The Geology of SG:U, “Air”

As promised, this is a look at the chemistry and geology presented in the pilot episode of Stargate: Universe, “Air” (part 1, 2, 3). Our heroes are on a spaceship with a life support system with a non-functional filtration system, and need to come up with a way to sequester the carbon dioxide. They head down to a sandy planet in search of calcium carbonate.

“How come our heroes couldn’t just hold the Stargate open to a planet with a nice, tasty atmosphere?”
That would violate the defined functionality of the Stargate established earlier in Stargate: SG-1 and Stargate: Atlantis. The Stargates prevent the transport of individual molecules, which is handy when the teams connect to space-gates (vacuum on the far side) or submerged gates (water, water everywhere!).

“Wait, if the problem is too much carbon dioxide, how come they’re looking for calcium carbonate? Won’t that just mean they have even more carbon to deal with?!”
Yes, but no.

As carbon dioxide dissolves into water, the water becomes more acidic. Calcium carbonate dissolves in pretty much any acid, and slews of carbonate ions running rampant will form bicarbonate. So, if you chuck a bunch of calcium carbonate into water and add carbon dioxide, the calcium carbonate will dissolve in the acidic water, and all the ionized carbonate will form bicarbonate instead.

This is a well-known phenomena (see here for instructions on how to demonstrate it), and it’s an acceptable hypothesis that shell sediments in the ocean help buffer the acidity from increased carbon dioxide in the atmosphere (see here for an older summary of a Science paper on the topic), so it’s within the realm of plausible science to use the chemical reaction for science fiction.

“…if lime reacts with carbon dioxide to make calcium carbonate, and then calcium carbonate reacts with more carbon dioxide to make bicarbonates, why not start with lime?”
Our heroes didn’t manage to bring the medical-grade lime with them; very unfortunate. Yes, the system would be more efficient if our heroes made lime-enriched water and let that react happily away with the carbon dioxide because then it would sequester carbon twice over, but lime isn’t as easy for novices to identify via field test as calcium carbonate. Calcium carbonate comes as three minerals: aragonite, calcite, and vaterite. They are polymorphs — identical chemicals but different structure — so all of them dissolve in acid. The standard test is to add a drop of 10% HCl, and if it bubbles merrily away, you found calcium carbonate (or drop the rock in the acid for more bubbles!).

“I saw no bubbles. I saw red.”
Eh, red is prettier, or the geologist had prissier field gear because she’s used to alien atmospheres and walking around with acid could be dangerous, or maybe they used something that reacts to changes in pH by going red (cabbage juice turns red in acids (pH below 7), purple when neutral (pH = 7) and blue (pH above 7) or green (pH above 9) with bases), or in the rush to evacuate the base they left behind the hydrochloric acid and had to improvise from the material they had on hand.

“But wait! The chemical reaction is reversible with heat, so why did they hunt for rocks instead of just boiling their old life support goo on the planet?”
Eh, they couldn’t find a cauldron to hold it that didn’t dissolve into muck when handling the goo, or the goo had other chemical reactions going on (you need more than just “less carbon dioxide” to keep a human happy and the montage did include some prep of a white foam) and would do Very Bad Things when heated, or the Ancient goo wasn’t even using this particular chemical reaction to scrub carbon dioxide, or… ie, the chemistry is good and the concepts are good, and the details fall within plausible exceptions for science fiction.

“Why’d they go hunting for calcium carbonate in dried-up lakes or oceans?”
Limestone, chalk, and sea shells on Earth are all high in calcium carbonate; when faced with an alien planet and limited time, the hope that alien sea shells are chemically similar is both plausible, and gives some sort of constraint to guide our luckless heroes.

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6 Responses to The Geology of SG:U, “Air”

  1. Mike says:

    Hello, very nice article. I happend upone this after cheking on replys to Joe’s latest rant on his blog, and I was quite pleased to read a little more on what was going on in this episode. You managed to break it down into easier cunks of data for me to actually grasp and retain, and who knows if I ever find myself on some kind of alien submarine that only surfaces once every once in a while or an alien ship it just might serve to stick in the old noggin hehe.

    If I mght add a posssible reason for why they were looking in the desert/ dry sea bed as I’m sure as an SGU fan your already aware but thought I’d mention it in case you missed it. The Destiny seems to realise that it’s in trouble and has some form of intelligence ( Such as the briefly mentioned “seeder” ships might need to be as they were most likely also unmanned while manufacturing Stargates) The ship seems to have data sent back from the seeder ships (or possibly also augments this with a planatery scan when in range) and drops out of FTL at the nearest Stargate inhabbited planet with resources there that her crew might need or be able to find to fix the latest urgent current soloution.

    Now to again augment your explanation too, it could also be simply stoppng there as the Aincents also knew that there was a high probibillity that sea shells might or would be chemically simmilar in other galaxy’s ect ect and the ship simply stops there in this case, however the wrters seemto give us hints that it’s in fact the destiny’s computer system deciding on the best course of action on it’s current situaton, to keep her new crew alive.

    Anyways thanks for the explanation, Cheers,

    Mike.

  2. Mika says:

    I’m glad you had fun learning, Mike!

    Good point about Destiny bringing the crew where they need to be to survive. My job as a science consultant is to provide the science that fits the story and supports the plot, so while the narrative is doing the heavy lifting, I fill in the science details that keeps the scifi plausible.

  3. iamza says:

    The predictable bubbling reaction to acid pretty much describes why I liked calcite so much when it came to first year mineralogy tests. Between that and the rhombohedral crystal faces… 🙂

    I must admit I did wonder briefly about the red reaction, and also who on the ship thought to pack glass flasks for careful chemical testing, but, eh, there was geoscience.

  4. Mathias Bage says:

    You’d think that The Ancients would’ve developed
    something better than simple chemical CO2
    sequestration. Today’s submarines sometimes utilize
    cryogenic tecniques for CO2 sequestration, where you
    only need energy input, AFAIK.

    I’m a little disappointed with the script writers
    in this case.

  5. Mika says:

    Iamaza — I always love easy-to-ID materials. Magnetite is another great one: heavy & magnetic.

    Mathias — interesting, I liked the solution in part because it seemed so clever to build a system that was so low-tech and easy to service without the trappings of nearby civilization to fall back on. I’m also not sure what you mean by using cryogenics for carbon sequestration; I know the old-skool (100yo) technique of extreme cooling to separate pure oxygen from CO2 & all other trace contaminants, but that’s to use as a fuel source since pure oxygen does like to explode. Checking out the IPCC special report on carbon sequestration isn’t turning up any newer techniques — is that the one you meant?

  6. Mathias Bage says:

    Only what I’ve heard from a guy at a swedish submarine manufacturer (Kockums) over the phone a some years ago. He didn’t say how they did it/were going to do it (I’m not sure it’s in production yet.) But I think some form of modified cryocooler would do the job.

    If you have the power (after munching on a nearby star) to propel a gigantic unmanned, FTL spaceship on autopilot like Destiny, you DEFINITELY have some power over to scrub your air from CO2.

    And if you’ve come to this technological level, then most of the other required systems of the ship (power, ventilation [when crew aboard]),
    propulsion, what have you) would also have to be self-sustaining or self-repairing, and so to exclude the relatively simple task of CO2 sequestration from the host of self-{sustaining,repairing} systems seems very unlikely in my view.

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