I'm sure they must have thought of this, but...
... have the Phoenix spacecraft designers anticipated any changes to Martian soil chemistry that might have been caused by the lander's rocket exhaust and made allowances for that in their soil-sampling analysis? I know that some rocket fuels (I assume they were using hydrazine) are extremely reactive and toxic, and would no doubt initiate bizarre chemical reactions in a soil whose chemistry is totally unknown to us. The heat of the exhaust would no doubt have chemical consequences, too. If anyone knows, I'd be curious to find out what steps were taken to avoid possible contamination of the soil by the spacecraft exhaust.