The sights one can see in space! I wasn’t quite sure I believed this scene when I first saw it yesterday. From the Cassini space probe, the moon Titan is imaged across the rings of Saturn from just above the dark horizon of Enceladus.
At the Planetary Society blog, Emily Lakdawalla has a time lapse piece of three polar water fountains at Enceladus.
A note: this isn’t what the unaided human eye would see. Cassini is equipped with imaging devices that can closely approximate human vision–and more. This technology also approximates what a decent backyard telescope would show you–if you were flying with Cassini. In actual viewing from Enceladus, Titan would appear as a small orange disk, usually smaller than our moon appears from Earth.
Cassini is there not only to snap pretty pictures–which is does quite well. It detects energy at certain wavelengths that also permit scientists to analyze the chemistry of Saturn and its moons. Astronomy once famously partnered physics, but now adds chemistry as well as geology as companions. A scientific polygamy, but a very moral one, to be sure.
Tuesday’s 270-mile swoop past Enceladus was designed to view the fountains backlit from the sun and detect nitrogen outgassing. Why is that important? Nitrogen is one of the two components of ammonia. When ammonia is dissolved in water, the freezing p0int lowers, and if Enceladus’s internal water has a quantity of ammonia, that would give us a temperature range for the interior we can’t detect directly. When ammonia is heated, the molecular bonds break and gaseous nitrogen is released. Determining how much nitrogen is sputtering from the moon’s south pole will further refine temperature estimates.
Christina Rosetti wrote of a bleak midwinter on Earth:
Earth stood hard as iron, water like a stone.
Her description fits the Saturnian system. On the surface, Enceladus is a bitter 320 degrees below zero, and water is in a permanent solid state, like the stony crust of Earth. But somewhere inside, there’s enough heat to melt ice–just like the Earth’s molten core. Before Cassini, it wasn’t thought that small bodies like Britain-sized Enceladus would be able to maintain a liquid interior. Moons should cool off–like our moon and even the planet Mars. Europa’s subsurface ocean was enough of a surprise, and that one is largely explained by tidal heating from nearby large moons. How can Mars be dead and distant outer solar system moons be geologically alive? Nobody saw that one coming.
Enceladus is much smaller than Europa. And the nearby moons of Saturn in no way rival the size of Io and Ganymede at Jupiter. We will probably have to wait for a probe to plunge directly into these geysers to determine exactly how and why the ice within melts. I’d love to be alive to see the internet feeds on that day. In the meantime, scientists look for whatever clues they can discern to view past the dark horizons of the unknown.