An interesting discussion as to the compressibility or incompressibility of liquids.

[Shadownailshot]

No it won't. It'll have the same density it does anywhere else, regardless of local gravity.

Both the water and the object floating in it would be heavier. Since buoyancy is based on the relative density of two substances, it isn't dependent on the strength of local gravity.

That doesn't make much sense. There's water on the moon, but it's mostly vapor. As a vapor it's less dense than as a liquid. This is not because of the lessened gravity? The higher gravity should pull things in more "tightly" if you will, causing more pressure on the surface of the liquid (water?) from the atmosphere, and this should cause a density increase.

[dragonfree97]

That doesn't make much sense. There's water on the moon, but it's mostly vapor. As a vapor it's less dense than as a liquid. This is not because of the lessened gravity? The higher gravity should pull things in more "tightly" if you will, causing more pressure on the surface of the liquid (water?) from the atmosphere, and this should cause a density increase.

Please correct me if I'm wrong, but it's not possible to compress a liquid, right? You can put it under pressure but not compress it.

[Serratus]

I'm with @shadownailshot on this one. External force would put more pressure on liquid causing it to either overflow (if we're talking about a cup 'o water) or pull the atoms closer together (in case of a whole planet). Density IS dependant on pressure and in turn - on gravity.

[Accelerando]

If there is any density change due to gravity, I doubt it will be much. Water at the bottom of the ocean, under the pressure of billions of cubic meters of the same laying on top of it, only experiences a 2% decrease in volume.

According to Wikipedia, anyway. If I be wrong, enlighten me.

[RoboRay]

That doesn't make much sense. There's water on the moon, but it's mostly vapor. As a vapor it's less dense than as a liquid. This is not because of the lessened gravity?

No, the lack of liquid water on the moon is not due to the lessened gravity. The water is mainly a vapor because of the higher temperatures (in sunlit areas) and incredibly low vapor pressure (due to the lack of an atmosphere).

The higher gravity should pull things in more "tightly" if you will, causing more pressure on the surface of the liquid (water?) from the atmosphere, and this should cause a density increase.

The higher atmospheric pressure exerted on the surface will increase the vapor pressure (or boiling point) but it will definitely not compress the water and increase it's density.

[RoboRay]

Please correct me if I'm wrong, but it's not possible to compress a liquid, right? You can put it under pressure but not compress it.

Right. If you could compress liquids, hydraulics wouldn't work.

[fnordianslip]

I haven't figured out whether or not the item in question will be more buoyant or not, but "billions of cubic meters" doesn't make for greater pressure. Its the head (depth) of water that counts, the volume is irrelevant.

[Shadownailshot]

No, the lack of liquid water on the moon is not due to the lessened gravity. The water is mainly a vapor because of the higher temperatures (in sunlit areas) and incredibly low vapor pressure (due to the lack of an atmosphere).

The higher atmospheric pressure exerted on the surface will increase the vapor pressure (or boiling point) but it will definitely not compress the water and increase it's density.

It does so with the Earth's atmosphere though. The higher densities at lower altitudes are caused by the pressure exerted from the mass of the atmosphere above. Fluid mechanics are fluid mechanics, and the pressure of the atmosphere that's exerted on the liquid water should act the same as the different altitudes for the air. It does this in earth's oceans to, the lower you go, the more pressure/density there is.

[nhnifong]

I think buoyancy in KSP isn't simulated in a realistic way anyways. The water's surface is like a colliding wall that considers all parts as points.

[RoboRay]

If gravity (and the pressure it exerted) compressed liquids, every drinking water container carried into space would either explode or become a projectile when opened.

Edited September 10, 2012 by RoboRay

[RoboRay]

It does so with the Earth's atmosphere though. The higher densities at lower altitudes are caused by the pressure exerted from the mass of the atmosphere above. Fluid mechanics are fluid mechanics, and the pressure of the atmosphere that's exerted on the liquid water should act the same as the different altitudes for the air. It does this in earth's oceans to, the lower you go, the more pressure/density there is.

The Earth's atmosphere is a gas, not a liquid. They are both fluids, yes, but compressibility is a defining characteristic about how they differ.

You can compress gasses.

You cannot compress liquids.

The pressure increases with ocean depth, yes. But not the density.

If the water density increased, submarines would find it pretty much impossible to maintain neutral buoyancy, for example.

Edited September 10, 2012 by RoboRay

[Accelerando]

Right. If you could compress liquids, hydraulics wouldn't work.

Hydraulics works because liquids tend to be very difficult to compress and the compressibility of water, at least, declines with increasing pressure.

[Winter Man]

No, the lack of liquid water on the moon is not due to the lessened gravity.

It is, in a kind of convoluted way. If the moon had enough more gravity, it'd be bigger and likely hold an atmosphere.

[Richy teh space man]

Ok.. I'm going to assume the oceans on all three solar bodies are going to be similar for maneouvering through..

Getting down there with the varying gravitys and atmospheres *in one piece* I think is what'll set them apart

[Shadownailshot]

If gravity (and the pressure it exerted) compressed liquids, every drinking water container carried into space would either explode or become a projectile when opened.

What would make you think that? That only happens when the container is compressing a gas. Leave the water out in zero G long enough, it would turn into a vapor if it weren't for surface tension.

But now that a moderator has commented, I digress.

[RoboRay]

What would make you think that? That only happens when the container is compressing a gas.

Well, since you bring it back up again...

You can either compress it or you can't. If you can, all the effects of that compression are going to be reversed when the source of the compression is removed. It's not going to compress and then just stay compressed.

Leave the water out in zero G long enough, it would turn into a vapor if it weren't for surface tension.

Uh, no. Zero G will simply not cause liquid water to evaporate. (If you are referring to being outside the spacecraft in a vacuum, that would be different, but it's the changes to the vapor pressure causing that, not the absence of relative gravitational acceleration.)

But ok, as the mod has said, this is not the thread for elementary physics, so I'll just let it go.

Edited September 10, 2012 by RoboRay

[PigletPG]

The Earth's atmosphere is a gas, not a liquid. They are both fluids, yes, but compressibility is a defining characteristic about how they differ.

You can compress gasses.

You cannot compress liquids.

The pressure increases with ocean depth, yes. But not the density.

If the water density increased, submarines would find it pretty much impossible to maintain neutral buoyancy, for example.

I know it's off-topic, but you are way wrong sir. It does get more compressed. Under enough pressure, water can turn to ice, even if it's way above freezing point. Hottest ice was at a few billion degrees in the Z machine if I am not mistaken... Which is a real thing.

[mossman]

That doesn't make much sense. There's water on the moon, but it's mostly vapor. As a vapor it's less dense than as a liquid. This is not because of the lessened gravity? The higher gravity should pull things in more "tightly" if you will, causing more pressure on the surface of the liquid (water?) from the atmosphere, and this should cause a density increase.

It's either vapour or ice on the moon because there isn't an atmosphere. Higher gravity does compress substances more, but really only gasses as solid and especially liquid substances are for the most part incompressible.

[Vanamonde]

It does get more compressed. Under enough pressure, water can turn to ice

Interesting detail: water is one of the substances that take up MORE space in solid form. Water expands as it transforms into ice, which is why ice floats: same mass but greater volume = lower density = bouyancy.

But how is this issue a controversy? The incompressibility of liquids is part of the definition of that phase of matter.

American Heritage: "The state of matter in which a substance exhibits a characteristic

readiness to flow, little or no tendency to disperse, and relatively high

incompressibility. "

Merriam-Webster: "a fluid (as water) that has no independent shape but has a definite volume and does not expand indefinitely and that is only slightly compressible"

Nasa.gov: "Regardless of gravity, a liquid has a fixed volume."

[calhoun1]

The volume is based on the size of the molecules of water. A single atom of Water in the liquid form is its size due to the nuclear forces holding it together.

[nhnifong]

The equation for buoyancy force is:

FB=ÃÂgV

where

ÃÂ -- density of the liquid,

g-- acceleration due to gravity, and

V-- volume of the displaced liquid.

Even if p and V do not change, g still increases, but g is completely balanced out by the increased gravity acting on the floating spacecraft.

So the extent that a spacecraft in an ocean on a more massive planet would float higher, depends on p*V. Since water is just a little bit compressible, the spaceship will float jsut a little bit higher.

[Shpaget]

I feel the need to input some science...

Water on the Moon can not exist in liquid form, but not because of low gravity but because low vapor pressure (no atmosphere).

You can boil water at room temperature if you expose it to vacuum. You can also have liquid water in microgravity and there are countless videos of ISS inhabitants playing around with floating blobs of water and juices (inside the ISS where the atmospheric pressure is significant).

Gravity on Earth forces water to be in liquid only by exerting pressure indirectly through the atmosphere.

Bulk modulus of water (the measure of resistance to uniform compression) is 2.2 GPa, which basically means a lot, but it is far from infinity which would be required to claim the water to be incompressible.

Increased gravity will cause the water to compress more, but again, only indirectly through the pressure.

Bulk modulus of steel is 160 GPa (70 times as much) but you don't hear claims that steel is incompressible.

[Johno]

I think that the problem here is that people use the word "incompressible" and mean two separate things. And the thing is, both sides are correct!

It is correct to say that (in general terms) liquids are for practical purposes incompressible. As previously stated, hydraulics work on that principle. Basically if you put 100mL of water into a syringe and press it with all your might, it will still take up 100mL under normal circumstances.

But the key word is that little bit at the end - "Under normal circumstances". If you have a lot of water and a means of conferring enormous pressures on it, you'd be able to see some compression take place. It wouldn't be very great, but over a large sample of water it'd be measurable.

Therefore: When we say water is incompressible, we're really meaning "practically incompressible." The amount of compression that can be achieved using the equipment we're likely to deal with on a normal day is so slight as to not really be worth considering.

But if you're talking about the entire planet's water . . .

[vexx32]

Temperature can be used to "compress" liquid. Generally, as a liquid cools, it "compresses", the exception being when you cool water below 4 degrees Celsius.

[Shadownailshot]

The volume is based on the size of the molecules of water. A single atom of Water in the liquid form is its size due to the nuclear forces holding it together.

Did you say atom of Water? Water is H2O, 2 hydrogens bonded to an oxygen. They're not held together by nuclear forces, they share electrons. I believe that's called a covalent bond, although I'm no chemist, so I'm not entirely sure on the terminology.