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Comment by pjc50 | Hacker News Reader

The gas has to be lighter than the atmosphere of the target planet. For planets with very thin atmospheres or planets with mostly-hydrogen atmospheres, that is hard to achieve. That leaves basically Venus and maybe Titan?

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Actually, on Mars it's relatively easy to have a gas be lighter than the atmosphere; the atmosphere is mostly CO2, so most common gases, including nitrogen, are lighter than it. The problem is that since the air is very thin, you don't get much lift for a given volume. So for Mars you need a very big balloon, but it can be filled with almost anything.

(Side note: balloons have actually been used on Venus already, on the Vega missions!)

A clarification: technically, filling a balloon with a lighter gas is actually pretty easy: the pressure inside the balloon is almost the same as that outside (if the balloon is made of a non-stretchy material like mylar that doesn't compress the air), so all you need to have the inside be lighter is to use a less dense gas. Mars atmosphere is mostly CO2, so even nitrogen could work as a lifting gas.

The problem of course is that while it's easy to use a gas that's less dense than Mars' atmosphere, it can't be very much less dense because you're starting from a very low density. So, you'd need a huge volume in order to lift anything substantial.

edit: looks like a sibling comment said the same thing first..

Of course! Presumably you could use heated gases in the gas giants (like hot air balloons). I found this rather old JPL page which suggests it could be possible: https://www2.jpl.nasa.gov/adv_tech/balloons/outer_jupisat.ht...

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