The engineers designing Pathfinder determined that the best latitude for a July 4, 1997 landing will be 15 degrees North, plus or minus about 5 degrees. The landing will occur during Northern Hemisphere Summer. The sun will be up the longest at 15 degrees North during the early part of July 1997. Likewise, Earth will be best seen from around 25 degrees North.
The engineers also determined that they could predict Pathfinder's landing spot within an elliptical area about 100 km wide by 200 km long along an axis that trends N 74 E. Thus, the Mars scientific community was asked to come up with a landing site that occurs near 15 degrees North and can have as much as 150 km uncertainty in the exact landing spot. Because of the difficulty of landing in the thin Martian atmosphere, scientists were asked to find a landing site below 0 km elevation. This would provide a thicker atmosphere through which to descend.
The Project Scientist for Pathfinder, Dr. Matthew Golombek of the Jet Propulsion Laboratory, convened a workshop at the Lunar and Planetary Institute in Houston, Texas, on April 18-19, 1994, so that members of the scientific community could come and suggest landing sites. In mid-June, 1994, the Pathfinder scientist teams met and narrowed the list of possible landing sites down to four key locations. They took a vote and finally listed these sites in order of preference.
The #1 choice and likely Pathfinder landing site is located at the junction of two major outflow channels, Ares and Tiu. The site chosen is at 19.5 degrees North latitude and 32.8 degrees West longitude, at an elevation of -2 km below the datum. This site is at the same latitude and about 2 degrees longitude east of the original Viking 1 site that was eliminated in June 1976, as being to hazardous. Pathfinder is expected to be able to land in terrain that is more rocky than at the Viking sites. The Ares/Tiu site was chosen mainly because it is likely to offer an abundance of rocks brought down by the channels. Sampling the chemistry of these rocks using the APXS on the microrover is expected to provide considerable new information about martian crustal composition.
The #2 site may be selected if new Earth-based radar observations planned for December 1994 - February 1995, determine that the #1 site is much too dangerous for a landing. The #2 site is located south of crater Trouvelot at about 10 to 17 degrees North, 11 to 20 degrees West, 0 km elevation. This site would allow sampling of cratered highlands rocks plus a glimpse of dark sandy material that was not observed at either Viking site and might not be present at the Ares/Tiu site. Finally, should both the #1 and #2 sites prove dangerous as a result of further investigation through February 1995, the following two sites are also on the list: #3= Maja Vallis Delta (18.8 degrees North, 52 degrees West, -0.5 km elevation), and #4= Maja Vallis Highlands area (13.5 degrees North, 53 degrees West, 0 km elevation).