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The title is my question. I'm writing a short story and I'm considering have the aliens develop radiation weapons for use against humans and I'm trying to figure out a way that they could be both naturally protected from it along with any technology.

If lead in the exoskeleton wouldn't be tenable in a realistic sense, is there any biological trait that could explain it?

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    $\begingroup$ There is not very much iron in human bones. $\endgroup$
    – Willk
    Jul 24 at 1:12
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    $\begingroup$ What kind of radiation? Lead is effective against gamma or x-rays, but can produce secondary radiation with charged particles and is not as effective against neutrons. A loosely-woven copper mesh is effective against microwaves. Lead-rich substances seem like unlikely components of an exoskeleton anyway, due to their poor strength to weight ratios. $\endgroup$ Jul 24 at 1:22
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    $\begingroup$ Why naturally protected, humans aren't naturally protected against bullets(explosives, etc whatever) as an example. Avoiding selfharm is done by specifics of design of weapon, by procedures of handling and using it. $\endgroup$
    – MolbOrg
    Jul 24 at 7:29
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    $\begingroup$ I'd expand what @MolbOrg says by noting that one normally develops weapons that can harm them so they can harm their opponents from the same species. Unless humans are somehow resistant to weapons the aliens normally use when fighting among themselves they would repurpose those weapons, not invent new concept, because designing new weapons is a lengthy process. $\endgroup$
    – Jan Hudec
    Jul 24 at 17:01
  • $\begingroup$ @MolbOrg Think pesticides, herbicides, fungicides, anti-biotics, etc. Humans have developed a very wide range of weapons specifically to kill our non-human enemies that we are naturally resistant too. Weapons that harm ourselves need to be limited in scope to not kill the attackers too, but using radiation weapons means the aliens can just denotate some dirty bombs over our cities and move in once we have all died. Irradiating the planet also prevents new human infestations from moving back in for many years. $\endgroup$
    – Nosajimiki
    Jul 27 at 14:01
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A small amount of lead - or even a large amount of lead - in an exoskeleton isn't likely to protect against gamma radiation, and even humans are protected against alpha and beta radiation by their skin. A lead exoskeleton would have to be impractically thick and heavy to attenuate high-energy gamma rays.

However, all is not lost. Terrestrial lifeforms are susceptible to radiation due to the sensitivity of DNA to modification by ionising radiation or the ions that it produces as it passes through a cell. However, even in terrestrial life forms, some are far more radioresistant than humans. This assumes that these aliens have DNA as their genetic material, an assumption that is not a certainty by any stretch of the imagination.

These aliens, which the OP desires to be radioresistant, may not use DNA as their genetic material at all. Without going into a lengthy speculation as to what mechanisms of inheritance they may use, it should suffice to say that the various possible mechanisms may be more or less radioresistant than DNA.

Should these aliens use a particularly radioresistant genetic mechanism, the dose of radiation required to cause illness or death may be such that illness or death is caused not primarily by genetic damage, but by destruction of other structures necessary for the processes of life... and these doses of radiation may be so high as to make even the most radioresistant terrestrial lifeforms appear positively fragile to these aliens, potentially on the order of a million Gray, a hundred thousand times that which is acutely lethal to humans, and over thirty times that of the most radioresistant terrestrial lifeforms.

So, these aliens may be highly radioresistant not due to any mechanism by which incoming radiation is attenuated, but rather by their cellular mechanisms being more radioresistant in themselves.

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Lead Not Required

Not only is lead not very effective for its weight, there is a much better approach that some organisms in nature actually use.

The only time cells are particularly vulnerable to radiation is when they are dividing. When a cell is not reproducing the chemical bonds in our DNA is quite strong and difficult for radiation to mess with, but once a cell starts to divide, our DNA has to go into a much more vulnerable state to allow for transcription to happen.

In mammals, like humans, radiation is pretty much always a problem because we always have cells dividing and reproducing somewhere in our bodies. So, for most mammal species, a dose of 350-1200 rads is fatal... however, most arthropods do not undergo continuous cellular reproduction. Because they have exoskeletons that they don't want to just out grow, they suspend nearly all cellular division when they are between molting cycles. During this period they average about 50 times as radioactive resistant as mammals with some species like the habrobracon hebetor wasp being able to survive over 180,000 rads. This is about the same amount of radiation resistance a human has when standing behind ~ 6.3cm of solid lead, 30cm of concrete, or 63cm of water... so not only is this WAY better than you would get by replacing your body's iron with lead, it would be even better than a human could practically design specialized armor against.

Just designing a radiation suit that can double a human's natural gamma radiation resistance weighs in at about 50kg.

So, the second you start to consider an animal with an exoskeleton at all, it is very like that you are looking at an alien that undergoes sprints of cellular division the way that insects on Earth do. If this is the case, then they should be able to use radiation weapons against humans just fine without any special add-ons like your leaded shell idea. They would just have to do their fighting when they are not molting.

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If lead in the exoskeleton wouldn't be tenable in a realistic sense, is there any biological trait that could explain it?

Actually water is a great radiation shield.

To quote Ranall Munroe's book "What If" on page 11.

"For the kinds of radiation coming of spent nuclear fuel, every 7 centimeters of water cuts the amount of radiation in half."

So if the aliens had an outer layer of water several cm thick they would actually be quite protected.

  • This could either be a totally inanimate fluid layer with a skin growing over it.
  • Or it could be a really sparse sacrificial tissue that contains a high percentage of water, and just falls off like dead skin when it gets old.
  • You could also have their inner skin have pores that secrete a thick gelatin like substance that coats their body many cm out.

Either way the design of the aliens would probably be quite chubby looking, and quite vulnerable to being punctured. Although, since the outer layer is sacrificial puncturing won't actually hurt them unless it gets really deep.

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    $\begingroup$ This answer fails to discriminate between different types of radiation, which is crucial. $\endgroup$
    – user77458
    Jul 24 at 16:47
  • $\begingroup$ 7cm per half resistance is still a lot of water for an organism to be carrying around on its exterior. While a water suit would be about 10% lighter than a lead suit, the differences in thickness would make mobility a pretty big issue to gain any significant resistance. $\endgroup$
    – Nosajimiki
    Jul 27 at 13:46
  • $\begingroup$ @BenCrowell He specifies in his citation: "the kinds of radiation coming of spent nuclear fuel". This implies that he means a combination of radiation types including gamma radiation. Anything you do to shield from gamma radiation will inherently also shield alpha and beta radiation... only way better. So, once you answer the gamma radiation problem, specifying types becomes a moot point. $\endgroup$
    – Nosajimiki
    Jul 27 at 13:52
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It depends on how strong the radiation weapons in question are. Let's assume that they're gamma radiation directed-energy weapons. Gamma is the kind that can't be stopped by dead skin cells (alpha) or a really thick jacket (beta).

The human body has 3.5-4 grams of iron in it, according to a quick Google search. I doubt that 7-8 grams of lead is enough to protect a human-sized entity from radiation being used as a weapon.

If you want these aliens to be rad-resistant to any notable degree, I'd probably move them away from carbon-based biology if I were you. Earth-style biology does not play well with high amounts of gamma radiation, and considering that it's being weaponized - i.e. actively used to harm people, meaning that the entities using it are specifically trying to subvert anti-radiation measures, it's not going to be something that can just be stopped with a bunch of shielding.

Now, if these aren't directed-energy weapons, it gets more interesting. If they're just scattering radiation into the atmosphere, an exoskeleton lined with a thin layer of some kind of metal would be enough to stop beta particles, and human skin can already stop alpha particles.

I'm not sure how they would filter out metabolized radioactive particles (i.e. eaten, breathed in, somehow inside their body, etc.) other than the iodine that humans already use to do so.

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