The Opportunity rover resides in Mars’s southern hemisphere, which is entering the planet’s summer season, a time when dust storms are common. During summer, sunlight warms the air closest to the Martian surface and leaves the upper layers of the atmosphere cooler. This mismatch causes warm air to rise, taking surface dust with it, and winds scatter the tiny particles around the planet. Scientists and engineers weren’t surprised when they spotted one such storm brewing on May 30. But within days, the storm had grown to record-breaking proportions.
With the atmosphere padded with dust, Opportunity could no longer use its solar panels to charge its batteries. It’s just too dark, according to Mark Lemmon, an associate professor at Texas A&M University who works on daily operations of the Opportunity rover. Lemmon is an atmospheric-opacity expert, which means he has spent nearly 15 years estimating how bright or dark the sky looks to rovers on Mars. The scientific term for this is “tau,” a measure of the opaqueness of an atmosphere.
Usually around this time of the Martian year, tau oscillates between 1 and 2. During an intense dust storm in 2007, the tau value rose to 5.5. On Sunday, as this storm swirled, Lemmon recorded a tau of 10.8 on Mars. The greater that value, the less sunlight permeates the atmosphere to reach the surface. Under these conditions, “the brightness of the sun was 99.9995 percent fainter than it normally would be,” Lemmon said. It was so dark on the surface that engineers could command the rover to take pictures of the sun without the use of its solar filters, which act as eclipse glasses.
“During the middle of the day like this, just like on Earth, it’s not advisable to look directly at the sun, but there’s so much dust in the atmosphere that you wouldn’t even have to squint,” Lemmon said. “That’s the scary thing: Even looking straight at the sun, it’s not bright.”
If humans were to stand next to Opportunity right now—clad in the appropriate protective spacesuit, of course—the sky would look overcast rather than pitch-black. But for a rover that depends on sunlight to work, it may as well be nighttime.
Here’s how the Martian sky looks to another rover on Mars, Curiosity, which runs on nuclear power. (NASA officials said Wednesday that the Curiosity team isn’t worried about that rover, which is located on the opposite side of Mars.)
NASA engineers haven’t heard from Opportunity since Sunday, which suggests that the rover, in an attempt to preserve its dwindling power supply, has slipped automatically into a mode that shuts off nearly all of its functions. This mode prevents Opportunity from taking more pictures for Lemmon to analyze, which means scientists don’t know how dark it is for the rover right now. “I doubt that it’s gotten much better than that 10.8,” Lemmon said. “And it might have gotten worse.”