QUESTION:
There has been much controversy about what happened to the water on Mars. 
How does this sound, assuming life on Mars is/was like Earth:

* Water on Mars (probably less than earth due to smaller size, less geologic
  activity, etc)
* Water has life
* Life releases O2 from water
* O2 oxidizes with rocks
* All the water is used up
* Life disappears/adapts to a form we can't detect

In a related question, Earth has lots of N2, Jupiter has lots of N in its
ammonia, where is all the N2 on mars?  It's fairly non-reactive and weighs
less than water.  Shouldn't the majority of the atmosphere be N2?

ANSWER from Jack Farmer on July 22, 1997:
Point 1: The scenario you present is reasonable, but leads to the prediction
of lots of oxygen in the atmosphere. We have no evidence for that today, but
some have suggested that it would have combined with iron in the crust to
form iron oxides or "rust", thus accounting for the red surface of Mars (see
previous discussion). The problem with this idea is that oxygen could have
also been released by the photolysis of water in the upper atmosphere, a
purely inorganic process. Therefore, the rusty surface of Mars is no proof
that photosynthesis ever originated on the Red Planet. Most of the life on
Earth is, in fact, chemosynthetic and does'nt necessarily even require a
connection to the surface. So, there is no really no compelling need for the
invention of photosynthesis in your scenario, or the oxygen liberated by
such biological processes. Life could have originated just as easily as a
chemosynthetic form and remained that way.

Point 2: If carbon-based life is there, we should in principle be able to
detect it. If it developed on some other basis, then all bets are off, and
there would no need for your scenario at all, which is notebaly
Earth-centric in its assumptions.

Point 3: The Martian atmosphere as measured by the Viking landers consisted
of ~95.3% carbon dioxide, 2.7% nitrogen, 1.6% argon, 0.13% oxygen, 0.7%
carbon monoxide, .03% water vapor, and trace quantities of inert (noble)
gases. This atmosphere is similar to the composition of the stratosphere on
Earth (only wetter), and also compares well to what we think the early
atmosphere of the Earth was like prior to the "invention" of
oxygen-liberating photosynthesis. - Jack Farmer