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.