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In a world very similar to ours, rain has a high concentration of simple carbohydrates in it, enough to sustain life on the surface. After some rainfall, this sugary rainwater is left behind and some organisms consume it while it's still dissolved. Other organisms wait for the water to evaporate and leave behind a crust of nearly pure sugar that can then be consumed. Fungi, animals, and microbes all live on the surface of the world, but there are no photosynthesizers found on the surface and all energy comes from the sky in the form of these sugary raindrops. The concentration of sugar in the rain can vary dramatically, from nearly-pure to saturated.

The world is very similar to ours, but atmosphere density and composition can be variable. I'd like to keep the $O_2$ concentration at about 20% to maintain Earthlike lifeforms. Sunlight intensity and temperature are both open to variation as well. Such a sugar should be produced by natural means- there's no god or intelligent being sprinkling sugar on the planet from the outside.

What could cause rain to have a high concentration of simple carbohydrates?

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    $\begingroup$ So kind of like a planet-wide Candyland? Sick. $\endgroup$ – Pleiades Dec 16 '17 at 2:37
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    $\begingroup$ Skittles rainbow.... $\endgroup$ – user6916458 Dec 16 '17 at 6:01
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    $\begingroup$ Wait until that sugar sits in a puddle and ferments. Paradise Planet! $\endgroup$ – Tony Ennis Dec 16 '17 at 15:35
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    $\begingroup$ C12H22O11 That is the chemical make up of sugar, so you need carbon,hydrogen, and oxygen which should be easy to have. However, convincing them to bond in that exact way would be hard. Lighting can bond atom, but not necessarily in the right order. $\endgroup$ – cybernard Dec 16 '17 at 16:38
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    $\begingroup$ So, not manna, then. Nifty idea! $\endgroup$ – phyrfox Dec 16 '17 at 16:52

10 Answers 10

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The rain is actually the blood of billions.

A species on a nearby planet is being harvested by a technologically advanced alien civilization and the specimens are drained of all their internal fluids and then the filtered fluid with only the sugars are deposited onto your planet.

The specimens themselves are made into powder to be consumed by other aliens as aphrodisiacs. The price of the drug is high so they want to harvest as much as possible with as little weight as possible.

The reason they throw the stuff on your planet is that there are alien laws that prevent dumping of waste in open space and your planet is the nearest to easily dump it on.

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    $\begingroup$ ...ummmm. yikes. $\endgroup$ – Dubukay Dec 15 '17 at 22:28
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    $\begingroup$ Definitely in the running for the Most Outrageous Answer of the Month Award. $\endgroup$ – a4android Dec 16 '17 at 0:21
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    $\begingroup$ If I upvote this, will I be called to court to testify about you? $\endgroup$ – kingledion Dec 16 '17 at 2:40
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    $\begingroup$ I'm not sure whether to upvote or facepalm or what is this even $\endgroup$ – pipe Dec 16 '17 at 11:08
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    $\begingroup$ I like this answer a lot, but I don’t think I can accept it because I’m trying to stay away from the “intelligent beings sprinkling sugar on it from above” as stated in the question- good frame challenging though! $\endgroup$ – Dubukay Dec 16 '17 at 17:07
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Your world has much more of an airborne ecosystem than Earth, including a large number of floating photosynthetic aeroplankton. These little plants float about turning sunlight into sugar and are collected in the clouds when water starts condensing. These water droplet collect the air-algae and fall, carrying their carbohydrates to the surface.

You are going to need some mechanism whereby they can't survive on the surface, otherwise they would colonize and you'd have normal plants. My first thought is constant cloud cover so no light, but you could also make them utilize some exotic chemistry existing only in higher altitude air.

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    $\begingroup$ This is essentially what I was thinking -- organisms generating sugar, though they could also spin out starchy filaments to be airborne (for dispersal and/or reproduction), and these starches could be partially broken up (digested) by the right wavelength of light, acidity in the air, or the right sort of mechanical energy to break them into constituent sugars, some of which being sucrose and fructose. If they feed organisms whose waste eventually feeds them in return, you have a sustainable cycle (along with what the local sun provides, of course). $\endgroup$ – Epanoui Dec 16 '17 at 18:32
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    $\begingroup$ Ozone layer, they don't use o2 but o3 and there's not enough of that on the surface. But how do they fly? $\endgroup$ – DonQuiKong Dec 17 '17 at 17:58
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    $\begingroup$ Perhaps their version of photosynthesis relies on a wavelength of light blocked by the atmosphere, such as UV-C? $\endgroup$ – Someone Somewhere Dec 18 '17 at 9:01
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    $\begingroup$ That leads to another ecosystem consequence: Low to no surface sunlight combined with simple sugars falling from the sky would highly favor fungal life. So plant would live in the sky, like in our oceans, and the surface would be forests and jungles of fungus. Then comes the question of whether and how any fauna would live, and what those fauna would be like. $\endgroup$ – Todd Wilcox Dec 18 '17 at 18:12
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    $\begingroup$ 95% of the questions on this site would need to be re-thought if we're including, but humans can still live there. +1 for making it rain sugar, not that we'd be able to enjoy it ;\ $\endgroup$ – Mazura Dec 18 '17 at 23:47
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How to build an airborne ecosystem

I will admit to plagiarising Josh King's airborne ecosystem. However, in my defense, the sugars have to either come from air or from space, so any answer has to incorporate those factors. I will explore how a plant could be made to have an airborne ecosystem, but not one of the ground.

To not have plants, you can't have sunlight

To not have sunlight, you must either a. be very, very far from a star or b. you must have heavy cloud cover. Turns out, you can do both together.

Venus has a series of high clouds made of sulfuric acid. These clouds cover the planet completely and are opaque to most forms of radiation. Despite being closer to the sun, the optimal light energy received on the surface of Venus is around 10,000. Meanwhile, direct sunlight on Earth is about 100,000 lux. Spacecraft that have landed on Venus are unable to make use of solar panels for energy, due to the low light conditions.

In order to create sulfuric acid, sulfur dioxide (from volcanos), oxygen and water are required. Since you wish to have a 20% oxygen atmosphere, this could work out well. There is no reason to have a majority carbon dioxide atmosphere as Venus does, a nitrogen/oxygen atmosphere with sulfuric acid clouds will work well.

However, there is a requirement to have a high atmospheric density for the sulfuric acid to float. Sulfuric acid haze exists above the 10 atm layer in Venus' atmosphere. If the pressure were lower than this, the sulfuric acid would collect on the ground from rain. Instead, sulfuric acid rain evaporates in the atmosphere at around 25km altitude and is recirculated. In order to have max cloud cover, the atmosphere must be well over 10 atm at ground level to ensure that minimal sulfuric acid rain pools on the ground.

Thick atmosphere can mean greenhouse effect

Depending on your atmospheric composition. If your atmosphere is primarily nitrogen/oxygen, the greenhouse effect will not be that significant. If it is carbon dioxide/oxygen it will be very significant. This will only affect the distance from the star of your planet.

With a nitrogen/oxygen atmosphere, your planet will need to be closer to the sun than the Earth. Due to the high albedo of the sulfur dioxide clouds, most solar energy will be reflected back into space. Venus, for example, receives less solar energy than the Earth, despite being closer, since so much is reflected.

Cloudtop organisms will need to process sulfur compounds

Sulfur-reducing bacteria exist on Earth; they convert sulfate ion (SO$_4^{2-}$) to hydrogen sulfide (H$_2$S). Sulfate ion will be available from sulfuric acid in solution in water droplets. Anywhere water droplets can float, these bacteria could find their food.

Here, an oxygen atmosphere is an advantage since it will recycle the hydrogen sulfide to water and sulfur dioxide, and the sulfur dioxide will further react with molecular oxygen and water to produce sulfuric acid.

Oxygen requires photosynthesis

In order to maintain this oxygen atmosphere, you will need a metabolic process creating oxygen. The only one I can think of that will work is photosynthesis. This is the weakest link in this ecology, in my mind. The photosynthesizers will need to float above the sulfuric acid clouds in order to get sufficient sunlight.

An alternative would be a photosynthesizer that uses long wave infrared light. This light could penetrate the clouds and mean that there would be some low-level photosynthesis happening on the planet's surface (a second food source, beyond the sugar rain). This needn't be too much biological mass, but it has to have been happening long enough to give the planet (with an active geological cycle to keep sulfur in the atmosphere) a 20% oxygen atmosphere.

Conclusions

You will need:

  • Sulfuric acid clouds to block sunlight
  • High-density atmosphere to keep sulfuric acid off the surface. As a bonus, this makes larger organisms able to float in the clouds (think salps)
  • To get a 20% oxygen atmosphere, you need either a nitrogen or carbon dioxide base. A nitrogen planet would be nearer the sun, CO$_2$ farther away to account for the greenhouse effect.
  • You need at least trace amounts of water to allow sulfuric acid formation
  • You need a sulfur source to keep sulfuric acid in the atmosphere. Volcanos are the usual culprit.
  • You need a photosynthesizer, possibly floating or at ground level using long wave infrared light to produce oxygen.

Of course, with all this, there isn't really sugar raining down from the sky; its more like dead bacteria. So you won't end up with a Candyland sugar-crust, but you would end up with plenty of fungus food on the surface, and I'd imagine the fungus could be the base of a complex animal food chain.

You could add some handwavium reason that either a. the sky bacteria just drop sugar for funsies or b. some sky amoeba-like predator eats bacteria and poops down sugar. But given how valuable sugar is as an energy source, neither of those explanations seems reasonable to me.

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    $\begingroup$ What if turbulent winds ground up the bacteria corpses so all that had a chance to fall was the denser sugar powder? $\endgroup$ – Muuski Dec 18 '17 at 20:32
  • $\begingroup$ I don't think you can have a CO2 base for the atmosphere if you want humans to be able to breath. It's not enough to just have 20% oxygen, you need to also not have more than ~6% CO2. $\endgroup$ – CactusCake Dec 18 '17 at 22:38
  • $\begingroup$ @CactusCake Per the OP, "atmospheric density and composition can be variable." $\endgroup$ – kingledion Dec 19 '17 at 18:05
  • $\begingroup$ @kingledion sure, but they also said they want to maintain earthlike forms, I think most would be poisoned at such high concentrations of CO2. Nathaniel's answer goes into more detail, I just didn't notice it before. I really do like this answer though. $\endgroup$ – CactusCake Dec 19 '17 at 18:14
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There is a chemical reaction (actually several reactions that all occur together) called the formose reaction that can produce sugars from formaldehyde. It is thought that this reaction might have occurred on the early Earth and played an important role in the origin of life. There might be a way to have the sugars produced by the formose reaction on your planet.

The main difficulty is in coming up with a plausible source of formaldehyde. This paper says it can be produced from carbon dioxide and water under UV light, i.e. in the upper atmosphere. Presumably the main reasons this doesn't happen on Earth are the ozone layer, and the fact that having lots of oxygen in the atmosphere means that formaldehyde will rapidly be oxidised.

I don't know for sure, but it might be possible that if your atmosphere is 20% oxygen and 80% CO2, and has plenty of water vapour, then you might be able to produce formaldehyde at a high enough rate that it won't all be oxidised. Then you could have sugars formed by the formose reaction inside water droplets in the atmosphere, which explains the sugary rain. The variability of sugar concentration would easily be explained by the fact that the formose reaction is quite slow and quite sensitively dependent on conditions, so the amount of sugar would depend quite strongly on how long the raindrops had been in the air, among other things.

Some caveats are in order though. If you don't want photosynthesisers on your planet then oxygen is a bit of an issue. Although the reaction that produces formaldehyde from CO2 also produces oxygen, that oxygen will be used up by the metabolisms of the organisms that consume the sugars. You would need an additional, extra source of oxygen in order to maintain an excess. (Though as Richard Tingle points out in the comments, this could happen if some of the organic matter doesn't get metabolised back into CO2 and instead gets buried and subducted, leaving an excess of O2. This is exactly how it works on Earth.)

Secondly, formaldehyde itself is quite poisonous to most (all?) organisms because it's quite reactive, and it's used as a disinfectant for that reason. The sugars from the formose reaction are also poisonous to humans, but I would not be surprised if some organisms can eat them. Similarly, an atmosphere with 80% CO2 would be lethal to most (all?) Earth animals, though plants and some microbes would be fine. So this planet's life might be somewhat Earth-like, but it would be adapted to this particular environment and most Earth life would not survive there.

Another caveat is that the formose reaction is catalysed by metal ions, which means your rain would either have to be salty (it's hard to imagine how that could happen) or there would have to be some other catalyst available in your atmosphere.

Finally, formaldehyde is not very stable in the presence of oxygen - pure formaldehyde is a flammable gas - so it's quite hard to imagine that formaldehyde and oxygen could co-exist in the same atmosphere.

Overall, I don't think this scenario is particularly realistic or likely, but it might be just about in the bounds of possibility, and I couldn't resist putting it forward given the specifics of what you were asking for.

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  • $\begingroup$ "Although the reaction that produces formaldehyde from CO2 also produces oxygen, that oxygen will be used up by the metabolisms of the organisms that consume the sugars." Isn't that fine. I though here on earth the amount of oxygen I consume is exactly equal to the amount of food I use for energy (down to the last molecule). Excess oxygen can be explained by the fact that not all the sugar is eaten. Just like how on earth excess oxygen exists because not all plant life was eaten (but instead ended us as oil etc) $\endgroup$ – Richard Tingle Dec 16 '17 at 15:10
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    $\begingroup$ @RichardTingle on a short time scale that's true. However, on longer time scales, oxygen is also used up by other processes, primarily oxidation of the crust. That would likely be true on any planet that supports life, which means that although on short time scales the oxygen produced exactly balances the oxygen consumed, on longer time scales there will be losses, and you'll need a source to offset them. It's kind of a subtle point, but to be realistic you'd have to take account of it. $\endgroup$ – Nathaniel Dec 16 '17 at 15:27
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    $\begingroup$ @RichardTingle though having thought about it, maybe you're right that the excess could exist on the sugar-rain planet for the same reason it does on Earth. We just have to imagine that some of the sugar doesn't get fully metabolised back into CO2 and instead gets buried as organic matter, ultimately turning into oil etc. just like on this planet. $\endgroup$ – Nathaniel Dec 17 '17 at 12:40
  • $\begingroup$ So can we just go ahead and say that ~3 billion years ago it maybe used to rain sugar on Earth (before there was so much oxygen)? You're getting hung up on having a viable planet afterwards, which is not part of the question... and everyone always seems to leave that part out. +1 $\endgroup$ – Mazura Dec 19 '17 at 0:06
  • $\begingroup$ @Mazura I'd say it's not impossible that this happened on Earth, but it's much more likely that it rained formaldehyde solution and the sugars were produced on the surface or in the oceans. I interpreted the question as wanting a stable ecosystem with native Earth-like organisms, but if that's not the case it does make it easier indeed. $\endgroup$ – Nathaniel Dec 19 '17 at 1:08
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We already havy sugary rain on earth.

We have several species of plant louses. Many of them are specialized to suck on trees, especially on conifers, but also on leaf-bearing trees. They are digesting proteins, but cannot use all the sugar they get with the blood of the trees. They loose aromatic sugar drops at their very back end, falling down to earth. Also, ants and bees collect these sugar drops from the louses. Beekeepers call this honeydew, it gives a rare, expensive and aromatic honey.

This honeydew falls down from the trees like a thin sweet rain. Did you ever touched something in the summer that feels stickily, your car, or a chair in the garden? I bet it was under a tree, covered with a thin layer of honeydew.

I imagine giant trees reaching up to the sky with their branches and leaves in the sun, above an everlasting cloud that inhibits photosynthesis on the ground. Myriads of louses drop sweet manna sugary rain.

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Sugar is produced by plants to attract pollinators (e.g. insects or hummingbirds or their analogues). It's also in the sap of plants (see e.g. "sugar maples"), and leaks whenever the plant is injured (see e.g. woodpeckers, see also e.g. sticky sap which leaks from evergreens).

Sugar can be fine (see icing sugar), even very fine.

When it's no longer needed (e.g. after pollination), or unwillingly by force (e.g. because of regular hurricanes) the sugar is carried off by the wind.

It's then carried in the atmosphere until rain drop condense on it, just like rain condenses on other types of dust on this planet.

[Disclaimer: I'm not a scientist ... this answer is only meant to be superficially plausible.]

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Liquid Sugar geysers, that spew the sugary liquid high into the atmosphere. The origin of the geysers could be animal plant etc.

Sugar volcanoes sound fun too.

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    $\begingroup$ While Sugar Volcanoes is a sweet idea, your answer just might be improved if you provided an explanation for the source of the sugar and how it part of a volcano. Go for it! $\endgroup$ – a4android Dec 17 '17 at 3:33
  • $\begingroup$ I was thinking visually it would be more dramatic, but as for the source .. well I got blank .. lol ... some also depends on the duration is it a one time event, or an everyday sight. $\endgroup$ – ArtisticPhoenix Dec 17 '17 at 6:08
  • $\begingroup$ I was thinking something like when you shake a can of soda and open it for the geyser but on a much grander scale. $\endgroup$ – ArtisticPhoenix Dec 17 '17 at 6:39
  • $\begingroup$ Suggest you add the geyser idea in an edit of your answer. That could involve high pressure gas generation. $\endgroup$ – a4android Dec 17 '17 at 7:59
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Ok, so no "probably-intoxicated-higher-power sprinkling sugar on the planet", but how about "probably-intoxicated-higher-power-engineered natural solution"? As in, it's plausibly self sustaining but stupidly improbable to have occurred naturally.

See, the issue I have with the idea of an airborne ecosystem is that they're fairly inhospitable to live near and/or improbable in any coherent evolutionary chain. It would have to evolve on the surface, have a strong enough selection factor to result in a sustainable airborne population by natural selection, but some how not be killed off by the selection factor before becoming airborne. It's kinda like the logic of a species of birds that evolved to never land because doing so results in instant death, the only way such a species could exist would be from avian paranoia. Wait, getting off track.

My solution is explicitly improbable. I call it the living weather balloon strategy.

Essentially you engineer a suitable photosynthesise strain that can do something like $CH_4 + H_2O + 2CO_2 + light => C_{3}H_{6}O_{3} + O_2$ then you wrap that around a sack of helium (See note 1), such that it has a neutral buoyancy somewhere near the top of your cloud layer, and figure out some handy way of making the things basically immortal. I figure any species that can bio-engineer an enzyme process that turns methane in to glyceraldehyde (given that they're kinda nothing alike) should be able to engineer a sufficiently robust host. They'd excrete (actually perspire is probably more accurate?) the sugar on their surface and it would get washed away by the same clouds that provide the water for the process.

Once you have a small army of these things, you release them in to the target atmosphere and leave for a while. They'll have fun turning two annoying greenhouse gases in to useful byproducts, exactly the sort of things you'd want to do if you were, say, terraforming a planet over the long haul. Obviously you're not going to wait around while your sweet (pun intended) beach balls clean up the atmosphere, maybe you stick a calendar reminder to pop back later and don't bother. So long as they're reasonably robust the losses due to damage should be negligible, the concentration of sugar in the rain would be proportional to methane concentration in the area and frequency of rain.

Oh and did I mention the neat alien planet vibe from the rafts of FLYING BALLOON WEED?! Like for real, how many planets do you come across with plants drifting through the clouds, probably casting greenish shadows when they pass in front of the sun? This is why I said probably intoxicated... you need a particularly odd frame of mind to fly around the galaxy dumping these things all over the place for giggles.

Note 1: I specified helium primarily because it side steps some tricky issues. You could actually use hydrogen instead as a lighter than air gas, maybe extracted from water. I would worry that sacks of hydrogen might be more vulnerable to damage (read: horribly unproductive combustion) though. The upshot though is that you could probably figure out a mechanism for them to reproduce via division or something, since the helium is a limiter for that, but that's a whole mess of more implausible stuff that I'll leave to someone else's imagination. To be fair, a hydrogen filled version does have a tiny possibility of evolving naturally, it's just babel fish levels of improbable and a finite improbability generator would be only slightly less intoxicated engineering than my proposal.

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    $\begingroup$ If they were full of hydrogen, then lightning becomes way more interesting. $\endgroup$ – TemporalWolf Dec 19 '17 at 20:12
  • $\begingroup$ @TemporalWolf that was my worry... but on the plus side, it wouldn't combust unless there was an oxider getting in to the sack.... $\endgroup$ – Kaithar Dec 20 '17 at 2:13
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You have flowers that release aerosolized sugar spores, rather than pollen. The rain simply encourages the plants to bloom. The plants derive their energy from bacterial processing similar to tube worms.

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  • $\begingroup$ Interesting, so like soluble fruit? I dig it. $\endgroup$ – Dubukay Dec 18 '17 at 20:48
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Your planet is located in a star system which contains a cloud of orbiting sugars, and interaction between the planet and the cloud mean that sugar is periodically deposited into the upper atmosphere, where it is slowly absorbed by atmospheric water and eventually rains down onto the surface.

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