Water Ice on Mars and Europa

Subsurface water ice on Mars, confirmed by Phoenix in 2008 at depths of just 5 centimeters, and Europa's global ocean—evidenced by Galileo's magnetic field data 20% stronger than expected—dramatically expanded the potential habitats for extraterrestrial life. Together, they suggest that liquid water exists on at least two other worlds in our solar system, with Europa's ocean estimated at 2 to 3 times the volume of Earth's oceans. This discovery is cheaper than the typical remote sensing mission yet yielded a 50% increase in known habitable zones within the solar system, outpacing #9's gravitational lensing in immediate implications for astrobiology.