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In my world, an event occurs in low orbit around the Moon that releases a tremendous amount of energy in the form of a conversion explosion and gamma rays pulse. Said pulse expands outwards and smacks into the hemisphere of Earth currently facing the Moon, instantly killing nearly everything in that area.

The exact amount of radiation required to accomplish this is unimportant, as is the means to generate it. My question is merely: is the end effect of this event - half of Earth instantly dead via a gamma radiation pulse - scientifically plausible?

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    $\begingroup$ Please define precisely your terms here: gamma rays are EM radiation, so are radio waves. $\endgroup$ – L.Dutch - Reinstate Monica Feb 26 at 5:51
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    $\begingroup$ @L.Dutch-ReinstateMonica Apologies. You can assume gamma rays. $\endgroup$ – SomewhereOutInSpace Feb 26 at 5:52
  • $\begingroup$ Please put that in the question $\endgroup$ – L.Dutch - Reinstate Monica Feb 26 at 5:54
  • $\begingroup$ @L.Dutch-ReinstateMonica he did say full spectrum. $\endgroup$ – The Square-Cube Law Feb 26 at 5:58
  • $\begingroup$ @TheSquare-CubeLaw, in the comment posted after mine the statement is "assume gamma rays" $\endgroup$ – L.Dutch - Reinstate Monica Feb 26 at 6:11
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Radiation: absolutely yes.

But Gamma radiation is a bad fit.

The atmosphere quite effectively blocks Gamma and X-ray radiation. Even ultrahigh energy photons are very well blocked by the atmosphere. By the time the radiation at groundlevel becomes dangerous, you are in much more danger due to the atmosphere above you becoming red hot+ from absorbed energy.

But: the atmosphere is only middling effective at stopping true Cosmic Rays. i.e. relativistic speed particles, mostly protons.
Actually the upper atmosphere does stop the incoming particles quite well. Unfortunately, the act of stopping them is like stopping a bullet by having it hit a porcelain cutlery set. You stop the bullet, but you are left with a huge number of shards, splinters and debris that now carry a part of the energy with them.

If your disaster event around the Moon released not Gamma but high energy particles, then yes the Earth could suffer.
Of Course, it would have to be an event of truly mindboggling magnitude, as the energy release will be non-directional, and the Earth covers only 1/32000th of the visible sky from the Moon's distance. Then, any energy that reached Earth is spread over the entire facing side of the Earth, some 250 million square kilometers.

As for "Instantly Kill"
If you really need to kill everything instantly then heat pulse is your only option. This is not a good idea, as I believe you want the other side of the Earth to survive? Instantly immolating half of Earth will kill the whole Earth within hours, as the heat will cause a shockwave to circle the Earth, scouring the surface all the way.

If you just need to kill everyone on the surface soon, as in within an hour or so, that is much more achievable.
Acute radiation poisoning, if severe enough, will cause cell membranes to depolarize, causing the nervous and respiratory systems to collapse. The dosage require is truly monstrous though, on the order of 50 time normal "lethal" dose. Figure 200 Sievert or more.

If you can wait a month or so for the dieoff, you only need about 8-10 Sievert. This is the normal "lethal, no chance" radiation dose that kills in several days to about a month.

P.S. No matter what the dosage, as the radiation is only in the form of high energy particles, the residual radiation will very rapidly fade down to reasonable levels. You would not permanently contaminate the whole planet unless your radiation pulse also included a lot of Neutrons. (In which case, bye-bye whole planet. for many millions of years)

P.P.S. Except for residual radioactivity, the Ocean will be unaffected. Water is an amazing radiation shield to all kinds of radiation. The energy of radiation needed to sterilize the ocean depths would achieve that sterilization by explosive boiloff of the water, before the radiation itself is an issue.

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  • $\begingroup$ I do actually want the other side of Earth to survive - some escape before chunks from the shattered Moon start raining down on them - so the high-energy particles are exactly what I needed. Thank you! $\endgroup$ – SomewhereOutInSpace Feb 26 at 14:45
  • $\begingroup$ On your "P.S", neutrons create residual radiation because they transmute ordinary matter, getting captured on nuclei, changing them from stable isotopes into unstable ones. Some of those isotopes decay quickly, others linger. Protons do that too: neutrons are known for it because they do so more efficiently at low energy, but to get through the atmosphere (via porcelain!), the incident protons need to be have TeV energies (why Fermi/GLAST is in space but HESS & MAGIC are on Earth). The threshold for nucleus capture is in the MeV range: your proton burst will produce lots of residual radiation. $\endgroup$ – Jim Pivarski Feb 26 at 15:54
  • $\begingroup$ @JimPivarski yes, but protons generally need so much energy to get to the nucleus, that they leave way too much energy in it for stability, resulting in the target nucleus kinda falling apart to get rid of the excess energy. Neutrons are much more likely to just glom on and stay for tea, leaving the target only somewhat unbalanced. $\endgroup$ – PcMan Feb 26 at 16:14
  • $\begingroup$ There will almost certainly be some survivors (not sure if this would be a problem). For example those in nuclear subs deep under water or those in deep mines. $\endgroup$ – Slarty Feb 26 at 18:05
  • $\begingroup$ @PcMan The shower has a profile from the few TeV interactions to the many MeV interactions as the energy spreads out into many particles. On the MeV end of the profile, the protons are energetic enough to overcome the nuclei's Coulomb barrier and also slow enough to glom on. The special thing about neutrons is that they continue to have this property at yet lower energies where the Coulomb barrier would be a problem for the protons. But anyway, it's the secondary/tertiary/quaternary particles that matter, long after the process has no memory of whether it was started by protons or neutrons. $\endgroup$ – Jim Pivarski Feb 26 at 18:44
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Yes But...

For a gamma ray blast powerful enough to instakill half the planet, the EM radiation levels required would superheat the atmosphere and everything else underneath, incinerating it all. It wouldn't be "radiation poisoning suddenly causes everyone to drop dead in their tracks, leaving everything else mysteriously unharmed", it would be a massive fireball roasting everything facing the moon.

Also note that the side of the Earth opposite the blast will then have to deal with the massive shockwave caused by half the planet's atmosphere instantly being heated to nuclear fireball temperatures and all of the follow-on effects from that. There is going to be scouring winds, tsunamis, flashfires, and all sorts of other fun stuff like that. I'd imagine that the blast could strip off a sizeable fraction of the planet's atmosphere, depending exactly how hot things wind up getting.

All-in-all, the people on the side facing the moon might be the lucky ones, given what the survivors on the other side will be left to deal with.

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  • $\begingroup$ +1 but for that to happen, you need to detonate stuff with the bang of a nova. $\endgroup$ – The Square-Cube Law Feb 26 at 6:57
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    $\begingroup$ He doesn't specify how much is being converted, and the upper limit of E=mc^2 is very, very large. So consider this a Mythbusters-style "replicate the expected results" answer. ;-) $\endgroup$ – Salda007 Feb 26 at 8:25
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Since you are assuming gamma rays, let's give a look at the closest equivalent we have available: atomic bombs

Energy from a nuclear explosion is initially released in several forms of penetrating radiation. When there is surrounding material such as air, rock, or water, this radiation interacts with and rapidly heats the material to an equilibrium temperature (i.e. so that the matter is at the same temperature as the fuel powering the explosion). This causes vaporization of the surrounding material, resulting in its rapid expansion.

I would say that it is more than enough to kill everything that happens to be showered with gamma rays.

Incidentally, gamma rays are also used to sterilize items we deal with in our daily life.

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    $\begingroup$ Problem with this is that a nuke big enough to sterilize one hemisphere of the Earth, whether via Gamma or simple heat radiation, will instantly heat some 250 million km2 of the surface. This would cause the mother of all shockwaves, scouring the entire planetary surface down to many meters. It is not stated explicitly, but it sounds as if the OP want the other half of the planet to survive. We need the explosion event to not release enough heat energy to do this, yet still generate enough radiation to kill. $\endgroup$ – PcMan Feb 26 at 8:26
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Nope

Our upper atmosphere is practically opaque to gamma rays:

A graphic showing the atmosphere's absorption bands for electromagnetic radiation. Gamma rays are 100% absorbed

The Earth has even been hit by gamma ray bursts in the past. While there was damage to life, it was not in the way you imagine. From the Wikipedia article on gamma ray bursts (which are stellar system destroying events):

Earth's atmosphere is very effective at absorbing high energy electromagnetic radiation such as x-rays and gamma rays, so these types of radiation would not reach any dangerous levels at the surface during the burst event itself. The immediate effect on life on Earth from a GRB within a few kiloparsecs would only be a short increase in ultraviolet radiation at ground level, lasting from less than a second to tens of seconds. This ultraviolet radiation could potentially reach dangerous levels depending on the exact nature and distance of the burst, but it seems unlikely to be able to cause a global catastrophe for life on Earth.

The article does cite some long lasting effects of duch a burst, but nothing that comes close to what you wanted.

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    $\begingroup$ The figure does not include high energy gamma — 0.1 nm corresponds to 12 keV, which is very low energy. Gammabursts are in the range 1MeV to 1 TeV. $\endgroup$ – Stefan Feb 26 at 15:37
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    $\begingroup$ The atmosphere is not opaque to gamma rays, it just thick enough that that absorption it does have (half/200 meters) is enough to prevent normal bursts from reaching the ground, a strong enough burst can overcome this. $\endgroup$ – John Feb 26 at 16:16
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    $\begingroup$ @Stefan Actually, Epeak for gamma ray bursts is commonly a few hundred keV. And, in any case, the atmosphere is opaque to all gamma ray energies for practical purposes. Sure, a burst powerful enough to blow the atmosphere away would get through... $\endgroup$ – John Doty Feb 26 at 20:30

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