A planet's lowest atmospheric layer is breathable for humans, with approx 21% oxygen and 78% nitrogen or other stable gas. The next atmospheric layer has large amounts of helium.

On this planet could a flat bottomed boat float in the oxygen layer, supported by open canopies that are floating in the helium layer and are partially "submerged" in the oxygen layer. I hope to mimic the concept of how a boat floats on water. Assume that ballast adjustments exist to maintain constant weight of barge.

Basically, instead of having to harness helium into a closed balloon to create an airship as we do on earth, could a planet with a layer of helium use open canopies to float an airship?

Also, mini question: would rain clouds float in helium layer or only oxygen layer?

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    $\begingroup$ How big is this planet? Helium is the second lightest gas, after hydrogen, and will easily escape the atmosphere. As a result, in order to retain a significant amount of it, the planet will need to have a high surface gravity and/or a very low atmospheric temperature. In our solar system, the only planets with a significant amount of helium in their atmosphere are the gas giants. $\endgroup$ – Pak Jul 16 '17 at 1:22
  • $\begingroup$ I think that you'll have a problem with "open canopies" of helium being lost as the ship moves. It's an imaginative concept but canopies that trap warm air – which can be replenished with a burner – might be more practical. $\endgroup$ – wetcircuit Jul 16 '17 at 3:13
  • $\begingroup$ @Pak I don't have a set planet size, but the planet needs to be able to support life similar to humans, so I don't think a gas giant would work. $\endgroup$ – Moreau Jul 16 '17 at 3:16
  • $\begingroup$ You are asking about floating ships and clouds. You want your planet habitable but with an helium layer. Can you narrow down your problem to a single one, or split your present question into more? As it is now I have the feeling it is too broad. $\endgroup$ – L.Dutch - Reinstate Monica Jul 16 '17 at 5:52
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    $\begingroup$ Such an atmosphere would mix very rapidly, i.e. it would never form. A stratification according to molecular weight of the gases only happens when the mean free path lenght becomes so long that heavier particles describe parabolic pathways. In two words: Um-possible. $\endgroup$ – Karl Jul 16 '17 at 18:36

This answer will initially concern itself with your mini-question. Namely, "would rain clouds float in helium layer or only oxygen layer?"

It is doubtful if rain clouds would float in the helium layer. Firstly, due to the low density of the helium rain clouds will be denser and heavier, if they formed there, and so sink to lower altitudes. Secondly, the helium layer seems to be too high for rain clouds to be supported. Remember rain clouds mainly form and float in the troposphere which is a lower part of the atmosphere. The description of this atmosphere suggests the helium layer is above its troposphere.

The troposphere is the lowest layer of Earth's atmosphere. The troposphere starts at Earth's surface and goes up to a height of 7 to 20 km (4 to 12 miles, or 23,000 to 65,000 feet) above sea level. Most of the mass (about 75-80%) of the atmosphere is in the troposphere. Almost all weather occurs within this layer. Air is warmest at the bottom of the troposphere near ground level. Higher up it gets colder. Air pressure and the density of the air are also less at high altitudes. The layer above the troposphere is called the stratosphere.

Nearly all of the water vapor and dust particles in the atmosphere are in the troposphere. That is why most clouds are found in this lowest layer, too. The bottom of the troposphere, right next to the surface of Earth, is called the "boundary layer". In places where Earth's surface is "bumpy" (mountains, forests) winds in the boundary layer are all jumbled up. In smooth places (over water or ice) the winds are smoother. The winds above the boundary layer aren't affected by the surface much.

Source: Troposphere

The structure of the atmosphere of this planet is unusual. On Earth a gas like helium would be found mainly in the thermosphere and, possibly, in the exosphere.

The thermosphere is located above the mesosphere. The temperature in the thermosphere generally increases with altitude reaching 600 to 3000 F (600-2000 K) depending on solar activity. This increase in temperature is due to the absorption of intense solar radiation by the limited amount of remaining molecular oxygen. At this extreme altitude gas molecules are widely separated. Above 60 miles (100 km) from Earth's surface the chemical composition of air becomes strongly dependent on altitude and the atmosphere becomes enriched with lighter gases (atomic oxygen, helium and hydrogen). Also at 60 miles (100 km) altitude, Earth's atmosphere becomes too thin to support aircraft and vehicles need to travel at orbital velocities to stay aloft. This demarcation between aeronautics and astronautics is known as the Karman Line. Above about 100 miles (160 km) altitude the major atmospheric component becomes atomic oxygen. At very high altitudes, the residual gases begin to stratify according to molecular mass, because of gravitational separation.

Presumably the planet formed in unusual circumstances, in terms of planetary science, and may have acquired its excess helium in a rare type of incident. Perhaps a nearby gas giant planet shed large amounts of its helium atmosphere and this was captured by your planet.

If lighter-than-air balloons or airships were going to stay aloft using open canopies, they have to somehow rise high into the helium layer before descending to capture enough helium to remain buoyant and airborne. This suggests that a balloon or airship would have to be launched upwards with a not unreasonable velocity to reach high enough for the helium capture to take place. This could be accomplished with a giant catapult or rocket propulsion.

However, it would be unfortunate if an airship or balloon failed to get enough helium and plummeted planet-surfacewards. Let alone being hurled high into the upper atmosphere before their perilous descent. Aviators both passengers and crew, will need breathing apparatus because it is likely there will be insufficient oxygen in the helium layer to sustain life.

It would be an ingenuous exercise to construct a model of this planet and its formation to explain how it acquired such a considerable layer of helium as part of its atmosphere.

In summary: open-canopy balloons and airships seem dubious at best. Methods for attaining altitudes necessary to collect enough helium appear to be too dangerous to be fit for the purpose. rain clouds are unlikely to float in the helium layer, and will be found in its oxygen-nitrogen layers in its troposphere.

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Theoretically yes, in practice no

You don't actually need a sealed balloon of helium for uncontrolled lighter than air flight. You can just fill a plastic bag and watch it float away.

Why do we seal the bag? Two primary reasons, first to stop the helium escaping and secondly to give us some control over the lift by reducing the lift volume without dumping gas.

Your canopy in the helium layer will be displacing air in the oxygen layer for lift, that much is fine, but if anything displaces the helium from your canopy, how are you getting it back again? In fact, how did you get up there in the first place?

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