August 25, 2021: The Voyager Probes Revealed That Titan Had A Captivating Atmosphere In Particular

In 1979, the Pioneer 11 spacecraft performed a flyby of the Saturn System for the first time in history. That flyby allowed us to determine that Titan was probably too cold for the development of life despite the presence of methane or organics in its atmosphere. In 1980 and 1981, the Voyager 1 spacecraft and the Voyager 2 spacecraft also performed a flyby of the Saturn System. In 1980, the Voyager 1 spacecraft revealed an intriguing moon with a dense and opaque atmosphere that contained a relatively significant concentration of methane. The destiny in terms of planetary exploration for the Voyager 1 spacecraft ended with the close flyby of Saturn's largest moon since the trajectory completely left the plane of the orbit of Saturn around the Sun with an upward movement for an interstellar journey. Later, on August 25, 1981, 40 years ago, the Voyager 2 spacecraft also performed a flyby of the Saturn System and obtained new views of Saturn and some of its moons. The planetary destiny of the Voyager 2 spacecraft didn't end with the flyby of the Saturn System. The probe benefited from a relatively strong gravitational boost that allowed it to continue its journey toward Uranus and Neptune.

The views of Titan in particular really captivated the imagination of planetologists and encouraged them to prepare a new mission to Saturn and Titan in particular. Ed Stone, the leader of the Voyager missions, had noted that if the Voyager 1 spacecraft had not reached its goal regarding Titan, the Voyager 2 spacecraft would have performed a closer flyby of Titan and would have ended its planetary destiny with the flyby of the Saturn System. Today, Voyager 1 and Voyager 2 continue their journey in the Outer Solar System towards other stars. In August 2012, the Voyager 1 spacecraft officially entered into interstellar space whereas the Voyager 2 spacecraft officially entered into interstellar space much later on November 5, 2018. That's a really long trip since the Voyager 1 spacecraft and the Voyager 2 spacecraft had left our planet in 1977. We continue to collect scientific data from the Voyager 1 spacecraft and the Voyager 2 spacecraft. However, they don't capture any image of asteroids, comets or Dwarf Planets in their path. The level of energy of the probes is progressively decreasing. We acquire the scientific data regarding the environment of both probes thanks to NASA's Deep Space Network (DSN) on Earth.

Prior to the Pioneer 11 flyby, the Voyager 1 flyby and the Voyager 2 flyby, we knew that Saturn's largest moon had a significant atmosphere. In 1944, the astronomer Gerard Kuiper had revealed that the atmosphere of Titan contained a relatively significant concentration of methane. The Voyager 1 spacecraft and the Voyager 2 spacecraft have allowed us to determine that Titan's atmosphere is dominated by nitrogen like our own atmosphere and that it also contains a few percent methane and lower concentrations of other hydrocarbons such as ethane, propane and acetylene. The atmosphere of the Orange Moon appears completely opaque so that planetologists were unable to discern surface features on that intriguing world in the period of the Voyager 1 and Voyager 2 flybys. Titan has a remarkable haze that makes the appearance of the moon almost uniform from outer space. Planetologists had hoped that they could see some surface features but the atmosphere of the giant moon appears completely opaque in the visible spectrum. A camera able to acquire data in the infrared or near-infrared spectrum was needed in order to discern surface features on Titan. A radar mapper also appeared useful to gather data regarding the surface of the Opaque Moon.

After the Voyager 1 and the Voyager 2 flybys in 1980 and 1981, planetologists were thirsty to know more about Titan, Saturn and some other moons of the Ringed Planet. That's why the researchers rapidly started to prepare a new mission to the Saturn System with Titan and the Gas Giant as major targets. Engineers in collaboration with scientists developed the Cassini spacecraft and the Huygens probe in order to obtain scientific data regarding Saturn and its numerous moons. The Cassini spacecraft was going to perform multiple flybys of Saturn and some of its moons and the Huygens probe was going to land on Titan, the largest moon of the Saturn System. Saturn contains an impressive ring system and is surrounded by a multitude of moons. The system of Saturn can be regarded as a Mini Solar System. The Cassini-Huygens spacecraft left Earth in 1997 and reached the Saturn System in 2004. The orbital dance of the Cassini spacecraft in the Saturn System lasted about 13 years from 2004 to 2017. A multitude of moons was imaged and analyzed during that period. The mission of the Cassini spacecraft in the Saturn System was really fruitful. The Cassini spacecraft ended its mission with a crash against the Gas Giant on September 15, 2017.

The mission of the Huygens probe was much shorter. The Huygens probe performed a parachuted descent in Titan's atmosphere on January 14, 2005. During that atmospheric dive, the probe collected a multitude of panoramic views of the landscape. The aerial views unveiled a dark or brown plain marking a sharp contrast with bright hills containing a network of dark channels. Those channels found at a relatively low latitude in the southern hemisphere of the giant moon are likely drainage channels related to meteorological processes. At the time of the observations performed by the Huygens probe, the dark channels were probably dry. One can advance that it must rain from time to time in the area. The rain of Titan appears radically different from the rain of the Earth since the rain of Titan is composed of hydrocarbons. Clouds of methane or ethane can develop in the Titanian atmosphere and like the clouds on Earth, those clouds can produce rainfall events. In the harsh environment of Titan where surface temperature can evolve around -179 degrees Celsius, -290 degrees Fahrenheit or 94 Kelvin, methane and ethane can appear in their liquid form on the surface.

The first aerial views captured by the Huygens probe were really surprising because the appearance of the plain in the black-and-white images was reminiscent of the appearance of a sea, lake or ocean. Some specialists had even thought, at first sight, that the dark plain marking a sharp contrast with the bright hills was a methane sea or ocean. The color image acquired from the surface clearly showed that the surface was devoid of any liquid. However, the color view obtained from the surface unveiled a landscape with pebbles or strongly eroded stones. Those pebbles or strongly eroded stones imply that the Huygens probe may have landed onto an ancient river, stream or brook. Strong rainfall events from time to time in the area are likely to generate rivers, streams or brooks. The aerial views appeared relatively familiar. Some individuals had noticed some similarities between the aerial views of the landscape and the aerial views of the French Riviera for instance. The boundary between the bright hills and the dark or brown plain was reminiscent of the coastline of the French Riviera. But the absence of sea or lake at the time of the observations was a surprise to many specialists.

Prior to the Cassini-Huygens mission in the Saturn System or prior to the Huygens mission on Titan in particular, some researchers had imagined, on the basis of the infrared or near-infrared views, that the relatively dark areas which mark a sharp contrast with relatively bright areas were probably seas or oceans dominated by methane. The views obtained from the Huygens probe as well as the radar views acquired from the Cassini orbiter clearly ruled out that hypothesis regarding the low or mid-latitudes. Prior to the atmospheric descent of the Huygens probe, many specialists had advanced that there is not the signature of a liquid surface in the relatively dark areas of the low or mid-latitudes. Yet, some specialists had imagined that the signature of oceans, seas or lakes of methane or ethane could be different from the signature of oceans, seas or lakes of water on Earth. That's why there was really some suspense during the atmospheric dive of the Huygens probe. The Radar Mapper of the Cassini spacecraft clearly revealed that Titan contained lakes, seas and rivers but those stable bodies of liquid appear in the high latitudes or in the polar regions of the giant moon.

The north polar region of Titan is by far the most humid area on Titan. A dichotomy in the distribution of lakes, seas and rivers on the surface can be noticed between the high latitudes of the northern hemisphere and the high latitudes of the southern hemisphere. The southern hemisphere contains a major lake or sea. That lake or sea whose shape is reminiscent of a foot is known as Ontario Lacus. The northern hemisphere contains three major pools of liquid found in its high latitudes. The largest pool is Kraken Mare. Kraken Mare is so big that it can be regarded as a sea. Ligeia Mare is the second largest pool in the northern hemisphere. Punga Mare is the third largest pool in the northern hemisphere. Many islands can be observed in Kraken Mare and Ligeia Mare. Drainage channels connected to those pools can also be observed. Titan looks like the Earth to a certain extent. That world contains canyons, fractures, dune fields, hills, mountains, lakes, seas and rivers like the Earth. Titan is so captivating that researchers have already planned a new mission to Saturn's largest moon in the 2030s. That mission known as Dragonfly will involve a rotorcraft to explore the environment of Titan in the low or mid-latitudes where dunes can be encountered.

The image above reveals a portion of the disk of Titan obtained from the Voyager 2 spacecraft on August 23, 1981. At the time of the observation, Titan was evolving at a distance of about 2.3 million kilometers or 1.4 million miles from the spacecraft. The image was generated on the basis of blue, green and violet frames. One can discern atmospheric bands in the opaque atmosphere in particular. One can also notice a detached haze layer in the upper atmosphere. Image credit: NASA/JPL/CICLOPS.

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