August 4, 2007: On August 4, 2007 the Phoenix Mars lander was launched from Earth to land on the surface of Mars. It was sent in response to the Mars Odyssey Orbiter’s discovery of the possibility of finding ice under the polar surface of Mars. Odyssey’s sensors detected a gamma ray signature from hydrogen concentrated around the ‘north’ pole during its mission. Phoenix would go there to find out.
Phoenix landed on Mars on May 25, 2008 and began soil experiments. The robotic arm from the lander scraped a shallow trench in the soil exposing a shiny substance that was later proved to be water ice. NASA announced the discovery of water ice on Mars late July 2008.
Update: On July 30, 2020, another Mars mission was launched with a more sophisticated rover called Perseverance with lots of scientific instruments on board. One of the main purposes of this mission is to determine if there is any evidence that life existed on Mars at any time in the past. Without the discovery of water in 2008, it is unlikely that the 2020 mission would have been launched. After all, water is life.