slow rotation. so Venus has lost a lot of water through atmospheric stripping shown by a much higher deuterium ratio than Earth, The heavy hydrogen is left behind and the lighter hydrogen is stripped away. DH20 ratio versus H20 ratio. These are split apart by EUV into hydrogen and oxygen. The light ordinary hydrogen it knocked off into space by the solar wind.

    • Alex T. May 11, 2019, 2:58

      Geoffrey, thanks for the nice explanation, but I suppose you understand wrongly my previous comment. I will try to explain again shortly:
      1. Pop-science Stereotype : to have strong magnetic field a planet have to have liquid , rotating, conducting core.
      My point – none can deny/exclude possibility that some planets can have solid state highly magnetized core inside.
      2. Thesis from discussed article: volcanic activity is important sign of magnetic field presence. My point was – a Volcanic activity does not mean automatically strong magnetic field.
      3. Pop-science Stereotype : a planet without strong magnetic field loose atmosphere. My point is – Weak magnetic field does not mean automatically that planet loose it’s atmosphere (compare Venus and Mars. ).
      Those three point we can met frequently in popular science articles as something that is 100% proven by facts, it is misleading.
      In every of this point lot of additional details and conditions has the same degree of importance. And most important that our present scientific data is very limited we need to collect much more data to build valuable theory.

  • Michael C. Fidler May 11, 2019, 18:49

    In search of the second Earth!!!
    At present, the Earth is a completely unique object among many thousands of famous planets. But somewhere there should be an exact copy of our planet.
    It would seem that after the recent completion of the mission of the Kepler space telescope, one should not expect any high-profile discoveries from this project. Almost 6 years have passed since the completion of the main 4-year mission of this telescope. A year ago, the latest catalog of planetary candidates for the main mission (DR25) was published. However, a fresh observational applicationHubble Space Telescope shows that the analysis of the data of the primary mission continues. The application under the number 15685 reports that recently a planetary candidate was found in the data of the main mission of the Kepler telescope, which is the most similar to Earth from all known today.
    The text of the application states the discovery of a planetary candidate with a radius of 1.1 +/- 0.1 of the Earth’s radius in a sun-like star with a radius of about 0.97 of the Sun’s radius. The period of treatment of the candidate is estimated at 365.4 days. For comparison, the Earth has a period of revolution around the Sun is 365.26 Earth days.
    PDF file of the application allows you to specify system parameters. The file states that the planetary candidate was found in the 2MASS-J19432996 + 5059289 system with coordinates 19 43 30 and 50 59 29. Simbad reports that in the Kepler target catalog this system is called KIC 12266812 space telescope Kepler DR25). KIC 12266812 system is 300 parsecs from Earth.

    Radius of 1.1 +/- 0.1 of the Earth’s radius.
    Sun-like star with a radius of about 0.97 of the Sun’s radius.
    Period of the candidate is estimated at 365.4 days.
    KIC 12266812 system 300 parsecs (1000 Light Years) from Earth!

    This seemed to bizarre to believe, but defiantly is real:

    http://simbad.u-strasbg.fr/simbad/sim-id?Ident=2MASS%20J19432996%2B5059289

    http://cdsportal.u-strasbg.fr/?target=%20KIC%2012266812

    http://www.stsci.edu/hst/phase2-public/15685.pro

    I’ve always had a fascination with the total solar eclipse that take place on earth and just how rare this is, a very unusual coincidence. So a little Sci-Fi contemplation: Earth is in a galactic zoo and quarantined, the great creators develop our planet for intelligent life, so they create a second earth analog at specific scientific fundamental formulas. Sounds crazy but civilizations that have been in existence for 10 billion years may find this more meaningful then populating or conquering the galaxy! This would be a better solution and organic then the Sagan’s patterns within Pi in Contact.

    • Nicky May 12, 2019, 3:48

      Wow! This is unbelievable!
      Shallue is also one of the authors. That means this planet is probably discovered by deep learning.

    • Nicky May 12, 2019, 5:19

      The properties of this candidate planet are better than I’ve ever dreamed for. Robovetter, the official Kepler transit vetter, does not have the accuracy to capture and validate habitable-zone Earth-size planet around solar-like stars. Deep learning, thus, provides a good method to discover these planets.
      Looking at this star, KIC 12266812, I found it to be likely evolved and old based on the recently updated DR25 Kepler stellar properties derived from photometry. The stellar radius (1.71 Rs) very likely puts the planet outside of habitable zone.
      See: Revised Stellar Properties of Kepler Targets for the Q1-17 (DR25) Transit Detection Run

      But then the hope came back when I looked it again with revised stellar properties using Gaia DR2. The stellar radius of KIC 12266812 is revised down to 0.978 +0.072/-0.065 Rs, just as what it is reported in HST proposal. Based on the Gaia DR2 stellar radius, effective temperature 5908 +/-207 K, and Stefan–Boltzmann law, the stellar luminosity is calculated which returns 1.044067 Ls.
      See: Revised Radii of Kepler Stars and Planets Using Gaia Data Release 2

      The stellar mass 0.981 +0.155/-0.127 Ms given by photometry and orbital period 365.45562 days given in the proposal place the candidate planet at 0.99399 AU. The insolation of the planet can be calculated as: 1.044067 / 0.99399^2, which puts the candidate planet right at where Earth is in terms of radiation. This is shocking, its ESI is 0.96.
      Being the only detectable planet in the system and having low stellar metallicity (Sun’s 40%), the planet is highly likely to have formed in situ and formed rocky like Earth.

      HST has observed this star on May 3. The result will probably be published next year or later, can’t wait to see its validity and impact on exo-Earth occurrence calculations.

      • Michael C. Fidler May 13, 2019, 10:37

        This is a 10.5 magnitude star that should be easy to monitor for other planet transits and any optical or radio SETI. Looking back at the solar system, would Earth and Venus even show good transits and the outer planets from Jupiter would be long term transits if they even did transit. TESS should be able to observe it when they start the northern sky imaging. The optical and radio SETI could be done now at amateur level equipment. Now if we could just find that large moon…

        • Nicky May 14, 2019, 2:28

          Yes follow-up observations will be much easier compared to Kepler-452, Kepler-62 and Kepler-442.

          exoplanetarchive.ipac.caltech.edu/cgi-bin/TblSearch/nph-tblSearchInit?app=ExoTbls&config=keplerstellar
          At this site, you can look up the light curves of KIC 12266812 by simply searching the ID. The transits of the potential Earth analog took place at 486.0136557, 851.3559907 and 1216.7902527 (BJD-2,454,833). The putative fourth transit (1582.2458727) is also supposed to be observed by Kepler but it does not have the flux data during this time for some reasons otherwise it would have helped a lot. I carefully examined all three transits detected in the light curves and they all look decent. This is quite a promising candidate I have to say. I wish I can upload pictures here.

          • Michael C. Fidler May 14, 2019, 9:25

            You can upload pictures, in windows just right click on image and click copy image address. Then in the comments in the comments, right click and click paste: https://www.nasa.gov/sites/default/files/thumbnails/image/lubin_graphic.jpg

            • Michael C. Fidler May 14, 2019, 9:40

              Well I’m the fool, there not pictures but data points. I need a course in how to use it first!

            • Nicky May 14, 2019, 16:32

              It’s not that hard to work with light curve data even if you’re amateur. The data points are the brightness of the star measured by Kepler every 30 minutes.

              First you can go in https://exoplanetarchive.ipac.caltech.edu/cgi-bin/TblSearch/nph-tblSearchInit?app=ExoTbls&config=keplerstellar
              Under “Time Series Lookup” you can search “12266812” and “plot” the light curve data. The graph you see is a scatter plot of raw brightness measurements (y-axis) over a period of 4 years (x-axis) during Kepler Mission.
              The x-axis represents the time in the unit of day when the data is collected starting on at 12:00 on Jan 1, 2009 UTC. For example, 1000 BDJ would be Sep 28, 2011.
              After plotting, you can go to “normalization” at the top and click “normalize” to mitigate the data noise and make the light curve easier to process. After normalization, you can go back to “plotting”. You will find “Y Axis Column” and you can change it by selecting “ndiv_PDCSAP_FLUX”. Click on “redraw”, all the brightness measurements are normalized to 1.0 which allows you to see the fraction of increase and decrease in brightness. Next you go to “Axes” which is right below “plotting” to adjust scaling to see a specific time period.
              Knowing that Hubble observed this star on May 3, 2019 and the orbital period 365.45562 days given in by Vanderburg, May 2019 – 365.45562 – 365.45562 – 365.45562 – 365.45562 – 365.45562 – 365.45562 – 365.45562 = May 2012. May 2012 is about 1220 days after Jan 1, 2009, so if you adjust the x-axis and y-axis manually (change “auto” to “manual”) to Min:1210 and Max:1230 and Min:0.9997 and Max:1.0003, you will an apparent drop in brightness right at 1216.8
              If you wanna go back 365.45562 days to the putative previous transit, just do 1216.8 – 365.45562 = 851.3 and adjust the x-axis to Min:847 and Max:857 and you will another drop in brightness right at 851.3. If you want examine the earliest one, 851.3 – 365.45562 = 485.8, adjust the x-axis to Min:481 and Max:491, you will see a less apparent but still obvious transit at 485.9
              Overall, all three transits are evenly spaced and they all look good.
              If you’re not use to find transit in light curves, you can try to look other planet candidates’ transit data points:
              Page 8 is Kepler-452b transits data https://exoplanetarchive.ipac.caltech.edu/data/KeplerData/008/008311/008311864/tcert/kplr008311864_q1_q17_dr25_obs_tcert.pdf

              Page 8 is Kepler-442b transit data
              https://exoplanetarchive.ipac.caltech.edu/data/KeplerData/004/004138/004138008/tcert/kplr004138008_q1_q17_dr25_obs_tcert.pdf

              Page 68 is Kepler-62f transit data:
              https://exoplanetarchive.ipac.caltech.edu/data/KeplerData/009/009002/009002278/tcert/kplr009002278_q1_q17_dr25_obs_tcert.pdf

        • Michael C. Fidler May 14, 2019, 9:12

          There is only one problem: Europe was still in the dark ages a 1000 years ago, with the Vikings running a muck. The Muslims were the center of knowledge and learning. So if any similarity to our twin earth would still be 900 years before any radio noise reaches us! :-(

    • Alex Tolley May 12, 2019, 9:37

      So the Magratheans have been busy. I do hope they added those nice crinkly bits in the sub-polar regions… ;)

      • Michael C. Fidler May 13, 2019, 10:46

        Doppelgänger a 1969 British science-fiction film later called Journey to the Far Side of the Sun but with a thousand light years journey!!!

  • Geoffrey Hillend May 11, 2019, 20:00

    Alex Toley, I don’t know what you mean by “solid state magnetized core.” It might be physically impossible for a planet the size of our Moon or larger to not have a liquid core.

    Earth’s inner core is solid and outer core is liquid. The liquid part is believed to cause the magnetism through the rotation moving it in circles. I don’t know about the inner core which is made of 10 percent nickel and 85 percent Iron but that part might be magnetized by the outer core? Venus probably has a liquid core and a solid one, but a very small magnetic field. I believe if we could increase the rotation speed of Venus and give it a fast rotation, Venus magnetic field would become stronger.

    A planet without the protection of a magnetic field has it’s upper atmosphere bombarded by more cosmic rays than one that has a magnetic field.

    Venus has lost considerable atmosphere through solar wind stripping maybe twice to three times the amount of what it is today. I just has a lot of atmosphere so it’s losses are not noticed over a short time but add up over a long time. A Scientific Study of Venus, Taylor. i don’t have the page number handy. Mars looses a quarter a pound of atmosphere every second due to solar wind stripping so over time it lost a lot. It’s atmosphere was much thicker in the past.

    • Alex T. May 12, 2019, 2:30

      Geoffrey, Alex T. Is not Alex Tolley :-)
      I am totally different person.
      Solid State planet core I mean core that cool enough , i.e. has temperature lower than Curie temperature (point) for specific pressure. I.e. core can form permanent magnet.
      Your declaration about strong “magnetic field”-atmosphere dependency does not hold even simple test by scientific facts – i.e. Venus than has surface atmospheric pressure almost two orders higher than Earth has.
      I know , pop-science sources somehow systematically ignoring this fact, they recognizing only Mars.

  • Geoffrey Hillend May 11, 2019, 21:01

    According to the giant impact hypothesis, when Theia collided with Earth, it’s iron core went into our Earth to give it larger Iron core which might help give it a stronger magnetic field. Venus might have a smaller iron core since it has no moon to give it any rotation speed or angular momentum.

    I will admit these are not absolutes since there still is the unknown factor, but these are supported by scientific principles. Also the mantle and crust of Earth and the Moon have the same composition anothosite or plageioclase feldspar which supports the GI hypothesis.

  • Bruce D. Mayfield May 12, 2019, 14:43

    Alex T. and Geoffrey Hillend, interesting discussion. You both make good points. To these I add questions re the Earth’s puzzling habit of flipping the orientation of its magnetic field at random intervals. See https://en.m.wikipedia.org/wiki/Geomagnetic_reversal

    Alex T., it might seem reasonable to assume that the solid inner core would also take on the magnetic field orientation from the dynamo above it in the molten outer core, but does it? Maybe it does to some extent, which might then act as a suppressor of field reversals, since the time for magnetic realignment would be much longer in the solid core as opposed to the fluid part. But my point is that whatever the magnetic properties of the inner solid core may be, they are too weak to prevent magnetic field reversals from happening. This suggests that completely frozen core planets will end up with very weak residual magnetic fields.

    • Alex T. May 13, 2019, 3:26

      Bruce, in connection to planetary magnetism – I do not have “ready to use” answers, but have lot of questions :-) Those questions mostly related to pop-science approach and mass-stereotypes than to science.
      If we will read respectable science sources related to the Earth magnetic field – you will met one very important word – PROBABLY.
      I.e. the Earth’s solid core probably paramagnetic due to high temperature, but aways you will met reference that it is current main stream hypothesis only, bur not proven fact.
      There is not scientific data what is the Curie temperature for the specific high temperature and high pressure inside the solid core. We know only that there is dependency between Curie temperature and pressure, it depends on alloy composition too. Many ferromagnetic materials shows increasing of Curie temperature with pressure increasing, but there is some alloys that show opposite dependency , but all this measurements was done for the maximum pressure order lower than supposed pressure inside the Earth’s solid core.
      By the way permanent magnet inside the Earth solid core can very well explain magnetic poles drift – like spinner top precession :-)
      But in this specific topic I mostly write about possibility that probably somewhere in the space there is planet that have strong magnetic field mostly produced by solid permanent magnet inside a planet core, you will not find scientific proves that it is impossible case.
      If we look carefully at our Solar System, we can find examples that fall apart of pop-science stereotypes.
      There the Venus, that has weak magnetic field, but very dense atmosphere. There is gas Giants that seams to have frozen cores, but relatively strong magnetic field.
      If we look at our Universe – there is neutron stars, that own super strong magnetic field , present time main stream explanation of this fact is next:
      neutron star composition is next – 90% neutrons, 10% all other type of matter including different chemical elements, atoms and molecules, but mostly plasma composed by charged particles , so according main stream – only fast rotating plasma is producing this magnetic field.
      Somehow none take in account neutrons, that electrically neutral, but own magnetic spin… And as sequence it is interesting question – what are magnetic property of neutron matter? How it changing under high pressure ? May be it has unusual magnetic property, for example we can imagine super-ferromagnet?
      Probably this is the main “secret” of pulsars and magnetars?

      • Bruce D. Mayfield May 13, 2019, 12:04

        Interesting comment Alex T. It’s (probably ;) true that assuming things out there must be the same as they are here will be too limiting. You might indeed be right that some frozen core planets can have strong magnetic fields, but that assumption also is one that can only be proven by finding such a world, which might never happen. But the only thing we KNOW FOR SURE is that life can exist on a planet much like Earth. (Thus the excitement even in this thread about finally finding a possible true Earth analog!)

        Your points about magnetic fields in giant planets are valid. As you also point out, the intense magnetic fields of (mostly neutral!) neutron stars is a real puzzle. Obviously, we have an enormous amount still to learn.

  • Geoffrey Hillend May 12, 2019, 21:28

    In the past billion years, Venus might have had an atmosphere of 300 bars instead of 90 bars if I recall what a read correctly based on a much higher DH20 (heavy water) in it’s atmosphere than Earth ratio and also oxygen loss due to atmospheric stripping. The heavy hydrogen in the DH20 remains behind and the lighter hydrogen escapes

  • Geoffrey Hillend May 12, 2019, 22:29

    https://blogs.ucl.ac.uk/science/2014/07/09/venus-is-losing-its-atmosphere/

    https://cosmosmagazine.com/space/electric-wind-stripped-venus-of-oxygen

    Maybe in the past Venus had twice it’s atmosphere of today, or 92 bars, but it has lost more atmosphere in the past than one bar or one Earth atmosphere if we include the past half billion years.

    • Bruce D. Mayfield May 13, 2019, 12:49

      Yes, since we can measure even CO2 being stripped away from Venus today its atmosphere must have been even thicker in the past. In any event, the most important molecule for life is water, and if a planet cannot hold onto its water, its game over for life as we know it. The fact that Venus still has a thick atmosphere falls way short as evidence that a magnetic field isn’t needed for a planet to be habitable.

    • Alex T. May 13, 2019, 16:17

      Geoffrey, In any case Venus has thick atmosphere for ~4.5 billions years and most probably will have it till the time when the Sun will became Red Giant…
      Your second link (about oxygen striping) is not about magnetic field, it is about electrostatic field, that is probably stronger on Venus…
      It seams the main problems of Venus not the weak magnetic field, but close proximity to the Sun. The hypothesis that Venus in the past had Oceans of water is hypothesis only, I believe it is absolutely false and Venus from the beginning was too hot for liquid water .

  • Michael Fidler May 14, 2019, 14:37

    Could this be what happened to Venus 700 million years ago and resulted on the earth’s Cambrian explosion? Venus deep oceans destroyed by giant comet impact, sending advanced animal lifeforms to earth.

    How Earth Life Could Come Back from a Sterilizing Asteroid Impact.
    Ejected rocks could serve as microbial lifeboats.

    https://www.space.com/life-on-earth-come-back-from-asteroid-impact.html?

  • Geoffrey Hillend May 14, 2019, 16:20

    Alex T. First, Venus had an Earth like atmosphere 3.8 billion years ago. The reason it that that the Sun increases brightness by 10 percent every billion years and the Sun was more than 30 percent dimmer at that time. Venus is thought to have oceans at that time. The runaway greenhouse effect occurred in the past 1.5 billion years when the Sun got brighter over time and the oceans evaporated. Ref. A Scientific Study of Venus, Taylor. I don’t have the page number handy since it was a library book I read three years ago. I assume a core same of some future mission of Venus could prove this if we found the evidence of water in the rocks since limestone is the result of the Urey reaction with rainfall and it’s combination with the land and water through river run off into the sea to make limestone build up at the bottom of the Sea. The seas were evaporated due to the runaway greenhouse effect and the solar wind stripping removed much water and oxygen.

    Second, the electric field of Venus is created by the Solar wind. The same thing occurs with Mars’s atmosphere because the solar wind is composed of fast electrons and protons which are ionized particles and have an electromagnetic field just like plasma. The electromagnetic field of the solar wind accelerates molecules an atoms like O2, and H out of the atmosphere. The reach escape velocity.

    • Alex T. May 15, 2019, 2:56

      Geoffrey, you again repeat very well know to anyone pop-science hypothesizes, but if you will read carefully some more respectable sources you will met one small, but very important addition – the word PROBABLY… even in wikipedia is used this word :-)
      I am sure our knowledge about Venus very limited much orders more limited than our knowledge about the Earth past, but our knowledge about our own planet very far to be complete, everything about the Earth history is covered by multiple hypotheses and PROBABLE, including the Earth’s magnetic field studying.
      I am sure there was no any life on the Venus, as well as liquid water, if there (Venus) was in the past some oceans it was oceans of liquid lead, tin, silicon, sulfur or tellurium etc…
      In connection to dimmer in past Sun, there is opposite hypothesis:
      https://www.space.com/14565-earth-climate-young-sun-paradox.html
      If shortly : how it is possible that in the period (~4billions years ago) when the Sun supposed to be dimmer all three planets Venus, Earth, Mars had liquid water? Fun, this shows brightly that something wrong with our present understanding of the Solar system history.

  • Geoffrey Hillend May 14, 2019, 16:26

    The electric field in Venus atmosphere has been built up by it’s interaction with the solar wind to be specific.

    • Alex T. May 15, 2019, 2:21

      To be specific, if we will directly relate to content of the article on the link that you posted , in this article is supposed that electric field of the Venus and the Earth is build up by interaction between UV radiation and atmosphere, magnetic field in this case has no relation to the discussed event.

  • Geoffrey Hillend May 15, 2019, 16:14

    I first read that the solar wind has a magnetic field built into it in my planetology college course book called The Exploration of the Solar System by William J, Kaufmann. He wrote that the solar wind has a an electromagnetic field built into it since it is composed of charged particles or ions. Charged particles when accelerated emit eletromagnetic radiation and also a lot of them create and electromagnetic field. When this magnetic field passes by a planetary atmosphere it will accelerate electrons in that atmosphere if it is perpendicular to the magnetic field. The acceleration of electrons in the atmosphere causes the electric field composed of electrons. Also some of the atoms high in the atmosphere get knocked out of the atmosphere by collisions with the solar wind particles so they escape. Some of the atmospheric particles get ionized by high speed collisions with the solar wind particles. The electrons get knocked out of oxygen or hydrogen etc and once this happens they become ionized so they can be attracted to the magnetic field of the solar wind and pulled out of the atmosphere. NASA explains it better than me: https://www.nasa.gov/press-release/nasas-maven-reveals-most-of-mars-atmosphere-was-lost-to-space

    The Sun gets hotter over time because it uses some of it’s hydrogen which is fused into helium. The result is there is less hydrogen so there is less radiation pressure caused by fusion of burning of hydrogen to balance the forces of gravity which contracts the gases of the star, so the star has to burn hotter to make up for the loss. This happens in all stars and is not guess work but based on the mathematics in nuclear physics, so stars always burn hotter and brighter after their birth at a rate of time based on their mass.

  • Geoffrey Hillend May 15, 2019, 16:26

    Excuse me, it is mostly the high energy photos like x rays and ultra violet radiation which ionize the upper atmospheric particles in a planetary atmosphere. Also the life belt where liquid water can exist does move further out as a star gets hotter so Mars was outside the life belt possible early one but it might not mean it was completely frozen. It warmed up at some point before the atmosphere was mostly lost by the solar wind and low escape velocity.

    • Alex T. May 16, 2019, 4:22

      Geoffrey, modern science studied Mars much more better than Venus, and those researches show that :
      – there is still water on the Mars, even liquid
      – geologic relief on the Mars shows bold signs that there was on surface lot of liquid water in the past (oceans, seas and rivers)… In same time …
      Due to harsh conditions on the Venus we have significantly less scientific data about this planet and most probably there where oceans of liquid … Sulfur, Plumbum, Stannum and tellurium, but not water…

  • Geoffrey Hillend May 16, 2019, 17:07

    The radar maps of Venus by the Magellan spacecraft don’t show any oceans of any kind including Sulfur, Plumbum, Stannum and tellurium. Sulfur, Plumbum, Stannum and tellurium.

    • Alex T. May 17, 2019, 3:10

      I can only confirm you notes, yes – No oceans was found in present time on the Venus, no sulfur, no water – it is scientific fact, what was there billion years ago big secret, that can be somehow studied by direct geologic studying of the Venus.
      In connection to Sulfur, lead, Tellurium oceans :-), it was was my exaggeration, but it is not disconnected from scientific data. If you want know more about the issue, you can google next topics:
      “tellurium on the Venus” or “Venus snow”.

  • Geoffrey Hillend May 17, 2019, 16:14

    The melting point of sulfur is 239 degrees F. The boiling point of Sulfur is 832 degrees F. The surface temperature of Venus is 872 degrees F. Sulfur probably will evaporate and become a gas. There is no doubt that scientists can learn something from the those elements including tellurium, but their knowledge of Venus past history is not so mysterious. “Recent spacecraft data show that water is still lost from Venus as hydrogen and oxygen in roughly the ratio of the proportion of two hydrogen atoms to one oxygen. ” Also “400 kilometer per second hydrogen protons from the solar wind impacts remove light and heavier atoms from Venus atmosphere with equal efficiency. ” p. 154, 155, A Scientific Exploration of Venus, Taylor. This is of course due to the electric field created by the solar wind and lack of a strong magnetic field by Venus core. Also the DH20 versus H20 is much higher in Venus atmosphere than Earth meaning Venus has lost a lot of water over the past half a billion years.

    Venus does not have plate tectonics or hard crust. The crust is soft due to the high surface temperature and lack of oceans. This is supported by a lack of argon 40 and lack of volcanoes. Several billion years ago Venus might have had plate tectonics and an ocean so the situation today might not be the same due to a greenhouse effect coming later when the Sun got brighter. http://theconversation.com/venus-has-very-few-volcanoes-weirdly-this-might-be-why-its-as-hot-as-hell-78363

    I agree that a direct study of from the surface of Venus by a spacecraft with a seismometer and a study of the rocks and soil at the surface and depth is needed if one day possible to be absolutely sure such a hypothesis is correct.

    • Alex T. May 19, 2019, 3:59

      The water still lost even from the Earth, not only from the Venus.
      In connection to greenhouse effect – I am sure it is complex result of :
      – the planet chemical composition ,
      – internal (core) thermal activity ,
      – proximity to the Sun,
      – probably, cosmic scale event (like asteroid’s impact)
      I am sure that Magnetic field plays no any significant role here.
      Let imagine the exactly same planet like Venus, but located on the orbit somewhere father from the Sun (for example on the Mars orbit or farther). In this case greenhouse effect can play “positive” role keeping planet’s surface temperature in liquid water range…
      Opposite, if we “move” our “ideal for life” Earth to Venus orbit it will became the similar hellish place like Venus… no life, no liquid water can remain in such place.
      Suppose that it is possible to build some theoretical planet’s model that will allow to have Earth’s like life on the Venus orbit, but for sure it will have parameters different from the Earth or Venus – it is obvious fact.
      I.e. there is very complicated combination of many different physical parameters, factors and historic events that maid the Earth habitable, the right combination in right moment , happened exactly in right place.
      For the different star system this combination most probably should be different, even if we will take in account Earth’s like life forms only.
      If return to discussed article – thermal activity of the planet is not the priority factor that should be taken in account when searching for ETI life.

  • Geoffrey Hillend May 21, 2019, 17:25

    Earth might lose it’s plate tectonics after a hundred million years if placed at Venus distance from the Sun. One does not have to take consideration of thermal activity when looking for ETI life especially if they have interstellar travel since they can go to any planet.

    Indigenous ETI is a different story though since on a planet without plate tectonics there is no carbon cycle and less replenishment of atmosphere over time. Mars is a good example.

  • Ronald August 15, 2019, 12:15

    Very late comment, just for the record (I did not have the time back then and then I forgot):
    With regard to planetary (and hence stellar) chemical composition and habitability, and in particular plate tectonics, there are a few interesting papers:
    – The Chemical Composition of Tau Ceti and Possible Effects on Terrestrial Planets, by Pagano et al., 2015.
    – THORIUM ABUNDANCES IN SOLAR TWINS AND ANALOGUES: IMPLICATIONS FOR THE HABITABILITY OF EXTRASOLAR PLANETARY SYSTEMS, by Unterborn et al., 2015.
    – Thorium in solar twins: implications for habitability in rocky planets, by Botelho et al., 2018.

    The first one (with regard to Tau Ceti) is particularly interesting, because it examines the higher Mg/Si ratio, which “would have a drastic impact on the rheology of the mantles”, i.e. much lower viscosity.
    “planets around Tau Ceti may have much more vigorous and long-lived mantle convection, which would in turn affect the activity of surface processes and (faster) bio-geochemical cycling.”.

    This also relates to the very first comment here by Abelard Lindsey.