Per this question I've been giving thought for some time to restricting the use of antimatter and it occurs to me that an object lesson in what can go wrong might go a long way to discouraging its use.

To that end what would the aftereffects of a reasonably large amount of antimatter going off look like after several hundred years?

For the purposes of answering the question one metric tonne of antiprotons broke containment on, for all intents and purposes, the surface of the moon three hundred years ago. Does the site still glow because of daughter isotopes or is it simply a big hole in the ground, how far would the debris be spread etc... The focus is really on what the site looks like, from L1, what legacy remains from this largest industrial accident in history.


5 Answers 5


Contrary to L.Dutch's statement above, annihilation of baryons eventually produces only gamma rays, but in the short-to-medium term you get all sorts of interesting daughter particles not all of which are unstable. Only electron-positron annihilation gives you nothing but gamma rays.

According to the source of all knowledge:

Thus, when a proton encounters an antiproton, one of its quarks, usually a constituent valence quark, may annihilate with an antiquark (which more rarely could be a sea quark) to produce a gluon, after which the gluon together with the remaining quarks, antiquarks, and gluons will undergo a complex process of rearrangement (called hadronization or fragmentation) into a number of mesons, (mostly pions and kaons), which will share the total energy and momentum.

In a vacuum, these annihilation products can travel quite some distance before they decay into gamma rays or stable particles such as electrons and positrons:

lifetime and range of unstable annihilation products in a vacuum, as shown in a magnetic nozzle of a beam core rocket

The mostly likely reaction involves the production of some neutral pions (shown as π0) which decay almost immediately into high-energy gamma rays, and also some charged pions (shown as π+ and π-) which have a short lifetime but are also travelling exceedingly fast and can therefore interact with nearby matter. The pions decay into charged muons (shown as μ+ and μ-) which are even more stable, and can travel for quite some distance in a vacuum. These, too, can interact with surrounding matter. The pions eventually decay into electrons and positrons (shown as e+ and e- respectively), the latter of which can annihilate an electron either produced by the decay chain or in surrounding matter producing more gamma rays

Those early charged pions have a kinetic energy of >200MeV, which does exceed the atomic binding energy of most light elements. The interaction cross section with a nucleus will be small, and the chances of the pion scattering off the nucleus will be reasonable, but some proportion of nuclei in the matter surrounding the blast might in fact be fissioned. Similarly, the >200MeV gamma rays coming out of neutral pion decays can also cause photodisintegration of nuclei.

Certainly, nuclei as light as aluminium might be smashed up by this process... possibly slightly heavier nuclei too, but I'm unclear on that. The resulting fragments may be radioactive, and they may be long lived unstable isotopes. The amount of induced radioactivity is obviously going to be nonzero, but it isn't clear how much there will be (and the question might be too hard to answer).

An uncontrolled release of bulk antimatter is also going to result in some of it being blasted away from ground zero by radiation pressure without necessarily being annihilated immediately, as you'd want if it were an antimatter rocket or warhead.

This means you've got an expanding shell of ionised antimatter that can also interact with surrounding material. Individual antiparticles will eventually interact with a matter particle, but obviously a single antiproton or antineutron cannot annihilate anything larger than a hydrogen-1 atom. Anything larger will end up being being transmuted into a different isotope or element, and the resulting release of energy by the annihilation will be soaked up by spectator nucleons which may in turn cause the atom to fission.

Does the site still glow because of daughter isotopes

Normal nuclear blasts don't have a radioactive glow in the aftermath, and antimatter blasts are no more likely to produce such an effect.

There's some scope for long-lived radioactivity, but in the absense of neutron activation and waste actinides from any fission byproducts, it seems likely that it will be much less. Even more so for the moon which has less metal than the Earth does, given its lower density, and it is often metals in the soil and rocks that undergo neutron activation to produce that long lived background radiation after a nuclear explosion.

or is it simply a big hole in the ground, how far would the debris be spread etc...

Some of the debris will certainly exceed local escape velocity. It will likely hit Earth, though I suspect that with only a tonne of antimatter (equivalent yield 40 GT) there won't be enough to cause an exciting meteor shower. The flash will probably be visible to the naked eye if you were looking in the right direction, and there might be some satellite casualties from debris.

You'll get little tiny bits scattered all around the moon's surface, but mostly what you'll get will be a nice big hole, not too unlike the other nice big holes already there.

Regarding the possiblity of a moon-shattering kaboom raised by the square-cube law, consider Tycho crater:

Tycho crater

Its a biggun... >80km across, and clearly visible from Earth (possibly with the aid of binoculars, if your eyes aren't great, but even so) but there are plenty of larger craters on the Moon, which remains resolutely intact.

I wasn't able to find much good research on the nature of the Tycho impactor, but having a fiddle with the charmingly old-school-looking lunar crater simulator, I was able to come up with something about 12km across and travelling at about 15km/s for a total kinetic energy of ~1.5 x 1023J, or ~36 teratonnes TNT equivalent. Even if it were wrong by two whole orders of magnitude, that's still a much bigger boom than the antimatter cooking off, and it'll do a much better job of shovelling regolith and rock around.

A tonne of antimatter will make a fearsome blast, but still small beans compared to a reasonably sized space rock. The moon isn't going anywhere.

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    $\begingroup$ "will be a nice big hole, not too unlike the other nice big holes already there." this one may be exceptionally glassy, as the heat is radiation generated (in contrast with generation by a massive impactor) $\endgroup$ Oct 21, 2021 at 13:58
  • $\begingroup$ @AdrianColomitchi I'm not sure how much glass you'd get after a largely-gamma-generating ground-level explosion in a vacuum. Maybe you'd generate a fireball in the ejecta which is a nice source of radiant heat, but I think that most of the early melt will be ejected from the crater. You might get tektites, of a sort. $\endgroup$ Oct 21, 2021 at 14:07
  • $\begingroup$ I'm betting for a heating in depth on the bottom of the crater. I feel the high energy gamma will have quite a small collision cross-section with the matter so I expect deeper penetration of... ummm... the heating element. Which means slower, more uniform cooling of the melt. $\endgroup$ Oct 21, 2021 at 14:34
  • $\begingroup$ @AdrianColomitchi you'll get fine ejecta falling back onto it, though. It is possible for big impacts to generate melting too... you'll end up with a different crater shape (maybe no central peak for a big nuke hole?) but the bottom could be quite similar. $\endgroup$ Oct 21, 2021 at 14:42
  • $\begingroup$ The induced radioactivity might be enough to physically damage materials, producing unusual amounts of fine dust or changing the surface albedo... $\endgroup$ Oct 21, 2021 at 17:14

Matter-antimatter annihilation produces gamma photons, nothing more.

enter image description here

When gamma photons interact with matter they can produce:

Photofission is a process in which a nucleus, after absorbing a gamma ray, undergoes nuclear fission and splits into two or more fragments.

Gamma radiation of modest energies, in the low tens of MeV, can induce fission in traditionally fissile elements such as the actinides thorium, uranium, plutonium, and neptunium. Experiments have been conducted with much higher energy gamma rays, finding that the photofission cross section varies little within ranges in the low GeV range.

Considering that the moon is not that rich in those elements, you won't have appreciable after glow.

As noted by PcMan:

As a general rule, antimatter does not bruise and infect other atoms with neutron bombardment the way nuclear fission or fusion reactions do. It just shreds the atoms, leaving either very much lighter atoms, or more usually just converting the lot into a strong flood of gamma rays that then just go on to massively heat up any unconsumed matter around it, making a huge thermal explosion not much secondary radiation. The residual glow-per-megaton of antimatter is a fraction of a fraction of the same size fusion explosion.

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    $\begingroup$ As comment because you already say the same: As a general rule, antimatter does not bruise and infect other atoms with neutron bombardment the way nuclear fission or fusion reactions do. It just shreds the atoms, leaving either very much lighter atoms, or more usually just converting the lot into a strong flood of gamma rays that then just go on to massively heat up any unconsumed matter around it, making a huge thermal explosion not much secondary radiation. The residual glow-per-megaton of antimatter is a fraction of a fraction of the same size fusion explosion. $\endgroup$
    – PcMan
    Oct 21, 2021 at 5:12
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    $\begingroup$ I upvote your answer and have question about secondary annihilation. Some antimatter would be scattered around due to radiation pressure. 1. Some would react not only with free protons, but atoms and molecules of moon surface. What consequences will this lead to? 2. Another part would be dropped to outer space. How far would this blowout be? Suppose there are no atmosphere. So anti-protons would fall to surface for some time (which depends on blowout distance and moon gravity). Could you estimate how long it would be? P.S. Unfortunately I'm unable to estimate so can't add my own answer. $\endgroup$
    – ADS
    Oct 21, 2021 at 7:14
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    $\begingroup$ Part of the reason antimatter is considered a potential fuel for long-term inhabited starships is precisely that there aren’t many byproducts. $\endgroup$
    – SRM
    Oct 21, 2021 at 13:02
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    $\begingroup$ @SRM wrong. it is considered a potential fuel because the annihilation byproducts include a number of charged particles, whose kinetic energy can be readily extracted for power generation or deflected for thrust. $\endgroup$ Oct 21, 2021 at 13:54
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    $\begingroup$ Don't forget thermal afterglow. Gamma penetrates deeply, so would heat the Lunar rock to considerable depth. This gives a situation similar to (but probably much larger than) what's seen in tufa after a volcanic eruption: ash/pumice deposits on the slopes of Mount Mazama (aka Crater Lake, Oregon) are still hot enough to boil water percolating into the formation after 5500 years. The surface of this crater might still glow faintly in visible light (and surely would, brightly, in long IR) after only a century. $\endgroup$
    – Zeiss Ikon
    Oct 21, 2021 at 14:20

I appreciate it that my fellow users have gone on extensive detail about the particles generated by matter and anti-matter interactions.

I am a big fan of both Starfish Prime and L.Dutch, but I must contradict both. You guys are focusing on the particles, but I wanna focus on the practical effects.

A metric ton of antimatter will react with a metric ton of matter to release 49,930,843,263,157 kWh, or about 50 petawatts-hours if we round it slightly up. $E = mc^2$ and all, converting joules to kWH. Actually I used a calculator because I am lazy.

For comparison, that is about 80x the sum of the yields of every nuke ever detonated.

Now take into account that the Moon has about 1% the mass of the Earth.

And then check this Kurtzgesagt video detailing what would happen if you detonated a 100 megatons nuke on the Moon.

That nuke would be a fraction of everything nuclear we ever detonated, and it would already be able to level every structure on the Moon due to Moon's small size and mass. I'm leaving an illustration of the shockwaves here because it is so beautiful.

Shockwaves from a Tsar Bomba-like nuke detonating on the Moon Source: the video I linked to a couple paragraphs above.

You throw in every nuke ever detonated at once on the Moon, then rinse and repeat eighty times...

For the record, the way the Moon gets damaged in this antimatter anihilation scenario is very different than if it had been nuked or hit by an asteroid. Practically all the energy will be in the form of gamma rays. For an analogy, the difference between the nuke and this scenario is the same as being hit by a bullet and looking through the beam of a particle accelerator with the same energy output on you. TL;DR a considerable portion of the Moon will be vaporized very fast, and a lot of matter will even escape her gravity well. That may cause some quakes. I don't know how violent those would be. I just imagine that the sheer amount of power being released might be enough to emulate the shockwaves of a large nuclear bomb.

There are two reminders that will last forever and which observers will be able to easily verify. The first is that any structure over the Moon built prior to the antimatter escape will be utterly destroyed. The second is that any map of the surface of the Moon from after the escape will have a really big crater that is not present in previous maps. How big? I imagine big as in visible with binoculars from the Earth.

Last but not least edit: I am not 100% sure, but I think the absurd amount of gamma radiation packed in such a tight place might cause photodisintegration, which could lead to some stuff becoming radioactive. I don't know if this would make the resulting crater significantly radioactive - on one hand most stuff exposed to gamma does not go radioactive, but on the other hand, you applied a divine amount of radiation to a very small volume of stuff.

  • $\begingroup$ Crater will be, but not a radioactive afterglowing crater, as the title suggest. $\endgroup$ Oct 21, 2021 at 14:51
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    $\begingroup$ Note that the energies mentioned in photodisintegration are in the order of 10MeV. The rest mass of a proton is 900-something MeV. O(10MeV) may happen, tho', since I don't think total photo-annihilation will take place. I also noted that the products of photodisintegration don't have generally O(100y) half-life, so the radioactive afterglow will subside faster than the requested "several hundred years". $\endgroup$ Oct 21, 2021 at 15:16
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    $\begingroup$ I've added a comparison to the Tycho impact in my answer. It was a lot bigger than a mere tonne of antimatter. The new crater will probably be a few km across, but not naked eye visible, I suspect (unless it was in a maria and had a contrasting colour, but even then...) $\endgroup$ Oct 21, 2021 at 15:49
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    $\begingroup$ @J... thanks, I don't know what's gotten into me today. I'll fix. $\endgroup$ Oct 21, 2021 at 19:45
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    $\begingroup$ @J... thank you! Fixed :) $\endgroup$ Oct 21, 2021 at 19:58

Possible, but extremely unlikely. You may want to put some ammonium nitra... err, pardon me... some well chosen elements susceptible to generate radioactive byproducts nearby and you will need lots of them, the paths to achieve this is very narrow.

The afterglow over hundred of years can be generated by the creation of massive amounts of slow decaying radioactive elements (not enough energy to do that, not even supernovae manage to get significant percentages of those) or decent amounts of radioactive isotopes with half-lives of tens or hundred years (not that many of those). All the isotopes with half-lives shorter than years (and there are lots of them) will show insignificant activity after half a century.

But let's see some details:

The annihilation with antimatter by itself is a "clean" process - you had mass, now you have lots of energy. As very energetic γ rays.

The interaction of gamma rays with matter will produce very little amount of fissionable elements - you will need heavy isotopes with very long half lives that are "pushed over the edge" and decide to hurry up under the jolt of heavy γ bombardment. But those that rush to do it will only result in daughter isotopes with very short half-lives, so the extra energy will only add to that of the explosion.
In any case, heavy nuclei are rare on the Moon soil, not enough supernovae explosions to create radioactive elements and Moons around in the last billion years or so.

There are two mechanisms on which creation of new radioactive species may still happen:

  • secondary neutron-antineutron pairs generation, with the capture of the neutron by a light nucleus - given that the neutron-antineutron pair have high energies, the capture of the energetic neutron is unlikely, so you can discount this one

  • fusion of light elements generated by the inertial confinement under the pure radiation pressure (see also NIF and HiPER). However, the energy you throw by a puny 1t of matter-antimatter explosion is insufficient for a massive production of heavy isotopes with medium half-life - you'd rather need a supernova explosion and then running around it to collect the products from an expanding shockwave spanning some light-years.


One way to achieve an afterglow would be for the energy released in the explosion to warm the place up so much that it emits thermal radiation. According to Wikipedia, a temperature of 4000 K would give a red glow, 5000 K would give a yellow glow, and so on.

The energy released by 1,000 kg of antimatter annihilating with an additional 1,000 kg of normal matter is given by E = mc2 ≈ 1.8 × 1020 J. If the accident occurs underground then the vast proportion of this energy could be converted to heat (otherwise more would be lost in the form of photons escaping). I couldn't find specific data about the moon, but we can make a wild guess that moon rock has a heat capacity somewhere on the order of 1,000 J kg-1 K-1. This means the energy released would be sufficient to heat about 4.5 × 1013 kg of moon rock by 4000 K.

For comparison, the moon itself has a mass of about 7.3 × 1022 kg, so either you'd be doing this to an asteroid instead, or the heat would dissipate to the rest of the moon via conduction and convection. But the explosion itself is also localised in a much smaller area which would be a lot hotter than 4000 K, so it would take some time for the heat to spread out to a mass of 4.5 × 1013 kg. The moon has a density of about 3.3 kg m-3, so if we assume the heat spreads out equally in all directions, it would form an approximate half-sphere of radius ≈ 19 km.

I'm not sure how to estimate how long it would take for the heat to spread out this far, or how long it would take after that for it to spread out further so that it stopped glowing; but if the glowing part of the surface has a radius on the order of 19 km it seems reasonable to suppose the glow would last for quite a while, and the centre of that radius would probably stay hotter than 4000 K for a while too. Given its size, the glowing area should be visible from space.


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