The dust storm continues at Mars, but both rovers continue to be power-positive. In fact, Opportunity is positive enough on power to have resumed taking measurements of "tau," the atmospheric opacity. The rovers measure the amount of light that passes through the atmosphere by placing a very dark filter over their Pancam "eyes" and taking a photo of the Sun. Lately, those images have been much darker and softer-edged than normal -- it's been awfully hard to see the Sun through all that dust. I asked Jim Bell, Pancam man, if he could give me calibrated versions of some of those Sun images to show you how its appearance has been changing with time for both rovers. I didn't quite have time to finish the work I wanted to do with these before the weekend, but I'll tease you for now with this animation of 23 "tau" frames from Opportunity, beginning on sol 1214. Note there's a big gap in time toward the end, when the sky got so dark that they couldn't even do tau measurements. For most of this animation, the Sun is nearly impossible to see. The images have been enlarged by a factor of 4.
NASA / JPL-Caltech / Cornell / animation by Emily Lakdawalla
Opportunity's dim Sun
The Mars Exploration Rovers take images of the Sun to determine the opacity of the atmosphere. Here, images of the sun taken since sol 1214 of Opportunity's mission have been calibrated to show how dim the Sun appeared through the dust storm. This is a work in progress...
The Spirit record is more continuous; I'll get to those next week, and will also expand the Opportunity animation.
Jim told me that both rovers have been instructed to follow a minimal activity plan over the weekend. "A few taus here and there, but mostly recharge and survive, recharge and survive. No communication with Opportunity until Monday, I believe. Two out of three communication [sessions] kept in for Spirit."
It's fun to compare that animation with the tau measurements being reported by Mark Lemmon. Here's Doug Ellison's graph of Mark's data, updated to sol 1245 for Opportunity and sol 1265 for Spirit. If you look at the graph and watch the animation, you can see the lumps in the graph reflected in dips in the visibility of the Sun in the above animation. I'd like to combine the tau images with a graph to make this more obvious -- another project for next week.
Data from Mark Lemmon / Graph by Doug Ellison
Atmospheric opacity (tau) for Spirit and Opportunity during the 2007 dust storm
For both rovers, typical values of atmospheric opacity, or tau, hover between 0 and 1. During June and July of 2007, dust storms lofted dust into the atmosphere, increasing the opacity to never-before-seen values above 5. At these levels, less than 1% of the sunlight that impinges on the top of Mars' atmosphere reaches the rovers' solar panels.
And, for completeness, here's the latest in the dust maps from the Mars Odyssey THEMIS team.
NASA / JPL-Caltech / Arizona State University / animation by Emily Lakdawalla
Mars' 2007 dust storm as observed by Mars Odyssey THEMIS
This animation is comprised of "dust maps" of Mars captured by the Thermal Emission Imaging System (THEMIS) on Mars Odyssey. The maps show the atmosphere's opacity at an infrared wavelength of 9 micrometers. The scale bar runs from a nearly clear opacity value of 0.05 to 0.40, which represents an approximately two-thirds reduction in the amount of sunlight reaching the ground. For the latest dust maps, visit the THEMIS website. This animation will be updated as the THEMIS team posts more maps.