QUESTION:
Do any of you think an atmosphere once existed on Mars? Could a natural
disaster such as an asteroid collision have ripped away the atmosphere that
was once there?

ANSWER from Bruce Jakosky on July 18, 1997:
Mars certainly had more atmosphere in the past than it does today.  We see
two types of evidence for this--geological and geochemical.

The geological evidence consists of types of features seen on the surface
that indicate that the atmosphere must have been thicker in the past. There
are valley networks that look a lot like river drainage systems on the
Earth.  For these to form, liquid water must have been more stable at the
surface of Mars than it is today; and, the easiest way to make water more
stable is by the presence of a thicker atmosphere.  Also, the oldest impact
craters are more heavily eroded than younger ones; again, the presence of a
thicker atmosphere is likely.

The geochemical evidence takes the form of the ratios of different isotopes
in the atmosphere.  These are molecules that have an extra neutron in the
nucleus, so are heavier than the normal ones.  The lighter molecules escape
to space more easily, so the atmosphere that stays behind has more of the
heavier molecules.  We see that the ratio of deuterium to hydrogen is
enhanced, as is the ratio of heavy nitrogen to lighter nitrogen.  In each
case, something like 90% of the gas must have been lost to space.

Certainly, asteroid collisions can strip off much of the atmosphere--not
from a single impact but from the many collisions that have occurred through
time. This process may have been a major player in removing much of the
early, thicker atmosphere.