Remember a few weeks ago when astronomers spotted that "ghostly snake" of a twister creeping its way across the face of Mars? Well, the windstorm featured up top makes that other storm look like an adorable little dust-up.
On March 14th, NASA's powerful HiRISE camera spied this Martian dust devil winding its way over the Amazonis Planitia region in the Red Planet's northern hemisphere (the same part of Mars that it spotted the earlier dust devil). But unlike its smaller sibling, which stood a measly 800 meters tall, this storm had some serious altitude to it — twelve miles of altitude, to be exact. [Hi-res available here]
Astronomers were able to determine the storm's height by examining its shadow in images like this one [click to embiggen]; just look at how massive it is. To put things in perspective, consider that on Earth, it's a big deal if a tornado reaches ten miles in height — and what you see here isn't actually a tornado at all. It's a dust devil. But dust devils here on Earth rarely exceed heights of even a few hundred meters. So how the heck did this dust storm get to be so enormous, and why do astronomers insist on calling it a dust storm instead of a tornado in the first place?
HiRISE scientist Paul Geissler has your answer:
Dust devils differ from tornadoes in their energy sources. Dust devils are driven by the heat of the surface, absorbed from sunlight and re-radiated to warm the atmosphere. The warm air rises and spins as it contracts, much as a figure skater spins faster as she draws her arms to her sides.
Tornadoes have an additional energy source: the heat given off as water vapor condenses into liquid rain. The condensing water vapor produces the visible part of a tornado, called the condensation funnel, which is made up of water droplets. On Mars, there is too little water vapor in the atmosphere to contribute significantly to atmospheric convection on local scales. The cloud that we see in this image is produced by dust particles, not raindrops.
As for the storm's prodigious height, Geissler says it's possible because the mass of an atmospheric column on Mars is less than one percent of a similarly sized column here on Earth; and less mass allows the spire to twist higher into the sky.
"Transfer of heat from the surface into this less dense atmosphere can produce more vigorous convection, which will penetrate higher into the Martian atmosphere than its counterparts do on Earth."
Or, as Yeats put it: "Things fall apart; the centre cannot hold... unless you're on Mars, in which case, climb on, blessed dust devil. Climb on."
You can check out more awesome images of the surface of Mars over at NASA HiRISE.