"The Martian dust storm has grown in size and is now officially a "planet-encircling" (or "global") dust event", NASA officials said in a statement.
The rover is on the red planet studying Martian soil at the Gale Crater.
Though Curiosity is on the other side of Mars from Opportunity, dust has steadily increased over it, more than doubling over the weekend. But the rover will go back to sleep if the batteries do not hold enough electrical charge, and the clock will set a new time to wake up for another battery check.
The 8.0 tau is the highest value that Curiosity has ever picked up in the area, the rover's science team wrote on Twitter.
The first Martian dust storm was recorded by French astronomer Honore Flaugergues in 1809. According to the last transmission received by the agency, tau was near 11 over its location. The atmosphere is so thick with dust, "accurate measurements are no longer possible for Mars' oldest active rover".
An artist's conception of a Martian dust storm, which might also crackle with electricity.
Martian dust storms are common, especially during southern hemisphere spring and summer, when the planet is closest to the Sun. But it's nearly Martian summer at Opportunity's location, and temperatures are not expected to dip below the danger level, NASA officials have said.
However, Martian dust storms don't usually cause a ruckus.
Coming back to the Mars Opportunity rover, there is still no signal from the strangled vehicle while a recent analysis has hinted that the long-term survivability of Opportunity rover in Mars' extreme cold conditions suggests that the rover's electronics, as well as batteries, might stay warm enough to function later. The European Space Agency also has two spacecraft in orbit (Mars Express and the ExoMars Trace Gas Orbiter). The Mars Reconnaissance Orbiter gave Opportunity's engineers an early warning about the approaching storm and acts like a weather satellite. One of the main questions they want to answer is why some Martian dust storms remain small and stall before a week has passed, while others grow and grow and last for months.
"We don't have any good idea", said Scott Guzewich, who is an atmospheric scientist working NASA's Goddard Space Flight Center situated in Greenbelt, Maryland.
Curiosity PanCam images of a rock the rover drilled into on May 20, 2018. "The largest impact is to the rover's cameras, which require extra exposure time due to the low lighting".
New photos from Curiosity show a wall of haze over Gale Crater that is up to eight times thicker than normal for this time on Mars, NASA officials said.