Through The Sandglass

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.]

Desert Moon

Clustered around a small television screen in the middle of the night in Cambridge forty years ago. One of the most memorable experiences of my life. I can't add anything to that extraordinary achievement, but I do want to pay my respects. The full moon in the desert is itself a wondrous experience, so many stars still visible. And then there is the landscape illuminated by its light. Sitting there alone, nothing, absolutely nothing, would have surprised me.

Streams of grains - some more amazing granular research

As I'm sure I've said before, to watch the sands of an hourglass in action is to observe all kinds of natural phenomena at work - and all kinds of questions to which we have no answers. It's not just the cascades and avalanches, but the liquid stream of grains itself that holds mysteries, interactions and behaviours too minute and too rapid for the eye and the brain to grasp. Traditionally, we have to resort to statistics, to averages, to clumps and groupings - to model at the level of individual grains is just too complex and there are too many of them. But now we are beginning to be able to peer into the nano-world of an individual grain and to simulate its interactions with its colleagues, thanks to the continuing, revelatory, research into granular materials. Friends have recently drawn my attention to two specific examples that I'll mention here.

The University of Chicago has long been a fluidised bed of granular behaviour research, with Sidney R. Nagel and Heinrich M. Jaeger as the uber-gurus of the bizarre (remember the Brazil nut effect and then the reverse Brazil nut effect), and now Jaeger, together with his students and colleagues, has yet again achieved something extraordinary. Look again at the flowing stream of sand grains in the hourglass: until recently, studies of so-called "free falling granular streams" tracked shape changes in flows of dry materials, but were unable to observe the full evolution of the forming droplets or the clustering mechanisms involved. But, as recently reported in Science Daily, with the aid of a very expensive and very high-speed camera, Jaeger has been able to track the formation of "droplets" in the granular stream and show that surface tension effects 100,000 times smaller than those that operate in liquids are at work. The dry material behaves as if it were a liquid with very low surface tension - as suggested in the graphic at right, above, and a short but dramatic video is available at the NSF site here.

"At first we thought grain-grain interactions would be far too weak to influence the granular stream," said John Royer, a graduate student on the team. "The atomic force microscopy surprised us by demonstrating that small changes in these interactions could have a large impact on the break up of the stream, conclusively showing that these interactions were actually controlling the droplet formation." So streaming sand not only looks like a liquid, but actually behaves like one, once again raising the question of exactly what form of matter this is. And, as with all this kind of research, it's not just of academic interest - the efficient filling of pharmaceutical capsules is but one (important) industrial process that could benefit. And it's also a further wonderful example of the deeper we look, the more complex things become and the more the questions that are raised. As Jaeger says, these "experimental results open up new territory for which there currently is no theoretical framework."

So Jaeger is breaking new ground in the experimental observation of individual grain interactions, but are we doomed to be unable to computationally model these things, overwhelmed by the sheer numbers, the scale of the problem in a different sense? Well, no, not if Dan Negrut, Toby Heyn, and Justin Madsen at the University of Wisconsin have anything to do with it. Working at the Simulation-Based Engineering Laboratory in Madison, they have developed both the theoretical and mathematical basis, and the power of parallel computing, for modelling the movements of huge numbers of grains. There's a report on their work on Science News, but the best thing to do is to go to their website and enjoy some of their simulations (you'll need to download the VLC viewer for which instructions are given). Not surprisingly, my favourite is their virtual hourglass (illustrated at left at the top of this post); this uses the relatively modest number of 25,000 individual "objects", i.e., grains, but it's incredibly realistic. There are other simulations involving hundreds of thousands of grains and the capability is growing all the time - Negrut hopes his simulation will analyze millions of grains in a single day, if not a matter of hours. To model the interactions of individual grains requires a firm theoretical foundation and the team has made great progress in establishing this; a planned collaboration with Professor Alessandro Tasora from the University of Parma will further refine this. Negrut kindly supplied me with a preprint of the group's forthcoming paper, A Parallel Algorithm for Solving Complex Multibody Problems with Stream Processors. Unfortunately, I have to confess that it's entirely beyond me, even the abstract outstripping my modest mathematical and computational capacity. Here's an extract - I recognised two sand grains in Figure 1 and was then terminally lost:

 

But among the simulations viewable on their website is one of work they are doing on a Mars rover moving over granular materials. This, as I recently noted (Stuck in the Sand) is currently a major problem for the Mars Rover Spirit, and its engineers back on earth. Thanks to being again alerted by a reader, I now find that the Rover's dilemma (sounds like the name of a pub) is being used to good effect to study details of the diverse granular materials revealed by its spinning wheel. As reported in Science Daily, "One of the rover's wheels tore into the site, exposing colored sandy materials and a miniature cliff of cemented sands. Some disturbed material cascaded down, evidence of the looseness that will be a challenge for getting Spirit out. But at the edge of the disturbed patch, the soil is cohesive enough to hold its shape as a steep cross-section." Ray Arvidson of Washington University in St. Louis, deputy principal investigator for the science payloads on Spirit and its twin rover, Opportunity, said "We are able here to study each layer, each different color of the interesting soils exposed by the wheels," adding that "The layers have basaltic sand, sulfate-rich sand and areas with the addition of silica-rich materials, possibly sorted by wind and cemented by the action of thin films of water. We're still at a stage of multiple working hypotheses." Below is an image of the revealed materials (NASA/JPL-Caltech/Cornell University).

 

There is, literally, no end to the extraordinary journeys that granular materials can take us on, so next time you watch an hourglass in action, think of low surface tension liquids, parallel computing and Spirit.

[I have to thank Richard Cathcart and Dominion Rognstad for drawing my attention to these topics - my theme for this blog is so wide-ranging that I can't possibly keep track of all the items of interest and I very much appreciate any suggestions and links that readers might send me.]

Stuck in the sand - a long way from anywhere

 

The image above is a geologist’s bad dream, the vehicle’s wheel churning uselessly in the sand. But this is also NASA’s bad dream, for the vehicle is Spirit, one of the tenacious and hard-working Mars Rovers that have been trundling around the red planet for five years more than they were expected to. Spirit had suffered a front wheel failure some time ago, so has been driving backwards ever since, dragging the non-rotating wheel behind it. But now Spirit is stuck in the sand – and look at how stuck that wheel is and how powdery the sand is. "Spirit is in a very difficult situation," JPL's John Callas, project manager for Spirit and its twin rover, Opportunity, said. "We are proceeding methodically and cautiously. It may be weeks before we try moving Spirit again. Meanwhile, we are using Spirit's scientific instruments to learn more about the physical properties of the soil that is giving us trouble." The project team are building a sand-filled replica model to experiment with possible solutions, but looking at that image, you can just feel the predicament. And then there is a concern that the rover has dug itself in so far that the chassis is high-centred - stuck on an underlying rock (I sympathise – been there, done that – but not on Mars).

For the early Mars landing missions, NASA sought the expertise of Ralph Bagnold (see my post on The Man who Figured Out how Deserts Work) and it was Bagnold who pioneered desert motor vehicle travel. Among his many contributions were, of necessity, ways of extricating oneself from soft sand. He started out using rope ladders (see illustration at left, below) to help his Model T and Model A Fords continue on their way, but he then hit on the idea of using lengths of metal roofing material (below, right) that evolved into the lightweight aluminium sand tracks that we use today.

 

I was personally grateful for this innovation on any number of occasions during our retracing of Bagnold’s 1938 Western Desert Expedition a couple of years ago. However good the driver, and however fast you drive over firm sand (with dramatically deflated tyres), there comes a point when the fickle and unpredictable behaviour of granular materials will do you in. A couple of examples below, the modern versions of Bagnold’s sand tracks being put to use on the left. An aside on the photo on the right: we were fortunate to have several women on the trip, two of whom were named Jane. The substantial lady in the foreground is one, and our Egyptian crew decided to distinguish between the two by naming one “Jane” and the other “Double Jane.” Double Jane, thank heavens, thought this was hysterically funny.

And all of this reminds me of a movie I recently watched on DVD – Japanese Story. This is ultimately a tragic tale, but I was for a while entertained by a female geologist (Toni Collette) in a leading role (a rare event, male or female). This is the movie in which, on being required to  act as chauffeur for a visiting Japanese investor in the Australian mining operation she works for, she delivers with fervour the classic line “I’m a bloody geologist, not a geisha!”  I soon began to realise, however, that the makers of the movie had not bothered to talk to a real geologist to figure out how this character might behave (she took no notice whatsoever of fantastic outcrops she was driving past, for example). But the ultimate absurdity was when she got her truck stuck in the sand. She revved the engine and tried to drive forward, all the time embedding the rear wheels up to the axles. Now I have a habit (which, yes, I am completely aware is pointless) of shouting at the TV, and I found myself hurling advice. But to no avail. She subsequently burnt out her winch trying to use a sand anchor. Now I appreciate that this scene was necessary for the dramatic progression of the story line, and the these two people had to spend time together marooned in the desert. But did it have to be at the cost of making geologists look incompetent?

Spirit could use help from Ralph Bagnold more than Toni Collette.

[Spirit image courtesy of  NASA/JPL-Caltech; see New Scientist article here; Ralph Bagnold photos by kind permission of Stephen Bagnold]

In Martian Memoriam

The sun is sinking rapidly below the horizon, temperatures are plummeting in the gathering darkness, snow is falling and ice begins to embalm everything exposed to the extremes of winter. The Martian winter.

And on the arctic plains of the red planet, Phoenix is dead.

As I was facing the final editorial deadline for the book, back in early June, one of the greatest frustrations was not being able to continue the story of the Phoenix Lander which had just arrived on Mars – it was one of those “at the time of writing” comments which had to be left at that. Since then, the Phoenix has joined, if not exceeded, the success of its comfort-loving colleagues, the Mars Rovers, which continue to trundle around the tropics. Phoenix is a hardy robotic field geologist which has worked diligently and delivered extraordinary science results – it has, deservedly, just won recognition from Popular Science magazine as an innovation worthy of the publication's "Best of What's New" Grand Award in the aviation and space category.

For much longer than was originally thought feasible, Phoenix has been scraping, digging, collecting, sorting, imaging, and analysing, with the most sophisticated laboratory on the planet, the sand (and dust, and ice) of Mars. It discovered water-ice in different forms. It found salts and calcium carbonate which support the case for liquid water existing on Mars, possibly relatively recently. It didn’t find little green men or even little green microbes, but it did show that some of ingredients necessary for the recipe of life are there.

  Phoenix observed its field area on all scales, recording the weather (dust storms, clouds and falling snow), the landscape, patterned in a way that is eerily evocative of our own Arctic tundra – and sand grains. Peering down its microscope, Phoenix recorded the grains in this image. About one-tenth of a millimeter across, these are the same minerals that make up rocks on Earth – and the rest of the universe. The iron content and weathering provide the rust-red color of the planet and its soil and some of the grains are magnetic.

The hard science of analyzing all of the results that Phoenix sent back will continue for years, but sometimes we just need to step back in wonder – this is a picture of microscopic sand grains on a planet a hundred million kilometers away.

Of course, by our standards, Mars is never balmy, but by now night time temperatures are expected to plummet to -184 F (-120 C). The ice that Phoenix has watched encroaching is still made of water, but the cold will be such that the carbon dioxide in the atmosphere will soon freeze, encasing if not entombing the lander. There is instrumentation on board that is designed to jump start Phoenix if, by any chance, it lives up to its name and resurrects itself next summer. But that chance is remote. For now we should just celebrate the remarkable life of this extraordinary field geologist.

[These images are courtesy of NASA/JPL-Caltech/University of Arizona/Texas A&M University/ Imperial College London. The Phoenix mission is another example of global collaboration. See NASA  and University of Arizona  for your own journey.]