Before astronauts can roam the gentle hills of Titan or glide through its colossal dunes, they must know where they are going.
Researchers have taken an important step in the exploration of Titan, Saturn's largest moon, producing the world's first map of frozen plains, twisted canyons and wrinkled shores. The charter of course, published Monday in Nature Astronomy, will form the basis for more detailed maps to come, which will help scientists decipher the Moon's past and plan for future exploration.
"Geological mapping," says David Williams, a scientist in planetary science at Arizona State University who has contributed to the production of the plot, "is basically another tool we use when we analyze planetary surfaces to try to understand their stories. "
The Cassini probe, which arrived in the Saturn system in 2004, allowed researchers to discover for the first time the thick yellow clouds of Titan (the atmosphere of the moon is thicker than that of the Earth, but almost all nitrogen and methane). The penetrating radar of the spacecraft has repeatedly swept the moon over the next 13 years, making more than 100 overflights, revealing lakes, rivers and other obvious signs of surface liquid, a first for a world other than Earth.
Williams and his colleagues started with radar imagery, where Cassini sent radio waves through the clouds to bounce off Titan's surface to generate a map. This method covered just under half of Titan's surface in crossing and downhill areas, but it provided a fine resolution that the team could use to identify the different types of moon lands, such as the plains. or the dunes. They then superimposed this radar data with data from other cameras, making the visible and infrared light more fuzzy, but covering the entire world, to learn how each form of relief appeared to each of Cassini's eyes. Finally, they used the global data from the other instruments to infer which types of land were between the swaths of the radar. "The main purpose of this map was to show the diversity of surface materials on a global scale," says Williams.
The map reveals a world shaped by liquid and wind, although at nearly 300 ° F below zero, no water flows on the surface of Titan. Instead, liquid methane and ethane fill the rivers and lakes of Titan before evaporating and raining again according to a strange analogy of the Earth's water cycle.
Even if it does not mean that the moon does not have H2O. On the contrary, the surface of Titan largely is H2O. "The global crust, like what people would walk on, which would form the continents on Earth," says Williams, "in the case of Titan, it would be ice-water."
The team identified six main types of terrain dug in the ice crust: craters, lakes, plains, dunes, mounds (hills) and labyrinths (canyons).
Here and there, some craters speckled the surface. Near the North Pole are the methane lakes, dark spots underlined by cut ribs. Going south, an explorer would encounter the most common relief of Titan: plains, flat expanses of ice crust covered with a layer of sand based on methane and ethane, cover nearly two-thirds of the planet. The equatorial winds stack the sand in dunes several hundred meters high, which extend for hundreds of kilometers, surrounding the mid-section of the moon and covering about a fifth of its surface.
Dotted with airplanes and dunes, there are "bumpy" hillsides of a few hundred meters high. Tides of cracked canyons dug by methane floods from a wetter time cover much of the south pole – a sign of climate change that Titan experienced as its orbit around Saturn changed slightly over time.
"In the course of Titan's geological history," explains Williams, "we observe a sequence in which the liquid accumulates at the poles and becomes more dominant at the equator and in both directions."
Further study of the map and other similar maps in the future could eventually allow researchers to also study the movement of liquids on a seasonal basis. "Ultimately, we hope such maps will help explain the methane cycle on Titan," said Imke de Pater, a scientist in planetary sciences at the University of California at Berkeley, who said not participated in the research.
And better cards are going well. Although Cassini 's dive into Saturn' s atmosphere in 2017 put an end to his data set, Williams said the identification and identification of key terrain was than the first step. He and his colleagues have gone further by dividing the country into finer categories, such as the plains and the clear mountains, and have produced a more detailed map. They plan to publish it through the US Geological Survey, where it will join other geological maps of extraterrestrial worlds, including the Moon, Mars, Ceres and Vesta. (Pluto and Mercury cards are also in preparation).
Although it is possible that many human explorers need these cards, a robotic scout may use them well before too long. NASA is developing a probe called Dragonfly that can fly over Titan by air, land on the surface and leave. The mission must leave the Earth in 2026 and arrive on the Moon in 2034, where it will look for signs of extraterrestrial habitability.
"This map will help inform this team of the nature of the geological units present wherever they plan to explore their drone," says Williams.