Enormous Martian Clouds Still Perplex Astronomers

On March 21, 2012, amateur astronomers noticed that strange things were afoot in Mars's southern hemisphere. Enormous cloud-like plumes were forming at very high altitudes over the Terra Cimmeria region, and continued to develop over the subsequent weeks.

Both the massive size of these plumes, which stretched up to 1,000 kilometers across, and their curious position in the upper atmosphere defy our current expectations of what is possible on Mars.

"No existing atmospheric model predicts the required conditions to form clouds or auroras of such magnitude," astrophysicist Agustin Sánchez-Lavega told me over email.

Sánchez-Lavega is the lead author of a new paper about the 2012 event, published today in Nature. In it, he and his colleagues outline two potential explanations for the mysterious phenomena—that it was either an extremely elevated cloud of dust or ice, or that it was an aurora. The authors note, however, that both of these preliminary theories still challenge and conflict with our understanding of Mars.

For instance, take the cloud hypothesis. Large plumes of Martian dust do frequently get kicked up into the atmosphere, and water and carbon dioxide often condense into cloud-forming crystals. Data collected both by spacecraft and Earth-based telescopes has consistently suggested that these clouds have an upward limit of about 100 kilometers, or 62 miles.

But the plumes observed in 2012 reached heights of 250 kilometers, well into the Martian ionosphere and exosphere. According to Sánchez-Lavega's team, it would have to be irregularly cold for water and carbon dioxide crystals to form at that height, and the presence of dust would require some kind of special, undiscovered vertical push off the surface. It's also not the right time of the sol for an event like this to take place—dust is most likely to ascend around noon, and the massive cloud plumes were observed on Mars's morning terminator.

"The cloud explanation we propose is an exploratory and open hypothesis," said Sánchez-Lavega of this scenario. "It has pros and cons."

The other explanation is that the protrusion was actually an unusually brilliant Martian aurora. While Mars's lack of a magnetic field normally precludes these charged light shows, the region over which the plumes were observed does have a history of producing auroras. This is due to a localized magnetic force called a crustal magnetic anomaly—a throwback to when Mars had a planetary magnetic field.

"The Martian magnetic field is different from that of Earth, whose origin is in a dynamo action in the interior of our planet," explained Sánchez-Lavega. "On Mars, the magnetic field is crustal with anomalies related to the presence of some magnetized substance buried beneath the surface."

So where Earth's auroras are generated by its powerful, core-driven magnetic field, Martian auroras are created by chunks of material that retain some of the planet's magnetic past, which then direct solar wind particles into the atmosphere, producing local auroras.

But even taking the presence of a crustal anomaly into consideration, the 2012 event was atypically bright and large. On top of that, an aurora of that magnitude would require a surge of charged particles from the Sun to fuel it, and there is no evidence of any particular influx of solar radiation during the event's appearance.

With this paper, Sánchez-Lavega and his colleagues have provided the first in-depth investigation of these bizarre Martian plumes. But astronomers need more observations and hypothetical models if this mystery is going to be unraveled any further. It goes to show that though Mars is the most studied planet in our solar system besides Earth, it still has a lot of secrets to spill.