# Sulphur Hexafluoride atmosphere for Ceres?

So, I have read that the escape velocity of Ceres is 514 m/s, and orbital velocity(for a low orbit) is 360 m/s...

Now, the RMS(Root Mean Square) velocity of Sulphur Hexafluoride at 72 $$^o$$F (23 $$^o$$C, far higher than Ceres could ever reach), is only 224 m/s. For Xenon at 23 $$^o$$C it's 237 m/s.

Could Ceres sustain an atmosphere of Sulphur Hexafluoride?

This is of course, ignoring the obvious problems of where the gas would come from in the first place. Also, is there a rough estimate of how many tonnes of gas one would need to create an atmosphere of 20 kPa?

• where did you read it? Where do the SF6 numbers come from? Can you add a link to the various sources or show how you did the calculations? Thanks, and Welcome to Space! – uhoh Jun 7 '19 at 6:14
• I would rather not. My understanding is that SF6 is a powerful greenhouse gas, putting 20kPa pressure of thus stuff on Ceres would have some unintended consequences. – Oscar Lanzi Jun 7 '19 at 12:35
• @OscarLanzi that's great news for the amateur terraformers! – uhoh Jun 7 '19 at 13:05
• Have a look at this answer to Heavy gas artificial atmosphere on Ceres? and see if you feel your question is answered there. If not, can you explain what it is that's you't like to know that's not answered there? Thanks, and Welcome to Space! – uhoh Jun 7 '19 at 13:10

Probably it will not work. The problem is that gas molecules have a wide distribution of velocities; most molecules in a gas are not really close to the rms velocity. Over the course of billions of years we should expect faster molecules to escape. At some point all the molecules have become "faster" at one time or another and make a run for it.

To prevent that, the body's gravity must be strong enough or its temperature low enough (e.g. Titan) to make the gas molecule R.M.S. velocity well below the escape velocity. This chart shows which gases are retained, primarily on bodies much larger than Ceres. Xenon, for instance, could be retained on the Moon if it were available, and $$\text{SF}_6$$ with a similar molecular weight would behave similarly. But on Ceres the escape velocity is below the range if the chart and that, combined with typical inner Solar System temperatures, would put the data for Ceres far below the boundary even for the heaviest gases. We therefore have no chance of retaining a $$\text{SF}_6$$ (or any other plausible gas) atmosphere on Ceres by gravity.

Moreover, $$\text{SF}_6$$ is a powerful greenhouse gas, so even if it could be retained we would have to limit it to trace amounts or risk adverse effects on the ice-covered dwarf planet/asteroid.

Appendix: How much of this stuff is there, anyway?

Another issue is that there just isn't a lot of sulfur hexafluoride around. The US Geological Survey recently reported that sulfur hexafluoride gas increased markedly in Earth's atmosphere ... to over 4 parts per trillion; and that is "primarily of anthropogenic origin" (quoted from the abstract of the reference). Earth atmospheric xenon is much more abundant than that according to Wikipedia, reaching parts per billion levels. Sulfur dioxide, another relatively heavy gas we Also see in the atmospheres of Venus and Io, is also more abundant on Earth. A corollary to these findings is that putting even a few Pascals of sulfur hexafluoride pressure around Ceres would require far more sulfur hexafluoride gas than is available on Earth.

The above data are for Earth, but they are likely to apply at least roughly to other bodies in the inner solar system where hydrogen and its compounds (aside from the Sun) are relatively poor in abundance. Comparing sulfur hexafluoride with xenon this would be all the more so if the sulfur hexafluoride on Earth is indeed primarily man-made.

• No, I've deleted, thanks for updating – GdD Jun 7 '19 at 15:16
• @OscarLanzi Would it be feasible to have a dome filled with a majority-SF6 atmosphere? Say, 1km wide, 200m tall? And have plants grow inside? – mzs.112000 Jun 9 '19 at 17:09
• How "air-tight" would it be? How to control temperature given the strong greenhouse effect? And why not use a more economical (and plant-friendly) gas such as $\text{CO}_2$ in the dome anyway? – Oscar Lanzi Jun 9 '19 at 17:18