" Red residue clinging to the scoop walls is from a sample collected earlier from a drift of windblown dust and sand called "Rocknest.

FORBES: First Curiosity Drilling Sample in the Scoop

Two days earlier, it had scooped some sand from a drift, sieved it to a fineness of less than a hundred and fifty microns, and dropped a portion half the size of a baby aspirin into a funnel on its back.

NEWYORKER: The Martian Chroniclers