NASA has agreed to help pick a landing site on Mars for a robot craft that the Soviets will launch in 1994, American and Soviet scientists announced Thursday at a joint news conference. Samuel W. Keller, the head of the NASA delegation that has been negotiating with Soviet scientists, said that the Soviets have picked four candidate landing sites after the United States provided detailed maps of the Martian surface. Keller said the Soviets have now asked for more data and details on the candidate sites. He made the comment at a news conference marking the end of a week of negotiations between representatives of the National Aeronautics and Space Administration and a delegation representing the Soviet solar system exploration program. The two groups met to work out areas of cooperation. Academician Valeriy L. Barsukov, head of the Soviet delegation, said that the four candidate Mars landing sites are all located within 30 degrees of the equator, but he declined to be more specific. NASA accumulated a vast amount of detailed images and maps of Mars during Viking spacecraft missions between 1975 and 1980. The Viking program put two craft in orbit of Mars and two on the surface. The Soviets plan to launch a spacecraft in 1994 that will orbit Mars and drop a lander that will scoop up soil and return it to Earth. Barsukov said that in picking a landing site the Soviets are considering such issues as a search for water and life, getting comprehensive samples of the geology, and gaining insights into the Martian evolution. Keller said other agreements reached during negotiations this week included: _Using the American Deep Space Network to help track and communicate with Soviet Mars probes. _An exchange of scientific personnel for work on specific missions. _An exchange of data on Venus, a planet extensively explored by both countries. Keller said the U.S.-Soviet studies are being combined to produce four detailed maps. _An exchange of lunar material gathered by the Apollo program for meteorite material collected by the Soviets. _An exchange of scientific instruments for use on planetary probes. Barsukov said that the Soviets feel that a manned Mars landing is ``inevitable'' and that it probably will take place in the middle of the next century. But before a manned mission to Mars is attempted, Barsukov said there probably will be new manned flights to the moon. He declined to elaborate.