The largest, shining object in Earth's starlit night sky, our Moon has long been the source of mystery, myth, and poetry--a captivating inspiration for those who stare at the sky in wonder. But Earth's Moon is a very real object--the only body beyond our own planet that we have actually set foot upon, leaving our lingering footprints in its distant, alien dust. Earth's Moon has been with us almost from the very beginning, and even though it is our planet's closest companion in space, it has nonetheless managed to keep some of its ancient secrets very well. In September 2015, astronomers released a new study that shed light on Earth's bewitching companion's ancient and secretive past. Although our Moon now appears to be unchanging, as if it has always been exactly the same as it appears now--going through its beautiful and, yet, familiar phases, and controlling our ocean tides--in the distant past, things were actually very different.
Ganymede: Ganymede is both the largest moon of Jupiter, our Solar System's planetary behemoth, as well as the largest moon in our entire Solar system. Observations of Ganymede by the HST in 2015 suggested the existence of a subsurface saline ocean. This is because patterns in auroral belts and rocking of the magnetic field hinted at the presence of an ocean. It is estimated to be approximately 100 kilometers deep with a surface situated below a crust of 150 kilometers.
Titan: Titan, the tormented, hydrocarbon-slashed largest moon of Saturn--and the second largest moon in our Solar System, after Ganymede--could possess a subsurface, salty ocean that may well be as salty as the Dead Sea on Earth. The salty water could begin approximately 31 to 62 miles beneath Titan's icy shell, according to recent estimates. Meanwhile, on Titan's smog enshrouded surface, "life as we do not know it" could swim in alien lakes and rivers that flow with liquid methane and ethane hydrocarbons--instead of water.