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Plate Tectonics: Necessary for Habitability?

Just how important is plate tectonics for the development of complex life? We’ve learned that its continual churn, with material pushing up from ocean rifts and being subducted as it meets continental shelves, can moderate the Earth’s climate. Increasing temperatures are tamped down through the capture of excess carbon dioxide in rocks, which reduces potential greenhouse conditions. Lowering temperatures will produce the reverse effect. The result is a mechanism for maintaining stable temperatures that some have seen as necessary for life.

“Volcanism releases gases into the atmosphere, and then through weathering, carbon dioxide is pulled from the atmosphere and sequestered into surface rocks and sediment,” said Bradford Foley, assistant professor of geosciences at Penn State University. “Balancing those two processes keeps carbon dioxide at a certain level in the atmosphere, which is really important for whether the climate stays temperate and suitable for life.”

And indeed, most of the volcanoes on our planet are found on the border between tectonic plates. Here, too, plates being driven deeper into the subsurface through subduction push carbon deep into the mantle as the cycle continues. But we also know that the Earth is the only planet in the Solar System on which plate tectonics has been confirmed. Planets without such plates are known as stagnant lid planets, coping with a crust made up of a single spherical plate floating on the mantle. Should we rule out such planets as candidates for life?

Penn State’s Foley, cited above, has been working with colleague Andrew Smye on computer models that probe the idea. The scientists wanted to learn whether climate regulation through chemical weathering could be sustained on a stagnant lid planet,a place where there is no subduction and the recycling of surface material back into the mantle would be limited. On such a world, the development of continents and their productive collisions would not occur, although as the paper points out, volcanism can still release some mantle CO2 to the atmosphere, allowing for at least some degree of surface recycling through lava flows.

Foley and Smye’s simulations are restricted to planets that are Earth-like in size and composition, given the steep uncertainties in our knowledge of volcanism and outgassing on planets with different mantle compositions. Their work models conditions on Earth-like stagnant lid planets in terms of weathering, CO2 outgassing, and changing heat retention in the crust. The question is whether chemical weathering can balance the CO2 being released on such worlds, and whether there is enough outgassing to keep the surface from freezing over. And we learn that ruling out life on stagnant lid planets would be premature.

For given volcanic outgassing in the right amounts and a suitable planetary composition, clement conditions on the surface can be produced and sustained, all without plate tectonics:

Models of the thermal, magmatic, and degassing history of rocky planets with Earth-like size and composition demonstrate that a carbon cycle capable of regulating atmospheric CO2 content, and stabilizing climate to temperate surface temperatures, can potentially operate on geological timescales on planets in the stagnant lid regime. Plate tectonics may not be required for habitability, at least in regard to sustaining a stable, temperate climate on a planet.

In fact, the authors find the potential for moderate climates that can last up to 5 billion years, after which volcanism and CO2 outgassing would cease and the climate would cool below the freezing point of water, inducing glaciation on the surface. Everything depends upon the planet’s supply of CO2, and that takes us back to its formation:

At CO2 budgets lower than ∼1020 mol, a planet’s climate is estimated to be in a snowball state for its entire history, while above ∼1022 mol weathering would become supply-limited. With supply-limited weathering, CO2 outgassing overwhelms CO2 drawdown, such that an inhospitably hot, CO2-rich atmosphere forms. Thus, the amount of carbon accreted to a planet during formation is critical for whether it can sustain habitable surface conditions in a stagnant lid regime.

The amount of carbon most conducive for habitable conditions turns out to be somewhere between Earth’s total amount of carbon to about 10 times less carbon than is found in today’s atmosphere, mantle and crust combined. Below that, the planet does not stay warm enough for liquid water to survive on the surface.

Image: This is Figure 5 from the paper. Caption: Time when the total degassing flux, Fd + Fmeta, falls below Earth’s present-day degassing flux (A) and below 10% of Earth’s present-day degassing flux (B). Labeled contours give this time in billions of years. To the right of the dashed line (shaded region), weathering will be supply-limited assuming an eruption efficiency of 0.1. Credit: Bradford Foley / Andrew Smye.

Thus carbon dioxide can still escape from rocks in a degassing process that takes it to the surface, a process that depends on the types and quantities of heat-producing elements found in the planet. The planet’s initial composition tells the tale. The work indicates that high internal heating rates favor long-term habitability, and points to young planets as more likely to experience sufficient rates of CO2 outgassing. In the hunt for biosignatures, the authors argue for planets orbiting stars with high thorium or uranium abundance, because these are the most likely to contain the necessary constituents to produce sufficient internal heat.

The paper is Foley, B. J. and A. J. Smye (2018), “Carbon cycling and habitability of Earth-size stagnant lid planets,” Astrobiology, Vol. 18, Issue 7 (2018), 873-896. Full text.

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{ 30 comments… add one }
  • Thomas R Mazanec August 8, 2018, 14:50

    What decided Earth having plate tectonics and other planets having stagnant lids? What factor is different?

    • Nicky August 9, 2018, 7:54

      Two conditions must be satisfied: Mantle driving stress reaches plate yield stress and plate is negatively buoyant enough to sink.

      • Thomas R Mazanec August 9, 2018, 12:59

        Is there some way of figuring out if these are very uncommon circumstances?

        • DCM August 10, 2018, 4:52

          Thousands of years of space exploration.

      • Ronald August 11, 2018, 2:40

        And this in turn is determined by the presence of abundant water in the mantle, and the chemical composition of mantle and crust. The latter concerns, in particular, the relative abundances of Si, Mg, Fe, besides Ca, Al, Ni.

  • John Walker August 8, 2018, 15:17

    I read this article a while back, and it struck me as problematic. The reason being that it essentially brackets water out of its considerations.

    When considering which boundary conditions lead to sustained plate tectonics and which to stagnant lid conditions several factors must be regarded. True, one is sufficient or insufficient internal heating. However, on Earth sized planets, it is thought that plate tectonics also require a hydrated mantle in order to sustain the process once it has been initiated . Hydrated minerals alter the flow dynamics of the upper mantle, not to forget the role of sea water in easing the subduction of basin(oceanic) plates. Water is the central element for plate tectonics on Earth mass planets, the planetary mass upon which this paper concentrates. Simulations have shown that more massive rocky worlds can potentially sustain plate tectonics long term even in the absence of water. However many if not most of these so called Super Earths will not be dry but rather innundated with global oceans which will in turn hinder carbon cycling despite the planets tectonic processes.

    The suggestion in the above article is that stagnant lid planets can create a carbon cycling regime and thus stabilize the thermic development of the atmosphere to a sufficient extent that long term habitability may ensue.

    But in that context one must ask the question why did a stagnant lid establish itself on these worlds itself in the first place?
    And it seems a lack of sufficient water is the likely cause.
    Thus neutralizing the habitability argument.

  • Robin Datta August 8, 2018, 17:20
    • tesh August 9, 2018, 10:02

      Sorry to drag this off topic but I found the link you posted quite interesting. In particular the notion (fact?) that civilizations (civ) come and go.

      If a civ disintegrates, the onward flow of tech development can be maintained by being absorbed by a parallel civ(s) that overlaps in time but are sufficiently removed geographically as to not be dragged into the disintegration. If there is no absorbing of the known tech, it will have to be newly developed and that will take time – a lot of time.

      My major concern is that as we are no longer sufficiently separated geographically to allow a “neighboring civ” to absorb the tech, we will be left with the other option – time (assuming there will be humans around).

      If how ever we have arks that could facilitate rebuilding then that would be a great bonus in terms of reducing the time required to rebuild. This is important as if (let say) a 1000-1500 years go by before we are at the present level again, we will have no memory/link to how civ broke down and any historical records will appear as detached from us as the Roman example the blog describes.

  • Rob Flores August 9, 2018, 1:47

    Why aren’t they looking at the mass of terrestrial planets cores to
    compared to total mass to find another factor for long term habitability…Because

    The collision with Theia, resulted in merging of the liquid cores and
    the shedding of large amount of upper crust. If the Earth’s core
    were only 60% the current Mass(no Theia impact), would it still be active today, or would it have sputtered like Mars’ core. If this cooling core had happened 700 Mya you can say goodbye to the cambrian explosion. Life would be in trouble even if the shutdown of the core
    occured only a few million years ago.

    Is the Earth’s ratio of Core mass/Total Mass unusual,
    that’s the rub.

    Also CO2 is not the only vital chemical that needs recycling to the surface. Most Life on Earth requires a substantial number of the
    periodic table of the elements.

    And How long is the Earth’s core projected to be able to drive
    plate tectonics.?

    • Alex Tolley August 9, 2018, 10:36

      Isn’t there evidence for tectonics on early Mars, as well as Venus? This would seem to imply that conditions were one suitable for both planets to have moving plates.

      • Rob Flores August 9, 2018, 11:44

        Point on regarding Mars, No complex life seems to have emerged on Mars while plate tectonics was active(there is evidence of four plates there). Too short an active span.

        Venus is a special case, since
        1) water left the stage long ago.
        2) it’s internal heat is there, but instead of arcs of volcanic
        action and subduction, its just remelts it’s surface every few
        eons.

        How good are those models regarding behaviour of varying thickness and composition of the core-mantle-crust. we
        have the one example, are we assuming the models are near
        true?

        • Alex Tolley August 10, 2018, 9:22

          My point was that there seems to be evidence that plate tectonics occurs, however briefly, on 2 of the 3 other rocky worlds around Sol. Mars, a much smaller world, seems to have supported PT for a while before it cooled.

  • Nicky August 9, 2018, 6:22

    I agree with you. If a planet has water in its interior and on the surface, it might naturally develop plate tectonics. However, the possibility of a habitable stagnant-lid planet remains due our incomplete understanding of planetary evolution and diversity of rocky planets.
    In fact, because the viscosity of silicate rocks is strongly temperature-dependent, the viscosity of planet surface is orders of magnitude stronger than that of interior, making stagnant-lid the natural mode of convection. Water is one way to form a localized weak viscosity zone in a rigid lithosphere and explain the presence of plate tectonics on Earth. But in the case of a planet with stronger lithosphere, water weakening becomes less efficient in driving plate yielding (plate breaking).
    Also, thermal modeling has shown mantle with higher iron content is less likely to have plate tectonics because it reduces driving stresses by reducing mantle viscosity. Higher internal heating (increasing radiogenic heat source concentrations) would also prevent plate tectonics through decreasing mantle viscosity, and if the heating is too low mantle would completely solidify.
    To have subduction, plates must be negatively buoyant enough to sink, so the composition and mineralogy of the lithosphere are important too as too much of Si and Na minerals would make subduction less likely to happen.
    Water isn’t the determining factor in deciding whether plate yields or not. Plate tectonics did not start to operate on Earth until 3.5-3.0 Ga, even Earth already has abundant water in 4.4 Ga. One reason is that Earth was highly active, and the Archean plate was just simply too buoyant due this excess heat.

    • Nicky August 9, 2018, 6:25

      Can rocky super-Earths initiate plate tectonics easier than Earth-size planets is debatable.

    • John walker August 9, 2018, 17:24

      Hi Nicky. Thanks for the general support and your additional exposition. Note the point I was trying to make was not about waters role in the initial stages of the tectonic process but rather in keeping it going (long term) on a 1E mass body as this has astrobiological relevance.

  • Alex Tolley August 9, 2018, 10:41

    Re: habitability on ocean worlds. Dr. Ramirez has a model for CO2 cycling and control for ocean worlds with ice caps. If this mechanism is possible and correct, it ensures that at least CO2 cycling is possible. As for cycling other elements that are needed for life, that is another issue.

    The Ice Cap Zone: A Unique Habitable Zone for Ocean Worlds

    • John walker August 9, 2018, 17:26

      Hi Alex. Interesting. Thanks for the link.

  • Geoffrey Hillend August 9, 2018, 17:47

    Ice caps don’t take much Co2 out of the atmosphere since carbon dioxide has a much lower freezing point than water. It is the rain that takes the Co2 out of the air in the carbon cycle-silicate cycle.

    Super Earth ocean worlds and even super Earths without large oceans probably don’t have plate tectonics due to adiabatic compression; the crust is too thick and the buoyancy of hot plumes in the mantle is lost so they don’t reach the surface so there is not enough volcanoes to have a carbon cycle.

    • Alex Tolley August 10, 2018, 9:15

      IIRC, the Ramirez paper has the CO2 sequestering mechanism via CO2-water-ice clathrates. These clathrates sink and accumulate at depth, much like the methane clathrates that are found in Arctic waters and are a current focus of natural gas mining as well as fears of contributing to a GW positive feedback loop.

  • Geoffrey Hillend August 9, 2018, 18:14

    Venus is one of those stagnant lid planets because it’s crust is too hot so it does not have plate tectonics which needs a solid crust. Also it has less volcanoes than Earth since the crust due to the softer crust; lava is less buoyant in a soft, hot crust so it is harder to reach the surface.

  • Geoffrey Hillend August 9, 2018, 18:21

    On Earth, the exposed volcanic basalt and silicates on the surface help to remove the Co2 as part of the carbon cycle.

  • Michael Fidler August 10, 2018, 2:05

    Water is deep in any planet that has had it on it’s surface – this is why.

    Did meteorites create the Earth’s tectonic plates?

    Modelling suggests that plate tectonics and the Earth’s magnetic field were the result of massive collisions during the “geologic dark age”.

    cosmosmagazine.com/geoscience/did-meteorites-create-the-earth-s-tectonic-plates

    The Hunt for Earth’s Deep Hidden Oceans
    By
    MARCUS WOO
    July 11, 2018

    Water-bearing minerals reveal that Earth’s mantle could hold more water than all its oceans. Researchers now ask: Where did it all come from?

    http://www.quantamagazine.org/the-hunt-for-earths-deep-hidden-oceans-20180711/

    Oxygen-Rich Iron Reservoirs Could Have Played a Crucial Role in the Creation of Life.

    A new study from the Carnegie Institution For Science shows that reservoirs of oxygen-rich iron between the Earth’s core and mantle could have played a major role in Earth’s history, including the breakup of supercontinents, drastic changes in Earth’s atmospheric makeup, and the creation of life.

    http://www.google.com/amp/s/scitechdaily.com/oxygen-rich-iron-reservoirs-could-have-played-a-crucial-role-in-the-creation-of-life/amp/?source=images

  • Michael Fidler August 10, 2018, 2:07
  • Geoffrey Hillend August 10, 2018, 16:49

    The problem with super Earths is that their large gravity may attract a big atmosphere which does not need a lot of Co2 to have a big green house effect. There might not be any polar ice caps on them unless solar wind stripping has reduced it to a pressure comparable to Earths one bar. Also Co2 clathrates would not contribute much to a carbon cycle compared to rain. The crucial problem is that are there enough volcanoes, volcanism and any plate tectonics to complete the carbon cycle and return the Co2 back into the atmosphere. If not then there would still be a big greenhouse effect and no ice caps.

  • Geoffrey Hillend August 10, 2018, 16:57

    I like the idea that a lot or most of the Earth’s water came from it’s mantle; the hydroxide anions in the silicate minerals in the mantle. From the paper: Why Venus has very few volcanoes (compared to Earth).

  • James Franklin August 10, 2018, 17:17

    I’m aghast, how can reputable researchers miss the two most obvious factors on Earth that may be defining features contributing to the active surface the planet experiences as plate tectonics.

    Many have already highlighted the first, H2O, however, some of the statements regarding its presence at Mars and Venus miss some pretty obvious factors many ignore..but hey ho.

    The second factor, and I personally think is a very important one, is the presence of the Moon. Whilst it may only have 1/6th Earth’s gravity, and the tides raised on the mantle are small, they must be a factor as the Earth rotates below the moon at 1026 mph (At the equator). Further, if the current formation theories are correct, then the Moon once resided only 16,000 miles from the centre of Earth, and we know that tidal interaction has slowed the Earth, slowed the Moon and is pushing them further apart. It is highly probable that the Moon’s proximity likely contributed to serious friction heating of the mantle and possibly the core, and perhaps this extra energy assisted in the starting of what we call plate tectonics, along with lubrication from copious H2O on and within the planet, perhaps the only reason we still have plate tectonics is the continued presence and influence of the Moon.

    Maybe all rocky worlds between a certain mass range all undergo plate tectonics in their early years, but as the planet ages, the surface cools and stiffens to the point where it seals in the heat, forming the “lid”. The presence of the Moon could assist in keeping the astenosphere pliable due to friction induced heating and cracking..it may not be a major factor anymore, and in eons ahead it may be insufficient to sustain it, but research should be done either way to lob this idea in the bin or polish to create a better understanding.

  • Ronald August 11, 2018, 2:55

    As I commented on another website, with regard to this paper;

    Interesting study. However, as the study shows, 4 gy of life-supporting conditions is already pushing it. And that is about where life on Earth became really interesting (Cambrian explosion).

    It would be even more interesting to have a comprehensive study on the effect of various element abundances (Fe, Si, Mg, etc.) on plate tectonics. I do not know whether such a study exists.

    I do know of an interesting study: The Chemical Composition of Tau Ceti and Possible Effects on Terrestrial Planets, by Pagano et al. 2015. They report: “The Mg/Si ratio of the star is found to be (…) much greater than for Earth (…). With a system that has such an excess of Mg to Si ratio it is possible that the mineralogical make-up of planets around Tau Ceti could be significantly different from that of Earth, with possible oversaturation of MgO, resulting in an increase in the content of olivine and ferropericlase compared with Earth. The increase in MgO would have a drastic impact on the rheology of the mantles of the planets around Tau Ceti”.

    • John walker August 12, 2018, 12:13

      Although I don’t claim any expertise in evolutionary biology, I dare remark on your noting of the extended time required on Earth to achieve a highly complex biosphere. It is important to consider that evolutionary processes on exostellar planets may be very diverse depending on radiation levels and variety of other factors.
      Your point on differences in planetary composition is well taken. As you say C/O and Mg/Si ratios but also Thorium abundance which is important for radiogenic heating. Relevant for todays theme of plate tectonics.

      Today it is thought that the relative abundance of such metals in the host star offers a close approximation of their abundance in the local planetary population. These easily measured stellar values may prove valuable in refining the location of individual habitable zones and in the selection of exoplanetary systems deserving of highly time intensive spectral analysis.

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