QUESTION:
How did NASA pick the landing site for the MPF? Many people from Eastern
Washington understand the catastrophic geology of the area--and many of them
don't!  Looking at the Mars landscape does, as someone mentioned, look like
the channelled scablands of Eastern Washington state.  My question is, has
the work of J. Harlen Bretz in the early 20's helped the geologists choose
and analyze the site of the landing?

Why wasn't a site chosen where you would be more likely to find life?

ANSWER from the Internet on January 22, 1997:
The rover will land in Ares Valles, at the mouth of an ancient flood
channel. The precise coordinate of the landing site are 19.5 degree
North, 32.8 degrees West.

NASA has selected an ancient flood plain on Mars as the landing site 
for the 1996 mission of Mars Pathfinder. Eons ago, when water flowed 
on Mars, great floods inundated the landing site, located on a rocky 
plain in an area known today as Ares Vallis. The site is 850 kilometers
(527 miles) southeast of the location of  Viking Lander 1.

Choosing a location has in the first place to do with engineering 
considerations which put constrains on the landing site. 

1) Since the spacecraft is solar-powered, the best site is one with
maximum sunshine and in July, 1997, the sun will be directly over the 
15 degrees north latitude region of the planet. 

2) The elevation must be as low as possible, so the descent parachute 
has sufficient time to open and slow the lander to the correct 
terminal velocity. The landing will be within a 100- by 200-kilometer
(60- by 120-mile) ellipse around the targeted site due to 
uncertainties in navigation and atmospheric entry.

In the second place the landing site must meet scientific constrains.
Ares Vallis, which meets the engineering constraints, was chosen after
a workshop earlier this year that involved the invited participation 
of the entire scientific community concerned with Mars. More than 60
scientists from the United States and Europe attended. The Ares Vallis
site is also a "grab bag" location, according to Matt Golombek, set at the 
mouth of a large outflow channel in which a wide variety of rocks are
potentially within the reach of the rover. Even though the exact 
origins of the samples would not be known, he said, the chance of 
sampling a variety of rocks in a small area could reveal a lot about 
Mars. The rocks would have been washed down from highlands at a time 
when floods moved over the surface of Mars. Several potential sites 
were listed where ancient flood channels emptied into Chryse Planitia,
having cut through crustal units and ridged plains where the water 
would have picked up material and deposited it on the plain.

Other sites that were considered included Oxia Palus, a dark highlands
region that contains highland crust and dark wind-blown deposits; Maja 
Valles Fan, a delta fan which drained an outflow channel; and the Maja
Highlands, just south of Maja Valles. All of the sites were studied 
using Viking orbiter data.

ANSWER from Mike Mellon on July 27, 1997:
Yes, the choice of landing site for Mars Pathfinder was motivated by the
desire to find a location on Mars where a wide variety of rocks would be
located, and in the case of Ares Valles, brought there by catastrophic
floods. Comparisons with the Channeled Scablands of Washington state was
beneficial to scientists and engineers for gaging what to expect at the
martian landing site. A number of important characteristics where considered
including; rock abundance and shape (critical to the survival of the
spacecraft on landing), rock types, surface type (rocky vs soil), and
topography. We hope to learn more about Mars and the history of water by
continuing to make close comparisons between features on Mars and Earth.

ANSWER from Cheick Diarra on September 19, l996:
The Mars Pathfinder mission's goal is not to search for fossils. It's 
scientific mission is to learn about the rocks on Mars and therefore help 
scientists understand the geologic history of the planet.  This goal was 
chosen four years ago, when the Martian meteorite discovery (with possible
life) hadn't yet been known.  By the time that discovery helped make the
search for life on Mars a hot topic, it was too late to change the
goals for Pathfinder.

ANSWER from Mike Mellon on August 4, 1997:
The Mars Pathfinder landing site was chosen for a variety of reasons
including spacecraft safety (successfully landing) and geologic interest. 
The geologic reason was to locate a wide variety of rocks for chemical
analysis. A good place to find such a wide variety is in a flood plain where
rocks have been brought in from many locations upstream.  The Mars
Pathfinder mission was not designed to search for any fossil evidence of
life on Mars and so this was not a criteria for choosing a landing site.
Future missions will use different criteria, some focusing on the search for
fossil life.

The present-day Martian polar caps are limited to poleward of about
80 degrees latitude, covering just a few percent of the surface. 
The rest of the surface of Mars is presently permafrost (permanently
frozen ground) where liquid water cannot persist. In the past Mars
was warmer. Although how warm is still debated, liquid water once
flowed over the surface in many regions (not restricted to just the
equatorial regions). The Pathfinder landing site is one of the warmer
regions on Mars, is considered equatorial (19.3 degrees North latitude 
nearly the same latitude as the island of Hawaii), and is a location 
where liquid water was once present, if only briefly. Better locations 
for searching for fossil evidence of life would include ancient lake 
beds, extinct hot springs, and even frozen organic compounds within the  
ice-rich permafrost. 

Mike Mellon