QUESTION: 
I would like to know what kind of power source is helping to run the
Pathfinder?                                

ANSWER from Guy Beutelschies on January 31, 1997:
The power source we use on Mars Pathfinder is the Sun.  We have solar
arrays on the Cruise Stage for the journey to Mars and arrays on the lander
for when we are on the surface.  For those times that the sun is not
available, like the entry and descent phase and the Martian night, we use a
Silver Zinc battery.  It discharges to provide power and then is recharged
by the solar arrays.

We also need some high power sources for a very short amount of time during
the entry and descent phase.  They are needed to fire pyrotechnic devices
such as separation bolts and cable cutters.  For this, we have Thermal
Batteries.  These batteries consist of chemicals that when ignited get very
hot and produce large amounts of electricity, but only for about 5 minutes.
By that time, though, we will be on the surface and we will not need them
anymore.

ANSWER from Richard Ewell:
The Mars Pathfinder is powered during cruise by a non-deployable solar array
that uses Gallium-Arsenide solar cells. This provides all of the power
during nominal cruise conditions. During Launch and the Entry, Descent and
Landing on the surface of Mars the spacecraft will be powered by a
rechargeable battery. The rechargeable battery has silver cathode, zinc
electrode, and potassium hydroxide electrolyte.

Once the lander has landed and the petals have been opened the lander is
powered by another solar array that uses the same type of gallium-arsenide
solar cells. During the night time and for peak power loads the spacecraft
is powered using the same rechargeable battery used during cruise.

The rover similarly is powered by a fixed array on gallium-arsenide solar
cells. Again peak power loads and night-time loads are supported by using a
battery. The battery for the rover isn't rechargeable and thus uses a
different chemistry.

--Richard Ewell, Mars Pathfinder Power Subsystem Engineer