Astronomers have been unable to explain the sudden but fleeting appearance of massive plumes high up in the Martian atmosphere. Rising to altitudes of over 155 miles (250 km), these hazy structures are forcing scientists to re-evaluate what they know about the Martian atmosphere.
Amateur astronomers reported the plume-like features on two separate occasions back in March and April 2012.
"I noticed this projection sticking out of the side of the planet," noted amateur astronomer Damian Peach in a BBC article. "To begin with, I thought there was a problem with the telescope or camera. But as I checked more of the images, I realized it was a real feature — and it was quite a surprise."
A surprise, indeed: The plumes extended way up to the top of the Martian atmosphere — about 100 miles (150 km) higher than similar features observed in the past. At such a height, where Mars's atmosphere is supposed to end and space begin, meteorological phenomenon such as this aren't supposed to happen. As noted by lead researcher Agustin Sanchez-Lavega of the Universidad del País Vasco in Spain in an ESA release, the "reported plumes are extremely unexpected."
Because scientists had never seen anything like this before, they decided to re-analyze more than 3,500 images captured by the Hubble Space Telescope and stored in amateur image archives. Sure enough, the astronomers discovered a similar occurrence in a Hubble image dated May 17, 1999. The images suggest that, while rare, these plumes do seem to happen on Mars.
When they do occur, the plumes only take about 10 hours to form, extending over an area measuring 620 by 930 miles (1,000 by 1,500 km) in both the north-south and east-west directions. They hang in the atmosphere for about 10 days, changing shape on a daily basis before finally disappearing. The apparent clouds extend up into Mars's ionosphere and exosphere where the atmosphere is incredibly thin.
The plumes were observed along the Martian limb, i.e. the observed 'edge' of the planet, at the night/day boundary. The misty structures discovered in 2012 were situated between 124 and 155 miles (200 and 250 km) above the Terra Cemmeria region of the Red Planet's south-eastern hemisphere.
The plumes could not be observed by orbiting spacecraft owing to an incorrect viewing angle and poor lighting conditions.
The astronomers who are currently studying the nature and cause of the plumes have come up with two different theories.
The first theory is that they're highly concentrated and localized patches of auroral emissions controlled by magnetic field anomalies in the planet's crust. And in fact, such phenomena have been recorded at an altitude of 80 miles (130 km). What's more, auroral emissions have been previously observed above these geographic locations; this particular spot on Mars is known for having a large anomaly in the crust's magnetic field.
"I think there's still some discussion about the magnetic field as a contributing factor because of the high solar winds at the time," noted amateur Florida astronomer Wayne Jaeschke in an ABC Science article. "We saw this feature as several large coronal mass ejections from the Sun were passing by Mars."
Trouble is, the math doesn't add up; the strength of this apparent auroral emission should have been far weaker than the brightness exhibited by these massive plumes. For this theory to work, the observed aurora would have to be 1,000 times brighter than aurora found on Earth. As astronomer Garcia Munoz told the BBC, "it is difficult to come to terms that Mars has such an intense aurora."
Another theory is that it's a hazy concoction of water-ice and/or carbon-dioxide-ice crystals mixed in with the fine Martian dust. Previous studies have shown that similar clouds can extend to a maximum height of 60 miles (100 km) when pushed upwards by major dust storms.
"One idea we've discussed is that the features are caused by a reflective cloud of water-ice, carbon dioxide-ice or dust particles, but this would require exceptional deviations from standard atmospheric circulation models to explain cloud formations at such high altitudes," noted Sanchez-Lavega.
Indeed, as the authors conclude in their study, "Importantly, both explanations defy our current understanding of Mars' upper atmosphere." Meaning that, if either of these theories is correct, science's present understanding of the physics in Mars's upper atmosphere is incorrect, or incomplete.
Hopefully these plumes will return in the near future so they can be studied further.
Read the entire study at Nature: "An extremely high-altitude plume seen at Mars' morning terminator".