Titan

FAS Astronomers Blog, Volume 33, Number 3.

The outer Solar System covers a vast distance from Jupiter (at just under half a billion miles from the Sun) out to Neptune (at 2.8 billion miles). As interesting as the outer gas giants (Jupiter and Saturn) and ice giants (Uranus and Neptune) are, it is the outer moons that might provide the best conditions for life to exist. For more information, see the previous article The Moons of the Solar System.

As of June 2024, Saturn has 146 moons – most are extremely small – the exception are the seven largest moons (Mimas, Enceladus, Tethys, Dione, Rhea, Titan, and Iapetus). Of these seven, six are smaller than the Earth’s Moon, the four Galilean moons of Jupiter, and Neptune’s Triton. However, situated well outside of Saturn’s rings and between the orbits of Rhea and Iapetus, we find the seventh, Titan, probably the most interesting moon in the solar system.

Here are a few basic facts about Titan.

• Titan is the second largest moon in the solar system with a diameter of 3,200 miles. This is just 70 miles or so smaller than the diameter of Jupiter’s Ganymede, but still larger than the planet Mercury. Titan’s width is 40% that of the Earth and around 1.34 times that of the Earth’s moon.
• Titan completes one orbit about Saturn in just under 16 days resulting in an alternating eight-day cycle of night and day. 
• Its orbit is very circular with an eccentricity of only 0.0288.
• Titan orbits almost in the same plain as Saturn’s equator and rings with a very slight inclination of around .35 degrees. Therefore, the rings might not be clearly visible from Titan’s surface (if one could see through the atmospheric haze). 
• Because of Saturn’s 26.7-degree tilt, Titan exhibits the same tilt when viewed from the Earth and from the vantage point of the Sun. This gives Titan seasons like those observed for Saturn, which take 29.4 years to complete one full cycle. 

(Facts from Wikipedia contributors. “Titan (moon).” Wikipedia, The Free Encyclopedia. 15 Jan. 2025. Web. 16 Jan. 2025 and from “Titan Facts.” NASA.)

Titan was discovered by Christiaan Huygens on March 25, 1655, becoming the first moon known after Earth’s moon and the four Galilean satellites of Jupiter. Huygens initially referred to his discovery as Saturini Luna (Moon of Saturn). After Giovanni Cassini discovered four more moons, it became known as Saturn IV (based on its position among the other four). John Herschel discovered two new moons in 1847 and coined the name Titan – naming it after the Titans of Greek Mythology. 

Much of what we know about Titan comes from the Cassini spacecraft that orbited Saturn between 2004 And 2017. Carried along by Cassini was the Huygens lander that touched down on the surface of Titan on January 14, 2005. This was the first (and, so far, only) spacecraft to land on another moon besides our Moon and to land on an object in the outer part of the solar system. 

Titan has a thick hazy orange atmosphere made up mostly of nitrogen (around 94%) with a small amount of methane (between 4% and 5%). This is similar to the Earth’s atmosphere, but with the Earth’s oxygen replaced by methane, albeit with a smaller proportion. Titan’s atmosphere is thicker than the Earth’s giving it a surface pressure 50% greater than the Earth at sea-level. It would feel like standing at the bottom of a swimming pool. Still, it is very cold on Titan’s surface with a temperature around 290 degrees F below zero. You wouldn’t need a pressure suit to walk around on Titan, but you would need an oxygen mask and a very warm winter coat. 

The solar wind breaks down the nitrogen and methane in Titan’s upper atmosphere and forms organic “gunk” – something astronomers call tholins. These tholins cause the orange haze in the atmosphere and slowly fall onto the surface creating dunes similar to sand dunes on the Earth. Shangri-La, found along Titan’s equator, is a dark colored lowland area filled with flowing dunes.

Titan has weather somewhat like the Earth. However, our water-based weather is replaced by methane. Methane raindrops are 50% larger than raindrops here on the Earth but fall at a 20% slower rate due to Titan’s lower gravity. Because of this, Titan is the only object in our Solar System other than the Earth with liquid flowing on its surface. This includes methane (and ethane) filled lakes and seas. Kraken Mare, Ligeia Mare, and Punga Mare are located at the north pole. In the south is Ontario Lacus, a lake about the size of one of the Great Lakes. Unfortunately, Titan’s larger seas are probably not great for surfing – the waves are thought to be quite small. 

Titan’s surface is composed of water ice that is hard as rock. Once again, Titan is similar to the Earth, but with rock hard water ice rather than the silicate rock surface we find here on the Earth. The bulk of Titan’s surface are plains that cover much of the northern and southern hemispheres. Although dunes make up much of the surface along Titan’s equator, there are also highlands such as Xanadu, which is a light-colored plateau of ice, and areas of hummocky, which are mounds or knolls extending above the ground.  

Radar and Infrared images of Titan’s surface by the Cassini spacecraft has yielded a complete view of Titan hidden surface. 

In 2019, scientists from NASA’s Jet Propulsion Laboratory and elsewhere published a global map of Titan showing its varied terrain including plains (65%), dunes (17%), and hummocky (14%). Surface features on Titan are named following rules set forth by the International Astronomical Union and articulated by the U.S. Geological Survey. 

There is speculation, based on data from the Cassini spacecraft, that Titan has an underground ocean of liquid water similar to that thought to be found on Jupiter’s Europa and Saturn’s Enceladus. A 2008 study found several surface features moved over time indicating that Titan’s frozen surface might be floating on an ocean of liquid water. In 2012, a study of Titan’s gravity also indicated the presence of an underground ocean.

The Cassini spacecraft concluded its mission on September 15, 2017 when it plunged into the atmosphere of Saturn. However, setting its sights on Titan is another mission called Dragonfly. Scheduled to launch in the 2027-28 timeframe, Dragonfly will reach Titan around 2034. Once there, its “8-blade rotorcraft” structure will allow it to fly around and visit various locations on Titan’s surface.