Phoenix digs five centimetres and touches ice
Phoenix landed on rockets at 68 degrees north and scraped down to white material. It vanished in four sols — ice, not salt — and in September the lidar watched snow fall.
On 25 May 2008 Phoenix hit the Martian atmosphere at about 5.6 kilometres per second. A heat shield took most of that speed, a parachute opened around 13 kilometres up, the lander cut loose from the backshell at roughly 900 metres, and twelve pulsed hydrazine thrusters flew it down, the first fully propulsive Mars landing since Viking 2 in 1976. Mars Reconnaissance Orbiter photographed the descent under parachute, the first time one spacecraft imaged another landing on another planet.
It touched down at 68.2 degrees north on the cracked polygonal ground of Vastitas Borealis, where Odyssey's neutron maps put the ice. It was the Mars Surveyor 2001 lander, cancelled after the 1999 failures, mothballed until 2003, then rebuilt and flown for about $420 million after Peter Smith's team at the University of Arizona won the first Mars Scout competition.
On 15 June the arm scraped a trench called Dodo-Goldilocks down about five centimetres, exposing bright, dice-sized crumbs. Four sols later they had gone: salt would have stayed, water ice sublimates. On 30 July an oven in the Thermal and Evolved-Gas Analyzer heated soil past 0 °C and the mass spectrometer saw water vapour. "We have water," said William Boynton, who led the instrument — "the first time Martian water has been touched and tasted."
The wet chemistry cells found perchlorate at a few tenths of a percent by weight: a strong oxidiser, it pushes water's freezing point toward minus 70 °C and wrecks organic molecules when heated — which reopened whether Viking's 1976 experiments had incinerated the organics it hunted for. The soil was mildly alkaline, around pH 7.7, and held calcium carbonate. In September the lidar watched snow falling out of water-ice clouds about four kilometres overhead, evaporating before reaching the ground.
Power fell with the arctic sun; the last transmission came on 2 November 2008, against a 90-sol design life. Images in 2010 showed the panels collapsed under a winter of carbon dioxide frost.