Introduction:
Titan is the largest moon of Saturn’s and is the second largest moon in our solar system. You may sometimes hear it referred to as “Earth’s Evil Twin” if you’re in Mr. Grossfeld’s Astronomy class, or as Alexander G. Hayes, Astronomer, refers to it as his “first research love” or “explorer’s utopia” and he states; “It has strikingly similar processes acting on its surface, generating landforms including lakes, channels, and dunes—everything you have here (Earth), you have there. Titan’s methane-based hydrologic system works just like Earth’s water cycle, but does so in a completely alien environment.” Despite Earth being my favorite solar object in our universe, it wouldn’t make for an exciting research paper but with Titan being very similar yet incredibly different from Earth, I decided to conduct my research project based on the interesting, largest moon of Saturn. In this paper, I will discuss topics surrounding the discovery, atmosphere, composition, missions that have gathered data about the moon and much more.
Discovery:
Born in 1629, Christiaan Huygens came from a wealthy and well-connected Dutch family, who served in the diplomatic service to the House of Orange. As a young boy, he showed promise in mathematics and drawing. In 1645 he went to the University of Leiden to study mathematics and law. Two years later, he went to the College of Breda. Inspired by Galileo’s discovery of Jupiter’s four largest moons in 1610 and his improvements in telescope technology, Christiaan Huygens and with help from his brother Constantijn, he applied himself to the manufacturing of telescopes, and soon after developed a theory of the telescope. Huygens discovered the law of refraction to derive the focal distance of lenses. He also realized how to optimize his telescopes by using a new way of grinding and polishing the lenses.
On March 25th, 1655, he pointed one of his new telescopes towards Saturn with the intention of studying its rings. He was very surprised to see that, besides the rings, the planet also had a large moon, now known as Titan. Interested in the measurement of time, Huygens discovered the pendulum could be a regulator of clocks. He became one of the founding members of the Acadèmie Royale des Sciences in 1666, publishing his work Horologium Oscillatorium in 1673.
Naming:
Huygens named his discovery Saturni Luna (Latin for "Saturn's moon"), publishing in the 1655 tract De Saturni Luna Observatio Nova (A New Observation of Saturn's Moon). After Giovanni Domenico Cassini published his discoveries of four more moons of Saturn between 1673 and 1686, astronomers fell into the habit of referring to these and Titan as Saturn I through V (with Titan then in fourth position). The name Titan, and the names of all seven satellites of Saturn then known, came from John Herschel (son of William Herschel, discoverer of two other Saturnian moons; Mimas and Enceladus), in his 1847 publication Results of Astronomical Observations Made during the Years 1834-1838, at the Cape of Good Hope. In Greek mythology, the Titans were a race of powerful deities, descendants of Gaia and Uranus, that ruled during the legendary Golden Age.
Fast Facts:
Atmosphere:
Our solar system is home to more than 150 (known) moons, but Titan is rather unique in which it is the only moon with a thick atmosphere. At the surface, the atmospheric pressure is about 60% greater than on Earth – roughly the same pressure a person would feel swimming about 50’ deep into Earth’s ocean(s).
Titan’s atmosphere is mostly nitrogen (approximately 95%) and methane (about 5%), with small amounts of other carbon-rich compounds. High up in Titan’s atmosphere, methane and nitrogen molecules are split apart by the Sun’s ultraviolet light and by high-energy particles accelerated in Saturn’s magnetic field. The pieces of these molecules recombine to form a variety of organic chemicals (substances that contain carbon and hydrogen), and often include nitrogen, oxygen and other elements important to life on Earth. Lower down in the atmosphere, the haze turns into a smog of complex organic molecules. This thick, orange-colored haze absorbs visible sunlight, allowing only about 10% of the light to reach the surface which makes it difficult to view from space. However, spacecraft and telescopes can see through the haze at certain wavelengths of light outside of those visible to human eyes. . The thick haze is also inefficient at holding in and then re-radiating infrared (thermal) energy back down to the surface. Thus, despite the fact that Titan has a thicker atmosphere than Earth, the thick global haze causes the greenhouse effect there to be somewhat weaker than it is on Earth. Because Titan is less massive than Earth, it’s gravity doesn’t hold onto its gaseous envelope as tightly, so the atmosphere extends to an altitude 10x higher than here on Earth – nearly 370 miles into space.
Some of the heavy, carbon-rich compounds settle to the moon’s surface—these hydrocarbons play the role of “sand” in Titan’s vast dunes. Methane condenses into clouds that occasionally drench the surface in methane storms. The methane in Titan’s atmosphere is what makes its complex atmospheric chemistry possible, but where all that methane comes from is a mystery. Because sunlight continuously breaks down methane in Titan’s atmosphere, some source must be replenishing it or it would be depleted over time. Researchers suspect methane could be belched into Titan's atmosphere by cryo-volcanism—volcanoes releasing chilled water instead of molten rock lava—but they’re not certain if this or some other process is responsible.
Surface:
The surface of Titan is one of the most Earth-like places in the solar system, despite vastly colder temperatures and with different chemistry. Here it is so cold (-290 degrees Fahrenheit or -179 degrees Celsius) that water ice plays the role of rock. Titan may have volcanic activity as well, but with liquid water “lava” instead of molten rock. Titan’s surface is sculpted by flowing methane and ethane, which carves river channels and fills great lakes with liquid natural gas. No other world in the solar system, aside from Earth, has that kind of liquid activity on its surface.
Vast regions of dark dunes stretch across Titan’s landscape, primarily around the equatorial regions. The "sand" in these dunes is composed of dark hydrocarbon grains that appear to look something like coffee grounds. In appearance, the tall, linear dunes are not unlike those seen in the desert of Namibia in Africa, for example. Titan has few visible impact craters, meaning its surface must be relatively young and some combination of processes erases evidence of impacts over time. Earth is similar in that respect as well; craters on our planet are erased by the relentless forces of flowing liquid (water, in Earth's case), wind, and the recycling of the crust via plate tectonics. These forces are present on Titan as well, in modified forms. In particular, tectonic forces appear to be at work on the icy moon, although scientists do not see evidence of plates like on Earth.
Missions:
As previously mentioned, Dutch astronomer Christiaan Huygens discovered Saturn’s largest moon, Titan, on March 25, 1655. It was nearly 300 years later, in 1944, when Dutch-American astronomer, Gerard Kuiper, discovered that Titan actually has a unique atmosphere. Kuiper made the discovery by passing sunlight reflected from Titan through a spectrometer and detecting methane. Further telescope observations from Earth showed that Titan’s atmosphere was dense and hazy.
The first spacecraft to explore Titan, Pioneer 11, flew through Saturn’s obit on September 1st, 1979. Astronomers on Earth had previously studied Titan’s temperature, and calculated its mass; Pioneer 11 confirmed those characteristics. Because of Titan’s extended and opaque atmosphere, scientists at the time thought (incorrectly, it turns out) that Titan might be the largest moon in the solar system but later confirmed that Ganymede, moon of Jupiter, was the largest moon in our solar system and is only 2% bigger than Titan. Pioneer 11 also saw hints of a bluish haze in Titan’s upper atmosphere, which scientists predicted the Voyager spacecraft would be able to see.
When the Voyager 1 and 2 spacecraft passed through the Saturn system in 1980 and 1981, they couldn’t see Titan’s surface because of its hazy atmosphere—images from that mission showed a featureless, orange world—but they did see the blue haze as a seemingly detached layer of Titan’s upper atmosphere. Just before Voyager 1 arrived in the Saturn system, some scientists speculated that the moon’s cold temperatures and methane meant that Titan might be home to oceans of liquid hydrocarbons. Unfortunately, the Voyager spacecrafts’ cameras were unable to penetrate Titan’s opaque atmosphere to get a clear view of the surface. Voyager did, however, reveal that Titan had traces of acetylene, ethane, and propane, along with other organic molecules, and that its atmosphere was primarily nitrogen.
Voyager 1 also finally provided a measurement of Titan’s surface temperature and air pressure and radius, revealing Titan to be the second largest moon in the solar system, not the largest, which is Jupiter’s Ganymede (both moons are larger than Mercury). The Voyagers’ also saw a distinct difference in brightness from North to South, which was assumed to be a seasonal effect that was later confirmed.
In 1994, NASA's Hubble Space Telescope recorded pictures of Titan using particular colors of infrared light that could pierce through the unique haze. The Hubble images showed large bright and dark areas, including bright region the size of Australia. The Hubble results didn't prove that liquid seas existed though, and the mystery about what was hidden below Titan’s haze remained until 2004.
The Cassini spacecraft, with the European Space Agency’s Huygens probe attached, became the first human-made object to orbit Saturn in 2004. Almost immediately, Cassini began observing Titan, peering through the haze for the first time. The Huygens probe detached from Cassini and parachuted through Titan’s atmosphere, landing on the surface on Jan. 14, 2005 (the first landing of a probe in the outer solar system). Huygens collected images and atmospheric data during its descent as well as from the surface, and transmitted that data to Cassini, which then sent the data to Earth. Cassini performed 127 close flybys of Titan over 13 years, using a suite of tools, including radar and infrared instruments to peer through Titan’s haze and finally give scientists a detailed view of the moon’s surface and complex atmosphere. Cassini-Huygens discovered that Titan has clouds, rain, lakes and rivers of liquid hydrocarbons, as well as a subsurface ocean of salty water.
The Cassini spacecraft’s numerous gravity measurements of Titan revealed that the moon is hiding an underground ocean of liquid water (likely mixed with salts and ammonia). The European Space Agency’s Huygens probe also measured radio signals during its descent to the surface, in 2005, that strongly suggested the presence of an ocean 35-50 miles below the rocky, icy surface. The discovery of a global ocean of liquid water adds Titan to the handful of worlds in our solar system that could potentially contain habitable environments. Additionally, Titan’s rivers, lakes and seas of liquid methane and ethane might serve as a habitable environment on the moon’s surface, though any life there would likely be very different from Earth’s life. Although there is so far no evidence of life on Titan, its complex chemistry and unique environments are certain to make it a destination for continued exploration.