# Can this planet have an ocean of water?

by willstaruss22
Tags: ocean, planet, water
 P: 96 Lets say there is an exoplanet with 0.5 Earth masses and is 0.8 Earth radii. This planet has an atmospheric pressure of 0.10 bar/10% as thick as Earths atmosphere with an average surface temperature of 285 K. Can this planet have a body of liquid water the size of an ocean with the low atmospheric pressure?
 Mentor P: 11,815 Saturation pressure at that temperature is ~16mbar, a significant fraction of your atmosphere would be H2O. The escape velocity at the surface is roughly 10km/s. This is similar to earth, indicating that the lifetime of the atmosphere is similar as well. Some hydrogen will be lost, but water can be present for billions of years. A magnetic field can help against solar wind, but it is not necessary, as Venus shows.
 P: 96 So how large a fraction are we talking, over 10 of the total mass of the atmosphere? Could an ocean as large as the Indian Ocean exist?
P: 384
Can this planet have an ocean of water?

 Quote by willstaruss22 So how large a fraction are we talking, over 10 of the total mass of the atmosphere?
You gave the volume fraction on surface. 285 K means 16 mbar saturated water vapour. Since the total is 100 mbar, it means 84 mbar is other gases.

This is the volume fraction. What the mass fraction is depends, on surface, what the molecular mass of the other gases is. Total mass of the atmosphere, depends also on the climate. The upper levels of atmosphere will be less than 16 % water, because some will rain or snow back down, but exactly how much less depends on the details of atmospheric composition and climate.

 Quote by willstaruss22 Could an ocean as large as the Indian Ocean exist?
Sure. An ocean bigger than Pacific could also exist. The water fraction is arbitrary, in a wide range.
 P: 96 Well my planet will have 73% nitrogen, 8% oxygen and 2% Co2 factoring the 16% water vapor. I was kind of going for simple life that is resistant to solar radiation producing oxygen in the atmosphere. I was expecting there to be 50% land 50% water on the planet.
Mentor
P: 11,815
 I was expecting there to be 50% land 50% water on the planet.
Should be fine.
The density is slightly higher than Earth's, indicating more heavy elements, and probably more radioactive decays in the core. It looks like plate tectonics is possible.

If oxygen is produced, something (living or not) should consume oxygen again.
 P: 96 Now as far as rotation is was going for 12 hours and going for a tilt of 28 degrees but im not sure how the climatic condition of these two will act in a 0.10 atmosphere. My thinking is the shorter days will even up the temperature differences to an extent and that the more extreme tilt will even the difference between the equator and poles.
 Mentor P: 11,815 More tilt will lead to more extreme seasons at the poles, in addition to the colder climate there.
 P: 96 Ok so do you think that with the conditions presented the liquid oceans could last a few billion years?
 P: 384 Mars, with 0,1 Earth masses, had running rivers, and probably oceans, for a long time. It seems likely that a planet slightly more massive than Mars could keep its water.
 P: 96 Ok so you said that there could be an ocean the size of the pacific ocean on this planet. So here is a question what about a global ocean with no land at all still keeping the atmospheric pressure at 0.10? I have heard that regular oceans and seas can exist in liquid form with a atmosphere like this so what about a total global ocean?
 Mentor P: 11,815 There is no limit on the size of the ocean (well, it cannot cover more than 100% of the surface :p). This is independent of the atmosphere - if oceans are possible, their size is not restricted.

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