QUESTION:
Are there fossil fuels on Mars that can power vehicles?  How can we
access them?

ANSWER from Cathy Davis on January 10, 1997:
In addition to solar energy, you can consider nuclear energy. At Mars, 
sunlight is only about have of what it is at the Earth. A small 
radioisotope thermoelectric generator (RTG) can be used to power 
small rovers on Mars.

ANSWER from Geoff Briggs on March 26, 1997:
We don't know whether there are any fossils on Mars and certainly no
knowledge of accumulations of organic matter that could be used as a fuel.
It takes a combination of profuse life and appropriate geologic processes
to bury organic materials to create coal or oil deposits and we see no
evidence of either.

The creation of fuel and, just as important, oxidiser on Mars is quite
straightforward in principle.  The martian air is mainly carbon dioxide.
Given a source of energy (a small nuclear reactor, perhaps) you could split
the CO2 into carbon monoxide, CO, and oxygen, O2.  These can be burned
together to drive a vehicle, even to power a rocket.

A more powerful fuel and oxidiser combination would be methane, CH4, and
oxygen.  We can make methane from carbon dioxide if we have some hydrogen.
We could take the hydrogen with us or we could find water (ice near the
surface) and then use our power source to melt the ice and electrolyze it
to hydrogen and oxygen.

If we can gain access to a plentiful source of water we could, of course,
use the hydrogen and oxygen as our rocket propellants or as fuel and
oxidiser for other kinds of engines.  It is relatively difficult to keep
the hydrogen liquid because it has to be kept so cold.

All these approaches depend upon having a source of power like a nuclear
reactor and, in effect, storing the energy from the reactor (generated over
a long period of time as we process the atmosphere or ice) in the form of
chemical energy in the fuel and oxidiser.  On Earth we have the 
convenience of lots of oxygen in our atmosphere so we only have to look for
fuels to burn in the oxygen.  On Mars the atmosphere is mostly carbon
dioxide which is chemically not reactive like oxygen.  However, a metal
like magnesium (used in flash bulbs once upon a time) can burn in CO2 and
so this might be another candidate fuel if we could extract it conveniently
from martian rocks -- probably not a very practical idea.

ANSWER from Ted Roush on April 8, 1997:
Probably the rovers used by the astronauts near the end of the 
Apollo program to drive around the moon are good examples of the kinds 
of vehicles we will initially have on Mars.  These vehicles had wheels 
and most closely resembled the cars or golf carts that we are 
familiar with on Earth, with one major difference.  On Earth the vast 
majority of cars use fossil fules (gasoline) to power their engines.  
On Mars fossil fuel will be difficult, if not impossible to get.  
As a result vehicles there will likely run on batteries.