[Water Planet Passwordl
Virginie Fayad
Every possibility that exists in reality had some serious problems, so I ultimately decided to go with an artificially constructed world so as to counter any issues that would come up in nature. In other words K 2 or 3 aliens created this place (for reasons that will be dealt with in the story).
At this point I don't know how the interior would have light (artificially?) but I'm hoping that all three environments- atmosphere, ocean, and inner shell world, could be habitats that a being could travel between and live in. To that end I was thinking tunnels from the inner shell world through the ocean past the surface of the ocean and out into the sky. Maybe that could work? If you don't think so then disregard the tunnels suggestion. I'm starting to think it would actually be detrimental instead of helpful.
Edit: This construct is an experiment by an alien species trying to invent new habitat for beings to live in. We can assume they know what they were doing but I allow for the possibility that the construct wont be stable forever.
Unless there is some sort of constant weather control in place, the inner side of the sphere will be filled with water pretty quick. Water will evaporate and become air humidity, even more so there than on Earth due to lower gravity. The humidity will disperse through the air and make it into the inner side, where it will condense and rain. Since liquid water is denser than air it will displace the air at the middle.
It is easier for water to go in than out, because even though the gravity at any point inside the sphere is lower than outside, the total energy cost to go outside increases the closer you are to the center. In other words, water goes in, but water does not go out. Depending on the planet's size, mass of the water and aperture of the entrances, it might take days or millions of years for all the water to get stuck inside the sphere, or for the sphere to be so full that no more water goes in.
Needless to say, the shell must be made of handwavium in order not to collapse, not only due to the planetary gravity itself, but also the weight of the water over it. The internal air pressure wouldn't support it for long since air is compressible and there is a huge escape path through the tunnel.
If you want a measure of the minimum mass the planet should have to hold an atmosphere and ocean, refer to this other question that you made. It is one of my favorite questions in this site. I think kingledion's answer is by far the best one.
A world similar to the one you propose was described in John Patrick Lowrie's Dancing with eternity. It has microgravity and there is water at its core. Due to the low gravity whales jump kilometers into the air and then fall back very slowly. The only explanation of how that planet came to be is that a raging nerd cut an existing moon's insides with lasers in order to prove a world building point and win a bet (I'm not kidding, it's part of the plot) (also, not world building as in this site, but as in actually sculpting whole planets). You won't be arrested by the sci-fi realism police if you do something similar.
To do that you basically need a surface gravity of about one gee. Anything less and the atmosphere can escape into space over time. You are particularly constrained by requiring an air atmosphere and (at the same time) a liquid water ocean on the surface. This restricts the sizes you can get away with.
Note that the microgravity is a dead duck. The gravitational pull at the edge of the air core is one gee (or whatever we need to maintain an atmosphere outside) and will decrease slowly as we get to the interior. It will only be microgravity near the center of the core itself.
But problem with that is preventing the gas in the interior from compressing. It simply will not stay at a nice even density - it will gravitationally collapse into a dense (probably liquid) inner core and a much less dense outer core. You'll end up with a mini gas planet inside a shell with no way to prevent them from destabilizing and "wobbling" into each other in the long run.
To get an Earth-like atmosphere outside the sea you need a roughly Earth-like density for the air at the surface. To get that with an open tunnel you need the density of the air inside the core to be the same (not more, not less, but the same) or there will be a flow of air from one to the other.
But a core density of $1.225\,kg\,m^-3$ (average surface air density on Earth) requires a radius for the core of $R_core\approx 2.7\times 10^10\,m$ which is about $4500$ Earth radii ! That's 45 times larger than the Sun's radius.
So what would happen the air in the core : it would collapse into a star (initially, probably) and then eventually become a black hole. Your shell would become a Dyson sphere orbiting a huge star at a distance I suspect would be inside the star's atmosphere. It would be vaporized.
One way, I think, is if the gravity is exactly correct. However, while I think the water might be in a unstable equilibrium, I have been unable to work out the mathematics. Let's assume that such a planet exist, will tides/ high waves result in spontaneous boiling of water; meaning that the water will soon or later disappear in space?
Depends on how you look at it. Europa doesn't have much of an atmosphere (and apparently it is mostly oxygen). It has a lot of liquid water, however, it is all under a crust of ice. Protecting it from being carried away. At least really slowing down the process. Gravitational forces from Jupiter generate enough energy to keep the water liquid beneath the surface.
The surface water will evaporate to form a pure water-vapor atmosphere giving you the much needed atmospheric pressure you need to maintain liquid state water. Gravity will make sure the water vapor does not escape into space. But the problem here is 1) the temperature(mostly due to distance from nearest star and partially presence of greenhouse gases and volcanoes) and 2) the amount/ratio of water present on the planet with respect to the size of the planet.
In an extreme hypothetical scenario, you can have a planet made entirely of water. The core will be ice due to the compression of surface water, liquid water on the surface closer to equator and ice landmass closer to the pole, and an atmosphere of water vapor.
Not in any stable sense, you could have liquid water on a world that had a very thin atmosphere but the boiling temperature of that water would be next to freezing, if the world is massive and cold enough you could have a semi-liquid slush of seawater with very little atmosphere but there's always going to be out-gassing from it where the sunlight heats it directly. In theory you could alter this further with chemistry, saltier water will be more thermally stable with a higher melting and boiling point, the more thermally stable the liquid the less you'll lose at any given time. But you're basically right eventually this system is going to result in any and all surface water boiling off into space over a long geological time. The last puddles of the ocean are going to take anywhere between hundreds of thousands and tens of millions of years to finally evaporate or freeze. All of this is assuming that the primary doesn't undergo catastrophic change first, like a nova or late phase red giant expansion, if that sort of thing happens all bets are off.
I saw this question and this question on the site a few days ago. It asks about escape velocity from the water-based planet in Interstellar and whether the black hole had any effect. Now, one question is unanswered whilst the other has an answer focussing on the effect of the black hole (it said the effect was non existent.
On a final note, I'll add that I'm an active member of the Movies & TV Stack Exchange. I'm asking this question here as we've had a plethora of questions there about issues like this and frankly none of us are physicists. Therefore, I'll cheekily request that answers be kept on the simple side!
Note: This was posted before the question was changed to explain that much of the planet's composition was unknown. As others have said, this means that there isn't really a great answer to be had. We cannot calculate the density or the mass of the planet; it will be nearly impossible to solve this accurately without making a host of assumptions.
If the black hole creates those huge tidal waves, then the black hole's gravity must have an impact on the planet and therefor it would be easier to escape the planet, if you were on the side of the planet that faces the black hole, wich the tidal wave indicates. If the people were on the other side of the planet the effect would be reverse, minus the fact that gravitational pull would shrink with the diametre of the water planet. And wouldnt be very hard to walk on the other side of the planet too? The gravity must be waing you down??
To leave a body near to a black hole, you need only sum all the available forces, and make sure the spacecraft can provide a greater force, in which case it can leave the system at any speed it wishes.
Somewhat of a two-part question. Firstly I'm interested to know if it is expected that a planet/asteroid has water/ice on it, what kinds of scientific instruments you need to identify the presence and the actual location of the liquid water/ice.
I am aware that this is more a geoscience question than directly space exploration. But, I was hoping someone might be able to shed some light on the relative benefits of just an orbiter mission, vs landing/analysis on the surface, vs a sample return mission.
EDIT: The focus here is on subsurface water, and so by location I mean the depth it occurs at. Additionally, the asteroid I'm actually interested in is Ceres. I kept the question general but any specific knowledge would also be appreciated.
The easiest way to tell if a planet has water on it is through spectral data. By splitting the light into its component wavelengths, you can identify the composition of an object's surface. This can be done fairly easily by Earth-based telescopes. However, this has two major downsides: it can only identify what's on the surface and it can only analyze the spectrum of wide swaths of planet.
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