Strange show spotted high above Mars surface remains mysterious!

BASQUE COUNTRY, Spain (PNN) - February 17, 2015 - Almost three years ago, the Red Planet put on a bit of a show for anyone with a telescope big enough, and an eye trained enough, to spot it. As the Terra Cimmeria region of Mars’ Southern Hemisphere rotated into view, a faint bulge rose above the smooth curve of the planet’s surface. It looked like a cloud, but it was too tall and too weird.

Starting on March 12, 2012, amateur astronomers reported seeing the odd lump on the Martian horizon. Reports continued to pour in over the next 11 days as the lump became even more obvious. It petered out some time before April 1, but a second occurrence was observed between April 6 and April 16. Each time, its form varied from day to day, and it was seen as dawn swept across the region - but not at dusk.

Although the Mars Reconnaissance Orbiter imaged the area daily, it did so in the afternoon, and nothing showed up as a result. But using the images that were captured by amateurs, a group of researchers led by University of Basque Country’s Agustín Sánchez-Lavega calculated the size of the fuzzy plume. It covered an area some 500 to 1,000 kilometers across and reached as high as 200 to 250 kilometers above the surface - that is, into Mars’ ionosphere and exosphere.

Of course, Mars is a very dusty place, and storms can kick up great clouds of dust. However, they’ve never been seen higher than 60 kilometers above the surface. Ice clouds - made of water or CO_{2 -} are also common, but haven’t been seen above about 100 kilometers. A patchy aurora is also a possibility, but even that has never been observed higher than 130 kilometers above the surface.

The researchers dug through databases of Hubble and amateur telescope images to look for any similar activity in the past over Terra Cimmeria. They found plenty of run-of-the-mill clouds, but nothing as wild as these recent images. A few Hubble images from May 1997 were close enough to warrant closer comparison, though the clouds weren’t nearly as impressive.

To see if it could have been an ice or dust cloud, the researchers first calculated the reflectivity of the plume at each of the several wavelengths at which it was imaged. In both the recent and 1997 images, ice clouds were a better fit than dust.

They then used an atmospheric model to see whether conditions might have been conducive to ice cloud formation. That showed it should have been too warm for that to happen. To condense water, the area would have either had to cool more than 50 degrees Celsius or somehow gotten a huge addition of water vapor. To make CO₂ ice particles, temperatures in the upper atmosphere would need to be more like 100 degrees Celsius colder than expected. However, the model did confirm that the winds necessary to pull up that much dust were even more unlikely.

So what about an aurora? That, too, is a common phenomenon above Terra Cimmeria. There is an anomaly in Mars’ magnetic field there that can interact with the charged particles of the solar wind to put on an atmospheric light show. The broad extent and rapid variability of the observed plumes would fit with this as well. But the plumes were much too bright, and no funky solar activity was detected that could have caused a flare-up.

The researchers do say that the aurora hypothesis could at least be tested fairly easily by continuing to watch the area, but we’re still left with a head-scratcher. Both of these proposed explanations, they write, “defy our current understanding of Mars’ upper atmosphere." That makes this all very interesting.