Extraterrestrial

The last [Sunday Sand post](https://throughthesandglass.typepad.com/through_the_sandglass/2011/02/sunday-
sand-bagnolds-grains.html) returned to the ever-fascinating work and exploits
of Ralph Bagnold; after writing it, I was catching up on arenaceous research
news and there he was again – on Mars, so to speak. A recent [press release](http://www.psi.edu/news/press-
releases/hansen.html) from the Planetary Science Institute and the Mars HiRise
imaging team announces “to their surprise” that seasonally repeated imagery
reveals that the polar dunes of the planet are constantly changing. Why should
this be a surprise? After all, dune systems on Earth are among the most
dynamic landscapes on the planet – why should Mars be any different? Well,
there are important differences, as we shall see, but that Martian dunes have
been “long thought to be frozen in time” is the real surprise – and is shown
to be unfounded. Yes, seasonal icing stabilises the dunes, but melting and
degassing causes instability – and the wind re-sculpts the surface.

Nor should we be surprised that Bagnold enters the arena of this debate for,
while his seminal work on the physics of windblown sand was first published
seventy years ago, it remains the foundation of aeolian research today, his
basic equations and analysis of processes refined but essentially unchanged.
And we should also not be surprised that his work applies as well to Mars as
it does to Earth – after all, he was called in as an advisor to NASA during
the planning of the early Mars missions and, in 1974, he published a paper
with Carl Sagan comparing transport on the two planets:

In this paper, as apparent in the abstract, the interest is in threshold
velocities of the wind – how strong a wind is necessary to move sand? This is
a complex topic, but Bagnold’s work on Earth reveals the keys to understanding
sand dynamics on Mars, as revealed by this recent press release – particularly
when combined with research results published last year.

But first, back to Bagnold basics. During his extraordinary expeditions, he
closely observed the characteristics of dune architecture and movement, and
came to appreciate that there were a number of fundamental questions that, at
that point, had not been answered. One of the most basic of these questions
was why do dunes form at all? Why is the sand not spread evenly over the
desert floor? Whether on Earth or Mars (or, indeed, Venus or Titan), dunes
appear to be self-accumulating , seeming to vacuum up sand from the bare
stony areas between them – they grow by attracting more sand. “Why did they
absorb nourishment and continue to grow instead of allowing the sand to spread
out evenly over the desert as finer dust grains do?” was one of Bagnold’s
questions. This was, he thought, something that “could be explored at home in
England under laboratory-controlled conditions” - and so began his rigorous
science. Two of the most important revelations of Bagnold’s work are the
process of saltation and the role of two different threshold velocities for
the wind.

First, saltation. From the Latin verb “to jump,” this is the process whereby
sand grains move in the wind by individual leaps, and, landing on a hard
surface, bounce off again; if a grain lands amongst other grains on the
surface of a dune, the impact kicks some of them up into the wind and the
crowd of flying grains grows. It is these two contrasting behaviours –
bouncing versus splashing – that explain the self-accumulating nature of
dunes. Over a hard surface of rocks and pebbles, the trajectories of
individual grains are high into the air, and they keep on bouncing. As soon as
they hit a soft surface of a dune, they kick off more grains, but the
trajectories are lower and shorter – the dune grows. Here’s Bagnold’s original
illustration of this:

So, saltation is the key activity of windblown sand. But how does it start?
Clearly, if the wind blowing over a surface of sand is strong enough, it will
nudge, roll, and pick up grains until the self-sustaining process of saltation
begins. The wind speed at which this starts Bagnold referred to as the “fluid
threshold” – and it represents a pretty strong wind. But , once grains are
saltating and being kicked up into the wind, it only needs a slower velocity
to keep the process going – lower the wind speed to the point where all grain
motion stops, and that is the “impact threshold,” the minimum velocity to keep
sand in motion – and it’s much lower than the fluid threshold.

So, back to Mars. The three sequential images at the head of this post show
clear changes in the dune as the thawing of winter carbon dioxide ice
destabilises the structure. The caption is as follows:

Three images of the same location taken at different times on Mars show
seasonal activity causing sand avalanches and ripple changes on a Martian
dune. The High Resolution Imaging Science Experiment (HiRISE) camera on
NASA’s Mars Reconnaissance Orbiter took these images, centered at 84 degrees
north latitude and 233.2 degrees east longitude. Dune fields at high
latitudes are covered every year by a seasonal polar cap of condensed CO2
(dry ice).

The sequential images, which each show an area 285 meters by 140 meters,
depict the before and after morphology of the dune in one Mars year, with
new alcoves and extension of the debris apron on the slipface, or steeply
sloping leeward surface, of the dune caused by the grainfall, and new wind
ripples on the debris apron.

The top image was taken first, in the Martian summer when the dunes were
free of seasonal dry ice. The middle image was acquired in the spring when
the region was covered by a layer of seasonal ice. Spring evaporation of the
seasonal layer of ice is manifested as dark streaks of fine particles
carried to the top of the ice layer by escaping gas. Gas flow under the ice
as the ice sublimates – changes from solid to gas – from the bottom
destabilizes the sand on the dune, and causes the sand to avalanche down the
dune slipface.

The third image shows the resulting changes revealed the following summer
after the frozen layer of ice was gone. Comparison of the middle and lower
images shows the correlation of seasonal activity with locations of change
of dune morphology.

The emphasis here is on the avalanches down the side of the dune. But these
are – hardly surprising – gravitational effects, even under the relatively low
gravity of Mars; what is perhaps more surprising is the brief mention of “new
wind ripples on the debris apron.” Conventional wisdom and standard Martian
climate models had held that wind speeds on Mars were rarely adequate to cause
sand movement; measurements from landers confirmed relatively modest winds –
and yet sand grains have accumulated on the deck of Spirit , the [stuck rover](https://throughthesandglass.typepad.com/through_the_sandglass/2009/12/free-
spirit-update-things-are-looking-worse.html), and now we see dynamic ripples.

Enter, firmly in the footsteps of Ralph Bagnold, Jasper Kok,
an atmospheric physicist at the National Center for Atmospheric Research in
Boulder, Colorado (previously at the University of Michigan). Last year, he
published the results of his work on sand transport on Mars and the key roles
of different threshold velocities. He pointed out that the focus of Martian
modelling had been on the fluid threshold, the velocity required to start
saltation, but that little attention had been paid to the impact threshold,
above which any saltation already happening would be sustained. His work
demonstrated that ““While it is very difficult for the wind to lift sand
grains, once the wind does become strong enough to start blowing sand on Mars,
the sand will keep bouncing, even when the wind speed drops by up to a factor
of 10.” Because of the thin atmosphere (and coupled with the low gravity),
while Martian sand grains need hurricane-strength wind speeds of 150 km/h to
start moving, they will keep bouncing over the surface at wind speeds of just
15 km/hour. The conspiracy of the difference in parameters between Earth and
Mars means that the fluid thresholds are little different – but the impact
threshold is far lower of the red planet: windblown sand processes are alive
and well – and, now, observable. Take into account different grain sizes and
differing saltation trajectories, and Kok’s work (see references at the end of
this post) also begins to explain the smaller dunes apparently typical of
Mars, and the complex relationships between sand and dust movement, not to
mention [dust devils](https://throughthesandglass.typepad.com/through_the_sandglass/2010/09/its-
all-about-size-martian-dust-devils.html).

So, once again, conventional wisdom is out the window, but the wisdom of Ralph
Bagnold endures; in Kok’s papers, the bibliography includes citations of
Bagnold’s work from seventy years ago – how often do you see that?

[Read Jasper Kok’s papers here
and
here,
and reports of his work at Physics World
and [Wired Science](https://www.wired.com/wiredscience/2010/02/martian-
dune-mystery-solved-by-bouncing-sand-grains/); for summaries of the recent
HiRise sequential imaging, see, for example, Mars Daily
and
[Wired](https://www.wired.com/wiredscience/2011/02/mars-
shifty-sand-dunes/). Images and more at http://hirise.lpl.arizona.edu/]

Originally published at: https://throughthesandglass.typepad.com/through_the_sandglass/extraterrestrial/page/2/