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. As a result, Spirit has gradually experienced a decline in power as the thin layer of dust has accumulated on the solar panels, blocking some of the sunlight that is converted to electricity. Spirit took the left image on martian day, or sol, 9 (Jan. 11, 2004), and took the right image nearly a year later, on sol 357 (Jan. 3, 2005), using the panoramic camera. The images show the camera's calibration target, which is used as a reference point for calibrating the colors on Mars. In the later image a semi-transparent layer of reddish martian dust coats the surfaces. The panoramic camera team's analysis indicates that the layer of dust on Spirit's calibration target is about 70 percent thicker than that on Opportunity's. 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 from 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).