Q) If Antimatter Mines are dropped on top of the Defiant, is it still considered to be defending, even though it does not roll any defense dice?
A) Yes. However, this is not the case when the Defiant moves into mines (Antimatter or Cloaked) on a later turn.
https://dl.dropboxusercontent.com/u/30223400/Official.jpg CGuild: http://boardgamegeek.com/thread/1166819/st-aw-official-faq/page/1 (Ships #4)
The U.S.S. Defiant's ability to change crits into hits applies in the
“modify attack dice” portion of the combat phase. It will not negate
crits that are set or re-rolled after this step from effects like Toreth, Drex, or a re-roll from a [target lock].
https://dl.dropboxusercontent.com/u/30223400/Official.jpg AParks: http://boardgamegeek.com/article/13890081#13890081
It can also be triggered by non-standard attacks, such as William T. Riker, Gelnon, Suicide Attack or having Antimatter Mines dropped on top of it, as long as the source of the damage is directly caused by an opposing ship. It will not negate crits inflicted by non-attack collisions such as obstacles, or previously deployed minefields.
https://dl.dropboxusercontent.com/u/30223400/Official.jpg AParks: http://www.boardgamegeek.com/article/14039148#14039148