NASA's Phoenix Mars Lander performed another successful "dig and dump" of Martian soil with its robotic arm and is ready to begin scooping and holding onto samples for closer analysis, mission scientists said on Wednesday.

"We're doing the first interactions between the robotic arm and the surface," said Phoenix principal investigator Peter Smith of the University of Arizona. "This is getting really interesting, this is what the mission's all about."

This was the lander's second "dig and dump" attempt, which involves the robotic arm scraping up some of the Martial soil with its scoop, then maneuvering it and dumping it at another site.

Mission scientists decided to do this second dump to practice using the robotic arm after they had trouble finding the dumped pile after the initial "dig and dump" maneuver on Sunday.

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The team now feels comfortable moving the lander's 7.7-foot (2.35-meter) arm and sent Phoenix instructions to collect its first sample today.

The $420 million mission aims to dig down into the soil to the layer of water ice thought to lie beneath.

The lander's instruments are designed to test the soil's composition and see if the ice was once liquid, which could have possibly created a habitable zone for Martian life at some point in the planet's past.

Smith says that the lander's instruments are ready to start sampling soil; the first will be the Thermal and Evolved-Gas Analyzer (TEGA), which heats up samples and analyzes the vapors that come off them to detect the composition of the soil.

"The TEGA instrument has got its doors open, ready to receive a sample," Smith said.

He added that one of the doors remained only partially open, as it was on Tuesday, but that this shouldn't affect the ability of the team to deliver a sample into the opening.

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Surface soil samples for each of the three instruments aboard the lander (TEGA, a wet chemistry lab, and two types of microscopes) will be scooped up from three sites just to the right of the initial dig site.

The three sites have been dubbed Baby Bear, Mama Bear and Papa Bear.

On Thursday, mission scientists will check to make sure Phoenix got its sample before sending it instructions to deliver the sample to TEGA.

"First thing, we want to make sure that we actually have a sample in the scoop," Smith said.

Phoenix has already used one of its microscopes to analyze some soil that was blown up into it by the thrusters during landing. Results from the microscope should come in the next few days.

One aspect of the soil that the scientists are especially interested in is a layer of white material that showed up just under the surface during the initial dig.

Mission scientists don't yet know what the material is, speculating that it could be salt minerals or the underlying layer of ice, which they also think was exposed by the thrusters during landing.

During a press briefing Wednesday, televised from the University of Arizona, mission controllers also presented images that showed the digging area around the lander, a topographic map of the flat landing site (which has only a 15-inch (38-centimeter) difference between its highest and lowest elevations), and the depth of the first trench that Phoenix dug.

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