Water is essential for life as we know it, so astronomers are very interested in finding planets beyond Earth that contain water. Besides Earth, there are several places in our solar system that are now known to have water in some form.
Mars
Mars is the planet most similar to Earth in our solar system. Observations by robotic spacecraft orbiting Mars have detected water in several forms:
- Water ice at the poles – Mars has ice caps at its north and south poles composed of frozen water and carbon dioxide. The northern ice cap contains about 1.2 million cubic km of ice.
- Underground ice – Radar measurements from orbit show large amounts of ice mixed in with soil in the ground at high latitudes. There may be up to 5 million cubic km of water ice underground on Mars.
- Frost – The surface of Mars gets cold enough for frost to form at night in some areas.
- Clouds – Mars has clouds made of water ice that form and dissipate each day.
- Traces of liquid water – Some features on Mars suggest small amounts of salty liquid water may flow on the surface today during warm seasons.
There is strong evidence that in the ancient past, Mars had much more water flowing across its surface in rivers, lakes and oceans. But most of that water is now gone. The cold and thin atmosphere means liquid water cannot last on the surface for very long. However, underground reservoirs of liquid water may still exist deep below the surface.
Europa
Europa is one of Jupiter’s largest moons. Observations by the Galileo spacecraft in the 1990s provided very strong evidence that Europa has a global ocean of liquid water beneath an icy crust. The ocean is estimated to be about 100 km deep and contain more than twice as much water as Earth’s oceans!
Europa’s ocean stays liquid because of tidal heating – the gravitational pull of Jupiter and the other big moons keeps Europa flexed and heated. The water stays entirely below the icy surface but in some places the ice may be thin enough to melt through, creating plumes of ejected water vapor detected by the Hubble Space Telescope in 2013.
Enceladus
Saturn’s moon Enceladus also has a subsurface ocean of liquid water. Plumes of water vapor and ice erupt from the south polar region suggest the ocean is likely in contact with the moon’s rocky core.
Cassini spacecraft flew through these plumes and detected salty water with traces of minerals, ammonia, carbon dioxide and methane. This indicates hydrothermal vents on the ocean floor, like those found deep in Earth’s oceans where chemosynthetic life survives.
Ceres
Ceres is the largest object in the asteroid belt between Mars and Jupiter. Observations from the Dawn spacecraft detected signs that Ceres may have significant amounts of water ice just below its surface, and possibly an underground ocean.
Bright spots on Ceres are deposits of salts like magnesium sulfate, which would be left behind by evaporating salty water. In some areas, water ice appears to be upwelling to the surface and sublimating into vapor.
Other Icy Moons
Several other icy moons also show hints of subsurface oceans, including Jupiter’s moons Ganymede and Callisto and Saturn’s moons Titan and Mimas. However, Europa and Enceladus are the clearest cases so far.
Conclusion
Besides Earth, Mars and a number of icy moons in our outer solar system appear to have or have had significant amounts of liquid water. This makes them extremely interesting places to search for evidence of life beyond Earth. Future spacecraft missions will continue to study these worlds in detail looking for signs of life in these watery environments.