As NASA prepares its Mars Phoenix spacecraft for an August launch, program officials are still trying to find a suitable landing site.

Using data from two spacecraft – Mars Odyssey and the Mars Reconnaissance Orbiter -- already orbiting the red planet, mission managers are searching for a landing zone near the northern polar region where there is a permanent ice cap.

Finding the right spot has not been easy. One favored area, for example, has already been vetoed because too many large boulders with spacecraft-eating potential were spotted there

The $386 million Phoenix lander mission will be the first to fly under the rubric of NASA’s “Scout” program. Once safely down on martian real estate, the lander is to use a sturdy arm to dig for subsurface water ice. The lander is outfitted with a set of instruments designed to help scientists glean clues to climate change on Mars, anjd search for potential habitats that might support life.

NASA’s Mars Odyssey has been orbiting the red planet since October 2001. One of its primary instruments is the Thermal Emission Imaging System – called Themis for short. This camera operates in the visible and infrared parts of the spectrum and has been busy determining the distribution of minerals on the surface of Mars.

A particularly cool feature of Themis is that in the martian night it maps heat radiating from rocks on the planet’s surface. And when Themis focused in on Region B – the top candidate for the Phoenix landing zone – it was an eye-opener.

“Region B was just speckled with little hot spots that we now know are rock and boulder fields,” said Philip Christensen at Arizona State University in Tempe, a leading Mars researcher and Principal Investigator for Odyssey’s Themis instrument.

“There are more big rocks than we’ve seen at any other landing site,” Christensen said. “I think there were a lot of puzzled heads wondering, wow, where did all these big rocks come from?”

In the quest to find a safe haven for Phoenix, Odyssey has been one busy bird, Christensen said.

“Half of the Themis bits coming from the spacecraft are devoted to the site survey for Phoenix. We are making a significant number of thermal maps of the candidate landing site areas,” Christensen said.

Christensen added that the collaboration – an imaging campaign — between Themis’ thermal data and the super-powerful Mars Reconnaissance Orbiter’s (MRO) High Resolution Science Imaging Experiment (HiRISE) has been going extremely well.

“We are using Themis data very successfully to narrow down the choice of sites, as HiRISE focuses on the areas where Themis data show the ground is largely rock-free,” Christensen noted.

It was the zoom lens of MRO’s HiRISE that aced out Region B as the Phoenix landing locale. Numbers of big rocks became a big issue with receipt of the first HiRISE shots late last year.

“Everything was looking good until [they received the] HiRISE images. They just blew us away. Mars just did it to us again,” said Ray Arvidson, a Phoenix science team member from Washington University in St. Louis, Missouri.

Why there’s an abundance of rocks in that zone is a brainteaser, Arvidson noted. Perhaps thick deposits of windblown dust are underneath the rocks, causing them to rise up on top of this dust mantle. Or climate changes have expanded and contracted the icy landscape, forcing large rocks topside, he suggested.

“Either way…we just didn’t expect Mars to have kept its rocks on the surface,” Arvidson said. “From a safety point of view, what counts is the number of rocks…and a lot of the rocks were bigger than the lander.”

The task now, Arvidson observed, is interpolating between the ground truth as revealed by HiRISE and the pre-dawn Themis data.

And the nice news is that three prospective Phoenix landing areas have been pinpointed.

Anxious to identify a happy address on Mars for Phoenix is Peter Smith, Phoenix Principal Investigator at the University of Arizona's Lunar and Planetary Laboratory in Tucson.

Smith said that Odyssey’s Themis nighttime infrared is delineating the locations of the hot rocks. Meanwhile, HiRISE images quantify local rock concentrations.

This doubling up of spacecraft has yielded a powerful combination of information to provide both coverage and high resolution of the “latitude girdle” between 65 and 72 north in which Phoenix is to land, Smith said.

“After finding three potential landing sites, we are quite hopeful that the scary, boulder-strewn surfaces first seen last October can be avoided completely,” Smith pointed out. “Data that will verify this claim are still coming down from Mars…but our next landing site meeting on January 22 will summarize the search and lead to the identification of our best sites.”

Alfred McEwen, principal investigator for the HiRISE instrument and Director of the University of Arizona's Planetary Image Research Lab, said HiRISE images acquired over the past few weeks — concentrated over the new candidate landing ellipses — have shown far fewer boulders than images of the previous candidates.

“So it’s looking good,” McEwen concluded.

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