“Everyone is a moon, and has a dark side which he never shows to anybody.” –Mark Twain
Back before the telescope was invented, Saturn was known as the Old Man of the Skies. The slowest-moving of the naked-eye planets, it’s the only one that would reliably be in nearly the same location, year after year. You can find it all summer, after sunset, by following the “arc” of the handle of the big dipper all the way until you run into the brightest northern-hemisphere star, Arcturus, and then speeding on to the very bright Spica. Saturn is right next door.
But everything got an awful lot more interesting once the telescope was developed. What was a bright, slowly moving point of light for all of humanity suddenly transformed into the ringed wonder we know today.
Saturn, as you know, has the most complex and spectacular ring system of any planet discovered so far. While you can see the ringed structure even through a good pair of binoculars, the best views come from NASA’s Cassini mission, presently orbiting the ringed giant and photographing as much of it as possible.
The best picture I’ve ever seen of its rings? That would be this one, taken from when the Sun was directly behind the planet.
But Saturn isn’t all by its lonesome out there. Just as we, on Earth, have our Moon to keep us company, Saturn has a family of its own. A very, very large family, with one member that’s unique in all the Solar System.
Sure, it’s big: larger than our Moon, larger than Mercury, and, at 5150 kilometers across, it’s the second largest moon in the Solar System.
We’ve known about it for a really long time. Discovered in 1655 by Christiaan Huygens, Titan was the first moon discovered to orbit a world other than Earth or Jupiter.
But unlike every other Moon ever discovered, and unlike even planets like Mercury and Mars, Titan is the only Moon known with an atmosphere so significant it’s even thicker than the one here on Earth!
While this was long suspected, it was Voyager 1, the first spacecraft to visit Titan, that really showed us just how severe this atmosphere was.
A thick nitrogen atmosphere, hundreds of kilometers thick, with a dense photochemical haze in the upper layers, obscures the surface from every single one of Voyager’s visible-light pictures. With a surface pressure that’s 60% greater than Earth’s despite being just a fraction of our size and having just one-seventh of our gravity, Titan’s atmosphere is actually more massive than our own.
But nitrogen is transparent to visible light; there’s something more interesting than just a thick nitrogen atmosphere at work here.
Thanks to the Cassini spacecraft, we were able to find methane molecules being broken apart by ultraviolet light from the Sun, producing other, more complex compounds, including ethane, acetyl alcohol, and even amino acids!
But that isn’t all that Cassini came outfitted with. In addition to visible light equipment, it’s also capable of seeing into the ultraviolet, which doesn’t help much, but the infrared as well! Shown in red and green, below, the infrared filters allow us to see down, through the thick clouds and haze, all the way down to the surface of the most planet-like of all the moons.
Initially, we couldn’t tell what those dark features on Titan’s surface were. Was it a vast ocean of methane, like our oceans on Earth? The chemistry, based on the temperatures and pressures present, would be close. Or is it just a case of differently-colored rock, like the maria of our Moon?
The great dark area, known as Shangri-la, was going to be the target of the first great experiment down to the surface. Because the Huygens probe, named after Titan’s discoverer, was launched from Cassini, and became the first spacecraft — in 2005 — to land on a rocky body in the outer Solar System.
Descending through Titan’s atmosphere and landing on the plains of Shangri-la, Huygens found mountains, valleys, strong evidence of past (but not present) liquid, and a dry, dark surface. Shangri-la was no lake.
But that doesn’t mean the surface of Titan looked very much like the surface of our Moon. To me, it looked much more like a rocky cove once the tide’s gone out.
But despite not finding any liquid at its landing site, Huygens and Cassini have learned an awful lot about the surface of Titan, and there is liquid there! There’s evidence that Huygens has heard a methane waterfall, methane rain, clouds and evaporation are Titan’s version of a water cycle, and thanks to the power of radar imaging, Cassini has confirmed that there are liquid methane lakes near the poles!
Based on what we know about Titan now, after nearly a decade of Cassini imaging, the cold regions near Titan’s poles have abundant liquid methane, but the warmer regions, near the equator, ought to be dry. Liquid methane in those regions wouldn’t last long and ought to boil off, so knowing what we know now, a methane lake near a tropical region of Titan would be a surprise.
Because of how rapidly methane evaporates, this 927-square-mile lake near the equator (and not far from the Huygens landing site) must be fed by an underground aquifer of methane, according to Cassini scientist Caitlin Griffith. According to the NASA press release:
“An aquifer could explain one of the puzzling questions about the existence of methane, which is continually depleted,” Griffith said. “Methane is a progenitor of Titan’s organic chemistry, which likely produces interesting molecules like amino acids, the building blocks of life.”
So, in a world flush with amino acids, there are underground stores of liquid that flow and pool near the equator, in an environment that’s not much different — save for being significantly cooler — than a very young Earth was. Are you thinking what I’m thinking?
Maybe, just maybe, Titan is another world in the Solar System where life, no matter how primitive, has found a way. Regardless of whether that’s the case or not, we’ve still made an amazing breakthrough learning about the planetary science of this unique world:
“We had thought that Titan simply had extensive dunes at the equator and lakes at the poles, but now we know that Titan is more complex than we previously thought,” said Linda Spilker, the Cassini project scientist based at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “Cassini still has multiple opportunities to fly by this moon going forward, so we can’t wait to see how the details of this story fill out.”
The quest for extraterrestrial life in the Universe may take us to places vastly different from anything we’ve ever experienced; I can’t wait to see how the details fill out, either!