What if we had a water giant instead of Mars?

Tom Kalbfus · 2014-07-20 12:48:21

What if our Solar System formed a little differently? Say instead of having a small red planet as the fourth from the Sun it was a water giant a planet similar in composition to Europa except there is 4 Earth masses of it at half the density of the Earth. That means we would be talking about a planet that is 25,600 km in diameter, its outer layers would be water. Now the question is, at this distance from the Sun, would this planet be a ball of water or of ice? Assume it has the other properties of Mars by default, that is the same rate of spin and the same axial tilt. the surface gravity of the planet would be about the same as on Earth, and it would probably hold onto a more substantial atmosphere as well. The atmosphere would have a greenhouse effect, but would it be enough to melt the ice, or would it stay frozen?

Spaniard · 2014-07-20 16:20:52

We know that Mars had seas. Thats mean near Earth temperature and pressure.
So, yes... with a enough thick atmosphere, a planet on Mars location could have enough greenhouse effect to have liquid water of surface.
Although without a magnetic field it could be possible that all hydrogen escape to space.
I think that gain life and "suffer" a "oxygen catastrophe" is a key to retain more hydrogen because even if dissociate because UV on high atmosphere, the presence of oxygen makes easy to because water. And water is mostly trapped on high atmosphere where the temperature is lowest.

Earth combine magnetic field and oxygen atmosphere. The optimal combination to retain hydrogen.

Tom Kalbfus · 2014-07-21 07:51:59

Hydrogen doesn't escape as easily, because despite having the same surface gravity as Earth, the escape velocity is going to be higher. Also I think if the ocean planet has salt in its ocean, then ocean currents could of themselves create a magnetic field, as we have found on certain Jovian satellites with lots of water. So lets see, the hydrogen disassociates from water and hangs around, recombining with oxygen once again to produce water, the oxygen doesn't go away either. Now the question is, would such a water world have life? I know alternate Earthlings could launch a probe to its surface, but bringing back samples due to its high orbital and escape velocities would be rather hard. There may be planets like this orbiting other stars.

JoshNH4H · 2014-07-21 08:51:41

Though technically possible, the existence of a planet like this is, on the face of things, rather unlikely. Having said that, water is quite dark (darker even than the surface of a planet) and this will tend to warm things up. Further, more water in the ground means more water in the air, which means more greenhouse effect.

A planet with four times the mass of Earth at the distance of Mars, however, would have likely held onto much of its Hydrogen and Helium and become a gas giant. But I digress. Is there carbon, or is the planet only water? What's the atmosphere made of?

The amount of water on the planet will tend to increase its thermal stability, which is good because thermal stability is positively correlated with temperature. Furthermore, I would say that the surface of the planet would have three basic options in terms of ice content:

-Icebergs, located almost exclusively at the poles
-Planet made completely of open water
-Planet completely covered in ice all the way down

Because ice is structurally weak compared to rock, in equivalent gravity a solid ice continent bordering water would be unstable, especially given that this means that both are near the melting point of water.

It's also unlikely that the planet would be made of saltwater, because this would require an ungodly amount of salt.

But then, the entire idea of a waterworld this big is fairly silly, so what do I know.

Tom Kalbfus · 2014-07-22 05:46:20

But Josh, are not Europa, Ganymede, and Callisto, as well as Titan essentially water worlds? Now imagine a world much like them but bigger.

Void · 2014-07-22 21:09:09

It is sad to see how little activity is happening here. I hope I am not the cause of that.

Anyway, I have done some thinking about minimal water worlds around "M" type stars, that is red dwarfs. And just a little about "G" type yellow stars.

I speculated that an Earth with twice the water perhaps, at an orbit between Mars and the Asteroid belt, and with a reasonable amount of a Nitrogen atmosphere, could support a sun pointing sea, if the planet was gravitationally locked. Most of the planet would be ice, but there would be a sea pointing at the star.

I have speculated that if life were there and if evolution were actually a acting fact, guided by a higher mind or by rules of physics, perhaps microbes in the sea.

Then fishes. Then flying fishes, to escape predators. Eventually flying warm blooded fishes. Eventually birds sort of. To nest, the warmer ice shelves would be a good place to brood young, away from predators. Then deposits of guano on the ice, then plants? Then a strange form of tree, where they are interlocked to hold against the wind, and they are on stilts, and not roots, where the stilts, grow from the plant and then push against the ground. Symbiosis, where the "Birds", bring fertilizer to the trees, and the trees shelter and perhaps even feed the birds. This would occur at the edges of the sea, on the ice shelf.

While seasons would not exist, oscillations in the weather would. At times fierce frigid winds would blow off of the dark side discharging thermal differential, and at other times, the winds would be still, allowing the temperatures to climb in the constant sun. Some melting would occur.

The Mona Loa bug exists in below freezing temperatures in Hawaii. It has antifreeze. Tibet has an insect with antifreeze as well that operates in below freezing temperatures.

Perhaps you can speculate on a world around a "G" type star. Would it have a significant moon?

Tom Kalbfus · 2014-07-23 06:58:03

On a water world orbiting a red dwarf, ocean currents would determine the length of day. The Sun would rise and set, because tidal locking only applies to solid bodies, bodies of water are easier to stretch with the tide, and thus provide less resistance. the solid core underneath the mantle of water might be tidally locked, but the ocean above it would move independently. A planet would have to rotate to keep one side always facing the star, the most recent example 16 light years away would require a rotation once every 9 days, that is enough to create a magnetic field due to rotation. As ocean currents moved away from the equator, they would move in the direction of the planet's spin, as ocean currents moved toward the equator, they would move in the antispinward direction. Some of these currents would cross the day/night terminator and if you were on a boat floating on the ocean, you would observe the sun to set as you would have no other frame of reference to measure it against other than the ocean itself.

The ocean and the atmosphere on top of it would super-rotate much like the upper atmosphere of Venus. Being water thousands of km thick, the watery mantle would encounter little resistance to its super rotation, the air on top would encounter no mountains to slow down its primary currents. So a planet like this would probably have ice caps of sea ice in its polar regions if cold enough.

Last edited by Tom Kalbfus (2014-07-23 07:03:11)

Void · 2014-07-23 08:16:50

A good conversation I think.

However we are modeling two different worlds. You are speculating on an extreme water world, and I am speculating on one that resembles Earth with
the exception of having just enough extra water to cover all or almost all land.

In the case you propose, it is possible that a hot core would drive ocean currents that would dominate surface flows, and so the ice pack.

In the case I propose, I make the case that there are several materials to pay attention to.
1) Ice, lighter than water.
2) Cold Fresh Water.
3) Warm Fresh Water.
4) Cold Brine.
5) Warm Brine.

It is intuitive that water ice will float on water in the planets gravitational field.

It is intuitive that brines and/or cold water will sink in the presence of lighter types of water, those that have less salt and/or are warmer.

While the fluid shell of the planet is in large part controlled by being in the gravitational field of the planet, it is also in the gravitational
field of the star that the planet orbits around. (I will keep things simple by leaving out possible influences from a moons influence).

So, in any moment in time separated from all other moments in time, so ignoring the orbit of the planet, and rotation, but looking at how the stars
gravitational field acts on it, the stars gravitational field will attract the cold and briny waters toward the star, and will allow the lighter items
to "Float" to the back side of the planet. So ice and surface water warmed by the stars light will tend to "Float" to the back side of the planet.

However if water is warmed by the star, and some of the water evaporates, then a more briny and heavier solution will result. On the "Dark Side"
of the planet, the fluctuations of temperature under the ice at the interface with liquid water, will cause freeze/thaw cycles where the freeze part
will squeeze cold brine out of the ice, which will fall to the ocean bottom, and then begin "Falling" along the ocean bottom towards the stars
gravitational attraction, to the day side of the planet.

But the spin of the planet will modify things a bit, but it is also true that if the core is gravitationally locked, the rotation of the planet
under the water against the water (Friction), will tend to drag the water along. As it happens, if the rotation of the planet is synchronous
with the orbit of the planet, in effect, there will be little disagreement between the average momentum of the water and ice, and the average
momentum of the planet.

In the case where a planet had a large mass such as Tibet protruding upwards, it is very possible that the planet itself would lock with that
land mass pointing at the star. If that land mass actually alone protruded above water, then that would be interesting. If that land mass
had a rift valley running through it like the Mariner Rift Valley, and it was flooded, then that sea could be warm, even though the bulk of
the exposed water would likely be much colder.

From the dark side Very Cold Ice, to the day side warmer ice at the terminator, as progressing toward the star even warmer ice, then finally
ice water, then somewhat warmer water, then a ring of land, then an inland sea, which would be the warmest water, particularly if the communication
of currents between that inland sea, and the greater sea was limited, but not absent.

Tom Kalbfus · 2014-07-23 10:00:46

Even the Earth's ocean currents move independently of Earth's rotation, if there is no land, the only thing that matters would be the relative motion of the water surface. If you are just sitting on the ocean surface, the ocean currents will carry you from the dayside to the night side and then back to the day side. You would notice no motion, as there would be nothing to compare the ocean's movement to, there would be no islands that you would be passing by. A tidally locked world does rotate after all, it rotates just enough to keep the same side facing the sun. If however an ocean current were to move away from the equator for instance, its momentum would carry it in the spinward direction faster than the planet rotates, so if you are sitting on this ocean as its current carries you, you would see the Sun set. As the current cycles back to the south, you would perhaps see the Sun rise again, your latitude would also change.

Void · 2014-07-23 10:12:41

OK, correct, if in a boat, on a water only world, you might drift to any place in accordance with the force of surface currents of water and wind.

JoshNH4H · 2014-07-23 22:22:30

Tom Kalbfus wrote:

But Josh, are not Europa, Ganymede, and Callisto, as well as Titan essentially water worlds? Now imagine a world much like them but bigger.

You are proposing a large planet, with a mass twice as large as Earth and a density half as large, or about 2.5 g/cm^3. Why doesn't such a body exist here? Because it's pretty much impossible. Because the notion that you could get that much Hydrogen on a planet without more accumulating and forming an ice giant. Plus, where does all that Oxygen come from? Assuming that the rocky part of the planet has a density of 5.5 like Earth and the watery part a density of 1 like, well, water, the planet is 2/3 water by mass.

Now let's look at the rocky portion. Let's assume that the unbonded metals in the mixture have a mean density of 8 g/cm^3 (E.g., Iron) and the oxidized metals have a mean density of 3 g/cm^3 (Rock). This means that the rocky portion is 50% metal by mass and 50% rocky material. This rocky material is 40% Oxygen, meaning that the rocky portions are 20% Oxygen (Compare to the watery portions of the planet, which are about 89% Oxygen by mass).

We find that the ratio of Oxygen to Metallic (Heavier than Oxygen, in this case) elements in a rocky planet is 1:4. However, in the planet you're positing it's roughly 2:1. Where is all the Oxygen coming from? It's different from the outer system, because the inner system was subject to heavy bombardment by the solar wind early in the planetary formation period which blasted away many of the volatiles. Those that remain were either protected within the planet or brought back on comets. Proposing a full Earth mass of water being brought back to a planet is silly bordering on ridiculous.

Tom Kalbfus · 2014-07-24 08:18:25

Josh, that same theory also suggests that gas giants shouldn't form in the inner Solar System, yet we find other star systems where gas giants are orbiting close to their star. The fact that we don't find something in our Solar System, doesn't mean that such things are impossible. A planet that was twice the diameter of Earth with one half its density would have 4 Earth masses and also 4 times the surface area of the Earth, since gravity is directly proportional to mass and inversely proportional to the square of the distance from the center, the surface gravity would be the same as on Earth. Now what would the escape velocity be as compared to Earth?

Ve = (2*G*m/r)^0.5 = 15,780.5 meters per second or 15.8 km/sec
G = 6.67E-11 m^3/(km*sec^2)
m = 4 Earth masses
r = 12,800,000 m
The Earth's escape velocity is 11,186 m/s or 11.2 km/s.
That means this planet's escape velocity is 1.41 times greater than Earth's as measured from its surface, this is for a planet who's mass is 4 times greater.
Neptune's escape velocity is 23,600 m/s, but it is also much further from the Sun than Mars.
Jupiter's escape velocity is 59,500 m/s.
Compared to the other two examples, this hypothetical planet's escape velocity isn't much greater than Earth's. Perhaps it wouldn't retain as much hydrogen as you think. As for it being two thirds water by mass, what's wrong with that? In order to have 2/3rds water by mass, its rocky core would have to be larger and more massive than Earth, the planet has 4 Earth masses after all!, so if it had an Earth sized core, it would be 3/4ths water, not 2/3rds, it would also have to be larger than twice Earth's diameter since water is less dense than rock and you would need greater volume and thus a larger diameter. So maybe 2/3rds is about right, it should have a larger than Earth core, and you have enough water to make the overall mass 4 Earths and the density half as much as Earth.

Last edited by Tom Kalbfus (2014-07-24 08:23:03)

JoshNH4H · 2014-07-24 12:38:01

Regarding core size, I did the calculations using assumed densities. You can take a look at the post, but it's really just a matter of algebra.

My claim is not that hot Jupiters are impossible-- clearly they exist. But rather that a planet is either massive enough to hold onto its Hydrogen or it's not. And if it is, you're going to have a whole lot more Hydrogen than a water planet, you'll have a full on gas or ice giant.

Tom Kalbfus · 2014-07-24 13:06:26

Is a 15.8 km/sec escape velocity enough for a planet to hold into most of its gaseous hydrogen where 11.2 km/sec is not? I didn't know the Earth was on the edge of being a gas giant. I would think Venus would be closer to being a gas giant than Earth, because its atmosphere is clearly 100 times thicker. You might get similar atmospheric pressures on the surface of a water world with a 15.8 km escape velocity, but I think at the Martian distance from the Sun, liquid water could clearly exist, unlike the case for Venus.

Lets assume this water world is sort of like Venus in that it has a 100 bar atmosphere at its watery surface, maybe the surface is too hot for human life, but something living might exist in its water, perhaps a heat resistant plant life producing free oxygen in the atmosphere, which at higher altitudes might be habitable for humans, this might be as close as we could get to a gas giant with a breathable atmosphere.

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#1 2014-07-20 12:48:21

What if we had a water giant instead of Mars?

#2 2014-07-20 16:20:52

Re: What if we had a water giant instead of Mars?

#3 2014-07-21 07:51:59

Re: What if we had a water giant instead of Mars?

#4 2014-07-21 08:51:41

Re: What if we had a water giant instead of Mars?

#5 2014-07-22 05:46:20

Re: What if we had a water giant instead of Mars?

#6 2014-07-22 21:09:09

Re: What if we had a water giant instead of Mars?

#7 2014-07-23 06:58:03

Re: What if we had a water giant instead of Mars?

#8 2014-07-23 08:16:50

Re: What if we had a water giant instead of Mars?

#9 2014-07-23 10:00:46

Re: What if we had a water giant instead of Mars?

#10 2014-07-23 10:12:41

Re: What if we had a water giant instead of Mars?

#11 2014-07-23 22:22:30

Re: What if we had a water giant instead of Mars?

#12 2014-07-24 08:18:25

Re: What if we had a water giant instead of Mars?

#13 2014-07-24 12:38:01

Re: What if we had a water giant instead of Mars?

#14 2014-07-24 13:06:26

Re: What if we had a water giant instead of Mars?

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