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Water Delivery to the Early Earth

Thinking about supplying a young planet with water, the mind naturally heads for the outer reaches of the Solar System. After all, beyond the ‘snowline,’ where temperatures are cold enough to allow water to condense into ice grains, volatiles are abundant (this also takes in methane, ammonia and carbon dioxide, all of which can condense into ice grains). The idea that comets or water-rich asteroids bumping around in a chaotic early Solar System could deliver the water Earth needed for its oceans makes sense, given our planet’s formation well inside the snowline.

We’ve just looked at Ceres, in celebration of the Dawn mission’s achievements there, and we know that Ceres has an icy mantle and perhaps even an ocean beneath its surface. At 2.7 AU, the dwarf planet is right on the edge of traditional estimates for the snowline as it would have occurred in the early days of planet formation. Obviously, the snowline has a great deal to do with various models about the accretion of solid grains into planetesimals.

If I dig around in the archives, I can even show you an image of a snowline. The star below is V883 Orionis, from a stellar class called FU Orionis, a young, pre-main sequence star capable of major variability in brightness and spectral type. The snowline is easy to find here because an outburst in luminosity has heated up the protoplanetary disk, pushing the snowline outwards.

Image: This image of the planet-forming disc around the young star V883 Orionis was obtained by ALMA in long-baseline mode. This star is currently in outburst, which has pushed the water snowline further from the star and allowed it to be detected for the first time. The dark ring midway through the disc is the water snowline, the point from the star where the temperature and pressure dip low enough for water ice to form. Credit: ALMA (ESO/NAOJ/NRAO)/L. Cieza.

As sound an idea as water delivery via objects from beyond the snowline seems, it’s always wise to question our assumptions, and indeed, the issue is strongly debated. For a second scenario for Earth’s water is available. This is the idea that enough water-rich dust grains can accumulate to form boulders of kilometer size, objects that can contain large enough amounts of water to explain the amount we have on Earth. This is the so-called ‘wet-endogenous scenario,’ in which water in the early, still accreting Earth occurs in the form of hydrous silicates.

An interesting take on this comes from Martina D’Angelo (University of Groningen, the Netherlands) and colleagues, with a second paper in the process from W. F. Thi (Max Planck Institute for Extraterrestrial Physics) and team. How to defend the latter scenario given Earth’s formation well inside the snowline? The answer may lie in sheets of silicate materials called phyllosilicates, which as I’ve learned in preparing this piece, include the micas, chlorite, serpentine, talc, and the clay minerals. Usefully, they have interesting properties when it comes to water, retaining it when heated up to several hundreds of degrees centigrade.

Thus D’Angelo, supported by earlier work, notes that there is a way to preserve structural water even in the warmer regions of the protoplanetary disk. D’Angelo’s paper, supported by the still unpublished work of Thi’s team, explores how water from the gas phase can diffuse into the silicates well before the dust grains of the inner system have accreted into planetesimals.

The paper explains the astrophysical models for protoplanetary disks and the Monte Carlo simulations used for studying ice accretion on grains that were used in this work. The simulations show water vapor abundances, temperature and pressure radial profiles that identify where in the protoplanetary disk hydration of dust grains could have occurred. The results show that the ‘wet endogenous scenario’ can by no means be ruled out. From the paper, addressing the simulation results for water adsorption on a forsterite crystal lattice:

Our MC [Monte Carlo] models show that complete surface water coverage is reached for temperatures between 300 and 500 K. For hotter environments (600, 700 and 800 K), less than 30% of the surface is hydrated. At low water vapor density and high temperature, water cluster formation plays a crucial role in enhancing the coverage… The binding energy of adsorbed water molecules increases with the number of occupied neighboring sites, enabling a more temperature-stable water layer to form. Lateral diffusion of water molecules lowers the timescale for surface hydration by water vapor condensation by three order of magnitude with respect to an SCT model, ruling out any doubts on the efficiency of such process in a nebular setting.

Image: Artist impression of a very young star surrounded by a disk of gas and dust. Scientists suspect that rocky planets such as the Earth are formed from these materials. Credit & Copyright: NASA/JPL-Caltech.

D’Angelo and colleagues believe that between 0.5 and 10 Earth oceans worth of water can be produced by the agglomeration of hydrated grains in an Earth-sized planet in formation, depending on size differences among the grains. The timescale in question fits easily into the time necessary for grains to eventually accrete into larger boulders within the early Solar nebula. Now needed are simulations of grain growth that will help the researchers understand how water is retained on grain surfaces through periods of accumulation and collision.

So it may be that we have twin processes at work, with delivery of water from comets and asteroids playing a role in bulking up a young world with a latent supply of its own water.

The papers are Thi et al., “Warm dust surface chemistry in protoplanetary disks – Formation of phyllosilicates,” submitted to Astronomy & Astrophysics, and D’Angelo et al., “On water delivery in the inner solar nebula: Monte Carlo simulations of forsterite hydration,” accepted at Astronomy & Astrophysics (preprint).


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  • Brett September 12, 2018, 13:45

    That seems right. The Earth has (obviously) one ocean’s worth of water on its surface, and possibly a lot more bound up in its mantle rocks making plate tectonics work. It would be pleasing from an astrobiological perspective, since it suggests that maybe Earth-sized planets’ water endowment isn’t just random, and they might be regulating their level of surface water.

    It would also work with the deuterium isotope data. IIRC the comets they’ve sampled don’t match Earth, suggesting the water came from meteorites instead.

  • Nicky September 12, 2018, 16:49

    That’s a good news for M-dwarf HZ planets.
    In the traditional snowline water delivery model, the snowline location is expected to be much farther away from the HZ planets around M-dwarfs than that of G-dwarfs. It is because of the extremely luminous PMS of M-dwarfs that pushes the snowline farther out, and plus the orbital period of HZ planets is much closer to the star.
    The snowline model thus suggests that *in situ growth* M-dwarf HZ planets might be drier than Earth.

    Aqueous hydration of silicate grains from water vapor inside snowline provides an excellent way to accrete water for M-dwarf planets. I don’t know how would extremely luminous PMS affect the abundance of water vapor inside snowline of M-dwarfs. Perhaps future research might focus more on it.

  • Nicky September 12, 2018, 20:11

    Noted that, this water delivery model is actually based on “equilibrium condensation model” proposed by John Lewis in 1980s on the assumption that the chemical reaction between water vapor and solid materials can hydrate the silicates.

    However, this model was later discarded because it was thought that the hydration process is kinetically inhibited. The timescale for water vapor to hydrate silicates is longer than the lifetime of the disk, so water delivery from asteroids and comets outside snowline becomes the mainstream theory. However, some recent models predict much shorter condensation timescale, like this one. I hope this study would reignites the debate of planetary water accretion process.

  • Alex Tolley September 12, 2018, 22:51

    Would not the late heavy bombardment or the formation of the Moon have stripped the Earth of its surface oceans, requiring water to have been added later, perhaps by comets? I can see that this asteroidal water might well account for the water in the mantle, but would it account for the surface oceans given the Earth’s history?

    • Nicky September 13, 2018, 12:25

      No such collision is actually energetic enough strip an entire surface ocean. Not even the collisions during Giant Impacts Phase, which includes moon-forming impact, can do that.

      Impact-driven volatile escape is done by blowing off with shock wave and heating up to cause hydrodynamic outflow. The loss of entire atmosphere and complete evaporation of an ocean can be easily achieved. However, very little evaporated ocean is lost through shock wave due the fact that ocean is hundred times more massive than atmosphere and initially exists below the atmosphere.
      Moon-forming impact heats up the entire atmosphere up to tens of thousand kelvins inducing catastrophic hydrodynamic outflow of hydrogens. Nevertheless, the atmosphere is also mixed with evaporated heavy materials like oxide and metal, so the escape is largely suppressed because not all the energy is contributing to hydrogens. And most importantly, the loss of the ocean through hydrodynamic escape requires much longer timescale (tens to hundreds of million years) than any impact heating can sustain (hundreds of years).

      See Abe, Y. (2011). Protoatmospheres and surface environment of protoplanets. Earth, Moon, and Planets, 108(1), 9-14.

  • Michael Fidler September 13, 2018, 1:02

    Interesting how nitrogen was brought to earth.

    “Scientists have long puzzled over the origin of Earth’s nitrogen because it has a different isotopic composition to nitrogen produced by the Sun, and is also different to the nitrogen found in comets.

    “For a long time there had been a theory that comets brought water and nitrogen to Earth, but there is growing evidence that the isotopic ratios in comets are very different to that in Earth’s atmosphere,” says the study’s lead author Dr Dennis Harries of the Friedrich-Schiller-Universität in Jena, Germany.

    The authors used an electron microscope to study crystals from two ancient carbonaceous chondrite meteorites named Yamato-791198 and Yamato-793321 which were recovered from Antarctica in 1979.

    They discovered an unusual mineralised form of nitrogen which matched Earth’s nitrogen.

    “We found nitrogen with a similar isotopic composition to nitrogen found in people and in Earth’s atmosphere, in a very unusual mineral which was detected in two meteorites,” says Harries.

    “This mineral shows us that there was another type of nitrogen in the early solar system billions of years ago, and this molecule was probably responsible for making the building blocks of life and bringing the nitrogen of our atmosphere to Earth.”

    “The discovery of this mineralized form of nitrogen is telling us something about how Earth got its nitrogen.”

    Strange discovery
    The meteorites examined by the authors contained a strange mineral called carlsbergite which is a compound of chromium and nitrogen.

    “That’s unusual because nitrogen is usually found as a gas in our atmosphere and is reluctant to enter mineral crystal structures,” says Harries.

    The authors discovered the carlsbergite by using an electron microscope to study crystals from the meteorites,

    “The crystals we’re looking at are only 100 nanometres in size, so they’re not visible to the naked eye,” says Harries.

    They then used mass spectrometry to measure the isotopic composition of the nitrogen atoms in these very small mineral grains.

    How it formed
    Harries believes there were high concentrations of ammonia in some parts of the solar nebula which formed the Sun and solar system 4.6 billion years ago.

    Under the right conditions, nitrogen can be produced out of molecules of ammonia.

    “We think the ammonia was locked up with water in ices and was somehow evaporated by shock waves, or by collisions of small bodies near the inner solar system,” says Harries.

    This generated extremely high temperatures causing the ammonia to react with minerals, forming carlsbergite.

    Harries speculates that the asteroids containing carlsbergite were flung into the inner solar system by gravitational perturbations caused by changes in the orbit of Jupiter early in the solar system’s history.

    Eventually some of these asteroids impacted the Earth.

    Most of the original bodies containing these ices would now be gone, but Harries thinks some ice could still be found when NASA’s Dawn spacecraft enters orbit around the main asteroid belt dwarf planet Ceres on March 7th 2015

    “This is the biggest body in the main asteroid belt, and may have retained some of its ices, a very interesting possibility which I’m looking forward to very much,” says Harries.

    “We’ve now found a tiny bit of this puzzle, allowing us to see what happened a long time ago just a little bit clearer,” says Harries.

    “Our next step will be to try to recreate these conditions in our laboratory to see how these things happened, how fast they occurred, and what was produced, so we can refine the conditions responsible for the formation of these nitrogen minerals that we observed.”

    • Michael C. Fidler September 15, 2018, 4:54

      I’m just wondering if Earth could of migrated from an area in original disc that had concentrated ammonia. Could the earth of had a super earth atmosphere in the beginning with lots of methane and ammonia that turned into gas and oil and nitrogen (N2) from the ammonia.

      Radio astronomers peer deep into the stellar nursery of the Orion Nebula:

      “Astronomers have released an image of a vast filament of star-forming gas, 1200 light-years away, in the stellar nursery of the Orion Nebula.”

      “The image shows ammonia molecules within a 50-light-year long filament detected through radio observations made with the Robert C. Byrd Green Bank Telescope in West Virginia. That image is combined with an image of the Orion Nebula—an object familiar to amateur and professional astronomers alike—taken with NASA’s Wide-field Infrared Survey Explore (WISE) telescope.”


      Read more at: https://phys.org/news/2017-06-radio-astronomers-peer-deep-stellar.html#jCp

      Methane and other compounds of natural gases could of existed on the surface, atmosphere and interior of earth. As the earth migrated inward or the sun increased in intensity or the large UV flares (even large impacts) would break the compounds down and form other long chain chemicals and amino acids.


      Just add water! ;-}

  • ljk September 13, 2018, 9:11

    So how did those water moons such as Europa, Ganymede, Callisto, Titan, and Enceladus (and probably many others) get all their water? Europa has twice as much water as all the oceans on Earth. Was it the same method described in this article, or something else? Or a combination of sources?

    • Eniac September 17, 2018, 19:05

      Those are well beyond the snow line, so volatiles would have played a big part in their aggregation. They would have been ‘baked in’

  • Robert September 13, 2018, 14:30

    In the stellar image above, is the apparent size of the star fairly accurate? Or is that an optical illusion? Thanks.

  • ljk September 21, 2018, 15:52

    The age of the oldest known animal on Earth is now at 558 million years before present: