The Finer Details Of A Global Ocean Planet

Question

Some statistics (forgive the redundancy):

Radius - 5,400 mi or 8,690.458 km

Diameter - 10,800 mi or 17,380.915 km

Circumference - 33,929.2 mi or 54,603.754 km

Mass - 7.33x10^24 kg or approximately 1.23x earths

Density - 2,672 kg/m^3 or approximately 0.484% (half) that of earth

Surface Gravity - 6.478 m/s^2

Escape Velocity - 10.61 km/s or 23,736 mph

I am attempting to make a relatively plausible ocean planet with which to inhabit with many different oceanic creatures. My main concern is that it would be very difficult to do in the way I desire (using the statistics listed), despite my trying to apply all of the intricacies of physics and planetary science I can understand to attempt to alleviate the skepticism one might have with my proposed planet.

I have yet to clarify the solar system it will reside in and its location within said system as I'm aware it could drastically change the viability of my planet. It is left open for any elaboration to help me create this world. I don't mind about where in the system it formed or how it could form, where it resides or orbits from, what type of star it orbits, if it has neighbors or moons or whatnot, or anything else really, just as long as those things are established in such a way that allows it to have life.

This planet will be somewhat larger and more massive than earth, with less density (nearly half as much as earth) and slightly less surface gravity/escape velocity. It will be made up mostly of water and covered by a very deep global surface ocean which extends several dozen km downward (preferably at least 54km) or until exotic ice begins to form, as depending on various planetary factors such as atmospheric pressure and density the exact depth of exotic ice formation can vary.

Most ocean planets we know of are covered in ice shells, however I do not wish for my planet to have one. There could perhaps be a relatively small ice belt/ring around the equator if possible, as polar regions would experience more heating from oceanic forces (without considering rotation and other factors). However, this is not a must have if it isn't feasible, as the ices depth would probably be relatively paltry compared to the depth of the planets ocean(s) if such a belt was present at all.

Less gravity will most likely require a denser atmosphere and/or a lower atmospheric temperature in order to maintain said atmosphere, with water vapor retention being a key factor. Thus the presence of and strength of the magnetosphere is important. This atmosphere must retain the ocean's water vapor and also supply the necessary conditions required to support life in the ocean below (oxygen among others). It could potentially be very violent and stormy at the surface of the water but so long as life can exist below the surface I don't particularly mind.

I am aware that rocky/metallic core planets are typically more dense and usually maintain stronger magnetospheres, whereas my larger and less dense planet could potentially lack as strong of a magnetosphere. So my planet could have a relatively smaller rocky/metallic core (whose actual size and overall composition I am unsure of as of yet) surrounded/sheathed by a mantle made of exotic ice(s), whose interactions with one another I surmise could create and maintain a magnetosphere.

The transportation/recycling of nutrients upwards from the mantle/core to the deep oceans (to sustain life) I presume is possible in such a scenario. Downwelling is also necessary to recycle these nutrients from the atmosphere downward to the deep oceans. This could be caused by atmospheric conditions and wind/ocean currents in the water to allow for the replenishment of oxygen and other stuff from the atmosphere to the deeper depths below. There will be a need for a strong ocean heat transport system and an active atmosphere with wind and storms (caused by thick and/or dense atmospheric conditions) as well strong global ocean currents and oceanic density variations (caused by temperature gradients and salinity).

Overall planet size, density, gravity, planet composition (one big concern), and atmospheric/oceanic pressure/composition (another big concern) are all key factors in the development of life and gigantic ocean and deep ocean lifeforms, as well as for humans to be able to explore and traverse the planet in special-built submarines and dive suits/scuba gear (technological feasibility not withstanding).

I have done extensive research on many topics related to these necessary features and have yet to come up with all of the details necessary to satisfy my desire to have a somewhat realistic/theoretically plausible world

So for starters...

Are the basic values I have given in the beginning at all plausible for an ocean planet hosting life?

If you guys could help me in understanding all of the fundamental/crucial aspects of my planet, things I haven't thought of or considered, factors that I mention that simply won't work under any circumstances, or anything else I must consider/alter to create a viable planet, it would be much appreciated.

I am aware that there are others who are more educated on these topics than me and despite my passion for world building and science I am starting to feel out of my depth here (pun intended). I hope to learn from you guys so that I can make my world more realistic and plausible.

I am willing to make some adjustments to the figures already stated if absolutely necessary but I'd like to try and stick to the general parameters listed already. I will consider all suggestions given to me.

Answer 1

Well lets try this "giving an answer" thingy.

There are a few immediate issues i personally see with the planet as you have described it. Those are, in order,

1. The density is to low
2. Oceans are to deep
3. Exotic ice wont form

Before addressing those, let me quickly talk about the rest of the system. I.e. the star and planets. While especially the star obviously has an impact on the conditions on your world, what really matters is what sort of atmosphere it can sustain. To simplify a lot here, you can make Worlds ranging from Venus to Mars livable if you have enough liberty with the atmospheric contents. So i really wouldnt worry to much about this right now.

Density

A Density of $2,6\frac{g}{cm^3}$ is really low for such a large planet. Look at this list for instance. The range is between $7\frac{g}{cm^3}$ and $3,3\frac{g}{cm^3}$. And that lower value has a margin of error of +-1 essentially.

If we look at a planet which matches your mass, say TRAPPIST-1g, the density is $4,186\frac{g}{cm^3}$.

Even within the solar system, Venus has a density of $5,24\frac{g}{cm^3}$ and Mars $3,93\frac{g}{cm^3}$. Looking at the list of exoplanets, we also see the trend that larger objects tend to be denser. Which intuitively makes sense and is reflected in the solar system. Earth is after all the densest planet, and that is not just because of Humans !

So this is my case as to why the density is unrealistically low for a planet of this kind.

Oceans

Oceans are just really overenthusiastic valleys. It is not a coincidence that the difference in height between the Challenger Deep and Mount Everest is about 20km. Ultimately most of the ocean is ~3500 meters deep. And most mountains are less than 1000 meters tall. Of course, that does depend on what you define as a mountain. But the general rule appears to be that the highest and lowest points on a planet should be within the same ballpark.

Of course, there are exceptions like Olympus Mons which is 27km tall while the deepest point on Mars, Hellas impact basin, is 4km deep. But Olympus mons is also like 600 kilometers in diameter.

Generally speaking, take the Gravity of your world, divide it by Earths gravity and that fraction is how high your tallest mountain and deep your deepest ocean should be.

For example, lets look at Ganymede. Ganymede's surface gravity is $1,4\frac{m}{s^2}$. While Earth is obviously $9,8\frac{m}{s^2}$. The fraction of this (Earth/Ganymede) is 7. The deepest point on Earth is ~12 kilometers deep, so this would suggest an Ocean depth for Ganymede of like 80-90 kilometers. Which is around the right ballpark. We estimate the depth to be ~100 kilometers.

Note, this method suggests the bigger a planet, the shallower the oceans will be. Which is just going to be the case because all that mass of the tectonic plates has to spread out.

If we do this math for your world, it suggests an ocean depth of around 7-8 Kilometers. Note, this is the maxima. Obviously the average ocean depth will be less. If Earth is to be trusted about $\frac{1}{4}$.

So if you want deep oceans, you need a small planet.

Exotic Ice

Just today, i looked at this exact same question. In my writing / World there is a rather extensive deep sea drilling operation and originally the Oceans were 30 kilometers deep. Which since then has been corrected to 14 for the maxima and ~4 for the average.

Regardless, i looked into Ice III. Which is the easiest exotic form of ice you can make. It gets made at a pressure of 350 MPa. With my original depth and Gravity ($10,2\frac{m}{s^2}$), this pressure would have been reached at the ocean floor. However, you cant actually ever make Ice III or any other such structure in an ocean. Why ? Because of the Temperature.

You see, Ice III (and all the other fancy ices) only get made at negative temperatures. For Ice III that is -23 C i believe. And that's an issue. Because Oceans tend to not freeze. At the Challenger deep we are at about 4 Degrees C. So even if you can reach the Pressure, you cant reach the Temperature.

You might ask, "But what if i contrive a reason for it to freeze ?". As in, what if we assume for some reason the Temperature goes negative ? Well in that case the Salt water is first going to turn into normal Ice and shoot up. But even if it turned into Ice III, it would still shoot up. Because water is not actually incompressible.

You can estimate the Density of Water using this equation;

$p_d = \frac{1}{1-(\frac{p*g*h}{B})} * p$

This equation (Note you have to solve this iteratively for a whole range of depths to actually get the real density, just solving it for one depth gives the right ballpark but usually undershoots. Also "B" is $2,2x10^9$), will tell you that for instance on my world at $10,2\frac{m}{s^2}$ and a depth of 30872 meters, the density of water is $1201\frac{kg}{m^3}$. Ice III has a density of $1160\frac{kg}{m^3}$. So it will shoot up immediately, where the Pressure goes below 350 MPa, and the Ice III dissolves.

And this basic issue is even true for smaller planets. On a planet with 1/10th the Gravity, this issue of Density vs Depth vs Temperature remains and makes it very hard to actually get any of the exotic ice forms. I am not saying it is impossible, some exotic Ice forms could be very dense. But you gotta remember, the deeper those oceans are the closer you are to the core and its heat.

So my judgment would be that getting any exotic ice in a natural environment is actually really difficult and should only be possible in very strange circumstances. Definitely not something you just see in the open ocean.

Actually answering the question

I think the values you chose are largely unrealistic and either overshoot or undershoot the mark. The notion of exotic ice (While i really love the idea and hate having to abandon it for realism) is also not really accurate as far as I can tell. For any sized world.

So it is not really realistic or a good start for a hard spec Evolution project if that is your goal. As a Soft Sci Fi planet it should pass. But it does not get the stamp of approval for realism on my part.

I would suggest bumping up the density to like at leas 4, lowering the ocean depth according to the gravity relation and see where you go from there.

Also side note, deep oceans sound cool in theory but when you go and see what would evolve down there the answer is "Absolutly nothing". Earths deep oceans look like a wasteland, imagine what something 5x deeper would look !

Answer 2

The primary issue with a world-wide ocean planet, is that the surface will be extremely poor in nutrients.

Without the continental runoff and the upwelling around the continents, there isn't a source of nutrients for the surface.

Without those nutrients, there is nothing to support a food chain in the upper, sunlit, layers of the ocean.

Without the marine snow falling to the dark depths, the only real sources of energy are geothermal vents, which will place heavy restrictions on the amount of life that can be supported.