By Irene Klotz

PASADENA, Calif., May 22 (Reuters) - A new chapter in Mars exploration opens on Sunday when a small robotic probe jets down to the planet's arctic circle to learn if ice beneath its surface ever had the right chemistry to support life, mission managers said on Thursday.

NASA approved the mission, known as Phoenix, after the Mars orbiter Odyssey found ice surrounding the polar caps in 2002. Five probes landed near Mars' equatorial zones, including the rovers Spirit and Opportunity, which discovered signs of past surface water. Odyssey found no sign of buried ice around Mars' equator.

"We're going way to the north," said Peter Smith, a planetary geologist at the University of Arizona in Tucson who heads the Phoenix science team.

On Earth, the arctic regions hold the history of the planet's climate changes, which are locked layer by layer into the ice core.

"This is where the history of life is preserved in its purest form -- organic molecules and cellular bacterial microbes and so forth," Smith said.

"We're wondering if this is true on Mars," he said.

Phoenix is not going to search for life directly, but it should be able to determine if the Martian ice was ever liquid. Liquid water is believed to be an essential ingredient for life to exist.

Among Phoenix's science instruments are small ovens to vaporize and chemically analyze the Martian ice, revealing, some of the processes the molecules underwent before reaching their present condition. Other sensors will study minerals in the soil and ice and image the shape and structure of individual grains in the soil.

"We're really trying to understand if the ice has ever melted, because it's liquid water that is required for a habitable zone," Smith said.

"We'll leave future missions the task of figuring out who's living there," he added.

The U.S. space agency faces a formidable obstacle before its new round of Mars studies can begin.

Phoenix has to land in a process that requires it to slow itself from 12,000 mph (19,000 kph) to zero in seven minutes.

"This will be a very nail-biting time for us," said Fuk Li, the Mars Exploration program manager at the Jet Propulsion Laboratory in Pasadena, which oversees Phoenix.

From 170 million miles (274 million km) away, flight controllers will only be able to watch and wait to learn Phoenix's fate. Radio signals from Mars traveling at the speed of light take 15 minutes to reach Earth, so by the time flight controllers learn that the probe has begun its descent it already should have landed.

Finally, Phoenix has to unfurl its solar power panels to begin collecting energy from the Sun. Otherwise its batteries will last just 31 hours.

"All of these events have to occur exactly as planned," said project manager Barry Goldstein. "The team is very confident in that we've done everything we can." (Editing by Tom Brown and Vicki Allen)

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