How did the Phoenix lander find water ice beneath the Martian surface?

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Mars An especially serene view of Mars (Tharsis side), a composite of images taken by the Mars Global Surveyor spacecraft in April 1999. The northern polar cap and encircling dark dune field of Vastitas Borealis are visible at the top of the globe. White water-ice clouds surround the most prominent volcanic peaks, including Olympus Mons near the western limb, Alba Patera to its northeast, and the line of Tharsis volcanoes to the southeast. East of the Tharsis rise can be seen the enormous near-equatorial gash that marks the canyon system Valles Marineris.

The Phoenix lander confirmed the presence of water ice beneath the Martian surface through direct physical excavation. Using its 2.35-meter robotic arm, the probe dug a trench into the soil of the north polar region of Mars.

Upon digging, the lander uncovered a white material that sublimed—transitioning directly from a solid to a gas—which provided definitive evidence that the substance was water ice. This discovery was further supported by the identification of minerals, such as calcium carbonate, which typically form in the presence of water.1