Soil Experiments

Attempts To Duplicate The Properties Of Martian Soil, Part 2

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   Our daily experiences with substances and materials teach us many things about their properties.  We sometimes will forget the simplest of them with time, as much of what we learn about sand and water is from our childhood.  Children often play with sand and water and have a very intuitive appreciation of their innate properties.  We can perform some simple experiments and quickly re-learn what we used to know about them, and perhaps pick up a few new things as well.

   Let's start with dry sand, a very simple system.  There are many types of sand, but none of them will take a good print when dry.  They will slump and reach a conic shape determined by something known as the "angle of repose".

   This is determined by gravitation, sand density, and how well the particles or granules clump together.  Dry sand is quite uniform in its properties, almost without regard to its source.

   This is plain white sand such as you might find in any sand pile or on any construction site.  The clumps are slightly damp, or have been dampened and then dried.  The rest is free running dry sand with very little salt content.  Notice how it flows down in smooth runnels.

  
   This is also dry sand, but from a different source.  I used this sand to simulate the Martian soil samples because of its very similar appearance to the color images and to the grain size shown in the microscopic images.

   You can see that it, too, runs freely.  It will form cones of the angle of repose, and they will be identical to the slope and smoothness of the white sand shown above.  This sand also has very little salt in it.

   In both cases, you will see that the sand that has been dampened previously will clump, but once dried, a small disturbance will break the grains free and the sand will run as freely as before.
   Even the presence of organic material will not appreciably change the free-running properties of dried soils.  Here is a mound of dried out compost.  Note that it, too, runs at the same angle of repose as the previous two samples.

   The major difference when you handle this soil is that it is "clumpier" and much lighter in composition.  Organic matter greatly reduces the mass of the soil per unit volume; i.e., the specific gravity is decreased, making a cubic volume of the soil much lighter and more capable of retaining moisture.

   So organic matter can be present and the sand can still be quite free-running when it dries out.  But in all these cases, there has been no significant level of salt in the sand or soil we have examined.
   Here, finally, we have beach sand.  This is very salty and is regularly immersed in (or exposed to spray from) brine in the form of seawater.

   This sand is dried and shows clumps, but once disturbed, the clumps break up easily and form individual granules that blow in the wind or slump in the characteristic conic angle of repose.

   In all these cases, dry soil shows the same slumping response, regardless of salt content.  This shows that the presence or absence of salt does not seem to significantly affect the properties of the dried soil, except in one way.

   Dried sand that has been in brine shows a greater tendency to stick even in the presence of small quantities of moisture.  This is because many salts are hygroscopic, which means that they draw moisture from the surrounding air and become self-adhering.  The moisture can provide a slight amount of stiction that would otherwise not be present.

    So, except for a slight tendency for the sand to stick slightly in the presence of salt and moisture, there is really very little difference in the properties of dried sand, regardless of the source in relation to its slumping properties.  It also means that the ability to take an imprint is not very different for various sands, and experiments with very fine, dry dust indicate that it, too, will slump easily and not take a print well.

    A similar experiment with larger granules of builders' sand showed no real difference, as did experiments with gravel or other small crushed stones.  All materials showed the same sort of slumping properties.

    In other words, the type of sand and the grain size does not seem to affect how it slumps, runs, or takes a print.  Salt also does not seem to make much of a difference.  There must be some other factor responsible for what we observe in Martian soil.

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