Getting to Know Titan, Rhea, and Iapetus

FROM THE LECTURE SERIES: A Field Guide to the Planets

By Sabine Stanley, Ph.D., Johns Hopkins University

Titan, Saturn’s largest moon, is a unique moon in the whole solar system. The missions and probes have discovered that its surface is similar to the Earth. Why has this moon, along with Rhea and Iapetus, attracted many astronomers and science-fiction authors?

Image showing Titan, largest moon of Saturn with dense atmosphere.
Titan is a giant moon with very low gravity and surface temperature but the possibility of life. (Image: Jurik Peter/Shutterstock)

Titan is the second-largest moon in the solar system and Saturn’s largest moon. It has a vast atmosphere due to its low gravity, reaching up to 1200 kilometers above its surface. As the only moon in the solar system with a substantial atmosphere, it has 98% nitrogen and 2% methane surrounding it. Titan is well-protected from the Sun and the solar winds, so the surface temperature reaches as low as -300 Fahrenheit. In 2004, the Cassini mission sent a probe through Titan’s atmosphere to land on the surface and collect data.

Huygens, the Probe to Titan

The Cassini mission sent probe Huygens to Titan. It took Huygens 2.5 hours to descend in Titan’s atmosphere. At about 70 kilometers above the surface, the atmosphere began to lose the haze, and the probe could take pictures. There were low-lying regions like dry lake beds and networks of drainage channels caused by liquid methane flowing on the surface. The probe saw small ice boulders that were smooth under the effect of liquid methane and ethane. Huygens lost power after 90 minutes of sending data.

The probe could float in case it landed on liquids, but it did not need to when landing on hard ice grains that form a kind of sticky sand. The 127 times that the Cassini mission flew over Titan revealed how similar its surface is to the Earth: rivers, lakes, dunes, mountains, flat plains, and volcanoes.

This is a transcript from the video series A Field Guide to the Planets. Watch it now, on Wondrium.

Titan’s Atmosphere and Surface

Titan’s atmosphere contains complex hydrocarbons necessary for forming life. The orange haze is made of tholins—complex organic molecules with tarlike properties—as a result of ultraviolet photons’ reaction with methane and nitrogen. Future missions will focus on photochemistry, which caught scientists by surprise.

Around the equator, there is Xanadu, a flat, bright region made of highly reflective water ice. Darker areas are perhaps dried-up seas covered with hydrocarbons. The dark spots around the poles are numerous liquid hydrocarbon lakes dominated by ethane, with depths of less than seven meters to over 200 meters.

Lakes on Titan

The gravity on Titan is only 14% of the Earth’s, and the lakes are less dense than our water lakes. Thus, any object falling into the lake will sink to the bottom and will have a hard time getting back up. The volume of the lakes show hundreds of times more natural gas and liquid hydrocarbons on Titan than on the Earth.

About 100 kilometers below Titan’s surface of frozen water ice, there might be a global subsurface water ocean. Thus, Titan is an interesting place to look for life.

Learn more about Saturn and the rings: Gravity’s masterpiece.

Rhea, the Second-largest Moon

Image showing Rhea, the second- biggest moon of Saturn.
Rhea is the second-largest moon of Saturn and has a diameter of around 950 miles.
(Image: Media Whalestock/Shutterstock)

Rhea is mainly made of water ice and rock, with a diameter of around 950 miles. Among the numerous craters, there is one large area with fewer craters, which shows resurfacing on the moon. There are also wisps on the surface, which are, in fact, fractures that form large canyons, as deep as hundreds of meters. They appear brighter because the material on the canyons’ walls falls off, and the fresher ice below shines brighter.

Rhea might have rings that absorb high-energy electrons in Saturn’s magnetosphere! They might have been formed from debris ejected from Rhea after an impact.

Iapetus, the Farthest Spherical Moon

Iapetus is much darker on the leading side than the trailing side. The feature, called Cassini Regio, is so significant that even the astronomer Giovanni Cassini could observe it in the 17th century. He noticed that the light moving around Saturn would fade and brighten during Iapetus’s orbit as Iapetus’s spin brought different sides of the moon into view from Earth.

The dark side is as black as coal, with the material being a thin one-foot coverage over cleaner ice. The bright side of the moon looks like shiny water ice. The dark material is believed to be initially debris coming from the moon Phoebe or from volcanic eruptions of hydrocarbons from within Iapetus. Regardless of how it was formed, it keeps getting darker due to a runaway heating mechanism.

Iapetus’s Features

Iapetus is tidally locked to Saturn, completing one orbit in over 79 Earth-days. When the darker side faces the Sun, it absorbs much more energy and heat, sublimates more ice, and gets darker.

The image shows Iapetus, Saturn's moon.
Iapetus has a large dark area that keeps getting darker and is covered with some kind of hydrocarbon. (Image: Andamati/Shutterstock)

Other than the dark spot, Iapetus also has a giant mountain range about 1400 kilometers around its equator. The mountains can get up to 20 kilometers high, which is very impressive on such a small moon. The mountain range might have formed when Iapetus was much younger and span faster, or might have been a ring around the equator that later fell on the planet and shaped the mountains.

These three impressive moons still have a lot to discover but are already clearly distinct and unique worlds.

Learn more about Exploring Mars from Space and the Ground

Common Questions about Titan and Other Saturnian Moons

Q: Can we live on Titan?

No. other than the lack of oxygen, Titan has a surface temperature of -300 Fahrenheit, a gravity of only 14% of the Earth’s, and almost no sun.

Q: Which Saturnian moon has a giant mountain range?

Iapetus has a giant mountain range about 1400 kilometers around its equator.

Q: Which is the second-largest moon of Saturn?

With a diameter of about 950 miles, Rhea is the second-largest moon of Saturn.

Q: Which Saturnian moon has a surface like Earth?

Titan’s surface is probably the most Earth-like of any world in the solar system. It has rivers, lakes, dunes, mountains, flat plains, and volcanoes—although they are cryovolcanos.

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