QUESTION:
As far as you know from the Viking missions, as well as the Pathfinder
mission, what do you expect the geotechnical caracteristics of the upper
layers of the soil in Mars to be (cohesion, friction angle)?  In so far as
the temperature can sometimes be higher than 0 degrees celsius, to what
extent does this fact affect the behaviour of the soils?

ANSWER from Bruce Jakosky on February 7, 1998:
There is a vast literature describing the physical properties of the Martian
surface and presenting results on technical parameters such as the angle of
internal friction, cohesiveness, etc.  Some of these results are discussed
in the book MARS (edited by H.H. Kieffer and others). Loosely speaking, the
dust is somewhat similar to a fine flour.  Of course, the rocks and crusted
material might feel similar to the surface in a terrestrial desert.

When the temperature changes from below to above freezing, there can be an
affect only when there is a significant amount of water present.  The
temperature can rise above freezing only in the top few millimeters of the
surface, and this region would quickly lose any water that it had.  Thus, as
a general rule, it is unlikely that there would be a big effect from
reaching the melting temperature.  Of course, there are places where water
might be abundant and where there would be large effects; no such places
have been identified yet, however.

ANSWER from Jeff Plescia on February 9, 1998:
The phyiscal properties of the Martian surface are not too different from
those of either the Earth or the Moon.

The best analog for Martian surface material is a moderately dense soil on
the Earth.

friction angles for Pathfinder site soils: 36.6 degrees
angle of repose: 34.2 degrees
cohesion: 0.238 kPa

I suggest reading the article in Science, vol 278, p. 1765-1768, December 5,
1977 on the physical properties of the Martian surface from Pathfinder.
References therein will direct you to Viking information.