Half a century after Galileo discovered the four large moons of Jupiter, an astronomer named Huygens pointed his telescope at Saturn in hopes of finding one in orbit around the ringed giant. He discovered five moons in total. Another astronomer, Cassini, discovered another four. Two more were discovered by William Herschel, whose son John gave names to all the satellites. He suggested they be named after the Titans, the brothers and sisters of Saturn. The largest of the satellites was simply called Titan. It is more than 96 percent of the total mass of the Saturnian satellite system, including all of the rings. It is the second most massive moon in the solar system singularly, and in relation to its gas giant, it is the most massive satellite — a titan indeed.
Known satellites of Saturn, to scale. (Click here to expand)
Until recently, not much has been known about the surface of Titan. With every other non-planetary object in the solar system, a lack of atmosphere has allowed us to view the surface directly. Titan is covered in a thick nitrogen atmosphere ten times denser than Earth’s, though under Titan’s lower gravity, the atmospheric pressure is only one-and-a-half times as high. The atmosphere is opaque, rendering Titan a featureless orange disk in visible light.
Titan looms before Saturn’s rings in this Cassini image
When the Cassini-Huygens probe entered the Saturnian system in 2004, it used radar to penetrate the atmosphere of Titan and map its surface. The mapping revealed dark swatches across the surface and liquid hydrocarbons flow freely in its lakes and rivers. The term hydrocarbon refers to hydrogen and carbon combined as molecules. In this case, the molecules are of methane or CH4. Liquid methane is abundant on Titan. In fact, there is an entire hydrosphere of methane, much like that of water on Earth. Titan is nine times further from the Sun than Earth and at this distance, the heavier methane condenses in clouds that rain onto the surface. The methane then collects in seas which then evaporate back into the atmosphere to rain again. Much of the larger bodies of methane are grouped around the poles, where the winter season keeps the lakes shaded from the Sun which lessens evaporation. The winter hemisphere also precipitates clouds more often, which then precipitate into the lakes. Titan shares in Saturn’s 15-year seasonal changes. Both are about halfway through northern summer, passing the equinox in 2010.
A gif of methane clouds forming in Titan’s atmosphere
Titan is a differentiated body made primarily of water ice. A planet or moon becomes differentiated when its interior is hot enough to be molten, which allows the heavier elements to sink to the center, forming the core, and lighter materials to float to the top, forming the crust. Titan has a rocky core surrounded by a “molten” mantle of ammonia-water slush. The surface is water ice, with a dusting of a sand-like material, thought to be the result of methane rain eroding the water-ice bedrock. Much of the surface data comes from the Huygens part of the Cassini probe, a lander designed for a Titanian landing. To date, Huygens is the only object to land on a satellite other than our own, and Titan is the only surface past Mars to be photographed from its surface.
The surface of Titan, photographed by the Huygens spacecraft
Like the other large moons of the solar system, Titan is tidally locked to its parent body. With an orbit of 16 days, it then has a rotational period longer than a week on Earth. Tidal stresses from Saturn’s gravitational pull, as well as heat differences during seasonal change, stir up the atmosphere, which in turn cause convective winds that blow the sand-like surface into elongated dunes. Wind speeds around the solstices are thought to be the main culprits, with the other seasonal winds being much too weak to move the surface material.
Being the only satellite with a substantial atmosphere, there is an idea of Titan being able to support life, though it wouldn’t be similar to Earth-based life by any means. It is thought that an organism could breathe hydrogen (H2) and exhale methane (compared to a human breathing oxygen, O2, and exhaling carbon dioxide). There is data to support this in the form of a lack of hydrogen in the lower layers of the atmosphere. However, Titan’s water is still solid on the surface, so it loses out on the great catalyst that is H2O. The presence of ammonia mixed with the water under the surface would act to slow down organic chemical reactions. In many ways, it is more analogous to an early Earth than a current one. But, in a couple billion years, once the Sun expands into a red giant, Titan could warm enough for liquid water and the hazy atmosphere could lessen to let in more heat. Life might yet exist in the solar system, but Earthly life will be long-dead by then.
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Sunlight gleams off of a hydrocarbon lake in Titan’s northern hemisphere
I've been fascinated by Titan for years and the thing that has stuck with me the most is the possibility of flight. Though Titan is larger than Mercury, it is not nearly as dense and therefore significantly less massive (compare a block of iron to a block of ice). Titan has much less surface gravity than Mercury, and even less than our Moon. Surface gravity is determined by the proportion of mass an object has to the distance from the surface to the center. While the Moon is smaller, its mass is closer to its center, so it has a higher surface gravity. (Density also comes into play, with rock being more dense than ice). Someone walking on Titan would then be able to jump higher than someone on the Moon, and given Titan’s thick atmosphere, if someone had large enough wings, it's conceivable that they could fly. Of course, the wing span necessary for a human’s mass would need to be many square feet, and they would most likely need some kind of boost to become aloft, but that's for the sci-fi writers to figure out.
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