What's more, it could settle the debate over whether Mars was once suitable for life.
As Phoenix closed in on the last miles of its journey, NASA scientists were gearing up for the "seven minutes of terror" that could make or break the U.S. $420-million mission. (Video: animation of the lander's expected turbulent touchdown.)
"Approximately 14 minutes before touchdown, the vehicle separates from its cruise stage," Barry Goldstein, Phoenix project manager at the Jet Propulsion Laboratory in California, said at a recent press conference.
"At this point we lose communication from the vehicle."
Once the craft reaches Mars's atmosphere, the next critical seven minutes make up what's known as the Entry, Descent, and Landing (EDL) phase.
Screaming down at about 12,600 miles (20,270 kilometers) an hour, the craft must open a parachute to slow itself for a three-minute glide to the surface about 70 miles (113 kilometers) below.
The craft's landing sequence then includes steps such as jettisoning its heat shield, extending its legs, and firing its landing thrusters.
"There are 26 pyrotechnic events, and each of those have to work perfectly for this to go as planned," Goldstein said. "Getting EDL communication [at touchdown]—that'll be the three seconds that I am really biting my nails over."
Risen From the Ashes
The tension for this mission seems especially intense, since Phoenix is not the first craft to attempt a landing at a Martian pole.
In 1999 NASA lost communication with the Mars Polar Lander as it entered the atmosphere above the planet's south pole. That lander's fate remains a mystery, but its hardware designs will be given a second chance—Phoenix is based on much of the lost craft's systems.
"We spent 15 years developing the hardware, and I really wanted some return from those," said Peter Smith, Phoenix principal investigator at the University of Arizona in Tucson, who first proposed "recycling" technology from the failed 1999 mission.
Engineers have put the so-called heritage hardware through a battery of tests, and NASA scientists say they have fixed all the known issues.
Teams using a variety of data also put serious thought into where exactly to set the lander down.
"Finding a place to land that was scientifically interesting and safe … has been a multiyear process," noted Ray Arvidson, chair of the Phoenix landing site working group at Washington University in St. Louis, Missouri.
The site, informally dubbed Green Valley, sits in a region of permafrost on Mars's northern plains that is analogous to northern Canada, the University of Arizona's Smith said.
The relatively shallow valley, which contains some of the highest concentrations of ice outside of the polar cap, is about 700 feet (213 meters) deep and stretches for 40 miles (64 kilometers).
A crater near the valley means that an impact pushed away most large rocks and spread out a soft cushion of fine particles 5 to 10 inches (13 to 25 centimeters) deep on top of the hard icy soil.
But "this is no trip to grandma's for the weekend," warned Ed Weiler, NASA's associate administrator for the Science Mission Directorate.
"Mars has been known to cause trouble, and I'll be worried until I hear the signal a few seconds after landing."
Search for Life
Still, NASA team members said that the scientific payoffs of a polar mission will be well worth the risks.
As opposed to the Mars rovers that have been exploring the red planet's geologic history, Phoenix will be taking samples that should reveal active processes.
Mars's northern ice cap expands and contracts with the seasons, which should allow scientists to analyze how water impacts the planet's soil chemistry.
And "we'll look at the properties of ice frozen into the surface with water vapor in the atmosphere to see if there's a communication there," Smith said.
But the biggest goal will be to look for signatures that Mars might once have been habitable. (Get full coverage of the search for water—and life—on Mars.)
"We're really doing a full geological and chemical experiment on the surface with the idea of finding if this is a habitable zone," Smith added.
The polar region offers the best hope, he said, because just like the refrigerator in a kitchen, polar ice may "preserve organic material and the history of life on this planet."
The consequence, however, is that the craft is not expected to last beyond the stated lifetime of the mission.
Unlike the Mars rovers that have roamed Mars's equatorial zone since 2004, Phoenix is touching down in a region that within months will be too cold and dark for the craft to maintain power supplies.
"Living in Hawaii would be wonderful, but we live north of the Arctic Circle," JPL's Goldstein said.
"In January [at the start of Martian winter] we'll go three to four months without any solar energy. At that point it's extraordinarily unlikely the craft will survive."