Since landing on Mars a year ago, NASA's pair of six-wheeled geologists have been constantly exposed to martian winds and dust. Both rovers have been coated by some dust falling out of the atmosphere during that time, with estimates of the dust thickness ranging from 1 to 10 micrometers, or between 1/100th and 1/10th the width of a single human hair. Of the two, NASA's Mars Exploration Rover Spirit is definitely the more dust-laden. The Opportunity rover, as shown here, appears to be collecting less dust, perhaps because of a cleaning by wind or even "scavenging" of dust by frost that forms on the rover some nights during the martian winter. (See image of frost on Opportunity PIA07108).
Opportunity took the left image on martian day, or sol, 23 (Feb. 16, 2004), and took the right image about 11 months later, on sol 346 (Jan. 13, 2005), using the panoramic camera. Both images show the camera's calibration target, which is used as a reference point for calibrating the colors on Mars. In the later image, the surfaces have become only mildly dusty compared to shortly after landing. Both images represent the panoramic camera team's best current attempt at generating true-color views of what these scenes would look like if viewed by a human on Mars. They were each generated using a combination of six calibrated, left-eye Pancam images acquired through filters ranging from 430-nanometer to 750-nanometer wavelengths. The diameter of the outer ring of the calibration target is 8 centimeters (3.15 inches).