For a while, Mars was beating Spirit while she was down, throwing a dust storm at the rover where it's bogged up to its hubcaps in fluffy soil. Although not nearly as bad as the dust storm of 2007, it did seriously darken Spirit's skies and reduce its available power by nearly two-thirds, as A. J. S. Rayl mentions in her latest rover update. Now, though, the sky has cleared substantially. Mark Lemmon posted to unmannedspaceflight.com this morning that tau, the measure of the opacity of the atmosphere, "peaked on [sol] 2007 around 2.6, fell dramatically from 2009 to 2011 and is about 0.9 on 2013. It was under 0.5 before the storm. The dust appears to have simply moved elsewhere, but of course some would have accumulated on the panels during the brief storm." That last bit is important -- when lots of dust is lofted into the sky, the hazard is that when it comes down, it may come down on the rover and its solar panels. But it appears things on Spirit are still pretty clean.
Fredk posted this illustrative animation on unmannedspaceflight.com yesterday. The contrast between the shadowed and sunlit areas on the Marsdial is an easy visual referent to how dusty Mars' skies are.
NASA / JPL-Caltech / Cornell / animation by fredk
Shadows brighten and darken with the dust storm
Six images captured by Spirit of its Marsdial calibration target from sol 1994 to sol 2012 illustrate the effect that a regional dust storm had on lighting conditions in Gusev crater. On sol 1994, skies were quite clear, so the shadow of the Marsdial's gnomon was dark. But as dust filled the skies, sunlight dimmed at the same time that more light was scattered into shadowed areas, making the shadow considerably lighter and more diffuse. By sol 2012, conditions were improving, and the shadow darkening again.
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