Sand grains
stirred up by the winds of Mars are tossed higher and farther than those kicked
up by winds on Earth, a new study finds. The results could help explain how
dunes migrate across the Martian surface as well as what whips up dust storms that
blow across the red planet.
Scientists
first noticed dunes on the Martian surface in pictures taken by NASA's Mariner
missions in the 1970s and have seen dust storms of all sizes spread across the
planet — one
major storm in 2005 was even visible through a simple backyard telescope. But
these features have puzzled astronomers because Mars has almost no atmosphere
and very weak winds that seem unlikely to be able to sculpt dunes or whip up
storms.
To help solve
this conundrum, a team of scientists recently conducted wind tunnel simulations
of windblown sand grains under the conditions found on both Earth and Mars to
figure out how the particles would behave on these planets with vastly different
atmospheres. Their results are detailed in the April 28 issue of the journal Proceedings
of the National Academy of Sciences.
Sand
collisions
Winds tend
to move particles in one of three ways. When the particles are heavy,
"they cannot be lifted off from the ground by the wind, so they remain
very close to the ground," a process aptly called "creep,"
explained study team member Eric Parteli of the Universidade Federal do Ceará
in Brazil. On the other hand, lighter particles can hang around in the
atmosphere for awhile and travel over long distances.
In between
these extremes, a process called "saltation" reigns. Wind turbulence
kicks up a sand particle and carries it along in a series of jumps, finally
dropping it to the ground where it collides with other particles and in turn
knocks them up into the air, Parteli said.
Saltation
is what creates the iconic sand dunes of the Sahara and other deserts on Earth
as the sand particles accumulate into mounds, usually with gentler slopes on
their windward sides. With the discovery of dunes
on Mars, scientists thought the process might occur on our red neighbor as
well.
"We
only know that saltation occurs on Mars because we see that there are
dunes," Parteli said.
But
scientists were unsure if the dunes were simply relics of Mars' past, when its
atmosphere was denser and capable of generating stronger winds than it is now.
The planet's current atmosphere is less dense than Earth's, and wind speeds 10
times faster than those on Earth would be required to pick up Martian sand
particles.
"So we
know actually that the winds of
Mars are actually very weak," Parteli told SPACE.com.
"They remain very much below the minimum threshold for saltation all the
time."
Dune
migration, also caused by saltation, has only been noticed on the red planet in
a few before-and-after images in the last couple years, which seemed to
indicate that saltation could still be going on, at least in some areas of the
Martian surface.
Parteli and
his colleagues used wind tunnel simulations that took into account the
difference between the Martian and terrestrial atmospheres (in terms of such
parameters as pressure and temperature) to see if Martian winds could generate
saltation events.
Their
experiments and models showed that the winds could in fact eject particles from
the surface. And not only that — the particles went higher and traveled farther
on Mars than on Earth, because Mars' gravity is weaker (approximately one-third
of Earth's gravity).
"The
particle is allowed to remain longer in the atmosphere," Parteli said. "It
is not pushed downward by the gravity so strongly as it is on Earth."
The
research was funded by the Conselho Nacional de Pesquisas (CNPq), the Comissao
de Aperfeiçoamento de Pessoal de Nival Superior (CAPES), the Fundação Cearense
de Amparo à Pesquisa (FUNCAP), the Volkswagenstiftung and the Max-Planck Prize.
Dunes
and dust storms
Though
Parteli and his team's results show that saltation can occur if the winds are
strong enough, the winds on Mars rarely reach above the proper threshold, which
makes saltation "a very seldom event on Mars," Parteli said.
Strong
winds could be confined to particular regions of Mars, said Kevin Williams of
Buffalo State College, who was not affiliated with the new study. These winds
could rush down from the polar ice caps and stir up sands in nearby regions, he
noted. Another possibility is that it might be easier for wind to move faster
in low-lying areas where the air is slightly denser, he told SPACE.com.
The new
study provides "progress in understanding the overall process of sand
saltation on Mars," Williams said.
The shapes
of dunes in certain areas of Mars could shed light on the winds there, as dune
shape is largely a function of wind direction. The most common type of dune (on
both Earth and Mars), called barchans, are shaped by winds blowing in just one
direction, while another type, called longitudinal, are formed by winds blowing
in two directions. The shapes of dunes on Mars have been likened to the video
game character Pac-Man,
horseshoe crabs and worms.
Wind speed
can also affect dune shape, with stronger winds producing taller, narrower
dunes than lighter winds.
Saltation
is also a suspected cause of the ruddy dust storms that can blow across the
Martian landscape. The saltating sand particles could eject dust from the
surface as they are dropped, Parteli said, though he noted that other factors,
such as temperature change, are also suspected causes. Ultimately, "how
dust storms form is actually an open question," he said.
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