One of the major factors that astronomers look for when trying to decipher whether a planet could host life is whether there is liquid water at the surface. We know that water is important for the evolution of life on Earth, so it makes sense that it could be important on other planets too.
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In our Solar System, Earth is the only known planet with large bodies of liquid water at the surface. Mars shows evidence of past water, as well as having ice caps at its poles that are made of water ice, and potentially water underneath its surface.
Venus and Mercury are not candidates for finding water, and the gas giant planets in the outer Solar System aren't either.
Hope of finding water in our Solar System is not lost though; some of the moons of the Jupiter and Saturn are prime candidates, maybe not for water at the surface, but below the surface.
Enceladus, a moon of Saturn, has a geyser that frequently spews out a huge plume of water, coming from below the surface. There is an icy outer shell to Enceladus, but beneath that shell there is thought to be an entire ocean, and Enceladus isn't the only one.
Europa, a moon of Jupiter is covered with a cross hatching of cracks, where the icy surface has ruptured, and water has rushed to fill the gaps. The subsurface ocean may be an environment suitable for life.
Another moon of Saturn, Titan, with its lakes and oceans of methane and ethane at the surface, potentially has a very salty ocean beneath its icy exterior.
Several new NASA missions have been proposed to explore these icy moons, and to investigate their subsurface oceans.
One is the Europa Clipper, slated for launch in 2024, an orbiter designed to explore Europa and help determine where a future probe could land.
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The goals are to study what the surface and potential ocean are made of, and understanding the features on the surface.
Another is the Dragonfly mission, with a launch planned for 2026, will fly around on Titan, landing in a number of interesting locations.
The goal is to investigate this organic-rich world, and see if there is any evidence for life on the moon, as well as understanding the chemistry and behavior of the atmosphere, surface, and potential subsurface ocean.
Even though we have to wait a few years for these missions to launch and get results, it's exciting to be exploring more of our own Solar System, and learning about the importance of water and its key to life.
So next time you're at the beach or a lake, imagine what that water would be like if it was under an icy shell.
- Eloise Birchall has a Masters of Astronomy and Astrophysics (Advanced) from the Research School of Astronomy and Astrophysics at the Australian National University.
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