QUESTION:
If the Pathfinder is not going to come back to Earth, how will the 
equipment on board test the soil and rock samples?

ANSWER from Greg Wilson on February 20, 1997:
During its mission the Pathfinder rover deploys its science instrument,
the Alpha Proton X-ray Spectrometer (APXS), onto rocks or soil on the
surface of Mars. You know that light travels in waves and at different
wave lengths (distance from crest to crest).  The APXS is designed to
"shine"  many different light wavelengths on rock and soil samples.  The
way the light is reflected off the rocks and soil is dependent of the
elemental composition of the target.  What we get back is energy levels at
the different wave length.  We call that a spectra.  From experiments, we
know what spectra look like for all kinds of rocks and minerals.  Data
collected by this instrument is transmitted from the rover to the lander
and then to earth.  Scientists on earth can determine from this data the
elemental composition of the rocks or soil. Images of rocks and soil taken
by the Imager for Mars Pathfinder (IMP) also contribute data which assists
in identifying composition and minerology of the surface in the vicinity
of the lander. Finally, the movement of the rover across the surface is
measured (e.g., torque output from the wheels)  and images will be taken
of wheel tracks both from the rover and lander.  This data will allow
determination of the mechanical properties of the soil (e.g., cohesion,
shear strength). 

A really good application of this can be found at:

http://galileo.ivv.nasa.gov/callisto/021897.html

ANSWER from David Dubov:
During its mission the Pathfinder rover deploys its science instrument, the
Alpha Proton X-ray Spectrometer (APXS), onto rocks or soil on the surface of
Mars. Data collected by this instrument is transmitted from the rover to the
lander and then to earth. Scientists on earth can determine from this data
the elemental composition of the rocks or soil. Images of rocks and soil
taken by the Imager for Mars Pathfinder (IMP) also contribute data which
assists in identifying composition and minerology of the surface in the
vicinity of the lander. Finally, the movement of the rover across the
surface is measured (e.g., torque output from the wheels) and images will be
taken of wheel tracks both from the rover and lander. This data will allow
determination of the mechanical properties of the soil (e.g., cohesion,
shear strength).

ANSWER from Howard J. Eisen on December 10, 1996:
The Rover carries a small scientific instrument called an Alpha-Proton-X-Ray
Spectrometer, or APXS for short.  The APXS shoots sub-atomic particles
called alphas at the atoms of a rock.  Those alphas strike the rock and
bounce back.  Sometimes it breaks some of the atoms such that they release
other subatomic particles called Protons or they release energy in the form
of X-rays.  Detectors on the APXS look for these particles and measure their
speed.  From that they can tell what the rock is made of.
   
If you throw a ball at a ball, the ball bounces back and you can see that. 
This is similar to the APXS seeing an alpha that bounced off a heavy atom. 
If you throw that ball into another ball, the two balls will scatter.  The
scattering which you see of both balls tells you that you hit another object
similar in size to yourself.  If the APXS detects other scattered alphas, it
knows the rock is made of light molecules.  Very precise measurement of the
scattering speed tells the APXS exactly what it hit.

Editor's note: it will take about 10 ten hours for the APXS to test each
rock it samples.