Astronomy Without A Telescope – So Why Not Exo-Oceans?


Well, not only may up to 25% of Sun-like stars have Earth-like planets – but if they are in the right temperature zone, apparently they are almost certain to have oceans. Current thinking is that Earth’s oceans formed from the accreted material that built the planet, rather than being delivered by comets at a later time. From this understanding, we can start to model the likelihood of a similar outcome occurring on rocky exoplanets around other stars.

Assuming terrestrial-like planets are indeed common – with a silicate mantle surrounding a metallic core – then we can expect that water may be exuded onto their surface during the final stages of magma cooling – or otherwise out-gassed as steam which then cools to fall back to the surface as rain. From there, if the planet is big enough to gravitationally retain a thick atmosphere and is in the temperature zone where water can remain fluid, then you’ve got yourself an exo-ocean.

We can assume that the dust cloud that became the Solar System had lots of water in it, given how much persists in the left-over ingredients of comets, asteroids and the like. When the Sun ignited some of this water may have been photodissociated – or otherwise blown out of the inner solar system. However, cool rocky materials seem to have a strong propensity to hold water – and in this manner, could have kept water available for planet formation.

Meteorites from differentiated objects (i.e. planets or smaller bodies that have differentiated such that, while in a molten state, their heavy elements have sunk to a core displacing lighter elements upwards) have around 3% water content – while some undifferentiated objects (like carbonaceous asteroids) may have more than 20% water content.

Mush these materials together in a planet formation scenario and materials compressed at the centre become hot, causing outgassing of volatiles like carbon dioxide and water. In the early stages of planet formation much of this outgassing may have been lost to space – but as the object approaches planet size, its gravity can hold the outgassed material in place as an atmosphere. And despite the outgassing, hot magma can still retain water content – only exuding it in the final stages of cooling and solidification to form a planet’s crust.

Mathematical modelling suggests that if planets accrete from materials with 1 to 3% water content, liquid water probably exudes onto their surface in the final stages of planet formation – having progressively moved upwards as the planet’s crust solidified from the bottom up.

Otherwise, and even starting with a water content as low as 0.01%, Earth-like planets would still generate an outgassed steam atmosphere that would later rain down as fluid water upon cooling.

As the Earth formed, water contained in rocky materials either 'outgassed' or just exuded onto the surface - as magma solidified, from the bottom up, to form the Earth's crust. And OK, this is just a nice image of a deep sea volcanic vent - but you get the idea. Credit: Woods Hole Oceanographic Institution.

If this ocean formation model is correct, it can be expected that rocky exoplanets from 0.5 to 5 Earth masses, which form from a roughly equivalent set of ingredients, would be likely to form oceans within 100 millions years of primary accretion.

This model fits well with the finding of zircon crystals in Western Australia – which are dated at 4.4 billion years and are suggestive that liquid water was present that long ago – although this preceded the Late Heavy Bombardment (4.1 to 3.8 billion years ago) which may have sent all that water back into a steam atmosphere again.

Currently it’s not thought that ices from the outer solar system – that might have been transported to Earth as comets – could have contributed more than around 10% of Earth’s current water content – as measurements to date suggest that ices in the outer solar system have significantly higher levels of deuterium (i.e. heavy water) than we see on Earth.

Further reading: Elkins-Tanton, L. Formation of Early Water Oceans on Rocky Planets.

14 Replies to “Astronomy Without A Telescope – So Why Not Exo-Oceans?”

  1. it seems earth is about the only place we know, that is filled with innumerable, essential, and favorable conditions for intelligent life. there must be enough water, and a floating continental tectonic plate to step up on, and get out of the disruptive ocean currents. Dolphins though intelligent and able to communicate with sound waves, cannot manipulate their environment nor acquire enough information to build a society, and ultimately leave their planet, unless they could evolve to live on land or dwindle inside caverns if on another planet. It seems we have no visitors landings from outer space, controlling our planet as if we were ants, because few civilizations have developed to venture into outer space in our galaxy. It should be simple for advanced aliens to use technology to control our minds by sending laser energy waves from deep outer space towards earth, if they existed within 100 LY ! Consider yourselves lucky !

  2. Uncle Fred, seems we have found a common interest! I will try to reply what I can today, but it is late here.

    1) “the sharp decline in terrestrial rock dating around 3.8 Ga must have an explanation.”

    Not necessarily, if sampling is spotty, compare with the fossil record and its difficulties to see gradual or punctuated change.

    But you prompt me to dig up material I found for the astrobiology course. Incidentally there are data that says it isn’t a sharp decline:

    “DID LHB END NOT WITH A BANG BUT A WHIMPER? […] Newly Discovered Mid-Archean Impacts […] Each of the 7 discovered impact and probable impact layers within the 3,472-3,230 interval (230 myr) … These layers suggest that Earth continued to be bombarded by large extraterrestrial objects late into the Archean, at least until 3.2 Ga.” [Note: conference proceedings only.]

    I don’t think it questions the LHB, which neatly ties into the Nice model of planetary system formation and its many other correct predictions, as much as LHB severity. Photosynthesizers did well at 3.5 Ga onwards!

    2) “that data point at ~ 3.1 Ga looks interesting, do you have the link on the paper?”

    Yes, it was in the actual course material. It is from Daniele Pinti’s chapter on “The Origin and Evolution of the Oceans” Fig 3.5. I see I likely switched times, since it was @ 3.2 Ga: “The star [marked “Fluid inclusions at 3.2 Ga (deRonde et al., 1997)” and 39 deg C] indicates the temperature at 3.2 Ga, obtained from homogenization temperatures of primary seawater inclusions from Ironstone Pods, South Africa (de Ronde et al., 1997)”.

    That ref is given as “de Ronde, …, (1997). Fluid chemistry of Archaean seafloor hydrothermal vents: Implications for the composition of circa 3,2 Ga seawater. Geochim. Cosmochim. Acta, 61, 4025-4042“.

    The text discusses contesting interpretation of the finds. However, since the new chert paper, which IIRC consolidates the chert data nicely and the chert data originally were subject to much the same critique, gets the same temperature window I am inclined to take the initial interpretation without having dug into the details.

    3) “Did you factor in the greenhouse effect of a dense Carbon dioxide atmosphere?”

    Ha, I even found this one in the by now impressive stack of unarchived papers in the course pile. ([Not my usual archive system, I’ll tell you! It has been an … extensive … course. New class, old mistakes at a guess.]

    The beef is that AGW has taught people how to model carbon dioxide in the atmosphere. Kasting et al thought that ~ 100 times more CO2 was needed to warm Earth in the initially weak sun (~ 70 % modern irradiance). Today it is claimed that ~ 3 mbar (~ 0.3 %) of CO2 was sufficient. (Also here for no paywall.)

    This is claimed to be consistent with “geological evidence [that] seemed to indicate that the atmospheric CO2 concentrations during the Archaean and Proterozoic were far too low to keep the surface from freezing.”

    I’m tired and can’t remember right now if I ever got a handle on the to and fro’ of the CO2 atmosphere. Some say it was dense, some claim differently. What this paper gives is a possibility to sustain the low pressure claim. So that is one consistent possibility at least.

  3. I have been somewhat mystified by why there are these isotope differences, such as between deuterium in comets and the inner solar system. However, it is clear that Earth’s oceans are not due to some comet “water balloon” which impacted in the Hadean period. Mars also appears to have a fair amount of water as well, though it is largely in ice. H_2O is also a rather common molecule in interstellar space, so it is not surprising that it should be present in the formation of planets.


  4. That really fascinates me is how the Hadrian period played out in regards to the formation of surface water. How quickly did Earth become temperate? Earth’s evidence for this period is scant. We have Zirconium crystals and a few rocks that survived the late bombardment but otherwise, little to infer on.

    I would love to get a sampling of terrestrial worlds that are still this period of their early life.

  5. Certainly, with oceans present, life is inevitable and in all likelihood should be relitively easy to detect. Atmospheric molecular oxygen (O2) would be a giveaway.

  6. The current model is that there is “an ocean below for an ocean above”, I believe.

    I.e. current Earth water mass may be twice the visible, or twice the ocean ~ 70 % surface or twice ~ 3*10^8*10^6*10^3 m^3 [Wikipedia & assuming ~ 1 km mean depth] @ 10^3 kg/m^3 or ~ 10^20 kg.

    This comes out at ~ 10^20/5*10^24 [Wikipedia] or a mere ~ 2*10^-5 of Earth mass. So meteorites could well have supplied the water. The maximum cometary delivery figure may vary with accuracy, I believe I’ve seen 20 % mentioned; in any case a now rather certifiable non-dominant amount.

    Certainly we can believe that volatiles survive what seems to be the last stage in planet accretion, when a large planetoid is busted up with another planetoid an order of magnitude smaller. Our own Earth had the Moon creating Mars sized impactor. Venus seems to have been retrograded by something similar. (And Mars north polar region low and flat is hypothesized to be due yet another analogous Moon sized impactor.) Both retain plenty of volatiles.

    To pitch in with Uncle Fred wish & data, it seems to me the LHB vaporization case isn’t all that good after said accretion was done with.

    First, absent ~ 400 km crust busters, at worst the oceans vaporized if at all for a mere 10-1000 years. Even with crust busters it is now IIRC claimed in some papers that there was a global Goldilocks crustal zone ~ 1 km down where liquid water survived, and life could have survived, outside of the impact. (Sorry, I seem to have misplaced that one reference.)

    Second, on “a few rocks that survived the late bombardment” I can repeat what I commented last week:

    “Another data point [besides Jack Hill zircons] comes from the recent found faux-amphibolite in Nuvvuagittuq, an igneous or meta-igneous rock derived from material @ 4.28 Ga (at least). It has undergone several thermal events @ 4.0, 3.8 and 3.66 Ga, but it follows that somewhere between 4.28 – 3.8 Ga:

    1) There were reservoirs of liquid water. (Wet produced minerals.)
    2) Archaean plate tectonics may have existed in the Hadean. (Both wet and dry produced minerals.)
    3) LHB impactors didn’t seriously affect the environment. (No oxygen anomalies.)
    4) Photosynthesis existed. (Preoxygen atmosphere sulfur photosynthesis drove a biological sulfur cycle, as witnessed by several isotope ratios.)

    While the ambiguities doesn’t allow us to test that life existed @ ~ 4.25 Ga as in the parsimonous [sic] hypothesis, this means that it isn’t a safe assumption to claim that life didn’t exist then!

    So life may have existed a mere 250 My after the Moon creating impact @ ~ 4.5 Ga, a reasonable amount of time. (About the same time it took from the first multicellular body plans to the first land living terapods.)”

    How quickly did Earth become temperate?

    That is a tougher one.

    There is an orphan data point from a rock water inclusion indicating ocean temp ~ 40 degC @ ~ 3.1 Ga from salinity data, IIRC.

    Shoring that one up is a new paper on understanding chert data, that rather persuasively (this layman thinks) shows maximum ocean temp ~ 40 degC @ ~ 3.2 Ga. (I have that reference somewhere, if you need it.)

    And of course we know that photosynthezisers, which today works at max ~ 70 degC and may be suspected to earlier have been able to sustain a lot lower temperature, thrived @ ~ 3.5 Ga.

    Putting that together, I would claim that ~ 3.5 Ga the temperature likely would have been close to ~ 40 degC by continuity.

    Before that, who knows? Assuming the carbon and sulfur data together indicates life, and seeing that the oldest developed protein folds works best @ ~ 20 degC (yet another paper; we drown in them, don’t we?), it may well be that ~ 4.25 Ga when photosynthesis and so proteins putatively existed, the temperature had dropped in the shine of the young and weak sun to a balmy 20-40 degC. Just what the doctor ordered!

  7. Oops. I meant to point out that point 3) “LHB impactors didn’t seriously affect the environment. (No oxygen anomalies.)” FWIW means, in a similar vein that it is likely unsafe to claim that life didn’t exist way back, that it is likely unsafe to claim that there _were_ crust busters or similar massive impactors.

    Now IM-not-so-humble-considering-the-exclamation-mark-O we should need evidence besides Moon statistics to say that it actually happened!

  8. So meteorites could well have supplied the water.” Another oops, meant to say planetoid accretion.

  9. Torbjorn, I was wondering if you meant planetoids.. (=

    I agree that relying on relatively limited Lunar sampling does not make for a completely open-and-shut case for the LHB. Still, the sharp decline in terrestrial rock dating around 3.8 Ga must have an explanation.

    Perhaps rock samples from other stable terrestrial bodies could shed some light on this. I would be very curious to know what samples from Mercury date out too. If older then ~ 4.0 Ga then this would call into question the LHB.

    Torbjorn, that data point at ~ 3.1 Ga looks interesting, do you have the link on the paper?

    When you extrapolated the temperature at ~ 3.5 Ga. Did you factor in the greenhouse effect of a dense Carbon dioxide atmosphere? Or would it have substantially thinned out by this point? There is a lot of processes going on here at this time and it’s difficult for a (even more) layperson like me to get a handle on atmospheric composition in the Early Hadrian.

  10. “It should be simple for advanced aliens to use technology to control our minds by sending laser energy waves from deep outer space towards earth.”

    Jimhension, I suppose these lasers travel on your electric plasma ether?

  11. Oh, I misread, “sharp decline in terrestrial rock dating” may imply rocks (tectonics), not impactors.

    Well, the same general problem of spotty sampling is relevant here too. Actually more and more rocks have been found, I comment on one, but there are also ~ 4 Ga rocks now and more. (Again, it is late, and the name slipped my mind. The Nuvvuagittuq papers goes through all the new rocks from the recent decade.)

    As for tectonics, which the Nuvvuagittuq rocks implies was continuing in the same mode from earlier, it is believed to have been more intense at ~ 5 times more heat flow from Earth. And there is an interesting paper on how the seeming varying rate may be explained by supercontinent cycles and how crust is formed and removed by plates coming together.

    So in this model there was no sharp decline in plate (rock) forming seen, and certainly none known to be tied to LHB timing. Apparently LHB didn’t make much of an impression on the crust either, in combination with the putative absence of oxygen (heating) anomalies and the seen & putative absence (depending on timing) of photosynthesis interruption.

  12. It sounds like I’m a total fan of LHB not being a serious obstacle, but the truth is I don’t know. It sure looks like it didn’t have to.

    Bacterias are hardy bastards. As a comparison, infectous buggers survive inside our bodies *with an advanced Gy old immune system* that does its very best to eradicate them! Think of what they can do in an environment that isn’t designed to kill them. (Without killing the host, natch. Though flu immune reactions may make one believe otherwise…)

    It may have been that bacteria saw the LHB and laughed! Or at least survived. “Hey, *more* nutrients! Om nom nom nom.”

  13. “the planet’s crust solidified from the bottom up”

    I am not a geologist, but this struck me as counter-intuitive. I would have expected the crust to solidify top-down, as surely heat more easily escapes through the atmosphere than through x km of rock plus atmosphere?

  14. Torbjorn Larsson, Did you revise your post? I didn’t know it could be done. I will have a look at those papers. Thanks.

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