There is this "tendency" on the part of some Mars "enthusiasts" to assume that the entire structure of Mars is unifrom, and to then generalize from an individual case (such as Lyot) to the entire planet.

If we look at the area surrounding Lyot, we can see other areas with dark subsurface "soil" exposed. I think Dan Spires was the first to point out that cometary hydrocarbons might be "pooling" on Mars.

There are plenty of other craters and comparing them shows differing amounts of water released by each separate small impact.

But in this case, what the original poster was talking about was not small impacts, but instead larger ones. The volatiles released in this case will be different (impact volatile release doesn's scale linearly), and there will be secondary reactions as well. These effects of these large impacts are seen in the banding (strata) exposed in different areas on Mars, and the process is chaotic about a 26 million year cycle.

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ep