Soil Experiments

Attempts To Duplicate The Properties Of Martian Soil, Part 4

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   When exploring the properties of terrestrial soils, we find that what meets our eye may not be what is truly present.  A sand dune can be desiccated and blow freely in the wind, but a short distance below the surface, a veritable ocean of water can be present.  In other words, dry sand can move easily over wet sand, and even hide the wet sand entirely.  This is paradoxical as finding that a desert is sitting on top of an ocean.  However, in the case of Mars, this may be exactly what is happening.

   On a typical beach, when you are near the shoreline, you can make a shallow hole and see water pooling below.  The upper sand does not seem to be greatly affected by this, and the water is moving slowly enough that it can effectively flow almost freely.

   This is the case in any water table- the water can appear to flow freely through areas where there is rock, sand, or almost any other mineral material.  The critical fact is that the velocity of the water is low enough not to displace or move the sand or gravel that it is flowing through.

   This is made clear by the fact that the changing tide changes the water level below the beach sand, but you do not see the sand flowing or shifting as this happens.
   This is the wet sand from the shore.  You can see the water just a short distance away in the background.

   What is interesting is that the sand is quite wet here, but there are white streaks of completely dry sand blowing across it and leaving lighter colored areas on the wet sand.

   Enough of this would result in a dry sand dune, but typically, the shore is too dynamic to allow this to happen so close to the water's edge.  Water, tides, wind, and other effects would quickly wear a dune away.
   If you walk a few short meters from the water's edge, you will encounter free moving sand and actual dunes on many beaches.  Does this mean that there is no water present a short distance below the surface?  Not at all.

   In fact, a shovel and a few minutes' work will reveal that the water is just as pervasive here as it is down where the waves are at work.  The only difference is the top layer of sand- it happens to be dry and the water from beneath is not wicking up fast enough to wet it.

   Take note of the sand ripples which are identical on Earth and on Mars- this is one very clear indicator that physics holds its own.  On either world, the laws of physics and chemistry remain firm.
   Here is a very curious inversion- wet sand on top, dry sand beneath.  Notice that there is a very clear crusting effect?

   The waves have washed over this area of dry sand as the tide approached.  The result is that the sand on top absorbed the water and formed a crust that is clear and separate from the dry sand below.

   Stepping on or deforming the crust produces broken "plates" that lift cleanly away from the soil beneath.

   Now, armed with a little knowledge about how sand and water can interact, let's examine the microscopic images and we will see for ourselves just what sort of sand we are dealing with.

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