On November 14th 1971 NASA's Mariner 9 became the first spacecraft to successfully orbit another planet. Its video-camera imaging system powered up, and American scientists eagerly awaited the first detailed pictures of Mars since the flyby of Mariners 6 and 7 just two years earlier.
Except what came back from the deep space telemetry were not pictures of the intricate canyons, craters, and mountains of Mars, but pictures of a blanketed world, a dust enshrouded mystery. Mariner 9 had arrived in the midst of one of the greatest global storms humans have ever witnessed on Mars.
It wasn't a total surprise for the scientists, earlier observations had hinted at a dust storm that began in late September of that year in the southern Noachis Terra landmass. But the extent of this system was astonishing. There was essentially nothing visible of the martian surface.
In another first for interplanetary exploration, the NASA engineers reprogrammed the spacecraft to wait the storm out. This was quite a feat, the computer was extraordinarily primitive by today's standards, as was the data storage. Here's a quote from the NASA NSSDC page detailing Mariner 9's specs:
"Spacecraft control was through the central computer and sequencer which had an onboard memory of 512 words. The command system was programmed with 86 direct commands, 4 quantitative commands, and 5 control commands. Data was stored on a digital reel-to-reel tape recorder. The 168 meter 8-track tape could store 180 million bits recorded at 132 kbits/s. Playback could be done at 16, 8, 4, 2, and 1 kbit/s using two tracks at a time."
In the meantime the Soviet mission Mars 2 arrived just two weeks after Mariner 9, but it didn't have the option to reprogram, and automatically sent its lander probe down to the surface and right into the dust. It's not clear exactly what went wrong, but barreling out of space and into a thick storm of micron-sized dust particles probably didn't help the probe's chances, and it summarily crashed on the surface.
By late 1971 and into January 1972 the storm abated, and Mariner 9 began to send back some spectacular images - a total of over 7,300 pictures that mapped the entire martian surface with resolutions ranging from 1 kilometer per pixel to as good at 100 meters per pixel.
The image here gives a sense of the magnitude of the storm. This was what the scientists began to see as the dust settled. The only visible features are the three great Tharsis Montes shield volcanoes, poking up through the haze in a line. The tallest of these reaches an altitude of over 18 kilometers. These peaks, and the enormous bulk of Olympus Mons had never been imaged by a spacecraft before, earlier flybys had missed them.
The late Bruce Murray (Caltech) was on the camera team and recalls, "there was a gradual clearing, like a stage scene, and three dark spots showed up." The Mars that came out of the storm was a revelation, from these colossal mountains to the great rift of Valles Marineris and the steep valleys of Noctis Labyrinthus.
It was good that the storm cleared when it did. These images and the global map from Mariner 9 paved the way for the extremely successful Viking missions, and helped pinpoint where the landers should try to set down.
Since then we've seen plenty of other dust storms on Mars. In fact, in 2001 NASA's Mars Global Surveyor was witness to another planet-scale event - possibly comparable in size to the 1971 storm.
You can see a time sequence of Global Surveyor full-globe images at Malin Space Science Systems, spanning June 2001 to September 2001. The whole planet was engulfed - a scene also captured by the Hubble Space Telescope in the image here (below).
With data from Global Surveyor we learned that although the dust reflects much of the sunlight hitting Mars during the day, thereby cooling the surface, the particles also absorb radiation and re-emit it as infrared (heat) at night. The upshot is that while the surface chills under its dusty blanket, the atmosphere actually heats up, by as much as 30 Celsius. This may be a clue to how such storms go global - a warmer atmosphere can drive stronger winds that lift more dust off the surface.
While Mariner 9's first view of the red planet may have been a disappointment, it was also a glimpse into the remarkable environment on another world. It's an environment that continues to fascinate, as well as provide a calibration point for our understanding of small rocky planets that are unlike the Earth.
Indeed, the thermal signature of planet-wide dust storms might be something we could keep an eye out for when the first direct glimpses of terrestrial-scale exoplanets are obtained. Even if we can't see these worlds as anything more than a point of light, their varying spectral character could be familiar, a sight we'd recognize from that stormy day on Mars in 1971.