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As I sit here in Colorado on a summer day, I keep seeing nasty afternoon storms blowing past.
As the Opportunity and Curiosity rovers sit on the surface of Mars, they're seeing significantly nastier weather: Cold, with 100% chance of dust storms.
Mars has an atmosphere that's about 0.6% as thick as Earth's. That's thin, but it's there. It's enough to generate local dust storms, picking up the very fine powder that's almost everywhere across the Martian surface and blowing it around. These grains are a pain — the dust gets into everything, and the grains tend to be loaded with static electricity, so they stick to surfaces as well, coating landers' solar panels and reducing power — but they tend to end quickly.
A storm started up recently on Mars (first seen on May 30), but instead of dying down, it grew. And grew. And holy wow, it grew. It is now officially a global dust storm, literally encompassing much of the planet.
These happen every few years on Mars. In fact, the first time humans ever sent a probe into orbit there (Mariner 9 in 1971), the planet was engulfed in a huge storm, and instead of lovely images of the surface all planetary scientists got for weeks were photos showing blurry nothingness.
The storm raging there now is also pretty bad. It's the worst Opportunity has ever experienced since landing in early 2004. That's a problem: Opportunity uses solar power, and the storm is so thick that sunlight has dropped hugely at the surface. It's only 0.002% as bright as it is on a clear day*, so the rover is getting a tiny fraction of the power it usually does (and that's not including the dust coating the panels). Scientists have suspended operations for Opportunity until this (literally) blows over. However, I have to note that it's cold there, and while engineers expect it to wake back up when the storm's done, there's a small chance that it may not be able to switch back on. Let's hope it does.
Curiosity, on the other side of the planet, is also seeing effects from the storm, but it uses nuclear power, so it's not as severely affected. It was able to take a self-portrait, and you can see the storm's effect in the air:
[Note: Curiosity takes a series of images using a camera mounted at the end of a robotic arm; these are stitched together to make the mosaic, and for aesthetic reasons only photos without the arm in them are used, so it looks like it was taken by someone standing there.]
You can see the sky is a butterscotch color due to the fine-grained iron-rich dust suspended in the thin air, and distant landscape features are obscured.
While this is dicey for the rovers, it's a boon for planetary scientists, especially atmospheric scientists. It's not clear (har har, clear) how the storms form, and why some grow larger while others die off. They tend to happen in the local summer, when the Sun warms the ground better. The warm air rises and can pick up the dust with it. This can form amazing and very large dust devils, but sometimes these storm systems grow to planetary scales.
This tends to happen in the southern summer. Mars has a two-Earth-year-long orbit, and it's also much more elliptical than Earth's. Southern summer also happens when the planet is closer to the Sun, adding to the warmth then. This is likely a big part of why some storms grow large.
With several spacecraft orbiting the planet (Mars Reconnaissance Orbiter, Mars Orbiter Mission, Mars Express, ExoMars, MAVEN, and Odyssey), this is a chance to record a lot of different kinds of data to better study this incredible phenomenon. If we want to send humans to Mars, even for short times, let alone to live there, we will very much need to understand dust storms. Because the air is so thin it won't blow over antennae or strand Mark Watney or anything like that, but it will absolutely impact life there.
Also, it's just a good idea to understand what other planets are doing. It helps us understand ours better, plus learning about the Universe in all its natural power is something we humans love doing.
*You may see some references to this as tau, a measure of how well the air blocks incoming light (also called optical depth). It's a combination of the physical thickness of the air, the density of stuff in it, and how well that stuff blocks light (called opacity). The amount of light that gets through is an exponential function of tau, so if tau = 2 then the light drops by a factor of e-2 = 0.13, so only 13% of the light gets through (e is the base of the natural logarithm, equal to about 2.718). Current conditions on Mars are tau =11, or a drop in light by a factor of e-11 = 0.0000167. In percentage, 0.00167% of the light gets through then. I rounded up a tad in the article.