By Richard A. Kerr
No one had ever seen anything like it in the quarter-century of exploration on the surface of Mars. It appeared in front of the Opportunity rover as if it had fallen from the sky, and its resemblance to a jelly-filled doughnut stoked the media's interest all the more. But the show's over, folks. NASA announced Friday that, once Opportunity turned to get a clear view of where it had roved from, it was obvious - as mission scientists had speculated - that a rover wheel had rolled over a rock, broken off a bit of it, and sent the chip downhill to where it was seen days later. The dark red "filling" could have formed geologically recently after erosion exposed the rock at the surface, scientists said, or it could have formed long ago deep within Mars. End of story. On to the next rock.