Consider that Mars has a much thinner atmosphere than Earth. A typical local-size storm on Mars reaches “10-15 kilometers” (about 6-9 miles) in height, Cantor said, while the recent haboob in the Phoenix, Arizona area on Earth reached “about 1.5-1.6 kilometers” (just under a mile) in height in early July. There’s just no comparison to a PEDE.

Last week though, Cantor saw signs indicating that the storm is beginning to dissipate. First, he found breaks in the veil of dust obscuring the planet that allowed him to see surface features that have been buried under the haze for six weeks. Then, by employing a measurement system that Mars scientists use to determine the atmospheric opacity, a parameter that indicates the amount of dust in the atmosphere, he assessed that the cloud is indeed thinning, and his latest Mars Weather Report was sunnier than it has been since May.

This dust opacity measurement, called a “Tau” in Mars science jargon, allows scientists to gauge and monitor dust in the atmosphere from both the Martian ground and from orbit. Acquiring a Tau measurement from the ground is preferred for surface missions, because rovers and landers are, after all, on Martian terrain, it is more accurate locally, and in the case of Opportunity because the local dust scene is what matters most to this solar-powered rover’s ability to live long and prosper.

Opportunity takes this measurement daily as part of her normal routine, using the Sun as the point of reference, which, of course, is a known, steady magnitude light source. Over the 14 years and six months of this mission, MER Athena Science Team member and atmospheric scientist Mark Lemmon, Senior Research Scientist of the Space Science Institute (SSI), who also works on the Mars Science Laboratory (MSL) rover Curiosity, has used this method to consistently determine what the dust opacity or Tau, which in turn helps the team estimate how much sunlight is getting through to the arrays.

But when the storm blotted out the Sun at Endeavour Crater and sent Opportunity into sleep mode, taking an atmospheric opacity measurement at Endeavour Crater was impossible. In the communiqué the MER team received from Opportunity on June 11th, the rover reported a Tau of 10.8, “the largest ever recorded on the surface of the Red Planet,” noted Mission Manager Scott Lever, of JPL. When the team did not hear from the rover on the next communication pass, Callas declared spacecraft emergency, because that level of opacity essentially meant the sky over the rover was near pitch black.

In fact, the jaw-dropping measurement overtook the record held by the Viking landers, which estimated a Tau of 9 during a PEDE back in the late 1970s; thus, it chalked up another record for MER, albeit one that was not desired.

Estimating the Tau from orbit is tricky, but it is possible. By using a terrestrial model tuned for the Martian atmosphere by colleague and MARCI science team member, Michael Wolff at the Space Science Institute, a model that he has used during the last 20 years, Cantor crunched the numbers based on the MARCI data in late July and produced a calculated estimate of a Tau that put guarded smiles on the faces of MER team members: the dust opacity at Endeavour had dropped substantially, to approximately 3.6 with a margin of error of 1, meaning the haze could be as low as 2.6 or as high as 4.6, “a huge uncertainty,” he admitted.