So then it's true! Missing thermos caps really DO go to Mars! I bet Mars has all my ballpoint pens and lone socks as well.
Seriously, wonder what it could be?
Spirit spent much of the past week studying a layered rock exposure known as "Montalva." During the study, the rover used the brush on the rock abrasion tool, the microscopic imager, the alpha-particle X-ray spectrometer that collects information about elemental composition, and the Moessbauer spectrometer that identifies iron-bearing minerals.
Sol 1070 (Jan. 6, 2006): Spirit unstowed the robotic arm and brushed the surface of Montalva, acquired a microscopic image of the rock, and placed the alpha-particle X-ray spectrometer on it for elemental analysis. Before shutting down for the evening, Spirit measured atmospheric dust using the panoramic camera.
Sol 1073: Spirit swung the robotic arm out of view to collect full-color, 13-filter images of Montalva with the panoramic camera and acquired 4 hours of worth of data using the Moessbauer spectrometer.