Yep Craig, this is where a little knowledge can get you into a lot of trouble. The Phoenix landing was prefaced by lots of Earth-based science, replicating Martian conditions in various labs. At the time, because a few friends scoffed that anyone could claim a white substance in the soil of a planet millions of kilometres away and never visited by Man was water, I did a very cursory Google search and found several papers relating to this. You could find them, I never bookmarked the links. By knowing the behaviour of a substance under those conditions, if you see the precise behaviour in the field then you can be pretty sure of what it is. Nothing's a stab, Phoenix wasn't a thing thrown together by engineers who hoped for the best when it got to Mars!
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