Helium Balloons Ascend in the Atmosphere of Earth But Descend in the Atmosphere of Jupiter!
We know that helium balloons ascend in out atmosphere (atmosphere of Earth) but they are expected to descend in the atmosphere of Jupiter (Jovian atmosphere). This is simply the result of different compositions of these two atmospheres. While Earth atmosphere is mainly composed of nitrogen (78% by volume) and oxygen (21% by volume) gases whose molecular weights are greater than that of helium (Mr(N2)≈28, Mr(O2)≈32, Mr(He)≈4), Jovian atmosphere is mainly composed of hydrogen (~90% by volume) and helium (~10% by volume) where the molecular weight of the main constituent, molecular hydrogen, is smaller than that of helium (Mr(H2)≈2, Mr(He)≈4). In other words, mean molecular weight of Jovian atmosphere equal to 2.2 is less than molecular weight of helium while mean molecular weight of Earth atmosphere equal to 29 is more than that of helium (although one should consider that particles of helium gas are individual helium atoms not molecules made of two or more atoms; therefore in the case of helium gas it is better to say atomic weight instead of molecular weight).
Lower molecular weight of the gas inside balloon makes the density of the balloon lower than density of surrounding atmosphere while higher molecular weight of the gas inside balloon makes the density of the balloon higher than the density of surrounding atmosphere. Based on physics laws, an object with lower density than density of a fluid will ascend inside it while an object with higher density than density of a fluid will descend inside it. This is due to buoyancy or upthrust which is an upward force exerted by a fluid on any immersed object. The magnitude of the buoyancy on an object is equal to the weight of the fluid displaced by the object (the weight of the fluid with volume equal to the volume of the object). For an immersed object, buoyancy pushes it upwards while weight of the object pulls it downwards. Whether the object moves upwards or downwards in this situation is determined by greater force. In the case of objects with lower densities than surrounding fluid, buoyancy will be greater than the weight of object and the object will rise while in the case of objects with higher densities than surrounding fluid, weight of object will be greater than buoyancy and the object will sink.