“The unending flow and ebb of Tethys,
of the sacred flood of Okeanos fathomless-rolling,
of the bounds of Earth that wearieth never of her travail,
of where the Sun-steeds leap from orient waves.”
— Quintus Smyrnaeus, “The Fall of Troy” (4th century AD)
Being a member of the first generation of Titans in ancient Greek mythology, Oceanus personified the impassable great World Ocean Okeanos that encircled what was believed to be a flat Earth, beyond which laid the eternal oblivion of Tartarus. In real life, Saturn’s moon Titan, appropriately named after the primordial deities of the ancient Greek pantheon, is slowly revealing the secrets of its previously enshrouded in darkness northern seas and lakes, thanks to NASA’s Cassini spacecraft.
One of the most fascinating and intriguing moons in the Solar System, Titan is the only planetary body to date, besides Earth, that has been found to hold large masses of liquid on its surface. In addition, the moon’s nitrogen-rich, thick atmosphere, replete with a hydrologic cycle that closely resembles that of our Home Planet, make this captivating world the most Earth-like place in the outer Solar System. Yet, at the same time, Titan is as alien as it is Earth-like. Its thick, opaque atmosphere has only trace amounts of molecular oxygen and water vapour, while its surface temperature of -290 degrees Fahrenheit (−179 °C) makes any water present there freeze rock-solid. Its seas and lakes which drive the moon’s hydrologic cycle are not made of liquid water, but under these surface conditions are instead filled with liquified hydrocarbons like methane and ethane, which on Earth are found in gaseous form, better known as natural gas.
The discovery of seas and lakes on Titan has been one of the most important scientific findings of the joint NASA/ESA Cassini-Huygens mission, which has been continuously studying the Saturnian system for the past 10 years in great detail, having entered orbit around the ringed gas giant on 1 July 2004. Ever since their discovery in 2005, these alien waters have also been an object of extensive study by the planetary science community during the course of Cassini’s 100+ flybys of Saturn’s largest moon. Yet, because of Saturn’s axial tilt, Cassini had until recently the chance to mostly study Titan’s southern hemisphere, which features only a few small lakes. Since Titan shares Saturn’s axial tilt of 27 degrees and the same 29.5-year orbit around the Sun, each of its seasons lasts for approximately 7.5 Earth years. When Cassini arrived at the Saturnian system, the northern hemisphere where 97 percent of the seas and lakes lie was enveloped in winter darkness. Only after 2009, with the approach of the vernal equinox, did Cassini start making its first detailed observations of the moon’s northern latitudes which received the warmth of the Sun again after almost 15 years of cold darkness, bringing the large northern seas and lakes into clear view.
Now the mission’s science team has targeted Titan’s northern seas once more, during the latest Cassini flybys of the mystifying moon on May and June, officially designated as T-101 and T-102 respectively. Following a carefully pre-programmed path by controllers on the ground, the spacecraft flew past Titan on May 17 and June 18, in order to conduct an important set of scientific investigations. The most important of these was carried out with the spacecraft’s onboard Radio Science Subsystem, or RSS, which is designed to study the overall properties and internal composition of Saturn and its moons, by both transmitting and receiving radio waves to and from Earth, while measuring the way that these signals are affected as they pass through the atmospheric layers of Saturn and Titan, or reflected by the latter’s solid surface. “Our instrument can measure exactly how well you could hear somebody talking, and the quality of the sound traveling through whatever is between you and the speaker,” says Sami Asmar, a member of the Cassini team, at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “By studying the changes in your voice as it goes through various materials, we’d learn information on the composition and characteristics of the door or the curtain behind which you’d be talking. For us, the materials are the rings of Saturn, or the planet’s [or Titan’s] atmosphere.”
The flybys’ primary goal was to more accurately map the chemical composition of the Titanian seas and lakes, which are thought to be composed of hydrocarbons like methane and ethane. Even though the surface temperature on Titan would allow for these chemical compounds to remain in a liquid state, scientists have never taken direct measurements of the composition of the Titanian waters. To that end, they directed Cassini’s RSS instrument to beam a radio signal from a distance of 1,860 miles (2,994 kilometers), during the May 17 flyby’s closest approach over Kraken Mare and Ligeia Mare, the two largest seas on Titan located near the moon’s north polar region. Despite being uncertain as to the outcome of this bistatic scattering experiment, Cassini’s science team was able to receive, with the large radio antennas of NASA’s Deep Space Network on Earth, the specular reflections of the radio signals which bounced back from the surface of the seas. “We held our breath as Cassini turned to beam its radio signals at the lakes,” says Dr. Essam Marouf, professor of electrical engineering at the San Jose State University and member of the Cassini team. “We knew we were getting good quality data when we saw clear echoes from Titan’s surface. It was thrilling.”
In addition to the technical accomplishment that this bistatic scattering experiment represented, the mission’s team also conducted another highly challenging technical feat with great success. While properly orienting the spacecraft in the right orbit throughout its high-speed loop around Titan, scientists on the ground beamed a radio signal toward Cassini, which passed through the moon’s thick atmosphere before being received by the spacecraft’s High-Gain Antenna. As part of this radio occultation experiment, Cassini beamed back an identical signal through Titan’s atmosphere, which was subsequently received by the ground stations of the Deep Space Network. Although the mission team had used this technique in the past during several Saturn occultations, it was the first time it was used on Titan, giving scientists the opportunity to study the temperature and density of the moon’s atmospheric layers in great detail. This radio occultation experiment was complemented by a stellar occultation observation with Cassini’s Ultraviolet Imaging Spectrograph, or UVIS, which studied the dimming of a distant star’s light as it passed behind Titan’s thick, hazy atmosphere.
The success of the May flyby was replicated during last week’s close encounter with Titan as well, which saw the spacecraft passing by the moon at a distance of 2,274 miles (3,659 kilometers). As part of this flyby’s science objectives, Cassini’s RSS instrument targeted an area between Kraken Mare and Ligeia Mare near the north pole, which has been shown to feature a series of rivers and channels, as revealed by previous Cassini radar observations. The bistatic scattering experiment of the June flyby was also accompanied by a new radio occultation event, during which Cassini successfully received a radio signal from Earth, which then beamed back as it passed behind Titan, adding to the wealth of data of last month’s observations. “This was like trying to hit a hole-in-one in golf, except that the hole is close to a billion miles away, and moving,” says Earl Maize, Cassini’s project manager at NASA’s Jet Propulsion Laboratory, commenting on the radio occultation of the May flyby. “This was our first attempt to precisely predict and compensate for the effect of Titan’s atmosphere on the uplinked radio signal from Earth, and it worked to perfection.”
Video Credit: Cassini Scientist for a Day/NASA/JPL-Caltech
Although processing of the data of the two flybys is still ongoing, with preliminary results due for release possibly by the end of the month, scientists nevertheless hope to gain important insights regarding the structure and composition of Titan’s atmosphere and surface, including its fascinating lakes and seas.“Likely, several different processes – such as wind, rain and tides – might affect the methane and ethane lakes on Titan,” says Jason Hofgartner, a graduate student at Cornell University’s Department of Astronomy, in New York. “We want to see the similarities and differences from geological processes that occur here on Earth. Ultimately, it will help us to understand better our own liquid environments here on the Earth.”
The highly successful Cassini mission has surpassed every expectation regarding its scientific value and return on investment, while providing a long list of revolutionary discoveries about Saturn and its hundreds of moons, including Titan. Scheduled to end its mission in 2017 with a plunge on Saturn’s cloud tops, the intrepid Cassini spacecraft will probably be our last robotic explorer of fascinating Titan for many decades to come, with no follow-up missions currently in NASA’s manifest. A mission concept that was proposed to NASA in 2009, which could study Titan’s seas up close, called Titan Mare Explorer, or TiME, wasn’t eventually picked up by the space agency due to budgetary constrains. More recently, NASA selected a concept for a Titan robotic submarine mission for further study, as part of the agency’s Innovative Advanced Concepts Program. Nevertheless, it would take many decades for such a mission to become a reality, provided that it will be given the green light for implementation in the future.
Yet the intriguing seas of Titan beckon. And contrary to the waters of the mythological Titan Oceanus, which were ultimately impassable, those of real-life’s Titan can be sailed and explored—if we choose to do so.