You won't find this on a restaurant's menu, but NASA's Phoenix Mars Lander has scooped up a sample of soil from the bottom of a trench called Stone Soup. This is after the lander reached a depth about three times greater than in any trench Phoenix has excavated, during its stay of 90 Martian days. The Phoenix landed on Mars on May 25.

Phoenix's main task is to scoop up a sample of soil from the bottom of a trench called Stone Soup, which is about 18 centimeters, or 7 inches deep. On a later day, the lander's robotic arm will sprinkle soil from the sample into the third cell of the wet chemistry laboratory. This deck-mounted laboratory, part of Phoenix's Microscopy, Electrochemistry and Conductivity Analyzer (MECA), has previously used two of its four soil-testing cells.

"In the first two cells we analyzed samples from the surface and the ice interface, and the results look similar. Our objective for Cell 3 is to use it as an exploratory cell to look at something that might be different," said JPL's Michael Hecht, lead scientist for MECA. "The appeal of Stone Soup is that this deep area may collect and concentrate different kinds of materials."

Stone Soup lies on the borderline, or natural trough, between two of the low, polygon-shaped hummocks that characterize the arctic plain where Phoenix landed. The trench is toward the left, or west, end of the robotic arm's work area on the north side of the lander. Stone Soup is in the center foreground of this stereo view, which appears three dimensional when seen through red-blue glasses. The view combines left-eye and right-eye images taken by the lander's Surface Stereo Imager on Sol 88 after the day's digging. The trench is about 25 centimeters (10 inches) wide and about 18 centimeters (7 inches) deep.

When digging near a polygon center, Phoenix has hit a layer of icy soil, as hard as concrete, about 5 centimeters, or 2 inches, beneath the ground surface. In the Stone Soup trench at a polygon margin, the digging has not yet hit an icy layer like that.

"The trough between polygons is sort of a trap where things can accumulate," Hecht said. "Over a long timescale, there may even be circulation of material sinking at the margins and rising at the center."

The science team had considered two finalist sites as sources for the next sample to be delivered to the wet chemistry lab. This past weekend, Stone Soup won out. "We had a shootout between Stone Soup and white stuff in a trench called 'Upper Cupboard,'" Hecht said. "If we had been able to confirm that the white material was a salt-rich deposit, we would have analyzed that, but we were unable to confirm that with various methods."  Both candidates for the sampling location offered a chance to gain more information about salt distribution in the Phoenix work area, which could be an indicator of whether or not liquid water has been present. Salt would concentrate in places that may have been wet.

While proceeding toward delivery of a sample from Stone Soup into the wet chemistry laboratory, Phoenix is also using its Thermal and Evolved-Gas Analyzer to examine a soil sample collected last week from another trench, at a depth intermediate between the surface and the hard, icy layer.