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Salinity

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

20 Nov , 2010

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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.



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jimhenson
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jimhenson
November 21, 2010 11:58 AM
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… Read more »
Torbjorn Larsson OM
Member
Torbjorn Larsson OM
November 21, 2010 1:33 PM
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… Read more »
Lawrence B. Crowell
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Lawrence B. Crowell
November 21, 2010 6:13 AM

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.

LC

Uncle Fred
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Uncle Fred
November 21, 2010 8:17 AM

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.

neoguru
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neoguru
November 21, 2010 9:13 AM

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.

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
November 21, 2010 9:33 AM
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… Read more »
Torbjorn Larsson OM
Member
Torbjorn Larsson OM
November 21, 2010 9:41 AM

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!

Torbjorn Larsson OM
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Torbjorn Larsson OM
November 21, 2010 9:48 AM

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

Uncle Fred
Member
Uncle Fred
November 21, 2010 12:35 PM
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… Read more »
Uncle Fred
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Uncle Fred
November 21, 2010 12:46 PM

“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?

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
November 21, 2010 1:50 PM
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… Read more »
Torbjorn Larsson OM
Member
Torbjorn Larsson OM
November 21, 2010 2:00 PM

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.”

MarkW
Member
MarkW
November 22, 2010 5:01 AM

“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?

Uncle Fred
Member
Uncle Fred
November 22, 2010 9:22 AM

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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