“This is less likely because of the low currents required for the mission clock, which makes the under voltage effects less impactful, especially at these low temperatures,” said Bugga. Smart concurs: “I don’t think that scenario would result in the batteries being destroyed either. As mentioned above, allowing the batteries to drain to very low voltage is not healthy for the battery, and will likely lead to some performance loss. But this one time occurrence is not expected to be catastrophic. In addition, the harmful effects of draining the batteries to low voltage is not as dramatic at very low temperatures, since the rate of degradation, which involves the dissolution of the anode copper current collector, will be slower.”

There is also the possibility that something, somewhere on the rover broke. “We have to accept the fact that this is a 14-and-a-half-year-old rover,” said Callas. “Remember my analogy, it’s the difference between your 17-year-old nephew being outside without a jacket and your 97-year-old grandmother outside without a jacket. We’re the 97-year-old grandmother.”

While there is no evidence for or reason to believe that something broke on the rover, there is just no way of knowing without any data from Opportunity. Barring a breakage or mission catastrophic failure, it all appears to come down to dust. Confounding things even more, there is just no way of knowing with any certainty just how much dust fell on this rover. Considering that the PEDE literally blanketed the entire planet though, it’s hard to fathom the rover wouldn’t be dirty with dust.

“Yes, it was a giant dust storm and there was a lifting center right nearby, but how much of that actually falls locally versus globally getting mixed into the atmosphere is not known, so that may or may not have been a major factor,” said Golombek.“For there not to be enough power to at least charge the battery sufficiently for it to wake-up, there would need to be an extraordinary amount of dust on the solar arrays, probably more than we have ever seen in the past.”

That’s exactly what Lemmon, Herman, Staab, and others have been thinking for a few months now. “My thought is that unless something is broken, Opportunity’s solar arrays must be covered in lots of dust, so much that it’s blocking well over half the light that’s hitting them, maybe as much as 80% or 90%,” said Herman.

“I think it’s more than half because of the results of some power simulations I ran,” Herman continued. The inputs were the known positions of the rover and of Mars, as well as Tau estimates from the orbiters. “I assumed various amounts of dust on the solar arrays, from enough dust to block 10% of the sunlight to enough to block 90% of the sunlight,” she said. “If less than half of the sunlight is blocked by dust, then the arrays should have produced enough energy to have allowed us to hear from Opportunity by now. So the only thing that makes sense – aside from something being broken – is that more than half of the sunlight hitting the arrays is being blocked by accumulated dust,” she said.

Actually, the storm may have kicked up large particles that could have settled on the rover’s arrays, and those large particles may be part of the reason the sunlight is being blocked, as reported in the last issue of The MER Update. “Many papers suggest this, and they show that the larger particles settle close to the lifting area,” Lemmon said.