Expected to end in September 2017, the Cassini-Huygens space mission has been orbiting the planet Saturn and its moons since July 2004. During the mission, one of the main targets of the unmanned spacecraft Cassini was Titan, Saturn’s largest natural satellite that is one and a half times larger than Earth’s Moon. In 2004, during one of the first Titan flybys, the ship launched the space probe Huygens, which landed on the icy satellite. The photos from the probe revealed a surface covered by dust and gravel, whose rounded shapes suggest erosion by the current of a now-dry river. Analyses of images released in August 2016, however, proved that great rivers still flow on Titan. They confirm that, based on what is known so far, this moon is the only celestial body in the Solar System aside from Earth with liquid constantly flowing on its surface.
“On Earth, water circulates in solid, liquid and gas states; on Titan methane can exist in all three of these physical states,” suggests astronomer Rosaly Lopes, head of the Division of Planetary Sciences at the Jet Propulsion Laboratory (JPL) of the National Aeronautics and Space Administration (NASA), the only Brazilian researcher participating in the mission. “On Titan, we see clouds, snow, rain, rivers and lakes of methane.”
Saturn’s largest moon has a very dense atmosphere. According to Lopes, this makes it difficult for the cameras that capture visible light, or for spectrometers — devices that identify the chemical composition of substances — to properly see the surface from space. “The best tool to penetrate this fog is radar,” says the astronomer. After dozens of flybys over this moon, Cassini’s radar obtained enough data to enable Lopes and her international team of collaborators to map the relief of roughly 60% of the surface. Published in a series of articles in the May 2016 issue of the journal Icarus, the conclusions summarize all that is known about the geology of Titan so far.
The mapping performed by Lopes and her colleagues indicated that rain and rivers of methane — a compound with one carbon atom and four hydrogen atoms (CH4) — sculpt the landscapes of the regions near the north and south poles of Titan. However, the relief of the rest of the satellite is mainly the effect of wind.
On Titan, the winds do not blow constantly and are not as strong as on Earth — there, wind speeds range from 1 meter per second (m/s) to 10 m/s, while here they can exceed 100 m/s. Over decades, however, the cumulative effect is enough to shape large fields of sand dunes at the equator and in the tropics of the moon. The Cassini images also suggest that winds reaching planetary scales carry part of the tropical sand and polar sediments to the immense, low-relief plains located in temperate zones.
An astronaut exploring the surface of Titan would be able to walk easily in a world where gravity is 10 times weaker than that on Earth, although she would need to use a special suit to protect herself from the cold temperatures (about -180ºC) and from the atmosphere without oxygen, composed principally of nitrogen and clouds of methane. She would also need flashlights, since the light reaching this moon is only one tenth of that reaching Earth, and an infrared visor to see through the thick, orange fog.
This fog consists of various compounds of carbon and hydrogen, such as ethane, propane, acetylene and other hydrocarbons. Chemical reactions triggered by sunlight convert these compounds into a dark soot of organic polymers called tholins that covers the entire surface of Titan, whose crust of mountains, valleys and basins is made up of water frozen as hard as rock.
The exact formula of the mixture of which the tholins are made is still a mystery, since the methane in Titan’s atmosphere acts as a barrier for Cassini’s spectrometer. “We can only analyze a few wavelengths of the light emitted by the surface, so we cannot determine the chemical composition of the substances that are there,” explains Lopes. She is working with Anezina Solomonidou, a planetary geologist who is currently a post-doctoral researcher at JPL, to combine the data from the spectrometer with that from the Cassini radar. “While the spectrometer provides clues as to the chemical composition at each surface point, the three different radar operating modes provide the temperature, topography, and an idea of the hardness and texture of the materials at each of these points.”
The temperature on Titan becomes low enough, especially near the poles, for the methane vapor in the atmosphere to fall to the surface like rain or snow. There, near the poles, an astronaut could use a boat to float along rivers of methane — the largest, at the North Pole, is as long as the Nile, in Africa. Titan’s rivers flow along the bottom of ice canyons, whose steep cliffs soar to over 500 meters in height — a landscape reminiscent of the Grand Canyon, as confirmed by Italian researchers in a study published in August 2016 in Geophysical Research Letters. These rivers feed methane lakes, some of sizes comparable to the Great Lakes between the United States and Canada. “We think that Titan’s canyons were formed by erosion caused by rivers,” says Lopes. “Most of these rivers are dry today though, and form structures we call polar labyrinths.”
At the same time, at Titan’s equator and in its tropics, the astronaut would behold vast fields of dunes as far as the eye can see. On this satellite of Saturn, the dunes reach up to 180 m high and form fields that resemble those in Egypt and Namibia, in Africa. The difference is that on Titan, the sand in the dunes is not made of silicates — a mineral containing inorganic chemical compounds — but rather grains of hydrocarbons (organic compounds) similar to tholins. “The sand in the dunes is produced in the equatorial region, but we do not know how,” says Michael Malaska, a JPL researcher and one of Lopes’ colleagues.
“It is fascinating how the composition of gases, liquids and solids on Titan’s surface is different from that found on Earth, even though the landscape there is similar to that here,” says Lopes. Titan is a very dynamic moon, unlike Earth’s, where practically nothing happens for billions of years. “The changes we have seen from year to year with Cassini are small, but we have evidence that, over decades, the chemical reactions in the atmosphere and on the surface of Titan, together with erosion by liquid methane and wind, cause its relief to change significantly.”
Lopes has been studying the geology of the planets and moons of the solar system at JPL since 1989. She collaborated on the Galileo mission, which explored Jupiter and its satellites from 1995 to 2003, and discovered dozens of volcanoes on the moon Io. She is still analyzing the data obtained by the mission (see Pesquisa FAPESP Issue nº 160). It was her special interest in volcanoes that led her to collaborate on the Cassini mission as well. Lopes identified mountain terrain on this moon of Saturn that appeared to be the product of activity by an apparently now-dormant ice volcano (cryovolcano). Unlike the volcanoes on Earth and Io, which expel incandescent rock lava, the cryovolcanoes are mountains of ice that, during their eruptions, expel a mixture of water, ammonia and methane, with a consistency similar to that of ice cream. “Studies measuring small differences during Cassini’s orbit around Titan suggest that there is an ocean of liquid water under the ice crust that forms the surface of this moon,” she explains.
It is likely that the cryovolcanoes transport material from the subterranean ocean to the surface. If the opposite occurred and part of the organic material on the surface penetrated Titan’s subterranean ocean, mixing with its water, that would be an environment conducive to the emergence of life, suggests Malaska. “Titan’s surface is very cold and many of the chemical reactions that characterize terrestrial life do not take place at such low temperatures,” explains the researcher.
Canyons, labyrinths and plains
Cryovolcanoes are extremely rare on the surface of Titan, and are less frequent than the labyrinths of polar canyons, the fourth most common type of landscape there. The third most common is ice mountain chains, most located just south of the equator, in a region called Xanadu. The second most common landscape consists of fields of dunes, concentrated in the Shangri-la region. The predominant landscape on Titan, however, is undifferentiated land, consisting of vast plains with gentle curves, concentrated in the temperate zones between the fields of dunes and the labyrinths of canyons. “Most of my colleagues did not want to study this region because it is flat and does not seem to contain anything interesting,” says Lopes. She disagrees, however. “If these plains cover most of the surface, we cannot understand the geology of Titan without knowing their origin.”
Since they appear smooth and flat, Lopes suspects that, in the beginning, this undifferentiated land consisted of large plains of ice, the remainder of ancient cryo-lava flows. Her research, though, has shown that the terrain of these plains consists of a layer dozens of meters deep of sediments containing material similar to that of the equatorial sand dunes. Examining the orientation of the shapes of dunes and other relief elements, she and her team reconstructed the preferred direction of the winds on Titan, concluding that the plains in the temperate zone were probably filled in by sediment brought from the tropics and from the poles via the wind.
“Titan research is still in its infancy,” says Lopes. “The best images have a relatively low resolution, but even then, there is so much data that we will not be able to map the entire surface by the end of the mission in September 2017.”
LOPES, R. M. C. et al. Nature, distribution, and origin of Titan’s undifferentiated plains. Icarus. V. 270, p. 162-82. May 15, 2016
MALASKA, M. J. et al. Material transport map of Titan: The fate of dunes. Icarus. V. 270, p. 183-96. May 15, 2016.