NASA's Cassini spacecraft captured this image of sunlight
reflecting off of a lake on the surface of Saturn's moon Titan.
Credit: NASA/Cassini

The evidence for the existence of lakes on Titan has been building since 2004, when the Cassini spacecraft, and its Titan-landing probe Huygens, arrived in the Saturn system and began collecting data.

First it was imagery from the Huygens camera as it descended through Titan's thick, hazy atmosphere: what looked like dark, flat, featureless regions defining apparent coastlines along solid terrain, as well as dentritic patterns like river channels draining into them.

Then it was imagery from Cassini's smog-penetrating infrared cameras showing numerous systems that looked for all the moon like giant lakes—on the order of size of Earth's Great Lakes. Cassini radar bouncing off the surfaces suggested that they were exceptionally flat, as one expects a watery surface to be.

Then there was confirmation of surface liquid in Titan's Southern Hemisphere—but not in the north, where the lake-like shapes were by far more numerous…an apparent land o' lakes.

Finally, what scientists hoped to see appeared, in a flash: sunlight, reflecting off of a Titanian lake called Kraken Mare, came shining through the moon's haze and was captured by Cassini's infrared camera. It's the same kind of reflection you see when sunlight blazes off the surface of the ocean before sunset. Astronauts see the same thing from Earth orbit, looking down at the ocean-lined limb of the Earth when the Sun is at the right position over it.

It required the right conditions; the Sun had to be in just the right position relative to the lake for the reflection to be seen. Ever seen the light of the setting Sun reflecting off the window of a distant house, though only for a moment when the geometry is just right? And only recently did the Sun rise over Kraken Mare's extreme northern latitude, after the 15-year dark of a Titanian arctic winter.

Titan is the only Solar System object known to have surface liquid, other than Earth. Jupiter's moon Europa is thought to harbor a vast ocean of liquid water, but under its outer crust of ice. Another of Saturn's moon's, Enceladus, also appears to hold liquid water inside, but its surface is as dry as the Earth's Moon. And yes, the Moon was recently confirmed to have surface water—but it's all frozen and mixed in with the lunar soil in a sort of dry cryo-mud.

The difference with Titan, however, is that its surface liquid is not water at all, but methane. It's too cold on Titan's surface for liquid water to exist there—but Titan's atmospheric pressure and temperature are right for methane to exist in its liquid state. So while on Earth we know methane as a greenhouse gas emitted by decomposing plants and the guts of cows—to name a couple of sources—on Titan it is the stuff of cloud, of rain, of river, and of lake.

What a wild world!

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Bumps and ripples in the otherwise flat ring system of Saturn cast long shadows at equinox. Image credit: NASA/Cassini

Hard to imagine what it would be like to float just above the rings of Saturn, but what a sight it must be! As a kid, one of my favorite astronomical pass-times was imagining the view from other places in the Solar System.

Now imagine a towering bulge of frosty mist rising up out of this super-flat plane of ice chunks, literally the size of a mountain. Such is what was beheld by NASA's Cassini spacecraft last month–albeit, from a distance–when it turned its cameras to Saturn's vast rings during the few days surrounding Saturn's equinox (August 29, 2009), giving us a view never before seen.

Equinox on Earth, when the Sun is positioned directly over our equator, happens twice a year. Due to Earth's tilted rotational axis, as we orbit the Sun the latitude over which the Sun shines directly cycles north and south between the latitudes of the Tropics. On its way north to warm our (Northern Hemisphere) summers or south to leave us in the chill, the Sun crosses the equator on the equinoxes (Fall and Spring).

The same thing happens on Saturn, with two differences. First, Saturn takes nearly 30 years to orbit the Sun, so equinox comes only about every 14 years. Second, Saturn has its system of rings that encircle the planet directly above its equator, serving as a visible extension of the equator. At Saturn's equinox, the Sun is not only directly over the equator, but sunlight strikes the rings edge-on, like a flashlight shining on a flat piece of paper from the edge, the light just grazing over the surfaces on either side.

When this happens, any deviations from the flatness of the ring system—bumps and ripples–cast long shadows across the rings, making the features much easier to see. The same thing is seen on that piece of paper with shadows from creases and bumps leaping across the page.

As seen from Earth, equinox on Saturn means the rings appear to vanish as we look at them edge-on. This behavior puzzled astronomers long ago before they understood the rings for what they are. During the August 2009 Saturn equinox, however, for the first time in history we had a bird's-eye view of the rings during equinox, from Cassini. Cassini has been in orbit around Saturn for five years now.

Cassini spotted a number of prominent shadows trailing bright spots and ridges—bumps and ripples of different sorts rising above the ring plane.

Some of the bumps–icy ring material kicked up by the gravitational disturbance of a small moonlet inside the rings–were measured at over two miles high, the height of the Rocky Mountains. Other rippling features, such as long ridges running along the direction the rings encircle Saturn, are waves created by the gravity of moons orbiting outside the ring system. Still other types of disturbances observed are possibly caused by the impact of meteoroids or chunks of ice with the rings.

Saturn's rings are tens of thousands of miles across, but are extremely thin—perhaps no thicker than the height of a four-story building! So a bump or ripple as high as a mountain is a big deal!

Ah, to be on Saturn, now that equinox is here…

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Artist concept of a geyser erupting on Enceladus.
Credit: David Seal.

A layer of the veil around Saturn’s moons was removed when Pioneer 11 and Voyagers 1 and 2 made flybys of Saturn in the '70s and '80s. The Saturnian moons, it appeared, were not the lumps of rock and dust that Earth's own Moon is made of, but objects containing no small amount of water ice. Not terribly surprising, considering the low temperatures of the outer solar system where ice-rich comets roam.

Visions of frozen alien landscapes, replete with icicles and ice cliffs and ice fields and ice ice ice! were conjured in my imagination, and in artist depictions of majestic ringed Saturn seen from moons like Rhea or Dione or Enceladus.

Four years ago, Saturn’s first permanent visitor from Earth–the Cassini spacecraft–arrived there, and since has been making extreme closeup examinations of Saturn, its rings, and its increasingly wondrous and beautiful moons. Cassini is almost literally ripping apart veil after veil of our ignorance of these little worlds.

Far from a contingent of enormous but simple snow cone balls, Cassini has shown us that some of Saturn’s moons are apparently alive with liquid motion. First, there were the surface “lakes” and “seas” on Titan, probably made of extremely cold liquid hydrocarbons like methane and ethane–the stuff that spouts out of the gas range in your kitchen. Lakes and seas and rolling waves of liquid natural gas are fine and dandy for an imagined shoreline scene–but take a dip in those "waters" and an actual water-based creature like you would freeze solid in seconds. Scenic, but not inviting for a swim…

But recent observations by Cassini have shown that Titan's frigid unearthly lakes and Enceladus' snowball exterior may just be additional veils that are now being lifted.

In March, Cassini flew within 30 miles of the surface of Enceladus and right through a plume of material venting into space from the moon’s interior—an enormous "geyser." Earlier observations had sensed the presence of water in the plume, giving rise to speculation that liquid water in some form might exist beneath Enceladus' surface—perhaps chambers of liquid heated by tidal stressing of the interior.

When Cassini flew through the plume, its chemical sensors "sniffed" more than just water in the stream, but a good deal of organic molecules as well…not unlike material found in comets, stuff left over from the formation of the Solar System that may have been the building blocks of life on Earth.

The other "water find" was that of a possible liquid ocean under the crust of Titan–similar perhaps to the deep liquid water ocean believed to exist under the surface of Jupiter's moon Europa. Unexpected "drift" in the locations of landmarks on Titan's surface is what suggests a liquid ocean–water with perhaps some ammonia–that the frozen crust may be floating on.

With all the liquid water and organic chemistry being revealed in the Saturn system (and elsewhere in the outer solar system), our imaginations can shift from the older standards of envisioning otherworldly landscapes of sculpted ice or even seascapes of liquid hydrocarbon lapping on shores of water ice sand, to something a little more, shall we say, "lively…"?

Benjamin Burress is a staff astronomer at The Chabot Space & Science Center in Oakland, CA.

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