Over the weekend I fiddled with the "tau" images that Mars Exploration Rover Pancam lead Jim Bell gave me, and I produced a couple of different ways to visualize the darkness of the rovers' skies. Typically, the rovers turn to find the Sun, cover their Pancam eyes with dark solar filters (like welders' glasses), and take photos, several times a day. These photos, when calibrated, provide a direct measurement of how much sunlight is getting through Mars' atmosphere to the rovers. The opacity of the atmosphere is referred to by the Greek letter tau (τ). Atmospheric opacity increases exponentially with increasing tau. So here are two animations, one for Spirit and one for Opportunity, covering pretty much the same time period. Opportunity's skies get darker faster than Spirit's skies do, and the opacity is also more variable. Spirit now has skies as dark as Opportunity's. For several sols, Opportunity wasn't able to muster enough power to take any tau measurements at all, so I substituted in images with an "X" to indicate that.
There have been more tau images returned to Earth from both rovers since I requested these from Jim last week. I hope to update these animations, but I won't pester Jim for new pictures daily.
Here's a different way of looking at the same data:
Both rovers are still OK. I've been using Midnight Mars Browser to check on their progress, and there were images returned from Spirit on sol 1269 (Sunday) and from Opportunity on sol 1249 (early this morning). For Opportunity, the newest images are still just focused on the sky, but Spirit is occasionally returning more science-y pictures. This series of forward Hazcam images documents some cool motions in the sand near Spirit's wheels: a track that its left front wheel made when it moved is being destroyed as wind shifts sand around.