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The alpha-proton X-ray spectrometer "kissing" Barnacle Bill The time necessary to accumulate a sufficient number of "counts" so as to accurately identify the elemental composition of a rock is ten hours. Thus, once Sojourner makes good contact with its target, it sits there, unmoving, overnight, as the alpha particles flood the surface of the rock. In this case, Sojourner is touching the "clean" side of Barnacle Bill, the side that has been scrubbed clean of fine sand by the wind. This side was chosen purposefully so as not to have the sand, which is of a different geologic history, contaminate the readings from the rock itself.
The fine grain silt drifts that form behind the tiniest of wind barriers are even more noticeable in this picture, taken later in the day, at a lower sun angle. |