QUESTION:
Would the angle of repose of the same material be different on Mars than on
Earth?  If so, what would that difference be and why?

ANSWER from Nathan Bridges on September 14, 1999:
In general, the angle of repose should be the same:

1) The angle of repose is approximately the angle of internal friction if
clean, dry sand is involved.

2) The angle of internal friction is the ability of soil or rock to
withstand a shear stress.  It is the angle (or, dimensionally speaking, the
ratio) between the normal and resulting forces when failure occurs in
response to a shear stress.

3) Both the normal and resulting forces will be dependent on g, not g^2,
g^3, etc.  So, g cancels out.

Now, if you have wet soil, that will affect pore pressure and probably
other factors.  How these would be different on Mars versus Earth I am not
sure.  Perhaps secondary factors such as these will affect differences in
the angle of repose, but I do not think it can be gravity per se.  Of
course, there is no wet soil on Mars today.

On Earth, the range of angles varies from ~30 to 39ƒ.  This is due to a
number of factors (some eluded to above) which can be complicated.
However, any subtle differences between Earth and Mars would probably be
masked by this wide distribution of values.  In other words, you would not
(and do not) see abnormally steep or shallow slopes on Mars compared to
Earth.