THEMIS helps Phoenix land safely on mars

On Sunday, May 25, 2008, the Phoenix spacecraft (a joint project between NASA and the University of Arizona) arrived at Mars after a 10-month cruise from Earth. Traveling initially at 12,500 miles per hour (5.7 kilometers per second), Phoenix made a hair-raising, nail-biting, 6-and-a-half-minute plunge though the Martian atmosphere to a smooth landing on icy dirt not far from the edge of the Red Planet's northern polar cap.

This was territory where no spacecraft had successfully landed. On Earth, the landing site (latitude 68 degrees north, longitude 233 degrees east) would lie not far from the town of Inuvik in Canada's Arctic. On Mars, the site, dubbed "Green Valley," lay on the rolling northern plains. It features water ice at depths of a few inches or centimeters; this ice was the spacecraft's target. The goal of the Phoenix mission was to scrape up some of the icy dirt and analyze it to determine its habitability for microorganisms.

The chosen landing area appeared smooth, flat, and what's most important, relatively free of rocks and boulders. The site's suitability was established in part through surveys by ASU's Thermal Emission Imaging System (THEMIS), a visible and infrared camera on NASA's Mars Odyssey orbiter.

In addition, THEMIS also provided mission controllers with data on the dustiness of the Martian atmosphere. This helped them decide when to open the parachutes and fire the retro-rockets to guide Phoenix to a safe landing.

Seeking safe harbor

"THEMIS' main contribution to Phoenix was identifying places where it's safe to land," says Philip Christensen, of Arizona State University's Mars Space Flight Facility in the School of Earth and Space Exploration. A Regents' Professor of Geological Sciences in the College of Liberal Arts and Sciences, Christensen designed THEMIS and is the instrument's principal investigator.

The Phoenix science team chose to land within a latitude band centered on 70 degrees north. Then, says Greg Mehall, mission manager for the Mars Space Flight Facility, "We collected temperature and imaging data sets over that entire band, all around the planet, for the last several years."

Christensen says the THEMIS team studied the region's geology. "We were using the THEMIS data to get a sense of the rock abundance," he says. "Rocks stay warmer at night, so by looking at nighttime infrared temperature data we were able to see warm, rocky areas Phoenix should avoid."

Christensen continues, "We saw a lot of these warm patches, so I raised a small caution. The intended landing areas were looking a good bit rougher than the Gusev and Meridiani landing sites where the Mars Exploration Rovers Spirit and Opportunity had landed."

Then the HiRISE camera on NASA's Mars Reconnaissance Orbiter imaged one of the THEMIS warm areas. "They found these incredible boulder fields," Christensen says. "Half the ground was covered with rocks the size of a kitchen table or bigger."

He continues, "That prompted us to systematically compare HiRISE images to the THEMIS nighttime temperatures. And we found a beautiful correlation between the warm patches and the places with lots of rocks."

Why couldn't the Phoenix team simply use detailed images from HiRISE to pick a site? As Christensen explains, "HiRISE takes very detailed images, but they cover only tiny areas. So there was no hope of imaging all the candidate sites in time."

With a wider field of view, however, THEMIS had little difficulty surveying the entire area, mapping its nighttime temperatures. "What we did," says Christensen, "was take all the THEMIS data, make mosaics of night infrared images, and look for places that were the coldest."

The scientists figured the coldest places probably had the fewest rocks, and those would make good places to start hunting for possible landing sites.

"Using the THEMIS data, we picked a couple of places where you could squeeze in the Phoenix landing ellipse," notes Christensen. "We selected the landing sites, and then HiRISE subsequently imaged those sites and confirmed they indeed have no rocks."

"Together, THEMIS and HiRISE made a great team," he says.

Something in the air

Besides landing site selection, THEMIS also helped mission controllers get Phoenix down safely by monitoring the Martian atmosphere. To plan the entry, descent, and landing stages for Phoenix, mission engineers needed to know the winds, density, and temperatures of the atmosphere at all altitudes from space down to the ground.

"For months, we used THEMIS to watch intensively for dust in the atmosphere," says Mehall. "The Phoenix landing team was using a web page we created for them that had constantly updated dust-opacity maps."

A huge global dust storm arose in June 2007, and that kicked atmospheric observations into high gear, says Kim Murray, data archivist at the Mars Space Flight Facility. "THEMIS couldn't see the surface."

THEMIS was designed primarily as a geological instrument, but every image captures atmosphere as well, Murray notes. "We're taking infrared images that record both the surface and whatever is in the atmosphere above the ground," she says. "All of that is in the image."

Since THEMIS began operations in early 2002, Murray explains, scientists have built up a "clear skies" model for the surface. "This allows us to subtract the surface component from the total in an image, and that leaves the atmospheric part," she explains. "The atmospheric component then gets further analyzed to extract the contributions from dust and ice. We also determine what altitude these are at."

After 2007's big dust storm, activity died down, and the atmosphere became a lot quieter and much more transparent as Phoenix' arrival date approached. (See video.)

Says Murray, "Our main support role for the entry, descent, and landing phase was the infrared atmosphere campaign."

It's a role the Mars Space Flight Facility had played before, beginning with the Thermal Emission Spectrometer (TES). This was an infrared-sensing instrument on NASA's Mars Global Surveyor orbiter, which operated from 1996 to 2006. While TES' main mission was mapping the mineralogy of Mars, its wavelength sensitivity let scientists use it to probe the atmosphere as well.

"Our atmospheric knowledge really began with TES," says Murray. "From March to November 2006, THEMIS helped the Mars Reconnaissance Orbiter team with its aerobraking maneuver as the spacecraft first arrived at Mars. At the time, we were lucky to have both TES and THEMIS working. That helped us build up our knowledge of the atmosphere a lot."

Says Christensen, "The good news for Phoenix was that they were landing in the northern spring, which is about the clearest time of year for the atmosphere. The probe got the smooth landing we all wanted."