InsideClimate: When ExxonMobil's Pegasus pipeline ruptured last month [3] in Mayflower, Ark., it was carrying diluted bitumen, a controversial form of oil from Canada's tar sands region. That was confirmed in a letter an Exxon lawyer wrote to the U.S. Environmental Protection Agency last week. But the letter contradicts public assertions by company officials that the spilled oil was simply "heavy oil," not tar sands bitumen. It also raises, once again, the question that surfaces after every spill involving oil...