Oceans on a more massive earth-like planet

[Guardsman Bass]

Somewhat inspired by the thread on the latest discovery, I had a question - would an earth-like, habitable planet that is more massive than earth (let's say, 3-4 times more massive) be much more likely to be "oceanic" (i.e., a greater percentage, or even all. of the surface to be covered by water)? I always figured the answer was probably yes, since gravity on a larger planet would reduce some of the extremes in elevation, combined with more out-gassing and the gravity to hold on to more volatiles and the like, but I'm not entirely certain.

[Kuroneko]

I don't know what kind of parameters would be 'typical', which makes discussing what's likely a bit nebuluous (most relevantly, just how much water is 'likely' to be on the a random Earth-like planet in the first place).

If the initial composition is similar to that of Earth, just scaled up to give a a planet larger by a factor of a (in terms of radius), then we have a³ as much radioactive material, a5 as much residual heat due to planetary formation, and similarly a5 as much heat generated in the settling of a heavy element to the core due to the gravitational gradient.

The surface area scales as a², and the surface gravity scales as a, which increases the energy cost of mountain formation. Thus, we should knock off a combined factor of a³ from the above in estimating how relatively orogenic the planet is.

This leads to the conclusion that although the maximum height of mountains will be lower, there should still be plenty of opportunity to form them, because higher per-area geothermal heat dissipation should lead to an increased tectonic activity. And I do not think we're saturating the maximum possible height on Earth, so for the marginal increases you're talking about, it's plausible to get something more mountainous than Earth.

Or so it appears to me, anyway. Perhaps a geologist would wander in.

[CaptainChewbacca]

Ocean size is determined by a number of things, including tectonics. A larger world would have a hotter interior (because volume is increasing by the cube while surface area increases by the square) and so it would be more geologically active. Any large event that happens underwater can displace large amounts of seawater, but large volcanic rifts can also 'swallow' considerable amounts of seawater to lower. I would expect that there would be less topographic variation across a higher-gravity world, but there's a lot more than just planetary mass determining the percentage of the surface covered by water.

[Guardsman Bass]

Thanks. Both your comments have been very helpful on this.

[Ariphaos]

I did a bit of research on the history of Earth's oceans, some of which is becoming a bit dated, but a few things stand out

1) The impact which created the Moon is thought to have stripped our planet bare of water - indeed any atmosphere.
2) What are now our oceans actually seeped up from the core, over the course of the next few hundred million years.
3) As water vapor formed again, ultraviolet rays occasionally break it up into oxygen and hydrogen. This process helps to oxidize the surface, but it also slowly strips water off of the planet, as hydrogen is not trapped well by our gravity.
4) Water levels will fall as the core cools, because a big component of keeping the water from seeping into the mantle is the heat it generates.

A bigger planet is going to have less water being stripped from impacts, have more water seep up from the core, lose less water to hydrogen escape, and maintain a higher water level due to an overall hotter core, given initial conditions.

Which are important, because Earth formed ~400 million years after the parent supernova spat out its raw materials. That's 400 million years of decay, versus a hundred million or two billion. All of that is cooling before the planet actually forms.

[Junghalli]

When talking about this it is absolutely vital to remember that surface area will not increase linearly with mass (i.e. a planet with twice the mass of Earth will not have twice its surface area). First of all because surface area of a sphere squares with radius (e.g. twice the radius = 4X the surface area), but secondly because the greater gravity of a more massive planet will pull its component material together more tightly. According to this assuming similar composition a planet's mass will increase by the cube of its diameter, e.g. Mars with ~1/8 the mass of Earth has ~1/2 its diameter. So to give an example, if Mars had the same proportion of water as Earth it would have 1/8 the water spread over 1/4 the surface area, so it would have an ocean depth equivalent to an Earth-mass world with 1/2 the Earth's amount of water. By the same token, a world with 8X the mass of Earth would have 8X our water spread over 4X our surface area, which would give it an ocean depth equivalent to an Earth-mass world with twice Earth's amount of water.

TL;DR: a planet that is more massive than Earth will have deeper oceans even if its oceans are scaled up in direct proportion to its mass (i.e. a planet twice as massive as ours had twice as much water in its oceans). So even if all other factors are equal yes, a more massive planet will be wetter.

[Guardsman Bass]

Even if, as Chewbacca mentioned, it's also quite tectonically active from the greater heat in the core (since the volume-to-surface area ratio is bigger), and thus you've got lots of subduction vents sucking water down into the mantle?

[Junghalli]

Even if, as Chewbacca mentioned, it's also quite tectonically active from the greater heat in the core (since the volume-to-surface area ratio is bigger), and thus you've got lots of subduction vents sucking water down into the mantle?

Don't forget that volcanoes release water too, and midocean rifts and volcanic islands also raise the sea level by physically displacing water (they're giant mountains and mountain ranges sitting on the ocean floor, and those mountains displace water). A more volcanically active world would probably also have more continental crust (since it builds up over time), and while this might mean more extensive land areas it would also mean higher sea levels (continents are giant highlands sticking up from the ocean floor and, again, those will displace water). Which effect would be dominant I'm not sure of, you might want to ask CaptainChewbacca.

Also, when continental crust covers too much of the surface plate tectonics will shut down (because continental crust is more bouyant than oceanic crust and doesn't really subduct well) and you'll get a stagnant lid geologic scheme. Assuming the planet retains conditions suitable for liquid water (plate tectonics is an important mechanism for regulating CO2 content and preventing runaway glaciation or greenhouse) this will eventually lead to an ocean planet with volcanic islands being the only land, as the continents will eventually be eroded away until they become shallow seas (I don't know how long this would take, but it takes hundreds of millions of years for mountain ranges to erode to sea level, and after that you won't have new sediment being deposited on the lowlands from the mountains). A limit of the surface being ~50% continental crust before plate tectonics stops has been suggested. Continental crust will be generated faster with a more active geology; any planet above ~2 Earth masses will probably have had plate tectonics shut down by the time it becomes comparable in age to Earth. My source for this is this paper, which includes a graph of fractional area covered by continental crust versus time.