FAQ
http://phoenix.lpl.arizona.edu/faq.php
"They" have thought of it...
Will Phoenix's descent thrusters alter the composition of its landing site?
Altering the chemistry of our landing site due to our thruster exhaust is unavoidable. The Phoenix Lander uses hydrazine, a hypergolic propellant that turns into ammonia during combustion. So essentially, we are spraying the surface with ammonia and a small amount of hydrazine that was not combusted. The way we get around that is by 1) knowing that we are going to be producing ammonia and 2) by designing the wet chemistry cells to carefully quantify the amount of ammonia in the regolith. We then use this information to interpret our other results.