There are actually a number of theoretical ways to build a quantum computer. You don't necessarily need phosphorous. Some use diamonds and one uses an einstein-bose condensate as the computational base. Atheon could use those instead.
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Its finally here the quantum minds of the vex theory: https://www.bungie.net/en/Forum/Post/165516310
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Found it. Although less abundant than other elements phosphorus is still found on Mars.
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I'm currently looking into resources on Venus and Mercury. This will narrow down my search vastly, thank you.