Soil Experiments

Attempts To Duplicate The Properties Of Martian Soil, Part 3

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   Now I will cover a few more observations of real soil systems here on Earth before making any sort of sweeping statements about Mars and its soils.  However, we will see how water has a major effect on the working properties of soils in the next few images.

   This is an image of wet beach sand very near the tide line.  You may have experienced stepping on "dry" sand only to remove your foot and see water welling up from your track.

   This is caused by a film of water that has formed between the grains of sand due to "wicking".  The spaces between the grains draw moisture in between because of something called "Van Der Waals" force.

   This is an electrostatic force that causes molecules to want to stick together, and also allows geckos to hang on glass plates with no adhesives.  Water molecules will fill the spaces and draw onto the surfaces of the sand grains, with the results that the grains are not actually touching.  Instead, they are held slightly apart by a microscopic distance by a film of water.

   Stepping on the sand compresses it and forces the water to emerge.
   On dry sand or damp sand, you can see the clear difference as a print is made.  The darker, damp sand is holding this print easily, but the dry sand is slumping back once the print is made.

   Also notice that the cracks in the white, dry sand show that damp sand is beneath it, but the top layer of dry sand is slumping into the cracks, showing that there are two distinct layers in the soil.

   The lower, damp layer can hold a print but the upper, dry layer cannot.  The resulting interaction shows cracks with dry sand sifting into the spaces.  

   Here is a dry print- it clearly has no definition.  The soil around it was damp at one time, but has dried into a hard shell.  Stepping on it produces this slumping dry print, but also cracks the surface layer of sand.  It almost looks like a crust is present on the surface.

   Is this sort of phenomenon only present in dry soil?  No.
   This is once more wet beach sand, and it shows a crusting effect even though both the upper and lower layers are wet.  What causes this effect is the fact that the upper and lower layers of the sand are of different composition.

   The effects of wind and water have sorted the materials in the soil so that the lighter sand formed a layer on the surface while the redder broken bits of seashell have formed the lower layer.

   When you step on this layer or apply pressure to it in any other manner, you will see that the top layer fractures cleanly from the lower layer.  This crusting can happen in both salty and non-salty soils, but is somewhat more pronounced in salty soils.

   So, what we see is that the presence of salt does not seem to have a great influence on the mechanical properties of the soil.  We have also seen that dry soils will slump while damp soils can take a print well.  Wet soils can also slump, but it takes a great deal of moisture for that to happen.

   Also, we see that crusts can form in dry or damp soils, and that it can be the results of soils that were previously damp but have dried, or from the sorting of the materials that the soil is made of.  This sorting process is called material segregation.

   Now let's look at subsurface water and a few more simple properties before we draw our conclusions.

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