Dust Devils' Powerful Updrafts Could Drive Mars Climate
Mars' atmosphere is often viewed as frigid and unchanging, but in studies of the red planet's aeolian processes, nothing could be further from the truth -- particularly where Martian dust devils are concerned. Continue reading →
Mars' atmosphere is often viewed as frigid and unchanging, but in studies of the red planet's aeolian processes, nothing could be further from the truth - particularly where Martian dust devils are concerned.
Aeolian, or wind-blown, processes dominate the Martian landscape; from space, aeolian features such as vast dune fields have fascinated planetary scientists. But another dominant atmospheric phenomenon studied by robotic missions in Mars orbit and on the ground is the dust devil, which often leaves its mark as dark curved paths in the dust.
Scientists are now beginning to understand how these swirling dusty vortexes are able to beef-up to the size of tornadoes we find on Earth and how they could impact the Martian climate.
"To start a dust devil on Mars you need convection, a strong updraft," said atmospheric science graduate Bryce Williams, of the University of Alabama in Huntsville (UAH), at the American Geophysical Union's fall meeting in San Francisco last week.
Dust devils, on Earth, are minor meteorological curiosities when the landscape is heated by sunlight. As the surface warms the air above it, the heat rises through the cooler upper layers. This convection can start to form a swirling vortex a couple of hundred meters high during an otherwise windless day. But their Mars cousins can dwarf their terrestrial counterparts, often becoming long-lived features reaching up to 12 miles high.
Now, Williams and supervisor Udaysankar Nair have been able to show how these fascinating funnels of air and dust are able to become super-sized.
"We looked at the ratio between convection and surface turbulence to find the sweet spot where there is enough updraft to overcome the low level wind and turbulence," said Williams. "And on Mars, where we think the process that creates a vortex is more easily disrupted by frictional dissipation - turbulence and wind at the surface - you need twice as much convective updraft as you do on Earth."
This conclusion was reached after studying meteorological data from Australian dust devils and comparing that with observations by NASA's Viking Lander mission. The researchers were able to create a 1-dimensional "planetary boundary layer model" that could identify the ideal conditions for dust devil formation in the Martian atmosphere and, as it turned out, for a dust devil to form on Mars, more powerful convection currents were needed at the surface layers.
This study is much more than just a curiosity about Martian dust devils, however. Considering the Martian atmosphere is, on average, less than one percent the pressure of Earth's atmosphere (at sea level), dust has a significant impact on the planet's climate. As dust devils provide a mechanism for kicking substantial quantities of dust into Mars' atmosphere, they could act as a global climate control of sorts.
"The Martian air is so thin, dust has a greater effect on energy transfers in the atmosphere and on the surface than it does in Earth's thick atmosphere," said Nair.
During the day, dust in the Martian air reduces the amount of sunlight that would otherwise heat the surface, but during night, the atmospheric dust emits long-wave radiation, warming the surface. Therefore, understanding how this dusty haze is transported into the thin Mars air via dust devil activity will aid Mars climate models, potentially helping us fit another piece of the Mars climate puzzle.
A large dust devil towers over the Martian surface on the plain of Amazonis Planitia, as seen in this High-Resolution Imaging Science Experiment (HiRISE) camera observation taken in 2012. The shadow indicates this dust plume reached a height of 12 miles although it was only 140 yards in diameter.
Mars plays host to a huge number of dune fields -- regions where fine wind-blown material gets deposited to form arguably some of the most beautiful dunes that can be found on any planetary body in the solar system. Using the powerful High-Resolution Imaging Science Experiment (HiRISE) camera on board NASA's Mars Reconnaissance Orbiter, planetary scientists have an orbital view on these features that aid our understanding of
and Martian geology. Here are some of our favorite Mars dunes as seen by HiRISE. Pictured here are shell-like "
" in the ancient Noachis Terra region of Mars.
Special thanks to Ari Espinoza of the
for helping to compile this list.
Dunes of many shapes, sizes and formation processes can be found on the Red Planet. Shown here are elegant "
" with deposits of larger rocks and possibly ices in their troughs.
These slug-like dark dunes are striking examples of "
" -- elliptical accumulations of fine material with no-slip surfaces. These domes contrast greatly with the often jagged appearance of barchan dunes. Found at the bottom of Proctor Crater, they are darker than the surrounding crater floor as they are composed of dark basaltic sand that was transported by the wind.
Looking like a wind-blown silk sheet, this field of "star dunes" overlays a plain of small ripples, another aeolian feature. The ripples move more slowly across the bottom of Proctor Crater, so the large dune field will travel
the smaller ripples. Dunes are continuously evolving and moving with the wind, ensuring that the Martian surface is never static.
These "transverse dunes" are undergoing seasonal changes. Likely entering Mars summer, this region of dunes is stained with pockets of subliming ices -- likely carbon dioxide. As the ices turn from solid to vapor, dune material slumps, revealing dark, sandy material underneath.
Resembling the mouths of a shoal of feeding fish, this is a group of barchan dunes in Mars' North Polar region. Barchan dunes betray the prevailing wind direction. In this case, the prevailing wind is traveling from bottom right to top left; the steep slope of material (plus dune "horns") point to the downwind direction. The HiRISE camera monitors barchans to see if they move between observing opportunities, thereby revealing their speed of motion across the Martian plains.
This is the same barchan dune field, zoomed out, a "swarm" of dunes covering the plains.
Not all barchan dunes "behave" and form neat "horny" shapes. They can become muddled and overlapping, creating "barchanoid dunes," as shown here.
This very fluid-looking collection of barchans is accompanied by a wind-blown ridge in the Hellespontus region of Mars but...
...only when zoomed out does the true nature of this fascinating region become clear. The prevailing wind is eroding the mesas (small hills) to the right of the image, carrying fine material downwind (from right to left), creating a startling pattern of barchans and a viscous-looking trail of sandy ridges across the plains.
The band Train sang about the "Drops of Jupiter" -- what about the "Drops of Mars"? Sure, they're not made of any kind of fluid, but they do make for incredibly-shaped dunes. These raindrop-shaped dunes are found in Copernicus Crater and are known to be rich in the mineral olivine, a mineral that formed during the wet history of Mars' evolution.
These craggy-looking dunes are old barchanoids eroding away through seasonal processes (sublimation of sub-surface ices) and the persistent Martian wind.
These linking barchan dunes are at the leading edge of a dune field -- grains of dust have been blown across a plain, deposited and left to accumulate in elongated arrow shapes.
Dome-shaped dunes and barchans seem to "reach out" and touch their downwind partners with slumped material.
Barchan dunes inside Arkhangelsky Crater in the southern hemisphere of Mars reveal a wind direction from top left to bottom right. Note the tracks of Martian dust devils over the dune slopes.
For more on the HiRISE camera,
. For more on Mars dune definitions,