Ecologies of desert planets

[rancke]

While working on an adventure set on Yori (C560757-A), a question occurred to me: Where does the free oxygen in a desert world's atmosphere come from? Free oxygen implies vegetation to free it and desert worlds imply a paucity of vegetation. Or does it? How does a world like Dune function? Or any other desert world with breathable atmosphere?


Hans

 

[Drakon]

Good question. To be honest, I do not believe it does work.

Oxygen is a very promiscuous element. It will join up with most anything. Without vegetation, your oxygen soon combines with something else, carbon or silicon or hydrogen or iron.

Water could be underground and you might have large caves of it, with plant life living off these underground oasis. But other than that, I don't think you can have a full planet desert with a breathable atmosphere, at least naturally.

Did Grandfather leave something behind to help?

 

[whulorigan]

While working on an adventure set on Yori (C560757-A), a question occurred to me: Where does the free oxygen in a desert world's atmosphere come from? Free oxygen implies vegetation to free it and desert worlds imply a paucity of vegetation. Or does it? How does a world like Dune function? Or any other desert world with breathable atmosphere?

It would seem to me to be one of the following:

1) There is some life-process occurring that produces O₂ as a byproduct (not necessarily vegetation);

2) There is a naturally occurring chemical process of some sort on-planet that produces O₂ as a byproduct;

3) The O₂ is a remnant of earlier terraforming efforts (either Ancient or some other sophont). In which case the O₂ will be slowly reacting with its environment and the atmosphere changing over time, apart from outside maintenance;

4) The O₂ is a result of ongoing current terraforming.

​

On Dune the sandworms (I believe) produced O₂ as a byproduct of biological processes.

 

[shaunhilburn]

It would seem to me to be one of the following:

1) There is some life-process occurring that produces O₂ as a byproduct (not necessarily vegetation);

2) There is a naturally occurring chemical process of some sort on-planet that produces O₂ as a byproduct;

3) The O₂ is a remnant of earlier terraforming efforts (either Ancient or some other sophont). In which case the O₂ will be slowly reacting with its environment and the atmosphere changing over time, apart from outside maintenance;

4) The O₂ is a result of ongoing current terraforming.

​

On Dune the sandworms (I believe) produced O₂ as a byproduct of biological processes.

Oxygen is very reactive and will vanish from the atmosphere after millenia if it is not replenished somehow. Normally plants do this, but they require precipitation or moisture from some source. Small seas should suffice. A planet doesn't have to be completely barren to qualify for hydro digit 0.

IS Form 9 contains 490 hexagons. Each of the 20 triangles contains 24.5 hexagons (15 full hexes, 18 half-hexes, and three 1/6 hexes). Each hexagon represents 0.2% of the surface area, and each triangle represents 5% of the surface.

A size 8 desert world can have an ocean the size of Greenland (2 hexes), yet still have 99.6% land surface. A sea the size of North America is nearly 5% of the planet surface. In these cases, the hydro digit can still be considered zero after rounding.

So where are the seas on Tatooine? They're never seen because they're on the opposite hemisphere from the camera. The lush jungles along their windward coastlines sustain the breathable atmosphere

 

[rancke]

Oxygen is very reactive and will vanish from the atmosphere after millenia if it is not replenished somehow. Normally plants do this, but they require precipitation or moisture from some source. Small seas should suffice. A planet doesn't have to be completely barren to qualify for hydro digit 0.

No, though according to BtC Yori has no surface water at all. But assume we're talking about a world with 4.9% surface water. Would that be enough to sustain the flora to keep the atmosphere of standard density?


Hans

 

[shaunhilburn]

But assume we're talking about a world with 4.9% surface water. Would that be enough to sustain the flora to keep the atmosphere of standard density?


Hans

I'd have to say no. I have a crude spreadsheet model that calculates atmosphere parameters based on planet size and coverage/photosynthetic efficacy of ice-free oceans and fertile land, O2 from phytoplankton, and O2 from forests/grasslands.

In its current state (it is crude), there are no successful what-if scenarios for a minimally breathable atmosphere - 72 mmHg piO2 - on a size 3 world. Besides, such a planet is too small to retain a breathable atmosphere of any sort over geologic timescales. It has to be artificially generated.

 

[BlackBat242]

Lichen - lots of lichen.

This is naturally found on rocks and even the soil in desert areas (see biological soil crusts), and so on.

They require very little water, but do process CO2.

 

[mike wightman]

Lichin that incorporates silica into it structure so you end up with razor sharp fractal pattern (to increase surface area) forests of the stuff.

 

[Highorbit_drifter]

I doubt you can justify the O2 with terran-like plantlife.

BlackBat242's idea about lichens is a good one. Their life cycle would have to be very different than terran lichens but that gives you ecological niches that would otherwise not be present in a desert environment. Think of Dune and the planetologist's death scene - strange chemistries underground that rely on very little water. The explosive release of gases (and maybe some sort of odd biological spoor that 'flavors' the local salts) could prove deadly if you've been dumped in the middle of no-where by Harkonnen agents.

Or a form of plankton that is airborn. Green tinted clouds and the occasional hail of encapsulated spoors with a drop or two of water, quickly processed by the local life.

All of this could be natural evolution as the seas retreated (or caused the seas to retreat), the work of Grandfather (a cop-out, IMO), or terraforming by early settlers.

 

[rancke]

Swamps would be classified as land, right? Might (many of) these so-called desert worlds with breathable air actually have huge tracts of swampland?


Hans

 

[atpollard]

Extremeophiles offer some amazing possibilities.
Endolith Microbes - living up to 7 km below the surface and reproducing every 10,000 years
Hypoliths - lives under translucent rocks in deserts - uses photosynthesis
Lithoautotroph microbes - eat rock
Xerophile Plants - some cacti and Lichens can survive up to three years between rainfall.

Pursuing the problem the other direction, there are some extreme areas that experience unusual precipitation.
Rainfall without surface water - the air is sufficiently humid that it rains periodically or even frequently, but the soil is a deep, poorly graded sand that almost instantly absorbs the water and allows it to flow deep into the aquifer. I have personally experienced sands that will absorb up to 35 inches (875 mm) of rainfall per hour, so a 1" (25 mm) rainfall will wet the surface for less than 2 minutes. While not technically a desert (too much rainfall), there could still be ZERO surface water.
Marine Fog - it never rains as we know rain, but water in the air will periodically form a fog that provides moisture to some hardy plants.
Infrequent Rains - the average annual rainfall is only 10 mm per year, making this an extreme desert world. However, most years experience an actual rainfall closer to 1 mm. About every 100 years, the conditions are just right and a wet season will deposit 900 mm of rainfall ... causing flash floods and creating small (or even large) lakes that might last for weeks as they are slowly absorbed into the soil and evaporate. Time for those Xerophile plants. Check out what happens to the Sonora Desert when it rains for some inspiration.

 

[esampson]

The photosynthetic process is to take molecules of CO₂ and combine them with the hydrogen from water molecules to form hydrocarbons. The water's oxygen atoms are where the free oxygen comes from. Using the standard photosynthetic process water is certainly required but that doesn't necessarily mean oceans would be required.

If memory serves me correctly (it's probably been a decade since I last read it) Dune might provide a fairly good example. As I recall there was actually a reasonable amount of water on Arrakis. It was simply all tied up by the little makers (and probably other lifeforms) so that you didn't have oceans. I believe there was also something going on with weather control to prevent rain from naturally forming since the water would be poisonous to the sand worms and upset the formation of Spice. There was some water in the atmosphere as I seem to remember that the Freeman had large devices to capture this water which was how they were filling their hidden caches of water.

As written I'm not sure Arrakis could naturally work as the amount of water vapor seemed to be way too low and the way the water was tied up would prevent oxygen production on the scale necessary to support people but it still illustrates the idea of a desert planet with a reasonable supply of hidden water. On Earth you can look at places such as the Atacama Desert which has quite possibly not seen significant rainfall in over 400 years and which has been in existence as a desert for 3 million years. Despite these conditions it has over 500 different species of plants and is capable of supporting creatures as large as guanacos (a creature roughly the size of a llama).

My guess is that almost certainly Atacama is not self-sufficient and gets large amounts of its oxygen from the rest of the planet but the point is that it is possible for plant life to survive in these extreme conditions to the point where they are capable of providing food to support some fairly good size animals.

If I had to envision an entire planet similar to the Atacama I would probably include things such as plants that seem to be almost like clumps of cotton or wool that catch water vapor from the air and store it underground in large root or tuber like structures. One plant might conceivably cover a very large area of ground. It would probably have some sort of mechanism within the fibers to move the captured water against the osmotic gradient so that it could be sent to its stores rather than evaporating through all the surface area of the plant.

Most likely there would be very long underground runners which could transport the water with less danger of evaporation and instead of a single body for storing water there would be multiple bodies so that if one becomes compromised the plant will continue to survive. The plant would also probably be capable of handling separation of a group of underground runners by simply having the separated runners function as an independent plant, possibly even being capable of rejoining the main plant.

Since the 'wool' structure would mostly be focused on the capture and transport of water photosynthesis would occur in structures that would emerge in various spots. They would have a broad but 'thin' structure, either a few very large leaves or else smaller leaves arranged in a very shallow layer, sort of like an umbrella. This would provide shade to the 'wool' underneath as well as capturing sunlight.

These plants would act as a sort of 'coral reef' providing a basis for complete ecosystems. Other plants would grow up in them/around them trying to make use of the water stores, although they would have evolved so as to not kill the underlying structure. Animals would live in/near the plants, likewise for the water. 'Faux wool' plants might evolve that try to capture some of these animals with 'sticky wool' so they can drain the moisture from these animals.

A secondary 'foundation species' might be something with very, very deep roots, capable of reaching down to deep underground aquifers. They would probably have a fairly tough and dry bark but even then would be covered in plant/lichen types which are adept at extracting small amounts of water.

And of course there's always the possibilities of some sort of airborne or land based 'plankton' which survives off of the atmospheric water vapor.

 

[aramis]

Dune's hardly devoid of life - or water - only of freestanding surface water. The initial novel notes at least half a dozen plant species - mostly growing in the rocky outcrops. Morning dew probably accounts for the plants' water. Oh, and the sandworm produces surplus oxygen.

Also, there's significant atmospheric water vapor - Arakis' air can be run through wind traps to extract water.

Then again, for comparison, Fairbanks, Alaska is in a desert... and is surrounded by wetlands. (The moisture largely stays put on the surface.)

 

[McPerth]

Dune's hardly devoid of life - or water - only of freestanding surface water. The initial novel notes at least half a dozen plant species - mostly growing in the rocky outcrops. Morning dew probably accounts for the plants' water. Oh, and the sandworm produces surplus oxygen.

IIRC (I read it years ago), when Paul Atreides asks the Fremen if there's enough water in Arakis to terraform it they don't answer, bet he deducts from their silece there is enough, though not as surface water bodies.

And, again IIRC, there are ice caps in Arakis poles.

 

[esampson]

. . .Also, there's significant atmospheric water vapor - Arakis' air can be run through wind traps to extract water. . .

I'm not sure I would go as far as saying 'significant' water vapor. I seem to recall that they spent lots and lots of time worrying about water evaporating (to the point of using creosote of the small amounts of skin exposed by the still suits).

I thought it was more that the wind traps and plants were just hyper-efficient at extracting what meager amounts of water there was. Still, that may be sort of a potato/potahto sort of differentiation.

The problem with the sandworms producing oxygen is simply where do they get it from? Normal photosynthetic process is to get the oxygen from water, but that is toxic to the sandworms. I don't think breaking down CO₂ works because of energy gradients (plants don't actually break down CO₂. They store it as hydrocarbons). That's why I said I don't think it quite works as written. Just not enough water on Arrakis, even with the hidden supplies. (And it should be noted that I still think Dune is a great read. So there's a scientific inaccuracy if you dig down deep enough. Even a lot of Clarke's stuff will break down if you dig deep enough. As long as there aren't glaring errors at the surface level I can be happy).

Which doesn't mean it isn't possible to have a hydrographics 0 planet with oxygen. Simple that I think that you need more water than appeared to be on Dune (even with its hidden supplies) that aren't capable of forming oceans (atpollard had some really good insights as well).

 

[whulorigan]

The problem with the sandworms producing oxygen is simply where do they get it from? Normal photosynthetic process is to get the oxygen from water, but that is toxic to the sandworms. I don't think breaking down CO₂ works because of energy gradients (plants don't actually break down CO₂. They store it as hydrocarbons). That's why I said I don't think it quite works as written. Just not enough water on Arrakis, even with the hidden supplies.

The O₂ may simply be a byproduct of some part of the sandworm's alien (unspecified) biology. Could it have something to do with the sand itself (SiO₂)? As the sandworm moves thru the dunes, does it break down the sand in some way to metabolize the silicon, releasing O₂? Remember, the sandworm's internal processes produce significant heat, IIRC.

 

[aramis]

I'm not sure I would go as far as saying 'significant' water vapor. I seem to recall that they spent lots and lots of time worrying about water evaporating (to the point of using creosote of the small amounts of skin exposed by the still suits).

I thought it was more that the wind traps and plants were just hyper-efficient at extracting what meager amounts of water there was. Still, that may be sort of a potato/potahto sort of differentiation.

The problem with the sandworms producing oxygen is simply where do they get it from? Normal photosynthetic process is to get the oxygen from water, but that is toxic to the sandworms. I don't think breaking down CO₂ works because of energy gradients (plants don't actually break down CO₂. They store it as hydrocarbons). That's why I said I don't think it quite works as written. Just not enough water on Arrakis, even with the hidden supplies. (And it should be noted that I still think Dune is a great read. So there's a scientific inaccuracy if you dig down deep enough. Even a lot of Clarke's stuff will break down if you dig deep enough. As long as there aren't glaring errors at the surface level I can be happy).

Which doesn't mean it isn't possible to have a hydrographics 0 planet with oxygen. Simple that I think that you need more water than appeared to be on Dune (even with its hidden supplies) that aren't capable of forming oceans (atpollard had some really good insights as well).

http://dune.wikia.com/wiki/Sandworm/Featured

Note that sandtrout feed sandworms by converting water to pre-spice mass.

 

[esampson]

http://dune.wikia.com/wiki/Sandworm/Featured

Note that sandtrout feed sandworms by converting water to pre-spice mass.

So perhaps the oxygen produced by the worms does ultimately stem from water. Sandtrout form a complex carbohydrate in the form of pre-spice, worms strip the oxygen from the carbohydrate to combine the carbon and hydrogen atoms to carbon dioxide to form new carbohydrates, lipids, and proteins.

It still seems like the process wouldn't produce enough oxygen because Arrakis simply didn't have enough water, but then the population density of its oxygen breathing animals could be very, very low (if we assume some of the hidden life forms that occurred in larger amounts such as sandtrout did not breathe oxygen).

 

[rancke]

It still seems like the process wouldn't produce enough oxygen because Arrakis simply didn't have enough water, but then the population density of its oxygen breathing animals could be very, very low (if we assume some of the hidden life forms that occurred in larger amounts such as sandtrout did not breathe oxygen).

Wouldn't that result in an atmosphere with very little oxygen?


Hans

 

[esampson]

Wouldn't that result in an atmosphere with very little oxygen?


Hans

Not sure why you would think that. Oxygen breathing animals use up oxygen, they don't produce it. Granted, they wouldn't be producing large amounts of CO₂, but that might be coming from other sources.