QUESTION:
If the gravitation of Mars cannot hold an atmosphere for certain, then how
did it have liquid water in the past?

The gravitation pull cannot change, unless the planet lost a huge portion of
itself. Maybe that is how the asteroids were formed?

ANSWER from Jim Murphy on August 6, 1997:
The gravitation of Mars can hold an atmosphere.  It is doing so today,
though the atmosphere it holds is much less massive than here on Earth (even
after taking into account the different sizes of the planets).

Liquid water could have existed in the past if the temperatures on the
planet were warmer, say, produced by an enhanced 'greenhouse' warming due to
gases which produce such an effect (carbon dioxide in much greater abundance
than is available today is one possibility).  Water vapor is an extremely
effective greenhouse gas, more effective than carbon dioxide, so if
temperatures were warm enough for liquid water, the water vapor this would
provide would help the 'greenhouse' persist and be amplified.

Additional carbon dioxide which might have been present in Mars' atmosphere
in the past could now be present in rocks on and below the surface.  If Mars
is less tectonically active than has been Earth, Mars would have been less
efficient in recycling CO2 into rocks and then back into the atmosphere. 
This would possibly have led to a cooling of the atmosphere, which would
mean less water vapor could be contained in the atmosphere, which would
induce additional cooling, ... you can see where I am going here.

SO, we don't need to change Mars' gravitational pull to assume a different,
more massive atmosphere in the past.  There are a number of additional
processes which might have played a role, more likely roles, in changing the
atmosphere from the past to present.