<div dir="ltr"><div>> A double-Earth would likely be a waterworld, from what I have read. Rock
after all contains water, and if you squeeze it enough it will be
released to the surface (plus cometary water). So double-Earth will at
least have eight times the water volume but just four times the surface
area.<br>
<br>
> Plenty of volcanism, and over time I think continents will develop.
Mountains would be half of our height (since gravity would be twice as
large), but that is still enough to get some to poke up.<br><br></div>Twice as much water is 8 km deep on average. 4 km above Earth ocean surface. And mountains half as big, not even Mont Everest would be above water.<br>
<div><br>
</div></div><div class="gmail_extra"><br><br><div class="gmail_quote">On Sun, Nov 17, 2013 at 1:13 PM, Anders Sandberg <span dir="ltr"><<a href="mailto:anders@aleph.se" target="_blank">anders@aleph.se</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="im">On 2013-11-13 19:24, Kelly Anderson wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
The loss of atmosphere is of course related to the loss of the magnetosphere.<br>
</blockquote>
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Is that really obvious? I know a fair bit of planetary science, and I always thought it was more UV-dissociation of water into hydrogen that does a Jeans escape or hydrodynamic escape that dried out the planet and led to most atmospheric loss.<div class="im">
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<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
For planets very close to their sun would the increased density of the solar wind require an even stronger magnetosphere to preserve an atmosphere? Would larger planets with greater gravity be able to hold onto an atmosphere more tightly?<br>
</blockquote>
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The solar wind intensity goes down as 1/a^2 where a is the orbital radius. The magnetosphere doesn't seem to scale in a simple way with planet size: I have tried to find some neat rule of thumb, but it mainly looks like big planets with fast rotation have a better chance of being more magnetic, but it all depends on iffy magnetohydrodynamic properties of the core. One possible scaling law I don't trust is that field strength goes as sqrt(density/period).<br>
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A heavy planet will have a shorter atmospheric scale height; 7400/g meters, where g is the surface gravity in Earth gravities. But a low atmosphere doesn't necessary help, it is temperature that matters most. However, the escape temperature for each gas scales as 1/R - large planets need to be hotter to lose as much atmosphere.<div class="im">
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<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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The dynamic interaction of atmosphere, magnetosphere and solar wind might make it difficult to have enough atmosphere for enough time to evolve intelligent life. Also, if the atmosphere is too thick, that seems like it would cause its own set of problems. You can't live on Jupiter for example, thought that is an extreme example.<br>
</blockquote>
<br></div>
Even thick terrestrial planet atmospheres cause a lot of complications - they distribute temperature evenly, they convect in ways different from our thin atmosphere (thinness in this case is all about how many optical length constants deep the atmosphere is - can an IR photon get through it without scattering?)<div class="im">
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<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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I don't know the necessary physics, but if you had a planet 2x the size of earth with the same proportion of water and atmosphere, but a much greater magnetosphere because of the size of the core, would you have problems with the atmosphere being too dense? Would the oceans be too deep in some sense? Would it be harder for continents to arise from the deep?<br>
</blockquote>
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A double-Earth would likely be a waterworld, from what I have read. Rock after all contains water, and if you squeeze it enough it will be released to the surface (plus cometary water). So double-Earth will at least have eight times the water volume but just four times the surface area.<br>
<br>
Plenty of volcanism, and over time I think continents will develop. Mountains would be half of our height (since gravity would be twice as large), but that is still enough to get some to poke up.<br>
<br>
Atmospheric density is the hard part: it depends on how the planet formed, and its temperature. It is to some extent a free variable which also affects the temperature that constrains it - it is a real headache in my worldbuilding program ;-)<br>
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Quick guess: the atmosphere would be thicker but not super-thick, as long as surface temperatures are fairly low and like Earth's. Might be unbreathable to humans (too high oxygen partial pressure) but not to local life.<div class="im">
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<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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So many questions. I'm sure there are people at NASA who have been scratching their heads about this stuff for decades.<br>
</blockquote>
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Indeed. I have to rush, but I have a library of papers and books on this. Am writing a short guide for worldbuilding.<div class="HOEnZb"><div class="h5"><br>
<br>
-- <br>
Dr Anders Sandberg<br>
Future of Humanity Institute<br>
Oxford Martin School<br>
Oxford University<br>
<br>
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