Erratic Rocks at Gale Crater, Comparison to Gusev and Meridiani Planum

Introduction: Currently on sol 2635, the MSL Rover Curiosity is climbing up Mount Sharp in Gale Crater to characterize the environmental conditions under which the encountered bedrock was laid down in the distant past [1]. Through extended imaging, remote and in-situ chemistry, as well as drill samples for mineralogy and wet chemistry, so far ~ 400 meters of stratigraphy were characterized from the foothills. In general, the bedrock was found to be very homogeneous over kilometers and in most cases fragments or pebbles clearly shared chemical similarity with the intact layers [2]. Currently the rover is in the clay bearing unit (CBU). During the mission several erratic rocks were found, either igneous floats or suspected cap rocks. Recently, a pebble was measured with the APXS [3], which shows little chemical similarity with the bedrock. Two hypotheses for its origin are discussed in this abstract. APXS results and context: The ~5 cm wide pebble dubbed GretnaGreen (GG) was found in close proximity to the Western Butte capping rock in the CBU, figure 1.