“It would take more Planck lengths to span a grain of sand than it would take grains of sand to span the observable universe.”
Geologists tend to pride themselves on their grasp of scale, and, of course, I enjoy the endless roles of sand grains as measures of scale large and small, but I have to admit that I have great difficulty getting my head around this. It appeared in an article by Seth Kadish on Wired, an article I found fascinating but challenging. The Planck length is “believed by physicists to be the shortest possible length in the universe. Beyond this point, they say, the very notion of distance becomes meaningless” – it is approximately 10-35 meters. The article, titled “We all might be living in an infinite hologram,” is intriguing, not least for the revelation of the existence of a device called the Holometer – something I might request for Christmas.
Here’s the piece, with thanks to Wired and Seth:
Quarks and leptons, the building blocks of matter, are staggeringly small. Even the largest quarks are only about an attometer (a billionth of a billionth of a meter) in diameter. But zoom in closer—a billion times more—past zeptometers and yoctometers, to where the units run out of names. Then keep going, a hundred million times smaller still, and you finally hit bottom: This is the Planck length, approximately 1.6 x 10-35 meters, believed by physicists to be the shortest possible length in the universe. Beyond this point, they say, the very notion of distance becomes meaningless.
How small are we talking? It would take more Planck lengths to span a grain of sand than it would take grains of sand to span the observable universe.
Still, the idea of a finite size limit may seem bizarre. After all, if you can define a distance, you can just cut that distance in half—ad infinitum, right? Not necessarily. One of the great discoveries of the 20th century was that at small scales, many physical properties, like angular momentum and energy, can take only certain discrete values, or “quanta.” That principle—supported by decades of experiments—is the foundation of quantum mechanics.
Which leads to a rather weighty question: If the properties of matter can be quantized, what about the fabric of spacetime itself? Is the universe a smooth continuum, as described by Einstein’s theory of relativity? Or if we looked really close, would it all dissolve into a mosaic of shimmering pixels like a computer screen? Is the reality we observe just a hologram made up of the tiniest of dots?
Probing down to the Planck scale with a particle accelerator would take an instrument the size of our galaxy. But scientists at Fermilab, near Chicago, have a surprisingly modest new device called the Holometer that just might yield some clues. Using a pair of solid state lasers and some precisely polished mirrors, they hope to pick up the telltale jitter of those hypothetical pixels—what’s called “holographic noise,” after the fuzziness of holograms. If they find it? Welcome to the Matrix.
[The title of this post is from Virgil's The Aeneid, the image, Shadow of the Dark Rift, courtesy of NASA]
Stevenage: for readers outside of the UK it may not ring much of a bell, and indeed, with no disrespect to Stevenagians, for most UK readers it is not one of our most famous and glamorous metropolitan areas. Located around 50 km north of London, Stevenage has Roman and Saxon roots and has been a market town for more than a millennium. Its name may originate from the Old English for ‘place of the strong oak’, but exactly why its coat of arms depicts a sword thrust through the heart of the oak remains something of a mystery to me.
But Stevenage has one claim to fame that originated close to a century ago and continues today: what is sometimes referred to as a military-industrial complex. The English Electric Company established a facility for making aircraft parts and engines there in 1918, and continued to do through the Second World War. Furthermore, according to the Royal Aeronautical Society, “it is also thought that… there was a secret explosive weapons establishment which designed and created sabotage devices.” In the 1950s and 60s Britain’s very own intercontinental ballistic missile, Blue Streak, was assembled at Stevenage and shipped to Australian desert where the requirements for its testing (along with nuclear devices) emptied the land of its native inhabitants and changed the outback forever. The remains of the first missile launched from Woomera on June 5th, 1964, were discovered not far from Giles Meteorological Station in Western Australia in 1980 and are on display there (after a hardly intercontinental journey of perhaps a thousand kilometers):
For more of the story of the British militarisation of the Australian desert, I recommend my next book, but enough advertising and back to Stevenage. The aerospace facilities there continue to thrive and are now the location for Airbus Defence and Space and Paradigm Secure Communications, housing “Airbus Defence and Space’s spacecraft design and build facility and the headquarters of Paradigm Secure Communications.” They are also now the location for the very large sand pit that is affectionately referred to as ‘Mars Yard’. As the European Space Agency reported recently:
A state-of-the-art ‘Mars yard’ is now ready to put the ExoMars rover through its paces before the vehicle is launched to the Red Planet in 2018.
ESA, the UK Space Agency and Airbus Defence and Space opened the renovated test area in Stevenage, UK, today.
ExoMars is a joint endeavour between ESA and Russia’s Roscosmos space agency. Comprising two missions for launch to Mars in 2016 and 2018, ExoMars will address the outstanding scientific question of whether life has ever existed on the planet, by investigating the atmosphere and drilling into the surface to collect and analyse samples.
Extended Mars Yard opening
The programme will also demonstrate key technologies for entry, descent, landing, drilling and roving.
Filled with 300 tonnes of sand, the 30 x 13 m Mars yard at the Stevenage site of Airbus Defence and Space mimics the appearance of the martian [sic] landscape. Its walls, doors and all interior surfaces are painted a reddish-brown colour to ensure the rover’s navigation cameras are confronted by as realistic a scenario as possible. … The yard will also be available after the rover has landed on Mars in 2019, to help overcome any challenging situations that might be encountered on the Red Planet.
The sand pit was honoured by a visit by a leading politician, the Secretary of State for Business – how often does a political photo-op feature suits in the sand?
The ExoMars rover represents the best of British high-value manufacturing… The technologies developed as part of the programme, such as autonomous navigation systems, new welding materials and techniques, will also have real impacts on other sectors, helping them stay on the cutting edge.
Not only is it hugely exciting that Europe’s next mission to Mars will be British-built, but it is incredibly rewarding to see the benefits of our investment in the European Space Agency creating jobs here in the UK.
Bravo for the sand pit!
Periodically, I become absorbed in the wonders of the images on the Mars High Resolution Imaging Science Experiment site. I recently became exhausted by the process of obtaining permissions for illustrations in the new book and escaped by browsing the catalog for a couple of hours - it becomes compulsive. So, while I return to the permissions, here a selection of the stunning dunes of Mars that I found particularly wondrous.
How could I not post this? The activities of our Martian geologist friend continue to amaze, the latest being sand sampling. All images and captions are courtesy of NASA/JPL-Caltech/MSSS, and the NASA Curiosity website, which describes the image above:
First Scoop by Curiosity, Sol 61 Views
This pairing illustrates the first time that NASA's Mars rover Curiosity collected a scoop of soil on Mars. It combines two raw images taken on the mission's 61st Martian day, or sol (Oct. 7, 2012) by the right camera of the rover's two-camera Mast Camera (Mastcam) instrument. The right Mastcam, or Mastcam-100, has a telephoto, 100-millimeter-focal-length lens.
The image on the left shows the ground at the location "Rocknest" after the scoop of sand and dust had been removed. The image on the right shows the material inside the rover's scoop, which is 1.8 inches (4.5 centimeters) wide, 2.8 inches (7 centimeters) long….
The team operating Curiosity decided on Oct. 9, 2012, to proceed with using the rover's first scoop of Martian material. Plans for Sol 64 (Oct. 10) call for shifting the scoopful of sand and dust into the mechanism for sieving and portioning samples, and vibrating it vigorously to clean internal surfaces of the mechanism. This first scooped sample, and the second one, will be discarded after use, since they are only being used for the cleaning process. Subsequent samples scooped from the same "Rocknest" area will be delivered to analytical instruments.
Sand Filtered through Curiosity's Sieve
This image shows fine sand from Mars that was filtered by NASA's Curiosity rover as part of its first "decontamination" exercise. These particles passed through a sample-processing sieve that is porous only to particles less than 0.006 inches (150 microns) across. The view from the rover's Mast Camera looks into the portion box and "throat" of the Collection and Handling for In-Situ Martian Rock Analysis (CHIMRA) tool on the end of the rover's arm.
The decontamination exercise involved scooping some soil, shaking it thoroughly inside the sample-processing chambers to scrub the internal surfaces, putting it through a sieve, dividing it into the appropriate portions, then discarding the sample. This image is downstream of the sieve. The portion box will meter out a portion about the volume of half a baby aspirin so that the instruments receiving the sample will not choke on a sample that is too big.
The decontamination procedure will be repeated three times. The rinse-and-discard cycles serve a quality-assurance purpose similar to a common practice in geochemical laboratory analysis on Earth.
This image was taken by Curiosity's right Mast Camera (Mastcam-100) on Oct. 10, 2012, the 64th sol, or Martian day, of operations. Scientists white-balanced the color in this view to show the Martian scene as it would appear under the lighting conditions we have on Earth.
This image shows the wall of a scuffmark NASA's Curiosity made in a windblown ripple of Martian sand with its wheel. The upper half of the image shows a small portion of the side wall of the scuff and a little bit of the floor of the scuff (bottom of this image). The prominent depression with raised rims at the bottom center of the image was formed by one of the treads on Curiosity's front right wheel.
The largest grains in this image are about 0.04 to 0.08 inches (1 to 2 millimeters) in size. Those large grains were on top of the windblown ripple and fell down to this location when the scuff was made. The bulk of the sand in the ripple is smaller, in the range below 0.002 to 0.008 inches (50 to 200 microns).
The full scuffmark is 20 inches (50 centimeters) wide, which is the width of Curiosity's wheel.
This image from the Mars Hand Lens Imager (MAHLI) is the product of merging eight images acquired at eight slightly different focus settings to bring out details on the wall, slopes, and floor of the wheel scuff. The merge was performed onboard the MAHLI instrument to reduce downlinked data volume.
The image was acquired by MAHLI with the lens about 4.7 inches (12 centimeters) from the target. The pixel scale is about 0.002 inches (50 microns) per pixel. The image covers an area, roughly 3 by 2 inches (8 by 6 centimeters). The image was obtained on Oct. 4, 2012, or sol 58, the 58th Martian day of operations on the surface.
'Rocknest' From Sol 52 Location
This patch of windblown sand and dust downhill from a cluster of dark rocks is the "Rocknest" site, which has been selected as the likely location for first use of the scoop on the arm of NASA's Mars rover Curiosity. This view is a mosaic of images taken by the telephoto right-eye camera of the Mast Camera (Mastcam) during the 52nd Martian day, or sol, of the mission (Sept. 28, 2012), four sols before the rover arrived at Rocknest. The Rocknest patch is about 8 feet by 16 feet (1.5 meters by 5 meters).
Scientists white-balanced the color in this view to show the Martian scene as it would appear under the lighting conditions we have on Earth, which helps in analyzing the terrain.
Once the first two scoops of sand have been used for the decontamination process, the next one will be delivered to the array of analytical instruments: I suspect that I am not alone in the eager anticipation of the results!
Countless words have been written – appropriately – over the last couple of weeks, in the blogosphere and the international press, about Curiosity. I have little to add, except for a personal note of awe and gratitude.
The landing was scheduled for about an hour before my flight landed in Singapore, and the first thing I did after disembarking was to hook up to Google News – and there it was, incredibly, awe-inspiringly, it had worked, and Curiosity was flexing its muscles on the surface of Mars. I will readily admit that I haven’t the faintest clue how this whole, mind-bogglingly complex, mission was planned and executed; I watched and relished the infectious celebrations in the control room with only a partial sense of the true emotions of every individual there.
And then – a picture is truly worth a thousand words – the images started coming in.
When I looked at that image, and understood that the hills in the distance were not on Curiosity’s fieldwork itinerary, my immediate reaction was “Well, why don’t I wander over there and have a look at that winding valley system, while you go off and do your stuff. We’ll meet back here this afternoon.” Curiosity feels, intimately, like a fellow field-geologist – because, of course, that’s exactly what the rover is. I now check out the mission site routinely to see what my friend has been up to, and to continue to celebrate this incredible achievement.
And, for me, the other cause for celebration is simply that, faced daily with the stories of the depravity, greed, and ignorance of our bizarre species, this provides a strong antidote, grounds for cautious, if perhaps fleeting, optimism. Thank you, NASA, JPL, Caltech, and everyone involved.
[All images courtesy of NASA/JPL-Caltech/MSSS]
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.]
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.]
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/]