Through The Sandglass

Reposing differently on Mars

Sand, Mars, and a challenge to conventional wisdom. What better ingredients for a post could I wish for?

The two images above show sedimentary fan deposits, the great  - yes, fan-shaped – aprons of detritus that accumulate at the foot of a gully, driven by gravity, often, on earth at least, assisted by periodic torrents of water. Water is an interesting factor: on Earth these things are often called “alluvial fans” because the occasional flood clearly plays a role.

These images are an opportunity for a quiz, since one is from Earth and the other is from Mars – which is which? Sorry, but I’ll maintain the suspense for a while. Are the fans on Mars alluvial – is water involved? The debate about the role of water in Martian surface processes continues, and it’s fascinating and the evidence for,against, ambiguous, is diverse. One of the lines of reasoning involves the slopes of fan deposits – they can be characterised by lower angles than expected and this is cited as a result of the lubricating effect of water. But this makes one critical assumption: that the slope of a deposit of granular materials (the angle of repose) is not affected by the low gravity of Mars – the angle of repose is independent of gravity. But is this assumption correct? Oddly enough, this question had not been seriously addressed until Maarten Kleinhans of Utrecht University together with Sebastiaan de Vet of the Institute for Biodiversity and Ecosystem Dynamics (IBED) at the UvA and colleagues from Delft University of Technology decided to do so. How? Well, as the University press release describes, they took a plane:

The research team used a parabolic flight campaign to mimic Mars’s lower gravity and test the effect on angles of repose of different materials. As the plane followed its roller coaster style path, slowly rotating cylinders containing different materials experienced one tenth of Earth's gravity, Martian gravity and the Earth's normal pull.

And here, from their paper in the Journal of Geophysical Research, is the gravity-reducing plane.

 
But, before we look at what they found, let’s back up for a minute and remind ourselves about angle of repose. In its simplest sense it’s the slope that sand, or any granular material, naturally settles at after it has been poured. It varies substantially with the nature of the material – grain size and shape and so on. The paper included this helpful illustration of “A pile of rounded gravel at the angle of repose, built by Gijsbert F. Kleinhans.”

Because it’s important in many contexts, I have written several posts on the angle of repose, for example in kitchen physics, and “From Bogart to Bugs,” And it’s one of many principles of physics that can be observed simply by watching the flow of sand in this blog’s icon, the sandglass. But the sandglass, simple though it may appear, illustrates the complexity of many things, among them the angle of repose. For there is not one simple angle of repose for a given material – there is a static one and a dynamic one. Essentially, the static angle of repose is demonstrated when a pile of grains is at rest, but, once they start avalanching down a slope, the constant angle of the avalanche is different, and that’s the dynamic angle of repose, always lower than the static.

There are different ways of studying all this (should you be inclined – and many people are), but a classic is the rotating cylinder. This is basically like watching clothes in the drier – a transparent cylinder is partially filled with granular material and rotated – the sand, or whatever stuff you chose, cascades down-slope constantly at it’s dynamic angle of repose. When it comes to rest, it does so at its static angle. This is exactly the equipment that the Dutch researchers took up with them in their gravity-reducing plane: nine cylinders half-filled with sand, gravel, glass beads, some in air, some in water, their behaviours captured by HD video cameras:

All kinds of clever corrections for the noise and accelerations of the aircraft itself were applied. Each stomach-churning parabola allowed for around 20 seconds of experiment at gravities of down to one tenth the normal earthly value – the gravity on Mars is just over a third of Earth’s.

As the authors write in their paper, “Our main result is surprising.” They found that, with decreasing gravity, the static angle typically increases by around 5 degrees, whereas the dynamic angle decreases by about 10 degrees. And this is true of all the materials and regardless of whether they were in air or water – the angle of repose is not independent of gravity. The difference between the two angles is critical to the natural behaviours of granular materials, and in these experiments it increased by roughly an order of magnitude. Why is this important? If the static angle is higher, then a slope can build more steeply, but, once stability is lost, then avalanches cascade down the slope at the dynamic angle – if this is low, then the avalanches keep going and involve more material.

So, bigger avalanches, lower slopes on Mars are possible without the lubricating effect of water. And, as the authors point out, this gravity dependency of the angle of repose has implications for a wide variety of phenomena on Mars and elsewhere.

Ah yes, the answer to the quiz: it was great fun juxtaposing these images (well, to me at least) because of their remarkable similarities. The image on the left is of gullies and fans in a severely eroded 100 kilometer impact crater in the southern polar region of Mars (courtesy NASA/JPL/University of Arizona). On the right is a Digital Elevation Model (DEM) image of gullies and fans on Svalbard (a.k.a. Spitsbergen) from the Europlanet HRSC-AX flight campaign. The laws of physics are compellingly universal.

 

[Many thanks to Brian Romans of Clastic Detritus for catching this paper for me. Paper citation: Kleinhans, M. G., H. Markies, S. J. de Vet, A. C. in 't Veld, and F. N. Postema (2011), Static and dynamic angles of repose in loose granular materials under reduced gravity, J. Geophys. Res., 116, E11004, doi:10.1029/2011JE003865.]

Martian floods of tears - are the TSIs ESRs? Or is it just the total drag?

It’s always sobering to realise that we have a more detailed view of the surface of Mars than we do of the floors of our own oceans. And, while the evidence for water’s role in the history of Mars and its geomorphology (or, I guess, more correctly, martiomorphology) is more or less accepted, the interpretation of that planet’s surface features is almost exclusively pursued through the spectacles of terrestrial (on-land) terrestrial processes. We look to the processes of rivers and floods on our continents to explain the surface features of Mars. So there’s a certain satisfaction when geologists break out of this mindset and propose ocean floor analogues from Earth to explain the – now definitely subaerial – topography of Mars.

Whether they are proved to be right or not really doesn’t matter at this point: Lorena Moscardelli and Lesli Wood have made the vital contribution of opening up a debate, stepping out of the box, challenging conventional wisdom. Both from the Bureau of Economic Geology at the University of Texas in Austin, they have published a paper in the July issue of Geology titled “Deep-water erosional remnants in eastern offshore Trinidad as terrestrial analogs for teardrop-shaped islands on Mars: Implications for outflow channel formation.” The “teardrop-shaped islands” are the TSIs and the ESRs are Erosional Shadow Remnants – but so what, what is this all about apart from more acronyms?

Well, first of all, let’s review what we know and what we don’t know about water on Mars. What we know is obviously that liquid water plays no role in today’s processes – there is evidence for water locked up in Martian minerals and the permanent polar ice caps contain frozen water (unlike the seasonal ones that form exclusively from dry ice, frozen carbon dioxide) – but a flash-flood is not one of the hazards that a Mars Rover faces. Then there is evidence of ancient shorelines – almost certainly lakes, possibly oceans. But when I say “ancient,” of course I mean serious deep-time – the idea of a warm and wet period on Mars refers to a time around three billion years ago. And finally we come to martiomorphology – there are endless features of the Martian landscape, imaged in superb detail, that look exactly like geomorphological hallmarks of rivers, floodplains, water-scoured gullies, deltas, alluvial fans and so on. There are always alternative explanations for individual features, but the collective evidence suggests flowing surface water.

But. detailed though our images are, and even though the Rovers Opportunity and Spirit (now, sadly, deceased) have, with unexpected grit and fortitude, explored and sampled a few square kilometres, a geologist or geomorphologist has yet to stride the field on Mars, and definitive interpretation eludes us. Which is where the fun lies.

One of the most closely scrutinised areas of Mars with respect to water-sculpted landforms is the gigantic valley of Ares Vallis – see, for example, this European Space Agency site (where the image at the head of this post came from). Like so many landforms on Mars, this is a seriously gigantic feature – a couple of thousand kilometres long, with valley walls a couple of thousand meters high. Its floor and the many tributary canyons provide enough for researchers to chew on for decades.

And that’s exactly what Moscardelli and Wood have been doing. Like their colleagues in the Martian scrutinising business, they must always bear in mind what they know of their own planet, but they have expanded this approach to considering the floor of our own oceans.

The topography of Ares Vallis and its tributary canyons bears many of the hallmarks of catastrophic flooding – canyon headwalls that seem to have been the site of massive cataracts that constantly eroded the cliffs backwards up the canyon and downstream from the cataracts, scoured floodplains. These landscapes are reminiscent of those of the so-called channelled scablands of Eastern Washington State, bizarre geomorphology carved by a series of gigantic floods released from glacial lakes as the climate warmed. Where, on Mars, all this water came from, is another matter, the subject of debate. The September 2010 issue of Geology contained a well-documented paper on the “Retreat of a giant cataract in a long-lived (3.7-2.6 Ga) martian outflow channel.” The research of a team from University and Imperial Colleges, London, argues that periodic groundwater release caused a long period of catastrophic flooding and erosion of the Ares Vallis region. And, when they say “long-lived,” they mean it – Ga is a billion years, so this period of landscape development lasted 1,100,000,000 years.

BUT. Among the distinctive and enticing landforms are TSIs - “teardrop-shaped islands,” clearly visible in the image below, taken from the paper by Moscardelli and Wood.

What could these forms be telling us? The teardrop – streamlined - shape is common in nature, and shows up in a variety of contexts on our own planet, not to mention in the aerodynamic profile of an airplane’s wing. Now, wonderfully, given that much of this work is available only to subscribers (of which I am one), Geology saw fit also to publish in the public domain, open-access, a short review of these landforms and their possible meaning on the floodplains of Mars. The illustration below is taken from that review (written by Devon Burr at the University of Tennessee), and it really is fascinating. For a start, it shows yardangs, on which Evelyn and I recently (and totally coincidentally) wrote a joint post.  
Teardrop-shaped, streamlined, landforms are common terrestrially and extra-terrestrially, and can be shaped by water or wind – they seem to be one result of fluid flow, regardless of the fluid. And, as Burr explains, this is for good reason – the physics of drag:

Streamlined forms are shaped during flow as a tendency to minimize total drag. Total drag is the sum of form (pressure) drag and skin (friction) drag. Form drag around a blunt body arises largely from flow separation in the lee of an obstacle, so that elongation of the form through in-filling of the leeward separation zone reduces the form drag. Conversely, skin drag acting tangentially to the obstacle surface is minimized by reducing the surface area by making the feature geometrically more compact. Consequently, minimization of total drag is accomplished through a combination of elongation and compaction. The result of these countervailing tendencies is the streamlined form…. Both erosional and depositional streamlined forms are observed in terrestrial floodscapes.

And there’s a key point: such forms can result from erosion (as with yardangs), or deposition, in the downstream lee of an obstacle.

Cut to the deep ocean waters off the Orinoco delta. Using clever seafloor imaging, Moscardelli and Wood describe the features shown below (taken from their paper): 
The dark circular features are mud volcanoes (liquefaction phenomena common in loose sediments in a tectonically active area) that seem to form the “anchor” for the TSIs – which are interpreted as ESRs; the mud volcanoes form the obstacle to massive flows down the sloping sea floor, leaving the streamlined form downstream from them protected from the erosional power of the flow and shaped by the physics of drag. Now, compare these with an example from Ares Vallis, where the TSIs are “anchored by impact craters:

Interesting, eh? As the authors conclude (after a detailed analysis documented in the paper) that “These observations suggest that the teardrop-shaped islands [on Mars] might have been formed as a result of catastrophic submarine mass movements similar to those documented within continental margins on Earth.”

There is, of course, one key difference between Earth and Mars that drives the drag equations: gravity. The gravity on Mars is significantly lower than that on Earth, and so such streamlined forms may result from different conditions of fluid flow. Given the fact that streamlined forms also occur on Mars in regions where an ocean is unlikely ever to have existed, and yet that water flow can be invoked as a mechanism for their formation, Burr concludes, “If the physical sedimentology in outflow channels on Mars is indeed similar to that in submarine environments on Earth (Komar, 1979), then the submarine analogy revived by Moscardelli and Wood might extend beyond the limited number of circum-Chryse TSIs. In other words, this analogy may argue for the effect of lower gravity producing submarine-style processes in Martian outflow channels generally, regardless of any hypothesized submarine context.”

So, who knows really what’s going on? The answer is, of course, nobody, but Moscardelli and Wood have opened up the debate, pointing the way to a broader understanding of sedimentary landforms in extra-terrestrial – and terrestrial – environments. As Burr sums it up:

Testing between these two interpretations—that certain streamlined forms on Mars formed in a submarine environment, or that all streamlined forms on Mars formed in an outflow environment in which particle physics mimics that in submarine environments on Earth—will require morphometric data from ESRs for comparison to Martian streamlined forms, as well as examination of the geologic context for the Martian examples. The question extends beyond Mars to other worlds. Titan has even lower gravity than Mars, a 10-times thicker atmosphere than Earth, and also shows streamlined forms (Fig. 1E). Could subaerial processes on Titan mimic outflow processes on Mars and submarine processes on Earth? Pinning down the true cause for the similar appearance between terrestrial ESRs and Martian TSIs would contribute to understanding the effect of reduced effective gravity on sedimentary landforms.

I’ll leave (assuming that anyone has made it this far) with a favourite image that raises a favourite question: what planet are you from?

 

[The paper by Moscardelli and Wood was picked up by Wired Science; the full reference is: Lorena Moscardelli and Lesli Wood, Deep-water erosional remnants in eastern offshore Trinidad as terrestrial analogs for teardrop-shaped islands on Mars: Implications for outflow channel formation, Geology 2011;39;699-702; for the cataracts analysis, Nicholas H. Warner, Sanjeev Gupta, Jung-Rack Kim, Shih-Yuan Lin and Jan-Peter Muller, Retreat of a giant cataract in a long-lived (3.7-2.6 Ga) martian outflow channel, Geology 2010;38;791-794. Burr’s open-access summary is here. For the origin of my final image, have a browse around the Mars HiRise image collection. The image of Ares Vallis at the head of this post is from the European Space Agency Mars Express site.]

Splashing around on Mars

The last Sunday Sand post 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 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, 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.

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; for summaries of the recent HiRise sequential imaging, see, for example, Mars Daily and Wired. Images and more at http://hirise.lpl.arizona.edu/]

It's all about size: Martian dust devils

 

I use this image as my desktop, so each day I gaze at it in wonder. It’s natural art and design, somehow mysterious, somehow sensual. This extraordinary – and much-publicised – image is from Mars, captured by the high resolution orbiter, and reveals the tracks of dust devils, whirling miniature tornado dervishes of wind engraving their way over the dunes. But exactly how these dark tracks form has been something of a mystery: is it to do with composition, that light dust is removed to reveal sand of darker materials beneath?

Ironically, more observations had been made of this phenomenon on Mars than on Earth – orbiters and rovers have gathered a gallery of images and movies of dust devils on the Red Planet:

 

Earlier this year, Dennis Reiss, a geographer at Westfälische Wilhelms Universität Münster in Germany, together with his colleagues, reported on their earth-bound fieldwork in the Turpan Desert of northwestern China. Dust devil tracks are small and ephemeral things here, whereas the Martian designs are larger and last far longer. But the workers in China were able to observe dust devils at work and examine the character of the dark trails left behind. What they showed was that the appearance of the tracks was dependent on the size of the grains. The sand grains outside the tracks were coated with fine dust that brightly reflects the light. Within the tracks the blast of the passing dust devil blew the sand along, tumbling and saltating, with the result that the dust was picked up by the vortex and clean sand grains were left behind. Smaller grains appear lighter than coarse ones – they reflect more light and their albedo is higher – and so the tracks appear darker than the surrounding sand.

So far, so good. But there are examples, as recently reported by Mary Pendleton Hoffer and Ronald Greeley at Arizona State University, of dust devil tracks on Mars that are lighter than their surroundings:

 

Reiss and his colleagues have now reported on light-coloured tracks from the Turpan Desert. These appeared in sand that had been darkened by a brief rain shower the previous night. But invoking rain on Mars is clearly a non-starter, so what is going on? Well, it turned out that the sand within the light tracks was no different from usual – millimetre-sized grains cleansed of finer material. But, the darkening of the surrounding sands was a result of the grains being cemented together by the rain into clumps up to a centimetre across. Big clumps = low albedo = dark appearance. The dust devil had broken these up, brightening the sand left behind in its track.  

OK, but there’s still no rain on Mars. However, there are forces that can clump sand grains together – electrostatic forces. Such clumping has been observed by the field geologists on Mars, the indomitable rovers.

In 1979, Greeley conducted lab experiments showing that charges build up on dry, wind-blown sand particles in a similar manner to the way charge builds up on a balloon when you rub it on your hair. Just like the charged-up balloon can stick to the wall, charged sand grains pull together to form delicate, “popcorn ball” aggregates.

“The destruction of aggregates on Mars would lead also to bright dust devil tracks,” Reiss said.

Greeley thinks the idea makes sense. “This is a plausible model for the formation of the bright tracks on Mars,” he said. “This is a very nice study, a very nice result.”

So, through questions raised on Mars we are better understanding processes on our own planet. The wonder of Martian calligraphy is, however, by no means diminished.

 

[See the amazing gallery of Mars high resolution images at the HiRISE site. Images courtesy of NASA/JPL/University of Arizona HiRISE]

  

Mesmerising Mars

 

Following up on bits and pieces of news, I periodically find myself on one of the Mars imagery sites, lost in wonder; it’s a time-consuming process. So, I thought, why not, again, lure readers of this blog to accompany me on these extraordinary journeys? Scientific Frontline, a site that I just discovered, compiles a wide variety of images into all kinds of galleries, not just of things extraterrestrial. It’s from there that the stunning image above comes, originally, of course, from NASA/JPL/University of Arizona and the Mars Reconnaissance Orbiter. The description is as follows:

Dunes within a crater on Mars are visible in this HiRISE image. This crater is located in the southern hemisphere where it was winter at the time this image was taken.
This observation documents new seasonal processes occurring on dunes at this latitude, as well as other interesting phenomena. The bright tones are interpreted as carbon dioxide or water frost. This is generally concentrated on the east-facing slopes of the dunes, which are in shadow and therefore cooler. Some dark spots on the dunes may be areas that have defrosted more than surrounding terrain.
Landslides and dark-toned streaks are seen on many of the west-facing dune slopes. The general dune morphology indicates formation by westerly winds. However, zooming in on the image shows smaller scale ripples that appear to have been formed by winds blowing from the south and north.
[Written by: Nathan Bridges & Kelly Kolb]

And, speaking of winter dunes, direct from the high resolution imaging site at the University of Arizona:

 

Mars has a vast sea of sand dunes in the high latitude region encircling its North polar cap, known as the North polar erg. These dunes are made up of basalt and gypsum sand grains. In some regions of the North polar erg where the sand supply is limited they take on an elongated crescent shape. The icy ground that the dunes are on top of has irregular polygonal patterns. In other areas with an abundant supply of sand the dunes are continuous (see also PSP_007494_2580).
The entire North polar erg is covered in the winter with a seasonal polar cap composed of carbon dioxide (dry ice). In the spring time this seasonal polar cap evaporates.
[Written by: Candy Hansen]

And then, off we go into the stunning landscapes of the planet (it’s clearly not geomorphology, but does anyone know the correct term?)

 

This observation shows a portion of the central sedimentary deposits in Pasteur Crater.
The deposits in this image now being eroded into knobs and ridges. The erosion is probably dominated by wind, as most of the ridges are parallel. This is common in wind-eroded features, with the ridges generally aligned with the prevailing wind.
At high resolution, layering is revealed in many of the knobs and outcrops. The horizontal layers indicate that the material was deposited uniformly over a broad area. Possible origins include volcanic airfall or lacustrine (lake) deposits. After deposition, the rock in this area has been fractured and faulted, forming a diverse array of cracks.
The mottled appearance of much of the image is caused by dark, featureless patches which may be wind-blown dust. These have interacted with lighter-toned ridges and ripples which are probably also formed by aeolian (wind) processes. In places, the dark patches partially cover the ripples, indicating that they have moved more recently, but they must be thin because the ripples frequently stand above surrounding dark material.
The ripples exhibit multiple interacting orientations in some places, producing networks of small ridges which reflect movement in winds from several directions.
[Written by: Colin Dundas]

We should not forget the European Space Agency project – the Mars Express high resolution stereo-photography and mineral mapping mission (whose lander, the Beagle 2, so ignominiously plummeted to the surface of Mars in 2003). The imaging orbiter was originally expected to have a relatively short life, but its operations have been extended several times and are continuing; its recent image of Orcus Patera, a mysteriously elongated crater, has been in the news last week ("scientists are baffled..."). I particularly like the image below (I have inserted a perspective detail) from early on in the mission: a dune field in the crater Argyre Planitia. I assume that the dark colour results from basaltic sands being constantly reworked by the winds as opposed to being covered in dust, but why is the dune field so compact and isolated? A reflection of the complex wind circulation patterns? Note that 3D images are available from the website.

 

These images, taken by the High Resolution Stereo Camera (HRSC) on board ESA's Mars Express spacecraft, show a Martian crater with a dune field on its floor.
The images were taken during orbit 427 in May 2004, and show the crater with a dune field located in the north-western part of the Argyre Planitia crater basin.

The images are centred at Mars longitude 303° East and latitude 43° South. The image resolution is approximately 16.2 metres per pixel.  The crater is about 45 kilometres wide and 2 kilometres deep. In the north-eastern part of this crater, the complex dune field is 7 kilometres wide by 12 kilometres long. In arid zones on Earth, these features are called ‘barchanes’, [sic: they mean, of course, “barchans”] which are dunes having an asymmetrical profile, with a gentle slope on the wind-facing side and a steep slope on the lee-side. The dune field shown here suggests an easterly wind direction with its steeper western part. The composition of the dune material is not certain, but the dark sands could be of basaltic origin.

Enough, I must stop – good luck!

 

 

Weekend break - other sand in the news

Put the words "Louisiana sand berms" (or even simply "sand") into Google news search, and it's apparent that the rhetoric around what is now being termed "The Great Wall of Louisiana" continues unabated (see, for example, The Seattle Times and the LA Times). But it's time for a break from this saga, a little light relief.

As the opening of the Football World Cup rapidly approaches (England's first game is against the US - the latter, of course, playing soccer rather than football), celebration in South Africa takes many forms. Including a collection of sand sculptures of the different stadiums hosting the games - above left, the brand new stadium in Durban.

And, in Moscow, six men have now been locked in a sealed complex of containers for 18 months (at least, that's the plan) in order to simulate a manned mission to Mars. One of the many facilities within the complex is a "Martian surface simulator" - otherwise known as a glorified sand box.

Oh, and please note that I now added, long overdue, a widget on the sidebar of this blog to link posts, should readers wish, directly to networking and bookmarking sites.

[Sand sculptures from Jon Saintz, originally published in The Observer; Mars experiment image from the project website, further information from many sources, for example the BBC news]

Dune mysteries and other Mars news

A friend sent me the link to this image (without annotations), yet another spectacular and provocative high-resolution image of Mars. The crest of each dune runs along the boundary between the light and dark blue-gray flanks, and the surface between the dunes is covered in boulders. However, I have a slight problem with this and would appreciate other views. Here's the caption that goes along with the image:

Dunes of sand-sized materials have been trapped on the floors of many Martian craters. This is one example, from a crater in Noachis Terra, west of the giant Hellas impact basin. The High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter captured this view on Dec. 28, 2009.

The dunes here are linear, thought to be due to shifting wind directions. In places, each dune is remarkably similar to adjacent dunes, including a reddish (or dust colored) band on northeast-facing slopes. Large angular boulders litter the floor between dunes.

(Image Credit: NASA/JPL-Caltech/University of Arizona;  http://www.nasa.gov/images/content/418311main_mro20100113-full_full.jpg)

Now, the fact that "large angular boulders litter the floor between dunes" is not the problem - on our own planet, one of the extraordinary things about dunes is the way in which they seem to self-accumulate and hoover up all the sand from the clear, rubble-strewn "streets" between them. It was Ralph Bagnold who set us firmly on the path to understanding why and how this happens - bouncing versus splashing. I saw this phenomenon very clearly in Bagnold's old stomping grounds of Egypt's Western Desert, as shown in the photo below - vast areas of pebbles and larger rock fragments, essentially swept clear of sand and encroached on by the flanking dunes.

If you cross the dunes, the only rocks to be found on them are either meteorites or surplus tossed out by a passing geologist. And therein lies my problem. Look closely at the Mars image - for example, the two areas that I've magnified - and very large boulders appear to be lying well up the flanks of the dunes, on the surface of the sand and clearly influencing the pattern of ripples around them. What's going on here - or am I misinterpreting something? (As is common, NASA frustratingly disdains providing a scale).

This is a good time to be celebrating the wonders of the Red Planet, because right now it's particularly close to us. As reported in Wired Science:

On Jan. 27, Mars will be closer to Earth than any other time between 2008 and 2014. A mere 60 million miles away, the red planet will be a great target for backyard telescopes, and will appear bright to the naked eye as well.

Every 26 months, the two planets’ orbits bring them closer together, sometimes closer than others. In 2003, Mars came within 35 million miles of Earth, a 60,000-year record.

Observers with a telescope will be able to see changes over the north pole of Mars as the carbon dioxide ice cap is nearing summer and evaporating into gas that affects the polar clouds. (If any of our reader-astronomers catch a nice image, send it our way!)

From the ground, Mars will look like an orange star almost as bright as Sirius, the brightest star in the sky. The display may actually be best on Friday, Jan. 29, when Mars will rise alongside the first full moon of the year, directly opposite the sun.

Shame about the weather and the light pollution here in London! [Friday night update - the London skies miraculously cleared - extremely cold and clear, a spectacular full moon and, yes, the red planet. Well worth a look.] 

From the surface of Mars, we have an update on the stuck rover, Spirit. In spite of some recent success in the attempts to extricate it (progress of a few inches in the direction it was originally travelling), NASA has now designated the Rover as a "stationary science platform" - it will likely stay where it is for the rest of its working life, the duration of which will very much depend on its ability to combat and survive this Martian winter.

"There's a class of science we can do only with a stationary vehicle that we had put off during the years of driving," said Steve Squyres, a researcher at Cornell University and principal investigator for Spirit and Opportunity. "Degraded mobility does not mean the mission ends abruptly. Instead, it lets us transition to stationary science."

Meanwhile, Spirit's twin, Opportunity, has fallen very much out of the limelight during the stuck-in-the-sand saga, but it continues to trundle effectively about, conducting geological fieldwork. Most recently, it's been drilling away at a rock that would seem to represent the interior of the planet and something different from anything we've seen before.

And, on a completely different note, I have just cottoned on to the fact that this seems to be "Sand Week" on the geoblogosphere. Ron Schott pointed this out as part of his great series of "Deskcrop" posts of different sands - "We'll find out who knows their sands." See also his recent Outcrop post from Death Valley with great images of a sand storm and beautiful nascent dunes blowing across the road.

"Free Spirit" update - things are looking worse

Since Spirit has been travelling backwards for a long time, dragging its right front wheel behind it, it's easy to lose track of orientations and what's going on where. The image below is taken from the front of the rover, but looking backwards, and the disabled wheel, the right front, is at the right. When I was reading about the failed attempts to move the rover, as I reported in the previous post, I hadn't realised that the stalled wheel that ended the test was the right rear wheel that had, up until now been functioning reasonably well. It now seems very likely that the right rear wheel has ceased to function and this leaves Spirit with only four working wheels, three of which are more or less buried.

 

It's therefore, as the New Scientist reported a couple of days ago, looking even more grim:

Now, Spirit's right-rear wheel is also having problems and may be permanently disabled.

The right-rear wheel stalled on 28 and 21 November during attempts to start driving the rover out of the sand trap. Each wheel has its own motor, and the rover team commanded Spirit to try to spin the wheel again during a series of tests on 3 and 4 December – but it did not budge.

In the previous stalls, the wheel had at least moved a little bit before unexpectedly stopping. But no motion at all was detected in the tests.

"That's troubling," says rover project manager John Callas of NASA's Jet Propulsion Laboratory in Pasadena, California. "It's conceivable we're witnessing a deterioration on the wheel."

If the wheel cannot be coaxed into working again, then Spirit will likely be trapped forever, Callas says.

"It was questionable whether we could get a five-wheel-driving rover out," he says. "If we have a four-wheel-driving rover [with] only one driving wheel on the right-hand side ... then extracting the rover from its current embedded location is unlikely."

Spirit could die if it remains stuck when winter arrives six months from now at the rover's location in Mars's southern hemisphere, Callas says.

In previous winters, Spirit rested on slopes that angled its solar arrays in a way that captured as much sunlight as possible. That allowed the rover to power heaters designed to keep its electronic innards from freezing.

But Spirit's solar panels are not currently at a good angle to maximise sunlight. Meanwhile, dust is slowly accumulating on the panels, reducing their efficiency. If the rover is unable to move, it could run out of power and die in the frigid Martian night.

"If we extrapolate current dust accumulation rates out to the next winter six months away, it does look troubling," Callas says. "There's a real possibility Spirit would not have power to survive the winter at its current attitude."

"Free Spirit" campaign - but it doesn't look good for the Mars Rover

 “It’s way out of warranty,” said Ray Arvidson, director of the Earth and Planetary Remote Sensing Laboratory at Washington University in St. Louis. “It’s like an old ’55 Chevy.” He was talking about the intrepid Mars Rover, Spirit, that has, along with its sibling on the other side of the planet, Opportunity, been trundling around for six years of a mission that was expected to last three months. In 2006, Spirit's right front wheel stopped working and so it continued backwards, dragging the wheel behind it. Then, in April of this year, having been working around an area known as "Home Plate", a miniature plateau just 80 meters (260 feet) across - see the image below - for more than three-and-a-half years, Spirit's wheels broke through a poorly cemented sand crust and the rover became stuck in the very fine loose sand beneath. And, as I wrote back in May, it was seriously stuck - the images were enough to strike horror into the heart of a field geologist and Arvidson described the situation as "Murphy's Law on steroids."

 

NASA have spent much of the last months setting up a test bed of fine sand of the the same character as that which has ensnared Spirit (see video), and working on possible minute manoeuvres to extricate the rover. These have now been put to the test - sadly with minimal results. The image at the top of this post is taken from NASA's report of a few days ago - and it looks awful. Total wheel movement during the attempt was 1.4 meters (4.6 feet) and "Analysis of data from the drive indicates that the center of the rover moved 0.5 millimeters (0.02 inch) forward, 0.25 millimeters (0.01 inch) to the left and 0.5 millimeters (0.02 inch) downward." Spirit was just spinning its wheels and the trial was stopped when the inactive right rear wheel stalled again. And, as if this weren't bad enough, communications with earth have been disrupted by problems with the Mars orbiting satellites.

But Spirit hasn't just been sitting there twiddling its robotic thumbs. As the New Scientist recently reported:

Spirit has been immobile for the past six months, but it has not been idle.

The rover's own observations have revealed the cause of its plight - a small crater filled with yielding, yellow-brown sand. The sand had been hidden beneath a dust-covered crust of weakly cemented sand particles.

Spirit's wheels punched through this centimetres-thick crust, exposing soil with the highest concentration of sulphate minerals ever found by either Spirit or its twin, Opportunity.

Sulphates form in the presence of water, so the find further reinforces the idea that, billions of years ago, the area surrounding Spirit was rich in the liquid.

Researchers suspect Spirit's stomping ground was once a site of intense hydrothermal activity. Pools of hot water and steam vents may have dotted the area, making it a good place for future missions to look for evidence of ancient Martian life, says Spirit scientist Ray Arvidson of Washington University in St Louis, Missouri. "If it were Earth with that kind of environment, it would be teeming with microbes," he says.

Nonetheless, the rover team is keen to move on. "It's an interesting area to be in over the summer," says Arvidson, "but we're ready to leave."

Meanwhile, the other rover, Opportunity, continues its successful ramblings - it's travelled 19 kilometers (nearly 12 miles) so far, 2.4 times the distance of Spirit's trek. But Opportunity has also had its problems with sand along the way - in April 2005, after speeding along at a reckless 200 meters per day,  it was stuck for five weeks on a ripple named "Purgatory Dune" but was successfully extricated. But lessons were learned: when, a year later it became embedded in a sandy spot nicknamed "Jammerbugt" (apparently the Danish for "bay of lamentation"), it was out and off again in no time.

For Spirit, the future is highly uncertain, with winter and dust storms approaching. The editorial in the New Scientist had the following comment about the '55 Chevy, something with which I think we would all agree:

NASA has been agonising over how to free Spirit from a sand trap for six months. That's double the design lifetime of the rover, which landed in 2004. Now, with winter on the way, NASA's hand is forced. Its plan is to backtrack slowly out of the mire. One could criticise the agency for investing too much effort in a mission that's past its prime, but that would be churlish: Spirit has achieved more than anyone dared dream, and could deliver yet more. Besides, it would seem strangely cruel to abandon the plucky rover to its fate.

 

Sand, glorious sand: Mars in high-resolution

[Just incredible. If you have a spare hour or so, go to http://hirise.lpl.arizona.edu/ and browse - and then browse some more. Each of the thumbnail images on the site is actually from a satellite pass, and clicking the "RGB color (non-map projected)" link downloads a long narrow image; I've simply compiled segments of some for this "montage." Thousands of geologically and artistically staggering images, all, wonderfully, in the public domain -  Credit: NASA/JPL/University of Arizona. A good way to mark a hundred posts on Through the Sandglass.]