QUESTION:
By using the in-situ rock field around the Sagan site as a measure of the
forces necessary to angle some large rocks all in the same direction, what
flood velocity (in miles per hour) and depth would have caused the patterns
of rocks we now see there?  Did the water go over the twin peaks?

ANSWER from Mary Urquhart on August 20, 1997:
Some of the large rocks at the Pathfinder landing site do appear to be
angled in the  same direction.   One explanation is  that flood waters
carried the rocks into those positions.  Others are that the rocks are
ejecta from a nearby impact crater or even that the large tilted rocks
are actually exposed bedrock (perhaps  exposed by the flood) and  that
we are seeing the native tilt (or dip) of the rocks.

If the rocks were angled by the flooding of Ares Vallis then the flood
would have to have been strong enough to move them into new  positions
but not strong enough to carry them with along with the flood  waters;
i.e., the flood would have to have moved the position of the rocks  to
one of  least resistance  to the  water but  without chipping  off the
sharp edges.  This  would suggest that the  rocks were not moved  very
far, or didn't move freely in the water (weren't tumbled but may  have
been  pushed  along  by  the  flood).    Boulders  deposited  by   the
catastrophic flooding of the  channeled scablands of Washington  state
(a terrestrial  analog for  martian catastrophic  flood features)  are
more  rounded  than  the  large  angled  boulders  we  see at the Mars
Pathfinder landing site.

Now to answer your question:  Flood velocities necessary to change the
position of rocks at the  Pathfinder landing site including the  angle
of rocks depends on the size of the rock.  Since at least portions  of
the rocks are buried in sediment, the apparent sizes of the rocks  can
only be used as a lower limit for the velocity and depth of the  flood
water.  At this early date  I don't know the actual size  distribution
of those angled rocks.  Work  on the data returned by Mars  Pathfinder
is just  beginning.   The first  results beyond  the very  preliminary
results we  have so  far won't  be available  until at least December.
Please  remember  that  understanding the  geology of  the  Pathfinder
landing site (like all scientific endeavors) will take time.

We have  had years  of studying  Viking data  to help  us develop  our
current understand of Mars, including the catastrophic flooding in the
area  of  the  Pathfinder  landing  site.    Work  on estimating flood
velocities from large scale surface features near the landing site was
done prior to the landing and suggests that the maximum flood velocity
could have  exceeded 100  m/s (or  about 220  miles/hr) in the deepest
part of  Ares Vallis  which would  have been  capable of  transporting
boulders 10 meters (or  about 30 feet) in  diameter.* By the time  the
flood waters reached  the broad terrain  of Chryse Planitia  the flood
velocities and  depth would  have decreased.   Hopefully  studying the
images from the  landing site will  help us understand  more about the
properties  of  the  flood  when  the  water  reached  the  area where
Pathfinder landed.

The twin peaks are a few hundred feet high.  Images of the twin  peaks
show some features that have  been speculated to be benchs  (which can
be thought of as water-level marks).  If this speculation is  correct,
then the  twin peaks  were not  completely covered  by water  at least
during a portion of the flood.

*For more details see "Paleohydrology and flood geomorphology of  Ares
Vallis" by G. Komatsu  and V.R.  Baker  in the Journal of  Geophysical
Research, Vol 102, Number E2, Feb. 25, 1997.

Mary Urquhart
Laboratory for Atmospheric and Space Physics
University of Colorado at Boulder