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In the story of my video game, I want a group of colonists to land at dawn on an alien planet and have to terraform or build protective structures before sundown (several earth days).

I want the death of the colony if they fail the lethal dose to be within less than an hour, so I would prefer to have a bad-science-instant radiation death instead of death from cold or creatures coming out at night.

I want the game to take place during the day.

The planet is a rocky planet with water.

Edit: There could be on a moon if required.

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    $\begingroup$ What if the planet doesn't become radioactive, but instead has a moon that is so radioactive that it will kill the colony as soon as it reaches a certain position in the (night) sky? $\endgroup$ – Anthony May 13 '18 at 8:33
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    $\begingroup$ Instadeath from radiation requires a stunningly high rad count, on the order of a gamma ray burst. Can they die within hours? $\endgroup$ – RonJohn May 13 '18 at 17:04
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    $\begingroup$ I'm not sure I agree. I was under the impression that a slow painful death wouldn't fit the plot, hence no slow freezing to death or being devoured one by one by animals. Dying from radiation poisoning over the course of hours is much more agonizing than freezing to death. In addition to vomiting, hair loss, bleeding from every orifice, and tooth loss, you would have to watch your fellow colonists also suffer through this at the same time. I would pick the freezing night planet any day of their solar week. $\endgroup$ – Anthony May 13 '18 at 23:03
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    $\begingroup$ Note that death by radiation in hours means central nervous system effects--you keel over in minutes. It also means roughly 30Sv of exposure. (The data at this level is obviously quite poor. This sort of exposure to a human only occurs with a criticality accident and there haven't been many of those in the world.) $\endgroup$ – Loren Pechtel May 14 '18 at 6:12
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    $\begingroup$ Does it have to be radiation? Radiation is a slow killer in most cases. Toxins on the other hand, can kill quite quickly. A plant which releases a nerve agent at night, and the chemical breaks down rapidly when it rises above a certain temperature. It could be as simple as having to huddle very close to a campfire to stay alive the first night. Maybe it's only lethal when it builds up in the bloodstream, so brief (<5 minute) forays at night would be fine, but permanent damage starts setting in within 20 minutes. Your body could get rid of the toxin during a couple hours away from exposure. $\endgroup$ – Martin Carney May 14 '18 at 16:10

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A highly radioactive gas that becomes much lighter when heated by the sun and rises to the top layers of the atmosphere or disperses into harmless levels.

I'm not sure if such a gas could exist and do that in such a dramatic and straightforward manner in an earth-like atmosphere, but i guess if it works for you it would be plausible in a planet with much lower atmospheric pressure (that could also change at times by moon gravitational fields or something), where the gas weight would be more significant in it's behavior.

Also such an event would be preceded by ominous windstorms, perfect for dramatic effect.

OR.. The gas is on higher layers of the atmosphere during the day and liquefies at night due to the temperature drop, causing a radioactive rainstorm with the radioactive liquid rapidly returning to gaseous form at sunup.

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  • $\begingroup$ Instead of a chemical that gets lighter a radioactive plant spore maybe? The rainstorm idea is great. $\endgroup$ – PStag May 14 '18 at 11:31
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    $\begingroup$ I don't really see how the spores add up in such a scenario... Maybe the plants emit the gas, like suggested in another answer, during decomposition, or just maybe..... as their perfume... (ominous smile) $\endgroup$ – Frixus May 14 '18 at 14:15
  • $\begingroup$ I thought spores like hot air balloons that heat up and rise during the day. But I much preferred the rainstorm idea anyway. If I put you in the credits would you want to go by Frixus? $\endgroup$ – PStag May 14 '18 at 17:46
  • $\begingroup$ I suggest adding some details about Pluto's Gate. en.wikipedia.org/wiki/Ploutonion_at_Hierapolis This cave emits toxic gases which settle in valleys like pools of death when the winds drop. The planet could be modified such that these toxic gases are emitted in much larger amounts over wider areas. Then also have the winds die down at night. Thus, dilution of the noxious miasma is minimized resulting in instant death. $\endgroup$ – Sandy Chapman May 15 '18 at 2:38
  • $\begingroup$ Another comment. A gas which is buoyant enough to float above oxygen and then reverse that buoyancy is incredibly unlikely. Radioactive gases either tend to be very heavy and sink or require being made radioactive via neutron radiation. Not impossible, but it's more likely the people are directly radiated rather than from some gas that only comes at night. $\endgroup$ – Sandy Chapman May 15 '18 at 2:43
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The ground worldwide might be rich in radioactive ores, but you have to dig at least half a mile down to find them.

However, many plants have roots that do go that far down. Those plants are radioactive. Staying away from them is enough to be safe by day.

But, by night, their flowers bloom, releasing radon, a radioactive noble gas.

The plants don't extract radon from the ground. It is a byproduct of the decay of thorium and uranium. Radon is a gas, and has a half-life of 3.8 days. The radioactivity will peak at night during blooms, but radiation levels might already be safe enough by day after the radioactive gas has spread thin.

In temperate areas, spring will be the most dangerous season of the year. In tropical areas, though, blooms might happen every single night, regardless of season.

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    $\begingroup$ Mmmm delicious radon honey. $\endgroup$ – Anthony May 13 '18 at 22:22
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    $\begingroup$ Why is the pollen contain radon? Pollen's a solid; radon's a gas. Unless the pollen is using the radon, it would make more sense to release radon through stomata. $\endgroup$ – tox123 May 14 '18 at 2:42
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    $\begingroup$ @tox123 you're right. I'll edit, thanks! $\endgroup$ – Renan May 14 '18 at 2:46
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    $\begingroup$ Won't work. "Radon is a gas, and has a half-life of 3.8 days." If you start with enough radon to kill you instantly, a tremendous amount of radon, now you have half as much which is still plenty to kill you. Radon gas is very heavy, which is why it accumulates in basements, and is unlikely to disperse easily. $\endgroup$ – Schwern May 14 '18 at 5:16
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    $\begingroup$ @schwern 3.8 earth days. On this planet days are "several earth days" if the radon was released at sunset it would have enough time to fall to a quarter naturally. And the plants would be absorbing the radioactive pollen through the night. $\endgroup$ – PStag May 14 '18 at 7:48
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What if the planet doesn't become radioactive, but instead has a moon that is so radioactive that it will kill the colony as soon as it reaches a certain position in the (night) sky?

Assume it is radioactive enough to have such an effect but weak enough that it has to be just high enough in the sky for it to be lethal. This adds an extra sense of urgency towards the end of the day, as they would be able to see the moon already having said impact a certain distance away and getting closer by the minute.

The camp could be in a box canyon with a high mountain creating a natural shield from the radiation which protects them from getting radiation poison before the moon is close enough to kill them instantly.

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    $\begingroup$ So what happens when the moon is high in the day sky ? Or does it follow an unusual orbit, that keeps the moon on the night side ? $\endgroup$ – Ghajini May 13 '18 at 10:02
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    $\begingroup$ @PStag: If the planet is tidally locked to the moon, that just means that, viewed from the planet's surface, the moon will always stay in (approximately) the same place in the sky. Which makes it useless for your purpose, since it will never rise or set (except in places where the moon is very close to the horizon, where it might bob up and down a bit if its orbit isn't perfectly circular). $\endgroup$ – Ilmari Karonen May 14 '18 at 5:52
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    $\begingroup$ @PStag: It isn't, at least not for long; the collinear Lagrange points are all unstable. (Also, that's not what tidal locking normally means.) $\endgroup$ – Ilmari Karonen May 14 '18 at 9:57
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    $\begingroup$ Can't work! We have a name for celestial bodies that give off that much radiation, so much radiation that it could slowly cook you. We call them STARS. Anything that radioactive would be glowing blue hot. $\endgroup$ – Aron May 14 '18 at 10:29
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    $\begingroup$ @Aron A small black hole would release a large amount of radiation and would not "glow blue hot". A small enough stable black hole won't suck things into it unless they actually fall into it, so it could maintain a stable orbit. $\endgroup$ – forest May 15 '18 at 0:42
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This solution is a bit extreme, but if your planet's solar system is doing something very exotic, you could get just that:

Have the entire solar system travel at relativistic speed through space

You can see some more specific details in this answer for a question of mine (predating worldBuilding.SE so asked on Astronomy.SE) about an even more exotic setup.

Basically, if the entire system is traveling very fast (90% - 99% of the speed of light, or even faster) relative to the galaxy it's passing through, than two separate phenomena create a single bright1, high-radiation circular patch in the sky:

  1. Light aberration and relativistic beaming means all the starlight reaching the planet is concentrated in a disk 'straight ahead' in the direction of the system's movement (size of disk becomes smaller the faster the system travels). This light is also strongly blue-shifted to a UV, X-ray and Gamma ray level (so radioactive and lethal)
  2. At those speeds, any interstellar particles the star hits will behave as cosmic rays (I encourage you to read the list of their effects - they are excellent for the epic instant death you need).

So, you have a very exotic planet (which is a good reason to try and build a temporary base on it - for research etc...) - which have a deadly "bright patch" in its night sky. During the day, there will still be insane atmospheric interference, lightning storms, winds etc. But the planet itself shields the base from the worst of the effect. But, a bit before the bright patch rises over the horizon things get much worse, and become instantly lethal when the patch is in the visible night-sky.

1: Note that this "bright patch" will likely not be visible to human eyes - it can be detected by instruments (and by its deadly effect), but with high enough velocity, it won't provide visible light.

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    $\begingroup$ Might be hard to land on a planet in such a system, though, unless you're departing another body in that system (in which case you'll already be adapted to the effects of this, or you'd never get as far as to space travel to begin with)... $\endgroup$ – a CVn May 13 '18 at 16:22
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    $\begingroup$ @MichaelKjörling, sort of - entering and matching velocity with the system requires FTL or an insane acceleration, but these are common in many sci-fi settings. After you reach the system and match velocity, the main difficulty is that your ship is also bombarded by that radiation and cosmic rays - so it better by shielded by first-grade handwavioum, but, aren't they all? ;) $\endgroup$ – G0BLiN May 13 '18 at 16:26
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    $\begingroup$ @VilleNiemi - keep in mind that this "bright patch" is very focused and with high enough velocity can be blue-shifted above the visible spectrum, so instruments will detect it, but it doesn't brighten the sky or provide any visible illumination. - I'll correct the answer's text to make it clearer, "bright" is maybe a poor word choice for this... $\endgroup$ – G0BLiN May 13 '18 at 16:32
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    $\begingroup$ The problem with this solution (and other similar ones, like a pulsar / black hole companion star emitting hard radiation) is that the radiation source will only be in the night sky for half the planet's year (and will only rise exactly at sundown on one single day of the year, if that). Which kind of makes one wonder why the colonists would decide to land on that exact day. $\endgroup$ – Ilmari Karonen May 14 '18 at 5:57
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    $\begingroup$ @LorenPechtel plausibly, though I reckon high-relativistic particles are actually less of a problem for the atmosphere than relatively calm solar wind is: cosmic rays just pass through the higher atmosphere without much effect, and in the lower atmosphere it doesn't much matter even if they hit lots of air molecules – the density is so high that the resulting ions won't get far. $\endgroup$ – leftaroundabout May 14 '18 at 9:46
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Have a magnetic anomaly by which the planet's magnetic field almost completely disappears on the night side. During the day, the solar wind of high-energy protons is deflected by the planet and focused to the poles. At night, the particles hit the planet's atmosphere, producing a shower of gamma radiation from the resulting collisions. The solar wind is just as strong at night as it is during the day because it takes it a few days to reach the planet so it doesn't vanish when the sun sets.

Night sky might have an inherent glow to it from the amount of radiation. Also the planet's poles would have an incredibly bright aurora, and the arctic circles would be uninhabitable.

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A highly radioactive asteroid belt, outside the orbit of the planet.

On the day-side, the asteroid belt is both further away and (very) partially shielded by the sun. On the night-side, the radiation is closer, and more direct/focused (at dawn/dusk the radiation is spread over a wider area due to the angle it arrives at, like the sun is in winter)

This would mean that the radiation would increase rapidly at a slowing rate until peaking at midnight, and then drop of slowly and an increasing rate - much like sunlight during the day.

But, really, radiation is a slow killer. Yes, less than an hour of exposure can be deadly, but it will kill you over a matter of days/weeks, not within that hour. With a poorly insulated planet set up right, you could go from ~20°C to ~-30°C in a couple of hours (The Sahara can drop from a 50°C day-time temperature to below freezing at night), and without proper protective measures the cold-shock will kill you within 10 minutes. Or the hypothermia will set in within 30 minutes, and kill you within 2 hours.

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The easiest way to get the effect is to have it coincidental to the daylight. There is simply a powerful source of hard radiation nearby at roughly the plane of the planet's rotation so that the surface is bombarded with radiation for half the day. The colonist then can choose which part of the day they can work to build their colony by timing their landing and oddly enough chose having natural light over nighttime.

That said, if they have spaceships and planet rotates fairly slowly they should be able to simply hop around the planet to avoid being exposed to the radiation. The ships would simply ferry people near areas soon to be exposed to radiation to an area that was recently protected from radiation.

Well, I am sure you have some workaround that sticks them to a single location and it is not really relevant to the question.

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  • $\begingroup$ Isn't this more or less the same as the Moon answer? $\endgroup$ – can-ned_food May 13 '18 at 20:11
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    $\begingroup$ @can-ned_food: Well, a radioactive moon (especially one so extremely radioactive as to produce instantly lethal radiation levels on the planet's surface) is rather physically implausible to begin with. A pulsar or a black hole with a jet pointed right at the solar system seems a lot more plausible, even if the precise alignment may require a bit of a lucky coincidence. $\endgroup$ – Ilmari Karonen May 14 '18 at 6:01
  • $\begingroup$ If the planet has a rotational axis tilted more like Uranus (97°) or Pluto (122.5°) instead of like Earth (23.5°) or Jupiter (3°) then A) the sun rises/sets throughout the year instead of the "day" and B) your pulsar-source could be above/below the solar plane, pointing into it, so that it affects the planet for half the "day" $\endgroup$ – Chronocidal May 14 '18 at 8:45
  • $\begingroup$ @can-ned_food Yes? I am trying to answer the same question, so my answer will be similar to other answers. Main reason I made a separate answer is probably (I have actually forgotten) because the moon itself is an unnecessarily complex assumption especially as regards to the link to the time of day. Any radiation source in direction opposite to the sun does better IMHO. $\endgroup$ – Ville Niemi May 15 '18 at 14:34
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You could have high daytime temperatures coupled with porous soil that continuously leaks radioactive gas. As it gets colder at night, the gas accumulates at ground level.

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Night is only correlated with the death radiation.

If your place is a moon tidally locked to a gas giant, night on the planet-facing side is closer to the star than the during the day. It may be the planet's magnetosphere is shaped so that it offers significantly less protection there.

If the solar system is being bathed in death radiation from an external source during part of the year night would be pointed straight at it. This means death happens for half a sidereal day rather than solar day and the effect would be exactly opposite half a (your planet) year later, but with a far enough orbit a half year might be long enough not to matter (100 Earth years on Pluto).

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Or swap day for night... The planet orbits a binary star system, where one star is as bright as normal day, and the other star is so bright it insta-kills.

Maybe make the other star a neutron star or something, so it doesn't show in visible wavelengths.

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  • $\begingroup$ Wouldn't the planet have to sit between the two stars for this to happen? Such a system would be too unstable to exist for long enough for a planet to form. $\endgroup$ – Martin Carney May 14 '18 at 16:02
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In line with Rennan's plant suggestion, I was thinking of highly radioactive Horta-ish animals that come out at night in droves. For some reason they sleep in lead houses during the day :-)

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The whole planetary system lays in a proximity of a place where intensive astronomical explosion occurred recently (less than a million years ago) like supernova or neutron star/blackhole collision. It would result in very high radiation from a matter that was ejected by the event.

The only problem that I see, is that the phenomenon would be seasonal: during some seasons the radiation would appear at nights and during some seasons it will be daily. You can at least put some protoplanetary matter in an inner orbit to prevent daytime-radiation from occurring during some seasons...

a simple sketch of the setting

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I like the "radioactive moon" idea, but you could do it the other way around: the planet is a moon of a highly radioactive gas giant. As long as the colony is facing away from the gas giant, it's protected from the radiation, but it will rotate towards it too.

Note that: 1) The times that the gas giant "rises" or "sets" is separate from day and night (when the sun rises and sets). 2) Most moons of gas giants have one face always facing the planet, so it doesn't rise or set, but you're permitting bad science.

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