From Science:
The countdown was excruciating: 7 minutes to landing–or annihilation. “Altitude 2000 meters.” Falling at more than 200 kilometers per hour with its parachute open, the $420 million Phoenix lander plummeted toward the martian surface. “One thousand meters. Lander separation detected.” One hurdle cleared, but Phoenix’s ill-fated predecessor, Mars Polar Lander (MPL), had passed that one too. “Five hundred meters, 400, 250, … 80, … 40.” Thrusters now blazing, Phoenix was slowing, but MPL had messed up at just this point, prematurely cutting off its thrusters while still 40 meters up, obliterating itself on red terrain. “Thirty meters, 27, 20, 16, … touchdown signal detected.” Cheers and applause erupted at mission control. “The Phoenix has landed! The Phoenix has landed!”
“Our 7 minutes of terror is going to be followed by 3 months of joy,” Phoenix project manager Barry Goldstein of NASA’s Jet Propulsion Laboratory in Pasadena, California, said at a press briefing yesterday, the day after the landing.
Although the landing was gratifying, it wasn’t quite a perfect 10. Phoenix nearly overshot its targeted landing ellipse, coming down on the edge of the 60-kilometer-long target zone near its far end. That was because, for reasons yet to be determined, its parachute detached 7 seconds later than planned. But the craft still found exactly what scientists had spent years looking for: a parcel of land that is as flat as a tabletop, a rock-littered vista with only a handful of mission-ending boulders in sight, and a crinkling of the surface at the landing site that speaks of the much-sought-after ice just beneath the surface.
The one real surprise in the early hours of the mission came from the icy crinkling, says Phoenix team member Raymond Arvidson of Washington University in St. Louis, Missouri. From orbit, the surface of the northern polar landing area appears to have a crazy-quilt patterning. Seasonal temperature cycling creates this “polygonal” design through the expansion and contraction of unseen ice just below the soil surface. Given the average polygon size of 5 meters seen from orbit, researchers had inferred a depth to the ice of about 5 centimeters. But from the landed Phoenix, smaller polygons are evident as well, perhaps 2 to 3 meters in size. That means that the area could be colder–or the ice dirtier or shallower–than expected, says Arvidson.
More here.