On the clear, moonless night on May 20, 2012, Wayne Jaeschke recorded a Martian phenomenon that he was sure couldn't be real: an unexplained cloud moving the Mars atmosphere. And now, two and a half years later, professional scientists who have studied it still aren't sure what it is, according to their report out in Nature today.

"At first I thought: maybe there's a pixel error in the live feed on my computer, or some dust has collected on a [telescope] sensor," said Jaeschke, a patent lawyer and amateur astronomer in West Chester, PA, of that initial sighting. But when Jaeschke reviewed the telescope footage from his backyard observatory the next day, the sighting seemed far less like an error and more like something inexplicable. Jaeschke had captured what appeared to be a vast, enormously bright plume of something that had formed at Mars's northwestern horizon. Whatever it was, the massive cloud was at least twice as high as anything ever before see—and unlike a recording malfunction, it morphed and rotated with the planet.

Jaeschke immediately brought his strange discovery (a "Martian stumper," he called it) to the attention of his fellow amateur astronomers, and the sighting was confirmed. Eighteen separate amateur observers had seen it across the globe.

Oddly, the rotating plume could only be seen as it crested the northern horizon (never the southern horizon.) By all reckoning, it was high enough in Mars's atmosphere that it should have been dispersed by solar wind—but it persisted for over 10 days. Jaeschke and his fellow amateur astronomers quickly reached out to professionals, and today a team of astronomers, lead by Agustin Sánchez-Lavega at the University of the Basque Country in Spain, has finally published their analysis.

Their conclusion? To date, this plume defies our understanding of Mars's atmosphere, and nimbly avoids all scientific explanation.

Best Guesses

After analyzing the various recordings of the 2012 Martian plume, Sánchez-Lavega and his colleagues have been able to confirm the rough height and size of the phenomenon—more than 120 miles off the ground, and up to 600 miles in diameter—but little more. "Unfortunately the Opportunity and Curiosity rovers were at a different location… and there were no spacecraft observing the Martian [horizon] during those days" Sánchez-Lavega says.

Even when the best guesses as to the origins or exact make-up of the plume are weakly supported. "I've heard about of four or five different possible explanations," says Bruce Jakosky, a planetary scientist who was not involved in the research, and head of NASA's new Martian atmospheric probe, MAVEN. "And honestly? I don't like any of them."

One idea that Sánchez-Lavega tackles in his analysis is that the cloud is just that: a massive, airy billow of water or CO2 vapor. But this theory is problematic for more than a few reasons. While the light reflected off small CO2 particles does indeed match what was seen in the various astronomical recordings (unlike dust), after decades of Mars observation, nobody has recorded a gas cloud even reaching half the height of the plume's.

In addition, scientists can't account for the rapid temperature fluctuation that would be required to thrust the gas so far into the outer atmosphere. "And at that altitude, you have to be puzzled by the fact that it didn't dissipate over the 10 days," says Jakosky.

The researcher's second-best guess is that the plume was an excruciatingly bright aurora—a phenomenon like Earth's northern lights, which are caused by blasts of space radiation smacking into the planet's natural magnetic field.

As with the vapor theory, auroral light could indeed match the recorded images—but almost all other support for this hypothesis stops there. First, to match the plume's luminosity, the 2012 aurora would have to have been over 1,000 times as bright as any aurora ever seen on Earth. And while the plume did occur over area known to have a particularly strong patch of magnetic field, the records of solar winds during the May 2012 period don't fit the picture.

Improbable Timing

"The takeaway is that we've seen something totally unexpected that doesn't have a simple explanation," Jakosky says. "But, we really can't guess at the significance of what we've found until we have some type of plausible explanation."

Luckily (thanks to a bit of auspicious, improbable timing) we may have an answer soon. MAVEN, the atmospheric NASA probe that Jakosky heads, has only just reached Mars last September—and is currently orbiting the planet and collecting its first round of data. Because MAVEN will be the first spacecraft to thoroughly sample the red planet's outer atmosphere (the poorly understood section where the mysterious plume was located,) "we may be able to come back anywhere from one to twelve months from now with new boundary constraints on what this phenomenon could have been."

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