Xenon is one of the most widely used propellants for ion engines (sic)- and I also just so happened to stumble on a graph that says the Moon could feasibly capture xenon into its atmosphere.

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I'd basically like to ask whether or not xenon from ion engines could be trapped in the Moon's atmosphere. Obviously the concentration would be insanely negligible. I just honestly want to know if I'm reading that graph right.

Note: I know near nothing about ion propulsion, my current understanding is that particles are accelerated using electromagnetic interaction on high density elements. If this basic understanding is wrong let me know. From what uhoh has said though it seems more likely the xenon would just embed into the surface if aimed at the Moon or travel extremely fast and escape.

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    $\begingroup$ Note that the velocity of ion engine exhaust is at least an order of magnitude larger than the escape velocity in this graph. It's less about "holding an atmosphere" than it is "shooting at the moon with a gun". $\endgroup$ Sep 5, 2019 at 21:24
  • $\begingroup$ Hehe... @Hohmannfan I made an edit after thinking more on uhohs response to me eariler. So the regolith would eat it essentially? Can i get numbers on xenon exhaust velocity? $\endgroup$ Sep 5, 2019 at 21:26
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    $\begingroup$ The range is 10 - 100 km/s, and depends greatly on the type of thruster used. I will make no claim to know what happens when it hits the regolith. High-velocity collisions are very unintuitive, and I will not go good for anything but actual research. $\endgroup$ Sep 5, 2019 at 21:31

2 Answers 2


Apparently there isn't much xenon, from any source, in the Moon's "atmosphere".

I put "atmosphere" in quotation marks because it's actually an exosphere, where the distance an atom or molecule travels between collisions with other atoms or molecules is greater than the scale height. In the Moon's case, it is much greater. Overwhelmingly, particles (atoms or molecules) in the Moon's exosphere are on suborbital trajectories, from the point where they were kicked (or bounced) off the surface to the point they re-impact.

The best measurements of lunar exospheric composition were made using the NMS instrument on the LADEE lunar orbiting spacecraft. NMS is a Neutral Mass Spectrometer with a quadrupole mass analyzer, capable of measuring species up to 150 units atomic mass (I think "units" is the current term du jour; for a while it was AMU, "atomic mass units", then it was "Daltons", but Dalton must have done something non-PC because it appears that term has been abandoned). The 150 unit spec is fortunate because the xenon isotopes average out at ~131 units, so they're within LADEE-NMS's range.

However, the quoted abundances of elements detected in the Moon's exosphere don't include xenon. In the Wikipedia article they quote abundances down to 17 particles per cc (for potassium — not your usual atmospheric constituent!). They quote other noble gases: helium, 5,000 - 30,000 particles per cc; neon, up to 20,000; and argon, 20,000 - 100,000. But no xenon, or krypton. The helium, neon, and argon are delivered by the solar wind, and krypton and xenon should be also (sorry — paywall) ... so where are they?

My theory is that the sublimation temperatures of krypton and xenon are high enough to make a difference. If you look at the sublimation temperatures of the noble gases, the light ones (He, Ne, and Ar) are all below 100 K. Those of the heavier ones are above 100 K. If the same mechanism proposed for trapping water in the permanently shadowed regions of the lunar poles is cold enough to trap water on eon time scales, then it could trap the heavier nobles also.

Should we look to the lunar poles as a source of Kr and Xe for future lunar colonies? Maybe! ... if they need it.

As far as the behavior of Xe atoms expelled from ion engines, as stated in the comments they are travelling much faster than lunar escape velocity — bye bye, xenon atoms! Unless — they impact the surface.

The speeds of the ion-thruster xenon atoms are not all that different from solar wind speeds, so they would behave as did the atoms impacting the Genesis spacecraft's collector wafers. (I was on the Genesis flight operations team as the unofficial "Mission Generalist" — I did all kinds of multi-disciplinary calculations and analyses nobody else wanted to do) Those impacting atoms were imbedded into the crystal lattice of the wafers. They would do the same into the crystals of lunar surface materials. But when the crystals get warm, and on the moon they get very warm during the day, lattice thermal vibrations mobilize those imbedded atoms: they can move around, slowly, pretty much Brownian-motion style. Eventually they make it to the surface of the crystal and are ejected. If the ejecting surface is on the lunar surface and the atom leaves with an upward velocity component, then voila! it becomes an exospheric atom. That ejection energy would be far less than the implantation energy.

Such an atom could be bounced around the lunar surface for some time, maybe even at times re-imbedded (shallower than the original event) and re-ejected, until one of its bounces takes it to a permanently-shadowed region. And there it stays.

This loss mechanism could result in atmospheric Kr and Xe abundances far, far less than those of the lighter noble gases.

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    $\begingroup$ This is a great answer! Once a Mission Generalist, always a Mission Generalist ;-) $\endgroup$
    – uhoh
    Sep 5, 2019 at 23:45
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    $\begingroup$ Thanks, @uhoh! Actually there is a mechanism for getting heavy atoms out of the permanently shadowed regions: cosmic ray impacts, or deflected (by local E or B fields) solar wind ion impacts could dislodge adsorbed atoms. But it would be a slow process, overwhelmed by the sequestering process, so the net heavy-nobles abundances in the exosphere would lag far below the light nobles that don't get sequestered. $\endgroup$ Sep 6, 2019 at 0:19


There certainly is human-provided xenon on the moon, albeit in infinitesimally small amounts. Don't think ion drives, think Apollo. There's no reason they would have purified every bit of xenon out of the air supply for the lunar lander and plenty of air was leaked to the surface of the moon.


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