During the 2012 and 2014 apparitions of mars, we talked a lot about a very curious detail imaged by several amateurs: an object projected beyond the terminator, that could only be at a very high altitude. Seven scientists and six amateurs co-sign this month a paper about it in the famous magazine Nature!
An extremely high-altitude plume seen at Mars’ morning terminator is a study written by A. Sánchez-Lavega, A. García Muñoz, E. García-Melendo, S. Pérez-Hoyos, J. M. Gómez-Forrellad, C. Pellier, M. Delcroix, M. A. López-Valverde, F. González-Galindo, W. Jaeschke, D. Parker, J. Phillips and D. Peach.
First it presents the result of our measures of the altitude of such “plumes” : they reach at least 200 kilometers above the surface, and probably more (up to 280 km), which is much higher than any known Martian cloud. Both dust storms and white clouds are not observed past an altitude of only 100 km!
Second we studied two hypothesis to try to find a plausible explanation. However, none of them looks completely satisfactory and the mystery is not solved!
- A cloud made either of water or carbon dioxide. This idea looks to be the more solid, but we would still have to explain how they can form so high. It’s coherent with the photometric behavior of the objects, being similar to that of the usual white clouds of the planet (brighter in blue). In any case the dust storm hypothesis can be ruled out because they are, to the contrary, brighter in red.
- Auroras. This hypothesis is backed by the fact that many occurrences have been observed above Mare Cimmerium, a region of the Martian ground that presents the strongest magnetic anomaly. However, they would then be much brighter than the terrestrial auroras, and this looks hard to justify…
If you don’t have access to the article I recommend the reading of two on-line materials:
Martian mystery cloud defies explanation, on Nature’s web site that describes briefly the main study, by Alexandra Witze
Martian Terminator Projections Observed by the HST , an amateur article in Communications in Mars Observations (I’m the author), quoted in the main study and that describes the plumes as they can be seen in HST images. I did not made any altitude estimations at the time but there are some interesting developments about the hourly and seasonal behavior of such “clouds”, their geographic repartition and photometric behavior. My feeling at the time was that of a cloud, although certainly a bit different than usual (I have been striked by how fast their shape evolve in a matter of minutes).
What conclusion for amateurs?
Once again, amateur observations do have a scientific interest. Image the planets, you can always meet surprises :).
In 2016, Mars will be observable again from Earth. However the configuration will not be very favorable to my eyes. Such plumes are mostly observed above and beyond the morning terminator during Martian southern winter. But the morning terminator can only be observed after opposition, and the Martian season will then reach late southern winter and if they are clouds, they should not form as easily. Let’s see!