SaddleBrooke Skygazers – December 2014

John Lauder

Where could we find water? Those of us who live in the desert certainly know water is important and future space travelers will need water. In our solar system, where could we find water?

The Goldilocks Zone is the region around our star where the surface temperature of a planet might allow liquid water. Venus, at the inside edge of this zone, has a thick carbon dioxide atmosphere which traps the sun’s heat. It became too hot before oceans could form and absorb the CO2. Venus got too hot before it got wet, so no water there.

Mars is small, about half the size of Earth. Its weaker gravity couldn’t hang on to its atmosphere. After the air leaked into space, the surface water had evaporated and leaked into space with the air. But, below the surface of Mars there could still be significant amounts of liquid water.

There are several moons that should possess underground water. Europa, orbiting Jupiter, has a surface of ice, probably tens of miles thick. Ridges on the surface imply that liquid water from below occasionally spills out onto the surface of the moon. The Hubble telescope captured plumes of water spouting from the surface of Europa. An ocean of water up to 100 miles thick probably lies below the icy surface. That would be more water than in all of Earth’s oceans combined. Based on gravity surveys, astronomers believe that two other moons of Jupiter, Ganymede and Callisto, may also hide underground oceans.

Enceladus, a small moon of Saturn, surprised astronomers when the Cassini spacecraft studying Saturn flew through a plume of salty water. Subsequent observations showed it came from the south polar region of the moon. Gravity studies eventually proved that there is an underground pool of water at the moon’s south pole equivalent to the volume of Lake Superior. Saturn’s largest moon, Titan, has a surface covered with liquid methane lakes under a thick nitrogen atmosphere. Gravitational studies again indicate an ocean of liquid water below the organic-laden crust.

Now for the strangest case! Mercury, the planet closest to the sun, has an average daytime temperature of 850 degrees on the Sun side of the planet. But with no air or water to carry the day’s heat to the night side, nighttime temperatures plummet to near 200 degrees below zero. At the poles of Mercury there are craters so deep, sunlight never touches their floors. Water vapor from colliding comets settles there and freezes, never to melt again. A planet with a daytime temperature of 850 degrees may have ice at its north and south poles!

Check out Rosetta’s successful landing on comet 67P/Churyumov-Gerasimenk!