re: For all the good it will probably do them.
Heating is one thing and cooking is another. Still carbon traces, etc. should still be found.
"changes in the soil's cohesiveness?" If this was on Earth, the reduced stickiness would be because the soil dried out. In other words, I'll bet that the most interesting substances pretty much evaporated while the dirt was sitting there."
Unfortunately, being that it is Mars, anything H2O related may have evaporated (even CO2), but remember, they still have a long way to go as far as digging. I know, they shouldn't heat it to death either, but I'm sure they know that.