Our home planet has been often described in glowing, nurturing terms. A cradle for life, right in the goldilocks zone. But our planet is actually right on the edge of habitability. If it were any smaller, and a little less massive, plate tectonics might never have gotten started. It turns out, life needs plate tectonics.

More...

Is it close to or far from the "other edge"?
Could life work on a planet a little bigger or massive or not?

Valencia et al. say that the continent cycle will become faster as planet mass increases, and seem to feel that this will not cause a problem for life.
More on this thread.

Grant Hutchison

I'm guessing the Big Whack knocked us over the edge. We got the benefit of another planet's radioactives. Without that, who knows?

__________________
"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

Wouldn't the radioactives per unit mass remain about the same?
In which case, it might be that just the added mass made the difference.

Grant Hutchison

Maybe order-of-magnitude the same, but I thought the smack scenarios were tilted toward largely lighter material being flung loose to form the moon, while heavier materials both stayed and joined. I think the fraction of (heavier) radioactives could have gone up. I certainly don't know if it could make a difference.

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Remember that a significant proportion of the Earth's naturally occuring radioactive elements is composed of the light element potassium.

Jon

How does Earth's percentage compare to the percentage of the Moon that's potassium?

__________________
"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

The Planetary Scientist's Companion gives the bulk Earth and bulk Moon potassium values as 135-225 and 83-102 ppm, respectively..

Jon

Right, so where is the internal heat of the moon due to the decay of 40K?

The point here I think that a new hypothesis is based on many other unproven hypotheses, like the cause of geothermal heat, the current state of Venus etc.

What would happen if this Earth would orbit the sun at the distance of Venus?

Mind that the evaporation of one gram water requires 2500 joules. That should have major effects on such a model.

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Quid est ergo tempus.
Augustinus

Additional big-whack considerations:

may have "blown-off" much of the early atmosphere.

mixed heavier metals into the new crust

antipodal bulge(s) from impact(s) may have formed original continental "seed."

There are other benefits to having a large moon but as far as just the initial impact, are there any other potential effects that I am overlooking?

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Iyam what Iyam

Andre:

I am afraid I am not sure quite what you are saying.

Are you disputing the calcuated bulk Moon composition values by people like Anders and Taylor? They are based on analyses of lunar samples, which all contain potassium.

As for internal heat, lunar seismology indicates a small molten core and there is a measureable heat flow.

What aspects of geothermal heat and the "current state of Venus are unproven?

Much the same as is suggested happened for Venus. The oceans would boil and eventually break down, leading to hydrogen loss. Oxygen would be tied up in the crust, as would carbon dioxide. Small amounts of water might persist in the uper atmosphere and as high temperature hydrated minerals like amphiboles.

Such as?

Jon

Never disputing data, but always doubting interpretation.

That's nice. Would it be proportional to the geothermal heat generation of the earth to explain it as radiogenic? Or would we need another heat contributor?

That's not how it works. Nothing is proven, there are ideas, hypotheses, etc which seem superfluosly consistent with some observations, but they may be completely inconsistent with other observations. How is the dispute going on the loss of spin of Venus? Is it still a game of colliding bodies, with highly doubtful physics? Or was it the chaotic zone of Laskar et al?

Another problem for Venus, if you do the math for climate temperature of carbon dioxide for Earth, you get in a range of 0.3 - 4.5 degrees per doubling of the concentration CO2 depending on feedback factors, the climate alarmist assume a positive feedback mainly of water vapor to boost it above the theoretical value of 1.2 degrees per doubling.

However, if you do the same for Venus you'd end up around 25 degrees per doubling, but without feedback of water vapor for instance. Wouldn't that raise an eyebrow?

The key word is "suggested", something else as "proven". This suggestion however, disdains a few problems, announced by that 2500 joule per gram water evaporation. Mind also that evaporation increases roughly exponentially with temperature.

So if we double the insolation on Earth to some 700+ watt/m2 by bringing it to Venus' orbit the grey body temperature would increase with a factor 1.2 (fourth root Stefan Boltzman), increasing the temperature to ballpark figures of 30C (blackbody) to 70C (including GHG effect. But the evaporation rate goes up exponentially and steals much of those incoming watts/m2 from the heating. Notice that the 700 watt can only evaporate some (700/2500) 0.3 gram of water per square meter per second and then there is nothing left to heat it.

The boost in the water cycle would increase the cloud cover and hence the albedo to values comparable to Venus, dropping the blackbody temperature likewise. Finally, the increased latent heat, released in the clouds, can radiate out more easily in the higher atmosphere. That cooling means that the precipitation falling back to earth does not carry the heat down again.

So with this mechanism you'll find in water vapor a tremendous negative feedback or air conditioner, fighting the warming with exponential effectiveness (why is water popular to extinghuish fire anyway?). This ensures that equilibrium would be reached instead of a runaway greenhouse effect. Earth may still be habitable but you got to love heat.

Better check the models and see of the 2500 joule is incorporated and the exponential increase of evaporation and the water cycle air conditioner with the increased cloud cover and out-radiation.

There is a very old study, that demonstrates empirically that air temperature above a sea surface has an asympthotic maximum value. I see if I can dig up.

__________________
Quid est ergo tempus.
Augustinus

Isn't water vapour a greenhouse gas?

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Join the Illuminati

The term greenhouse gas is the misnomer of the century. Water vapor has radiative properties, meaning that it can absorp and radiate electromagic energy. The key word is "radiate". That's what is causing us to scratch car windshields in a clear winter morning, even when the actual temperature is one or two degrees degrees above freezing. That's what water vapor also does in the top of the atmosphere.

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Quid est ergo tempus.
Augustinus

What aspects of the interpreted bulk composition of the Earth and Moon do you doubt?


What do you mean?


And that is not how it works either. Ideas, hypotheses are not of equal merit, but are tested against observations. Those that survive the testingwhile not "proven" have better standing than those which don't. Some ideas are much harder to test than others, such as those regarding the loss of Venus's spin, and remain speculation. But that is not germane to the issues here. Which are "geothermal heat" and the "current state of Venus" What aspect of geothermal heat are "unproven" (your word)? Just because some problems regarding Venus (like its rotation) are difficult does not man to say that there are many things that are not known reasonably well (like its general surface morphology, and the detailed morphology and compositions of the 9 landing sites).

Your point is? if you are positing an ATM view, I suggest you do so in the appropriate forum.

Sure, That would be the initial effect. But don't having that more water in the atmosphere will lead to it more being lost through dissassociation. Eventually essentially the whole of the surface water can be lost.

But what is the relevance of all this to plate tectonics and planetary size, the subject of this thread?

Jon

Currently your post is a bit unclear in the lay out, Jon,

Overal A few observation.

Doing the math on "greenhouse" effect for Venus and finding highly unrealistic results, is that against the mainstream? You can do the same math, end up with the same result but are you ATM then? How about that mainstream in the first place?

About the spin of Venus. Here is that division all into different cabins again. Astrophysists do the spinning, climatologists do the temperature, geologists do the most remarkably geography, magnetologists keep wondering about the lack of it but is there any coordination in between? Yet it's all about the same planet and no effect is stand alone.


You might like to take notice of Correia et al about the spinning history,

http://www.imcce.fr/Equipes/ASD/prep...enus1.2002.pdf
http://www.imcce.fr/Equipes/ASD/prep...enus2.2002.pdf

but when you do, you might notice the complete absence of any thermodynamical effects, like the preservation of energy for instance.

edit:
forgot to mention that the modellers need the water for their runaway wet greenhouse model while I show that the opposite happens, stabilization and negative feedback. If the water disappears, by ultra violet decay then Earth just ends up with an dry green house effect of 380 ppmv CO2, or just about nothing. Talking about carbon, how did just about all Venus carbon get in the atmosphere anyway? Note that the carbon in Venus atmosphere is in the same order of magnitude as the estimates for the total amount of carbon on Earth.

The relevance to habitability? Just trying to demonstrate that we know very little yet. Perhaps that Venus would have been habitable with a stabilizing moon like Earth, to avoid the chaotic zone following resonance between precession and obliquity cycles. The same may be true for gas giants, which might not have been gas giants if they had a stabilizing moon.

__________________
Quid est ergo tempus.
Augustinus

A large moon for Venus might cool the surface of Venus 5 degrees c = 9 degrees f, but my guess is less effect. Total energy reaching the surface is about the same reguardless of tilt on the axis. The extreemly thick atmosphere may reduce the heat loss of Venus more than the carbon dioxide. Neil

Don't forget that it not only has a lower proportion of 40k, but a lower overall amount as well. That may not be enough to make up for the lower amount of heavy metal isotopes. The internal heat can't get through the relatively thicker crust of light minerals.

__________________
"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

The idea is slightly different. I try to demonstrate that the condition of Venus is not caused by solar enegry/greenhouse effect and that it has nothing to do with a possible habitable zone.

Perhaps recheck the Correia et al study that I linked too. Earth's Milankovitch cycles are largely determined by the gravitation of the moon, causing the precession cycle of the equinoxes (26ka) to be much quicker than the obliquity cycle (41ka). Without a moon, if Venus was to be spinning, the precession would have been dominated by the much weaker solar gravitation only, that would have slowed down the precession cycle to eventually get in resonance with the obliquity cycle, (the chaotic zone) causing wild perbutations of the spinning. That may have stopped Venus spinning eventually. That's all in Coreia et al 2002. But where did the spinning energy go? They ignore that completely. Care to calculate how much heat must have been generated from stopping the supposedly initial three day rotation? Any idea where the heat of Venus might come from?

The point is that having a robust moon, stabilizing the spinning and preventing chaotic zone resonance is much more important than the size of the planet or the distance to the sun.

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Quid est ergo tempus.
Augustinus

Sorry, but Andre seems to be leading us into woo, rather than any actual discussion of solid and accepted science on the issue, I get hesitant to get too involved in science discussions that take that turn. I apologize if I'm mistaken and if reasonable argument is persuasive that such is not the case, I will happily retract my statement and make a more personal apology.

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Iyam what Iyam

Sorry about that. I have fixed it.

This thread is not primarily about the atmosphere of Venus but about planetary size, so I will refrain from detailed discussion.

But to my knowledge the mainstream view is that the super greenhouse of Venus can be correctly modelled. So what you are arguing is the contrary. I suggest you start a separate thread to discuss this. It could be interesting so I would encourage you to do so.

Where it is relevant you will find that most planetary science is highly interdisciplinary. With Venus, people interested in surface mineralogy and landforms are very interetsed in the atmosphere, because of the importance of the later to the former. Atmospheric chemists are very interested in the surface chemstry, as that appears to buffer atmospheric composition. Spin and the magnetic field may well be closely related, and so the list goes on.


but when you do, you might notice the complete absence of any thermodynamical effects, like the preservation of energy for instance.

Interesting papers, thank you. Conservation of energy seems covered by the sections titled "dissipative effects", "dissipation", "friction", etc. As for thermal effects, that is not the topic of the paper. You don't clutter papers up with irrelevancies. But I suspect that they are very low as rate at which energy is dissipated is also very low. If you wish to discuss the rotation of Venus further, why don't you start a new thread?

Venus is so hot that most of the carbon that on earth is sequestered as organic carbon and carbonates has ended up in the atmosphere. Venus is too hot for life so we don't get organic carbon. it is too hot for an ocean, so we don't get limestone. Any carbonate that was present would break down in the presence of quartz to form wollastonite and carbon dioxide. I suspect even a hot dry earth would not get hot enough for this to happen, as you need temperatures approaching 500 dgrees for this. But CO2 would get a bit higher through weathering of organic matter, at last until the oxygen was consumed.

I am sceptical about arguments about the neccessity of large Moon for habitability, but as you say, we know very little.

Gas giants are gas giants because of where they formed in the solar system, not because they lack large moons.

Jon

There should be very little fractionation between potassium 39 and 40 because the proportion mass difference is very small (1/40). It is only the lighter elements (e.g. H, C, O, S) that show strong fractionation.

The Moon is strongly enriched in Th and U compared to the Earth. Lunar bulk Th is 125-224 ppb and U 33-663 ppb. Terrestrial bulk Th is 51-54 ppb and U 14-15 ppb.

The moon has less geothermal heat because it's generates less because of a smaller mass and is able to lose it more effectively because of a lower mass-surface area ratio.


Jon

Well, that too.

__________________
"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

Problem is that people do not think outside their specialism box. For instance Correia et al require the dense atmosphere for their braking mechanism, but the moist greenhouse hypothesis requires a relatively thin atmosphere for the outgassing of the hydrogen.

Well if you don't want to do the math, you might check my back of the envelope gross estimation:

Mass of venus M: 4.87 E+24 kg
radius R: 6,050,000 meters

Inertial momentum sphere assuming uniform: I = 0.4 MR^2: Venus: 7.13 E+37 kgm^2/s^2

Assume initial rotation 72 hours gives an omega an angular speed of (W) 2.4E-5 rad/sec

Rotational energy E=0.5 * I W^2 is 2.1 E+28 kJ

Specific heat capacity for iron / basalt is 0.46 / 0.85 cal/g So lets assume 0.8 for a planet is about 3.4 Kj/Kg. So the conversion of the spinning energy to heat is enough to heat the planet 1300 degrees K uniformely. If the spinning rate was 24 hours it would have been 11,500 degrees K

of course the heat would not be generated instantaneously but the heat transport within the planet goes also very very slow, moreover there is an extremely effective isolation blanket around the planet. this would have given very interesting effects at the surface. Now might there be some of those effects still visible perhaps?

Please do indicate where I leave the straight and narrow of the physics or consider an apology.

__________________
Quid est ergo tempus.
Augustinus

As stated initially, and restated now, that which you suggest is most appropriate in ATM not in the discussion of mainstream science and understanding. If you are willing to demonstrate where your considerations are accurately reflected and represented in the mainstream astronomical/planetary body of material, I will happily withdraw my objection and offer a personal apology.

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Iyam what Iyam

You're moving the goal posts, you have not stated initially that what I suggest is most appropriate in ATM. Jon used "ATM" for the first time. Having the physics right, against what is the most focal, erroneosuly perceived to be the mainsteam, is quite something different that what I think is defined as woo.

Moreover we have covered a tremendous amount of ground here, it may be appropriate to specify what exactly is considered to be woo.

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Quid est ergo tempus.
Augustinus

ATM = Woo, at least IMO.

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Iyam what Iyam

If everybody thought like that we would still be living in caves.

__________________
Quid est ergo tempus.
Augustinus

The rules of BAUT use a different definition.

__________________
"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort