QUESTION:
I have looked at the soil and rock analysis, however there is no mention of
moisture/water content of soil. In one of the color photos of the rover with
its wheel tracks going from the lander to Barnacle Bill and Yogi (photo
#82018), it looks as if the soil is quite moist. Is this moisture or not? 
Since there is water vapor in the atmosphere, with such low temperatures, it
has to condense on the soil and rocks.  I am very anxious to find out if
there is sub-surface water on Mars and what rover tracks apparently exposed
looks like moist soil to me.  What is a NASA scientist's interpretation?

Viking photos did see condensation of water vapor overnight and its
evaporation during the day. Wouldn't we expect similar phenomena at the
Pathfinder site? Would this be evidence for microbial life?

ANSWER from Bruce Jakosky on August 14, 1997:
The instruments on Pathfinder and on the rover are not able to measure water
directly.  For instance, the APXS experiment on the rover measures the
abundances of elements, not molecules, and is not sensitive enough to detect
the relatively small amounts of water that would be present.

There is water in the atmosphere, but the atmosphere is cold enough that it
cannot hold much water, so it only saturates at night.  As a result of the
presence of atmospheric water, there is adsorbed water in the soil. Adsorbed
water is bound relatively lightly, in a monolayer or less of water molecules
on the individual soil grains.  There is not enough water for the soil to be
considered "moist".

So, what are the tire tracks?  Most likely, the wheels disturb the soil and
roughen it up.  This exposes material underlying the surface.  The surface
probably has some bright dust on it, so this exposes a darker, subsurface
crust of material.  The same thing happens in my garden when I take smooth,
well-tamped, soil, and break it up with a hoe--the newly roughened surface
is darker.

ANSWER from Bruce Jakosky on October 7, 1997:
While there is considerable water in the Martian soil, I would not call the
soil "wet".  Rather, the water is most likely in two forms--chemically bound
water (water of hydration, chemically bound to the minerals) and physically
bound water.  The physically bound water, also known as "adsorbed" water,
consists of water molecules attached to the soil grains by van der Waals
forces.  Typically, the bonds are stronger than in liquid water, and there
is no more than a single layer of water molecules covering the grains.  If
the atmosphere were near saturation, the number of layers of water on the
grains would increase.  If it got to be more than a few molecules thick,
then it would behave like liquid water and we would consider it to be liquid
water.

Another way of saying this is that the atmosphere is not saturated with
water vapor for most of the day.  If liquid water were present in the top
few millimeters of the soil, it would quickly evaporate and diffuse into the
atmosphere.  The water molecules that are left behind are bound more
strongly to the grains, so they do not evaporate as readily.

Given the primary function of liquid water in living organisms--as a medium
through which nutrients can diffuse in and waste products can diffuse out--
it is unlikely that adsorbed water can do the job.  This does not rule out
the possibility of Martian life.  However, to find the life, you should look
for places where water is both present and stable as liquid.  This might be
deep below the surface (where temperatures are higher and diffusion to the
surface is inhibited) or near the most recent volcanism (where liquid water
might be present in hot springs or hydrothermal systems).