Martian Touchdown

“Phoenix has landed! Phoenix has landed! Welcome to the northern plains of Mars!”

Phoenix team members celebrate Phoenix’s successful landing on Mars.
Credit: NASA

With that declaration, Richard Kornfeld, EDL communications lead at NASA’s Jet Propulsion Laboratory, affirmed that NASA’s Phoenix lander had successfully reached the surface of Red Planet, only the sixth interplanetary vehicle ever to do so. (EDL stands for “entry, descent and landing.”)

Phoenix will be the first lander ever to explore Mars’s polar region. NASA’s Mars Polar Lander, a precursor to Phoenix, crashed when it attempted to land in the planet’s southern polar region nearly a decade ago.

Engineers at the Jet Propulsion Lab in Pasadena, Calif., received a signal from the vehicle at 4:53 pm PDT, precisely as expected. The signal was relayed by Mars Odyssey, an orbiting spacecraft, to the Goldstone, Calif., antenna station of NASA’s Deep Space Network.

Two hours later, the spacecraft sent back its first pictures: grayscale images, most of which showed various parts of the spacecraft itself, plus a few that offered a glimpse of the surrounding terrain. The first image Phoenix took was of its two solar arrays – both fully deployed. Had the solar arrays failed to open, Phoenix would have run out of power within hours, before it could conduct any science activities.

This image shows one of the two Phoenix solar arrays fully deployed. Had the solar arrays failed to open, Phoenix would have been unable to conduct scientific experiments.
Credit: NASA/JPL-Caltech/University of Arizona

Phoenix represents a departure from recent NASA landed missions to Mars in a number of ways. Unlike Pathfinder/Sojourner, Spirit and Opportunity, which all slammed into the martian surface, using airbags to cushion their impacts, Phoenix employed pulsed thrusters to slow its descent. When its three foot pads made contact with the ground, the vehicle was traveling a mere 2.4 meters per second (just over 5 miles per hour). Before Phoenix, the last spacecraft to accomplish soft landings were the two Viking landers, thirty-two years ago.

But what most distinguishes Phoenix most from previous Mars missions are its destination and the science investigations it will conduct. The Phoenix landing site is in the planet’s arctic north, in a region where orbiting spacecraft have detected water ice just inches beneath the surface. Previous landers have all explored Mars’s equatorial region.

“The arctic region is really sensitive to climate change on a planet,” said Peter Smith, Phoenix principal investigator, of the University of Arizona. Mars’s polar regions undergo dramatic climate change, alternately warming and cooling, because of changes in the planet’s orbit that occur in cycles lasting tens of thousands of years. It’s possible that during a recent warming phase, water that is now locked up in frozen, buried ice temporarily became liquid. If it did, it would have left behind telltale chemical signs that Phoenix will be able to detect.

Phoenix will not be looking for liquid water per se, Smith said, “because we think it’s too cold now. We’re looking at the changes in the soil caused by the liquid,” changes that may have occurred during a previous warm period in the martian northern plains.

An approximate-color, close-up image of the ground near the Phoenix landing site shows the characteristic patterned polygonal terrain that scientists expected to find there.
Credit: NASA/JPL-Caltech/University of Arizona

Phoenix will also look for organic molecules in the soil and ice it scoops up with its robotic arm. The Viking landers looked for organics, too – again, they were the last missions to do so – but they failed to find any.

“We think organics must exist at Mars,” Smith said, “because they’re brought in by asteroids and comets that have clearly impacted the planet over its history. And so what happened to those organics, and are they preserved in this region?” If there is evidence of organics and liquid water, he adds, “this is what we call a habitable zone on Mars, and therefore very worthy of further exploration.”

Evidence of liquid water in Mars’s past has been discovered by the two Mars Exploration Rovers, Spirit and Opportunity. But, Smith says, if Phoenix discovers signs of liquid water, it will be a more compelling find. “This would be very recent water, the last few hundred thousand years, last million years. The salts that you see down on these dry lakebeds [where Spirit and Opportunity have been exploring] might be 4 billion years old, so that’s 4 thousand million years; we’re looking at the last 1 or 2 million years.”

Ice “also preserves the history of life,” Smith said. No-one is suggesting that there is life in the martian arctic – not yet, at least. And the scientific instruments onboard the spacecraft are not capable of detecting martian life, even if living organisms are present in the samples it analyzes. But, Smith points out, “This polar region, both north and south, is 25 percent of the area of the planet; it’s a big, big unexplored area.” If Phoenix were to discover that these polar regions have been habitable in Mars’s recent past, “why would you not go back?”

Related Web Sites

Rendezvous with Mars
Mars Cold Goes Down Deep
Paving the Road to Mars
Piecing Together Life’s Potential