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Exoplanet Geology: A Clue to Habitability?

Because we’ve just looked at how a carbon cycle like Earth’s may play out to allow habitability on other worlds, today’s paper seems a natural segue. It involves geology and planet formation, though here we’re less concerned with plate tectonics and feedback mechanisms than the composition of a planet’s mantle. At the University of British Columbia – Okanagan, Brendan Dyck argues that the presence of iron is more important than a planet’s location in the habitable zone in predicting habitability.

We learn that planetary mantles become increasingly iron-rich with proximity to the snow-line. In the Solar System, Mercury, Earth and Mars show silicate-mantle iron content that increases with distance from the Sun. Each planet had different proportions of iron entering its core during the planet formation period. The differences between them are the result of how much of their iron is contained in the mantle versus the core, for each should have the same proportion of iron as the star they orbit.

Core mass fraction (CMF) is a key player in this paper, defined as the extent of planetary core formation as a function of total planet mass. We start with similar precursor materials, but variations in the core mass fraction point to the differences in the silicate mantle and the surface crusts that should result on each rocky world. CMF itself “reflects the oxidation gradient present in the proto-planetary disc and the increasing contribution of oxidized, outer solar system material to planetary feedstocks.” We can use CMF as a marker for how a given planet will evolve.

Can we expect a similar growth in iron content in the mantles of planets around other stars? Evidently so. From the paper:

Oxidation gradients have been observed around other main sequence stars… and similar gradients in mantle iron contents are thus expected in other planetary systems possessing rocky differentiated planets… Consequently, even if each rocky body in a multi-planetary system forms from similar precursor material, variations in their core mass fraction will generate silicate mantles and derivative surface crusts that exhibit distinct compositional and petrophysical differences. Hence, variations in CMF may have a disproportionate role in determining a planet’s geological evolution and its future habitability.

Image: Brendan Dyck (University of British Columbia – Okanagan) is using his geology expertise about planet formation to help identify other planets that might support life. Credit: NASA/Goddard Space Flight Center.

To explore how core formation influences both the thickness and composition of a planet’s crust, Dyck and team developed computer models simulating mantle and crust production in planets through a range of core mass fractions. As we saw on Tuesday, Earth’s CMF is 0.32, while Mars’ is 0.24. Dyck’s models investigate core mass fractions between 0.34 and 0.16.

So here we have some interesting observables to juggle. The modeling shows that a larger core points to thinner crusts; smaller cores produce thicker crusts that are more iron-rich, along the model of Mars. And now we circle back to plate tectonics, which is dependent upon the thickness of the planetary crust, remembering that plate tectonics is thought to be critical for a carbon cycle that can support life. The conclusion is apparent: We may well find numerous planets located within the habitable zone whose early formation history makes them unable to support water on the surface.

Adds Dyck:

“Our findings show that if we know the amount of iron present in a planet’s mantle, we can predict how thick its crust will be and, in turn, whether liquid water and an atmosphere may be present. It’s a more precise way of identifying potential new Earth-like worlds than relying on their position in the habitable zone alone.”

These conclusions again point to the critical nature of chemical composition in stellar systems, which is a key area of research made feasible by new instruments like the James Webb Space Telescope. Assuming a (fingers crossed) safe launch and deployment, JWST should be able to measure the amount of iron present in exoplanetary systems, which will offer clues as to whether life is possible there.

The paper is Dyck et al., “The effect of core formation on surface composition and planetary habitability,” in process at Astrophysical Journal Letters (preprint).

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{ 25 comments… add one }
  • Alex Tolley May 6, 2021, 10:51

    Our findings show that if we know the amount of iron present in a planet’s mantle, we can predict how thick its crust will be and, in turn, whether liquid water and an atmosphere may be present.

    This should be a readily testable hypothesis as we start to get spectroscopic data on rocky exoplanet atmospheres.

    I note that there is a rather weasel-worded whether liquid water and an atmosphere may be present. This seems to offer an escape if the hypothesis fails. I also wonder if it has anything to say about water worlds.

    It seems to me that finding direct evidence of planetary surface conditions is still a better method to search for life as it uses only one proxy, rather than a proxy to a proxy.

  • ljk May 6, 2021, 14:28

    Volcanoes on Mars Could Be Active, Raise Possibility of Recent Habitable Conditions

    May 6, 2021

    Evidence of recent volcanic activity on Mars shows that eruptions could have taken place within the past 50,000 years, a paper by Planetary Science Institute (PSI) Research Scientist David Horvath says.

    Most volcanism on the red planet occurred between 3 and 4 billion years ago, with smaller eruptions in isolated locales continuing perhaps as recently as 3 million years ago. But, until now, there was no evidence to indicate whether Mars could still be volcanically active.

    Using data from satellites orbiting Mars, the research team found evidence of an eruption in a region called Elysium Planitia that would be the youngest known volcanic eruption on Mars, said Horvath, lead author on “Evidence for geologically recent explosive volcanism in Elysium Planitia, Mars” (https://doi.org/10.1016/j.icarus.2021.114499) that appears in Icarus.

    “This feature is a mysterious dark deposit, covering an area slightly larger than Washington D.C. It has a high thermal inertia, includes high‑calcium pyroxene-rich material, and is distributed symmetrically around a segment of the Cerberus Fossae fissure system in Elysium Planitia, atypical of aeolian, or wind-driven, deposits in the region. This feature is similar to dark spots on the Moon and Mercury suggested to be explosive volcanic eruptions,” Horvath said.

    “This may be the youngest volcanic deposit yet documented on Mars. If we were to compress Mars geologic history into a single day, this would have occurred in the very last second.”

    The majority of volcanism in the Elysium Planitia region and elsewhere on Mars consists of lava flowing at the surface, though there are numerous examples of explosive volcanism on Mars. However, this deposit appears to be different.

    “This feature overlies the surrounding lava flows and appears to be a relatively fresh deposit of ash and rock, representing a different style and time period of eruption than previously identified pyroclastic features,” Horvath said.

    “This eruption could have spewed ash as high as 10 kilometers into the Martian atmosphere but likely represents a last gasp of erupted material. Elysium Planitia hosts some of the youngest volcanism on Mars, dating around 3 million years ago, so it is not entirely unexpected. It is possible that these sorts of deposits were more common but have been eroded or buried.”

    The site of the recent eruption is about 1,000 miles (1,600 kilometers) from NASA’s InSight lander, which has been studying tectonic activity on Mars since 2018. Two Marsquakes have been localized to the region around the Cerberus Fossae and recent work has suggested the possibility that these could be due to the movement of magma at depth.

    Full article here:

    https://www.psi.edu/news/recentmarsvulcanism

    • Michael Fidler May 6, 2021, 23:10

      Superlasers Shed Light on Super-Earth Mantles.

      10 March 2021

      By compressing iron oxide to pressures expected inside a large and rocky exoplanet, scientists discovered that such mantles could layer, mix, and flow in ways very different from those inside our planet.

      However, the fact that the material properties of iron oxide and magnesium oxide diverge at high pressures means that super-Earth mantles could layer, mix, and flow in entirely foreign ways.

      https://eos.org/articles/superlasers-shed-light-on-super-earth-mantles

      Magnesium oxide might go metallic in super-Earths.

      22 NOVEMBER 2012

      https://www.chemistryworld.com/news/magnesium-oxide-might-go-metallic-in-super-earths/5658.article

      Interesting article on Martians, reminds me of the movie Dreamcatcher from 2003, that we watched again last night.

      YESTERDAY
      Scientists Claim to Spot Fungus Growing on Mars in NASA Rover Photos.
      “Fungi thrive in radiation intense environments.”

      https://futurism.com/scientists-fungus-growing-mars

      Fungi on Mars? Evidence of Growth and Behavior From Sequential Images.

      May 2021Advances in Microbiology.

      Fungi thrive in radiation intense environments. Sequential photos document that fungus-like Martian specimens emerge from the soil and increase in size, including those resembling puffballs (Basidiomycota). After obliteration of spherical specimens by the rover wheels, new sphericals-some with stalks-appeared atop the crests of old tracks. Sequences document that thousands of black arctic “araneiforms” grow up to 300 meters in the Spring and disappear by Winter; a pattern repeated each Spring and which may represent massive colonies of black fungi, mould, lichens, algae, methanogens and sulfur reducing species. Black fungi-bacteria-like specimens also appeared atop the rovers. In a series of photographs over three days (Sols) white amorphous specimens within a crevice changed shape and location then disappeared. White protoplasmic-mycelium-like-tendrils with fruiting-body-like appendages form networks upon and above the surface; or increase in mass as documented by sequential photographs. Hundreds of dimpled donut-shaped “mushroom-like” formations approximately 1mm in size are adjacent or attached to these mycelium-like complexes. Additional sequences document that white amorphous masses beneath rock-shelters increase in mass, number, or disappear and that similar white-fungus-like specimens appeared inside an open rover compartment. Comparative statistical analysis of a sample of 9 spherical specimens believed to be fungal “puffballs” photographed on Sol 1145 and 12 specimens that emerged from beneath the soil on Sol 1148 confirmed the nine grew significantly closer together as their diameters expanded and some showed evidence of movement. Cluster analysis and a paired sample ‘t’ test indicates a statistically significant size increase in the average size ratio over all comparisons between and within groups (P = 0.011). Statistical comparisons indicates that arctic “araneiforms” significantly increased in length in parallel following an initial growth spurt. Although similarities in morphology are not proof of life, growth, movement, and changes in shape and location constitute behavior and support the hypothesis there is life on Mars.

      https://www.researchgate.net/publication/351252619_Fungi_on_Mars_Evidence_of_Growth_and_Behavior_From_Sequential_Images

      • Robin Datta May 7, 2021, 14:28

        If the Mars fungus story pans out, identifying and sequencing its nucleic acids (or equivalent thereof) should be the highest priority in science, addressing issues of panspermia and xenobiology. It could clarify world-views in a major way.

      • Gary Wilson May 7, 2021, 18:12

        The paper describing life on Mars in the form of black fungi will have to be vetted very carefully. They have sequential photographic evidence which is interesting but not proof of life. A lot more work to do but very interesting.

      • Andrei May 7, 2021, 18:47

        Whatever it is they gotten images of, it simply cannot be fungi. Where’s the trees they need for the symbiotic mycorrhiza?
        What the image show is the same ‘blueberries’ that the previous twin rovers Spirit and Opportunity took images of several times.

        • Henry Cordova May 7, 2021, 20:56

          Magic Mushrooms? Genus Psilocybe? The fungus among us?

          The mind boggles.

          Just because something looks like a fungus doesn’t mean it is a fungus. Even if it IS alive.

        • Alex Tolley May 7, 2021, 22:50

          I agree these cannot be fungi. Complex fungi must live in an ecosystem, as they are saprophytic and feed on carbon fixed by primary producers. It is like those sci-fi movies where there are only a few species in an ecosystem rather than an evolved complex web of relationships between many species. Even a perfectly mowed lawn without any other invasive plants covers a rich ecosystem in the soil below the turf.

          Whatever they are, they are not fungi as we understand them, and I would very much doubt they are evidence of life. As Andrei says, we have seen the “blueberries” before and these may be something very similar.

      • Michael Fidler May 7, 2021, 23:50

        Mars might support microbial life, deep underground.

        May 7, 2021

        A new study from scientists at Brown University suggests that the Martian subsurface might be a good place to look for possible present-day microbial life on the planet. It’s an idea that has also been suggested in in other studies, but the new research, published April 15, 2021, in the peer-reviewed journal Astrobiology, finds evidence that rocks below the planet’s surface could produce the same kinds of chemical energy that sustain microbial life underground on Earth.

        https://earthsky.org/space/mars-microbial-life-ingredients-below-surface

      • Mike Serfas May 11, 2021, 12:52

        The “Fungi on Mars” paper reminds us of many of Mars’ weird wonders. (Figure 46 may be the most perplexing and trippiest of them all). Somewhere in it, there might be evidence of life. But their foremost claim, that the spheres grow, as evidenced in Figure 8 … seems obviously wrong. Figure 8 shows two scenes of the Martian blueberries or puffballs, whatever they may be. On the left they are largely covered by sand and on the right it has blown away, revealing more of each sphere. I see no evidence that anything moved, grew, or replicated. How does the careful annotation and analysis in Figure 41 miss that the objects they are measuring are buried in sand?

  • Mike Serfas May 6, 2021, 16:30

    This model would seem to nudge up the odds of finding life on Mars a little. I could be so wrong, but I think it says that *any* planet is subject to the overall trend of a “water line” in the protoplanetary disk (citing https://arxiv.org/abs/1801.05456 I think), and that planets that are a bit far out and cold by our standards are going to have less reduced iron so a smaller core and more iron in the mantle. In other words, any star system can have a Mars! It’s not just a freak combination of small size and poor position, but sort of a “design feature”. If (big if!) primitive life can be transferred between planets by impacts, then Mars might have received organisms preadapted to its geology from a planet of some passing star. (I wonder if other systems have their own zodaical lights from dust that originated on their own versions of Mars)

  • Robert Flores May 6, 2021, 16:39

    Nice study, by Dyck et al.

    The crux of habitability beyond HZ orbit is indeed the size of a
    planets iron core relative to overall size. Beyond the thickness
    of upper the crust the other crucial matters are the amount of time the core induces a magnetic field and is the energy source of plate tectonics.
    Assuming (big assumption) it takes 4.5 B years for complex intelligent life to emerge on average, as the paper states many solar nebulas will not be iron rich enough to create an Earth near twin in terms of internal heating. It’s also telling that they don’t mention Venus. Do we not have a good handle on the Venusian core as we can’t rely on a magnetic field
    morphology to get a reliable Core/Non-core ratio? Also Mercury has an incredibly large iron core compared to it’s size, and it’s still active as Mercury does posses a global magnetic field. Maybe there is a Rule here that can be developed, that Planets vary in iron core sizes as some formula related to distance to it’s host star. Of course the complicating factor as far whether a core is active is the overall size of the planet controlling the heat loss of said planet.

    • Abelard Lindsey May 8, 2021, 16:22

      Venus also has a thick crust and, I believe, generally less dense than Earth, meaning less Iron. Not only does a thick crust prevent plate tectonics. The lack of plate tectonics leads to where the planet has global “resurfacing” events every few hundred million years, which also makes the planet uninhabitable.

      Is it possible that the big impact that made the Earth and Moon what they are also thinned Earth’s crust enough such that plate tectonics were possible? If so, this certainly says something about the commonality of habitable planets in the galaxy.

  • Christian G May 6, 2021, 16:42

    Interesting….Does this mean plate tectonics can cause a planet in a poorer orbital position more likely to be habitable? And what does it mean for planets around M-dwarfs?

  • Geoffrey Hillend May 6, 2021, 17:34

    I agree with this paper based on the idea that an iron rich mantle is less viscous. The mantle of Mars is more viscous is made mostly of silicates like it’s crust, but Mars can’t have plate tectonics because it is also too small, so it lost a lot of its internal heat and volcanism.

    We can assume that any exoplanet with the same size and density of Earth should have the same iron rich mantle and mafic rocks. The mass divided by the volume is the density, so we can make some assumptions about exoplanets mantles by the density. Of course having the spectra will give us the chemical composition of the surface rocks and crust, so the mantle can be inferred.

  • tesh May 7, 2021, 11:21

    https://www.popularmechanics.com/space/moon-mars/a36356445/mushrooms-on-mars-nasa-photos-life-on-mars/

    sorry for being OT but the above is an interesting read… Maybe you’ve already covered this – sorry if that’s the case.

    • Mike Serfas May 8, 2021, 3:02

      There is a difference between looking at mushrooms on Mars and looking at Mars on mushrooms. :)

      There are a few species of fungus that grow well in Earthly deserts – see https://mushring.com/mushrooms-that-grow-in-the-desert/ for illustration – but even these rely on the occasional presence of water. Fungi make their living with mycelial networks – a puffball is not simply a round ball, but a round reproductive structure supported by a network of moist threads absorbing nutrients from the soil.

      • Michael Fidler May 8, 2021, 9:56

        So how did the fungus survive in the radioactive hot Chernobyl.

        Radiotrophic fungus.

        Radiotrophic fungi are fungi that can use radiation as an energy source to stimulate growth. Radiotrophic fungi have been found in extreme environments such as in the Chernobyl Nuclear Power Plant and on the exteriors of Low Earth orbit spacecraft.[1]

        Most known radiotrophic fungi utilize melanin in some capacity to survive.[2] The process of using radiation and melanin for energy has been termed radiosynthesis, and is thought to be analogous to anaerobic respiration.[1] However, it is not known if multi-step processes such as photosynthesis or chemosynthesis are used in radiosynthesis.

        https://en.wikipedia.org/wiki/Radiotrophic_fungus

        Well the magic mushroom’s on Mars and Proxima Centauri b may have a different story to tell.

        • Mike Serfas May 8, 2021, 21:31

          Thanks for pointing this out! This is an extremely interesting topic of inquiry, which suggests ideas that conflict with all our preconceptions: survival of extreme radiation, autotrophic fungus, and absorption of X-rays by low molecular weight materials. Still, for just such reasons, I’m a little skeptical of the last two things.

          The preprint Wikipedia cites ( https://www.biorxiv.org/content/10.1101/2020.07.16.205534v5 ) does describe absorption of space radiation by these fungi. However, the actual absorbance is still lower than Martian regolith, and not quite double the non-melanized fungus. I don’t see anything that they weighed the fungus or examined cell wall thickness directly, so I wonder if it simply is a bit richer in heavier elements. (Human melanin can bind iron and other transition metals) They say it would take 2.3 meters of the melanized fungus to bring Mars radiation levels to those of Earth – a 1.7 mm layer absorbed just 2% of the radiation.

          Another way to put it: if I naively assume a sievert means 1 J/kg is absorbed, and Mars offers a dose of 0.67 mSv/day ( https://www.universetoday.com/107093/how-much-radiation-would-you-get-during-a-mars-mission/ ), then what I come up with is that for every kilogram of idealized flesh, the total intake is 0.7 mJ per day, or 0.0000002 “Calories” as they appear on a food label. (There is much more energy in UV, even hard UV, because the blackbody radiation, representing lucky ultra-fast collisions in the solar photosphere, trails off exponentially with increasing frequency) Now yes, things are more complicated whenever sieverts are involved – there would probably be a different “tissue weighting factor” since the fungi may absorb more radiation (per kg?) for whatever reason – but I have a hard time seeing how this can end up amounting to a food source.

          I don’t know that always has to be the case, though. Standard images of X-ray crystallography show how materials really can bounce off X-rays according to the arrangement of their atoms. Could there really be some melanin metamaterial that you can grow within an organism, that by clever optics would be as strong a dye for X-rays as DNA is for the ultraviolet?

          • Alex Tolley May 9, 2021, 13:56

            A quick perusal of the biorXiv paper confirms the comments on the BiorXix article page that the experiment was poorly done and the conclusions likely unwarranted.

            1. The ISS fungal control was a bare agar plate, not a non-melanized version of the fungus. Therefore there are other causes for any radiation attenuation that have nothing to do with melanin absorption.

            2. The growth differential in the early stages with the Earth control could be due to other factors than gamma radiation. We have no way of knowing as the experiment and control conditions are so different.

            3. Type of radiation. The inexpensive radiation detector [Pocket Geiger Type5, X100-7 SMD (Radiation Watch, Miyagi, Japan),} detects both gamma rays and beta particles. The autotrophy argument requires the use of gamma rays. However, we don’t know if the radiation experienced by the fungus on the ISS was gamma, beta, or a combination of both. If it was all beta, then the autotrophy argument falls apart if using this paper as proof. As Mike Serfas suggests in his comment, the solar gamma radiation might be quite low, and I would argue likely low compared to that in the Chernobyl cooling ponds.

            The authors admit the shortcomings of the experiment but as they cannot quickly repeat the experiment correctly, their conclusions are unwarranted even for using this fungus as a general radiation shield although the idea has merit in the use of living things that can grow from space resources. This seems similar to the idea of growing shaped packing of mushroom hyphae to replace styrofoam.

            [Note that Cordwainer Smith indicated oyster beds as a protective living shield in the hull of starships for the pilot and crew to protect them from the pain of space.]

        • Alex Tolley May 8, 2021, 23:23

          The 3 referenced fungi in the Chernobyl cooling tanks are more like yeasts than mushrooms. However, the idea that such fungi will produce melanin that can be used to increase growth rate is interesting. How this works is relevant. If it acts to fix carbon and allow the fungus to become an autotroph vs whether it just increases metabolism is important in the Martian context. Nevertheless, an interesting phenomenon.

        • Michael Fidler May 9, 2021, 11:29

          Here is some information on Aspergillus niger or ‘black mould’.

          What is Aspergillus niger?

          https://www.news-medical.net/life-sciences/What-is-Aspergillus-niger.aspx

          And some nice electron microscope pictures of this mould.

          https://www.sciencephoto.com/media/14616/view/aspergillus-niger-fungus-sem

          I remember one of my first experiment’s was putting some damp bread under my bed many moons ago. There was all sorts of mold growing on it after a week and it had a very interesting odor. The most unusual mold had stems maybe 1/2 inch long with a cluster of spore pods on the end that was very similar to Aspergillus niger.

          • ljk May 10, 2021, 10:29

            Your parents must have been just thrilled with that “experiment.” :^)

  • ljk May 18, 2021, 13:51

    http://astrobiology.com/2021/05/mars-astrobiological-cave-and-internal-habitability-explorer-macie-a-new-frontiers-mission-concept.html

    Mars Astrobiological Cave And Internal Habitability Explorer (MACIE): A New Frontiers Mission Concept

    Source: astro-ph.IM)Posted May 13, 2021 12:03 AM0 Comments
    ©ASTRO-PH.IM

    Mars Astrobiological Cave and Internal habitability Explorer

    Martian subsurface habitability and astrobiology can be evaluated via a lava tube cave, without drilling. MACIE addresses two key goals of the Decadal Survey (2013-2022) and three MEPAG goals.

    New advances in robotic architectures, autonomous navigation, target sample selection, and analysis will enable MACIE to explore the Martian subsurface.

    Charity M. Phillips-Lander, Ali Agha-Mohammadi, J. J. Wynne, Timothy N. Titus, Nancy Chanover, Cansu Demirel-Floyd, Kyle Uckert, Kaj Williams, Danielle Wyrick, Jen Blank, Penelope Boston, Karl Mitchell, Akos Kereszturi, Javier Martin-Torres, Svetlana Shkolyar, Nicole Bardebelias, Saugata Datta, Kurt Retherford, Lydia Sam, Anshuman Bhardwaj, Alberto Fairen, David Flannery, Roger Weins

    Comments: This paper was submitted to the Planetary and Astrobiology Decadal Survey in August 2020

    Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:2105.05281 [astro-ph.IM] (or arXiv:2105.05281v1 [astro-ph.IM] for this version)

    Submission history

    From: Charity Phillips-Lander

    [v1] Tue, 11 May 2021 18:27:20 UTC (2,240 KB)

    https://arxiv.org/abs/2105.05281

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