QUESTION:
Why are Isidis Planitia, east of the Elysium Planitia, and the
confluences of
Ganges, Caprii, and Eos Chasams good places to look for water?  Which
one would
be the best? 

ANSWER from Jeff Plescia on October 20, 1998:
Gangis, Capri, and Eos Chasmas occur at the eastern end of
Valles Marineris.  They have a morphology - scour marks, tear
drop shaped islands, erosion - which suggests they were formed
by the flow of water. The scale of these features indicates that
the water volume and flow speeds were very large.  The source
of the water is uncertain.  For Gangis Chasma it must have
come out of the ground at the western end of the Chasma.  For
Eos and Capri is may have come out of Valles Marineris.
It has been suggested that Valles Marineris may have once been
filled with water forming a giant lake.  When the lake emptied
the water flowed out of the east end of Valles Marineris,
carved some of the chamsa and then flowed north into the
southern end of the Chryse basin.

Isisdis Planitia is an ancient impact basin. It has a few small
valleys around the margin, but does not have extensive evidence
for water.

The chasma would be a better place to look for water.  However,
most of the water from Gangis, Eos and Capri flowed north into
Chryse, so that might be where it sank into the ground.  Both
the Viking 1 and Pathfinder landing sites are in this general
area and the idea was to go to a location where water may have
been.