Jon,
There may or may not be ice at the bottom of Endurance Crater. The "dune field" in Endurance is believed by most people to be a simple sand (or silt) dune deposit. It is possible that it is nothing more than a dune deposit, perhaps it was deposited during one of the global dust storms that envelope Mars from time to time. It may have been deposited relativly recently or more likely sometime long ago (millions of years).
Maybe it was deposited at a time when Mars was going through a warmer period and water vapor or other cohesive vapors mixed with the dust in a storm, these dunes formed, and then become more solidified when the temperatures dropped. For example: If it snows on Earth and the temperature is between say 30 and 20 degrees F and then the winds blows the snow around we typically see snow drifts form. If the temperature drops to say 10 to 0 degrees F the drifts usually become more solid or cemented and remain stable during subsequest snow storms as long as the temperature remains below freezing. Perhaps the dunes at the bottom of Endurance are "fossilized sand or silt dunes that formed in conjunction with a cohesive material or vapor.
The "ice" refers to what myself and some other people believed they were seeing. The Endurance dune field appears similar to ablated ice or ablated ice-like/evaporite material. If the rover had performed a simple RAT sample on the "dune material" we would likely understand much more about this material. If it is an ablated briney ice and not simple sand/silt dunes (fossilized or otherwise), then the implications of this material could be instrumental in explaining the spherule formation.
