The amateur and professional astronomy communities alike are busy scratching their heads today; headlines from the ESA (European Space Agency) claim that scientists are baffled. Now, such a claim is common among the mainstream media, and we can usually assume they mean, simply, that they don’t know how to explain whatever thing they’re reporting on. In this case though, scientists are indeed baffled by what they’ve found.
That’s because it makes little sense, but then when it comes to the Red Planet, this isn’t uncommon.
In a report on their website, the ESA have provided photographs taken by the Hubble Space Telescope that show a large plume of dust billowing off the surface of Mars. You might now be wondering just what the big deal is, and, well, the big deal is this…Mars’ atmosphere isn’t thought capable of doing such a thing (see caveats below).
OK, here are the details. ESA scientists reviewing Hubble images from March and April of 2012 noticed a plume of dust on two separate dates, but coming from the same general location. Dust plumes aren’t altogether unheard of on Mars. Both professional and amateur astronomers have captured images of plumes in the past, but none were ever this big. The images in question show what looks like just a small puff of smoke sort of hovering just above the surface of the planet, but in reality, those clouds are more than 250 kilometres high. By comparison, previously observed dust plumes have never exceeded 100 kilometres and most have been well under that.
The weird thing is, no one really know why these dust plumes are happening, and since these new ones are so much larger than previous, there are a lot of questions being asked.
There’s actually a lot to explain here, so here’s a crash course on Mars’ atmosphere and dust.
Mars has an extremely thin atmosphere, about 1% that of Earth’s. This is why the future Mars astronauts will still have to wear EV suits when on the planet. Well, that’s one of the reasons. That atmosphere consists of about 96% carbon dioxide, much like Venus, and because it’s so thin, there’s very little wind, and what wind there is, is never more than what would be a gentle breeze here on Earth.
That doesn’t mean that Mars doesn’t see its share of dust storms though. Mars is covered in an extremely fine particulate dust. The grains of that dust are about 30 μmetres (micrometer) in diameter or about the size of a human skin cell. To give you an idea of how small that is; a human egg is approximately 130 μmetres, paramecium are typically no larger than 200x60 μmetres, and a grain of sand on Earth is between 62.5 μmetres and 2 mm. The point is, the fine dust on Mars is of the very smallest particulate.
The fact that Mars’ dust is so fine means that it’s relatively lightweight and easy to kick up into a cloud. In fact, Mars is known to undergo planet-wide dust storms that can last for up to a year. The reason for the longevity of the storms is that the dust is so light it stays airborne for a very long time once picked up. The thing is, a dust storm and a localised dust plume are two very different things.
There have been a number of possible causes put forward. Some from the ESA and NASA suggest that it might not be dust at all, but moreso a trick of light due to ice particles floating in the atmosphere. Though this doesn’t really answer our biggest question.
Others have suggested that it could be an auroral effect or auroral emission. Aurora are somewhat common on the Red Planet, as they are here on Earth. For those unfamiliar, auroras are caused by charged particles, mainly electrons and protons, entering the atmosphere from above causing ionisation and excitation of atmospheric constituents, and consequent optical emissions. In short, it too is a trick of light, however it’s possible that the excitation of particles in the atmosphere of Mars could be creating an electric charge in the extremely fine dust, which might carry it aloft in localised areas. Sort of like Martian static cling.
Other’s still have suggested the plumes might have been caused by impactors; relatively small meteors striking the surface and causing shock waves on impact which could carry the dust into higher altitudes. Though as with lightning, meteors rarely strike the same place twice, at least in such a short span. (Yes, I’m aware that lightning does in fact regularly strike the same locations multiple times.)
It could even be solar winds, which are streams of charged particles (electrons and protons) projected from the sun at roughly 900 kilometers per second, though that’s sort of a long shot.
Of course, it’s virtually guaranteed that we’re going to see some wacky ideas put forward in the coming days for why this might have happened. Aliens, Mars ROVER (which is nowhere near the location of the plumes), Elvis…someone might even suggest that Mars just farted. Any way you slice it, these dust plumes are indeed a baffling development, it’s possible and maybe even likely that we’ll never know what happened, but we’ll keep watching the Martian surface and let you know if you need to worry.
 A. Sánchez-Lavega et al. An extremely high altitude plume seen at Mars morning terminator. Nature, 16 February 2015.