Titan (moon)

Saturn’s largest satellite, Titan, is the only satellite in the solar system with a thick atmosphere. Astronomical observations made from Earth established some time ago that Titan has a density slightly greater than compressed ice, indicating a composition primarily of ice but with a relatively small rocky core. Observations made by the Cassini-Huygens spacecraft show a surface with multiple hydrocarbon dunes that could be breeding grounds for primitive living organisms.

Overview

Titan is Saturn’s largest satellite, with a diameter of about 5,150 kilometers. It was discovered by Dutch astronomer Christiaan Huygens in 1655. Its atmosphere (whose density is several times that of Earth’s atmosphere) was discovered in 1944 from spectral analyses of sunlight reflected from the cloud cover. The spectral data indicated the presence of methane gas (CH₄). Additional Earth-based observations in 1973 showed a reddish, hazy atmosphere, which was assumed to be photochemical smog created by ultraviolet light from the Sun acting on the methane and other hydrocarbon compounds.

Because Titan is such an unusual satellite, the Pioneer 11 spacecraft flew by Titan in 1979, followed by Voyager 1 and 2 in 1980 and 1981, respectively. Unfortunately, their instruments were not sensitive enough to penetrate Titan’s thick atmosphere, although it was learned that its major constituent is nitrogen, with methane smog making up less than 5 percent of the atmosphere. Hazy smog is formed as the methane is catalyzed by ultraviolet light from the Sun to form more complex organic molecules, similar to the manner in which photochemical smog is produced from unburned fuel in the exhaust emitted by vehicles in Earth’s large cities. Because Titan is quite far from the Sun and relatively little of the available light can presumably penetrate the thick clouds, the surface temperature is predicted to be about 94 kelvins (-290 degrees F, or -179 degrees C).

Calculations indicate that at these temperatures, methane should condense from the clouds and fall as rain. The denser created in the atmosphere, such as ethane (C₂ H₆), would also eventually settle on the surface as a layer of malodorous slime. It was thus assumed in the 1980s and early 1990s that the icy surface is covered by either an ocean of liquid methane or a hydrocarbon swamp, with frequent rainstorms of methane.

In 1994, scientists used the Hubble at wavelengths (where the haze is more transparent) to map some of Titan’s surface features according to their reflectivity. Although details were not resolvable, light and dark surface features were recorded over Titan’s sixteen-day rotation period, and one bright area the size of Australia was documented. Definitive conclusions about the nature of the dark and bright areas could not be ascertained, but the images proved that the surface is not a global ocean of methane and ethane, as had been assumed; at least part of the surface is solid. Although definite conclusions could not be made, scientists surmised that the bright areas are major impact craters in the frozen surface. Information gleaned from this research provided important background information for the international Cassini mission, a robotic spacecraft sent to study Saturn and its satellites. In addition to data gathered from flybys, Cassini released the European Space Agency’s Huygens probe, which parachuted to the surface. Images of Titan from the Hubble telescope were used to locate an optimum landing site and to predict how Titan’s winds would affect the parachute as it descended through the atmosphere.

The Cassini spacecraft was launched in October 1997 for a seven-year voyage to rendezvous with Saturn. Beginning to orbit Titan in 2004, it flew 1,192 kilometers above the surface, using infrared cameras and radar to produce detailed maps. It detected irregular highlands and smoother dark areas, including one large region about the size of Lake Ontario (232 by 72 kilometers), so reminiscent of a lake that its perimeter even exhibits sinuous drainage channels leading to an apparent shorelike boundary. Because the surface temperature is so cold (94 kelvins), the lakes are presumed to be liquid methane and ethane fed by streams of dark organic gunk washed by precipitation from the highlands. It is assumed that methane evaporating from the lakes replenishes the methane in the atmosphere, from which it eventually precipitates and returns to the surface as rain, mimicking the hydrologic cycle on Earth. The fact that this feature appears in Titan’s cloudiest region, where presumably storms are intense enough that methane rain reaches the surface, gave credence to the lake hypothesis. Furthermore, Titan’s cold temperature would require a long time for liquid methane on the surface to evaporate; thus, a methane-filled lake would remain stable for a long time.

The Huygens probe landed on January 14, 2005. As it descended, it recorded the temperature, pressure, wind speed, and atmospheric composition at regularly timed intervals. It also radioed back more detailed images of the surface, showing dark drainage networks leading into the smooth areas but relatively few craters, as expected. The bright spots appear to be “islands” around which dark material had flowed in the past. Other images show areas evocative of water ice extruded onto the surface and short, stubby, dark channels, which could be springs of liquid methane. After landing, Huygens probed the surface, which has the consistency of wet sand covered with a thin crust, possibly consisting of hydrocarbon sand and ice mixed with small amounts of solid methane. First pictures of the surface show a plethora of small erosion-rounded pebbles, assumed initially to be rocks or granite-hard ice blocks, on an orange-colored surface. They were later determined to be mixtures of water and hydrocarbon ice. One image shows tendrils of surface fog, presumed to be ethane or methane.

The concentration of methane in Titan’s atmosphere is puzzling because ultraviolet light from the Sun should dissociate the methane into carbon and hydrogen, which would either react with the nitrogen to form ammonia (NH₃) or be dissipated into space. More complex organic molecules, such as ethane, would also be created; being heavier, they would settle to the surface. Early Cassini-Huygens calculations deduced that Titan's methane would remain in the atmosphere for fewer than one million years. Consequently, methane must be injected from some surface source. Scientists conjectured that perhaps it is outgassed from the methane in the icy crust or from a methane volcano. Detailed observations identified one area where ice and methane may be rising to the crust from a subterranean heat source to form a methane volcanic emitting methane gas. Since Titan is too small to have a molten interior, the heat source driving the release of methane gas has been suspected to be tidal heating, the frictional force generated as this massive satellite revolves in its about Saturn. Several dark surface markings having straight boundaries with preferred orientations suggest the presence of internal tectonic processes.

Analyses of close flybys of Titan made by the Cassini orbiter in 2008, as well as conclusions based on data gathered by flybys made between 2004 and 2006, provided suggestive evidence for the possibility that Titan’s surface may have active cryovolcanoes, perhaps spewing water, ammonia, and methane. Cassini recorded variations in brightness and reflectance (the ratio of reflected light to the incident light upon a surface) in two separate regions of Saturn’s largest satellite using its Visible and Infrared Mapping Spectrometer. In one region, the reflectance increased significantly and remained at the elevated level; in the other, it rapidly increased and then tailed off again. Both indicated vapor or liquid being ejected out of an active vent. Such a mechanism would explain why Titan continues to maintain a thick methane atmosphere when, without replenishment, it should have been greatly diminished over the passage of geologic time. The first evidence of a cryovolcano on Titan was discovered in 2010.

From 2006 to 2011, Cassini measured Titan's tides, leading to the conclusion in 2012 that the moon likely has a subsurface layer of liquid water. This could mean two things for the presence of methane on Titan's surface: one, that ammonia-water could bubble up through the crust and free up methane from the icy surface, and two, that the ocean could serve as a methane reservoir. The presence of water is not an indicator of life, however, as it is believed that life is more likely when liquid water is in contact with rock, and the measurements could not determine whether the ocean's floor is made up of rock or ice.

By 2013, scientists concluded that Titan's methane era may only last some tens of millions of years more. Titan's lakes do not appear to be changing in size, and methane should evaporate quickly; this suggests that most of the lakes' liquid may be ethane, which evaporates more slowly. In addition, the lakes do not seem to be filling, as there is only occasional hydrocarbon rain, and methane does not seem to be replaced from the interior. Thus, scientists have speculated that Titan's supply of methane may have come from a giant outburst from its interior eons ago.

Knowledge Gained

Using radar, Cassini continued to map Titan’s surface while orbiting Saturn during an extension of its originally planned four-year mission. The hundreds of observed dark areas are believed to be lakes of liquid methane or ethane more than 12.2 meters deep, while shadowy dunes running along the equator are assumed to consist of complex solid organics. Titan’s surface seems to contain many gigantic organic chemical factories producing complex hydrocarbons in an abundance surpassing all of Earth’s oil reserves. The amount of liquid on Titan’s surface is important to ascertain because methane is a strong greenhouse gas; without atmospheric methane, Titan’s surface would be even colder. Liquid methane on the surface could remain, at most, tens of millions of years before dissociating and reacting to form heavier hydrocarbon compounds. It is believed that the atmospheric methane may be constantly supplied by volcanic eruptions from the mantle.

In late December 2008, after much analysis, a group of researchers were ready to publish a scientific article in the research journal Icarus, reporting the first image taken of a liquid on a planetary surface other than the Earth’s. The Phoenix Mars lander had provided clear evidence about six weeks earlier of subsurface water ice at its far north landing site, but when that ice was exposed, it fairly quickly sublimated into the gas phase and became part of the Martian atmosphere. However, an image taken by the Huygens probe after reaching the surface of Saturn’s satellite Titan appears to have clearly recorded a droplet of methane near the edge of the robotic spacecraft itself. The small droplet might have been created by the heat emanating from the probe when it condensed humid air to temporarily form liquid methane. In several other images, splotches that appear and then are not seen in subsequent images of the same area are believed by the authors of the Icarus paper also to be droplets of methane.

Titan has an extensive atmosphere, including a methane layer extending 696 kilometers above the surface. There, the methane molecules are dissociated by ultraviolet light to form ethane (C₂ H₆) and acetylene (C₂ H₂). Cassini images show two thin haze layers. The outer haze layer, floating about four hundred kilometers above the surface, is where additional molecules (such as hydrogen cyanide) are formed from carbon, hydrogen, and nitrogen. About two hundred kilometers above the surface, there is a thick global smog of complex organic molecules produced by chemical reactions among the dissociated by ultraviolet light. This haze layer absorbs about 90 percent of the incident sunlight, leaving only an orangish haze to reach the surface. Although Titan has a dense atmosphere, it is relatively inefficient at reradiating infrared radiation, thus producing negligible greenhouse warming. With Titan's unique atmosphere and its status as the only planetary body apart from Earth to contain liquid rivers and surfaces, NASA scientists remained eager to learn more about the moon. In 2019, a new mission to further investigate Titan was announced.

Context

The Cassini-Huygens mission was a joint venture of NASA, the European Space Agency, and the Italian Space Agency. Enough data was gleaned to keep researchers occupied for years to come. Scientists were also presented with new images and data to research when the James T. Webb Space Telescope was launched in 2021. In 2022, scientists working the Keck Observatory in Hawaii teamed up with scientists examining images from the Webb to use their data to further explore Saturn and Titan.

Titan’s surface temperature (94 kelvins) appears to make the satellite a place inhospitable for life to evolve. This environment, although colder, is remarkably similar to that found on Earth billions of years ago, before life began adding oxygen to the atmosphere. Four billion years ago, Earth was covered with warm, shallow seas containing hydrogen, ammonia, and methane gases. From this primordial soup, driven by ultraviolet light and lightning discharges, complex hydrocarbons, including amino acids, formed. Over time, the amino acids linked together to form proteins, eventually creating one that was able to replicate itself. At that point, life was created, and molecular evolution became biological evolution.

If life could evolve in Earth’s primordial soup, it seems reasonable to suppose that the pools of organic gunk on Titan’s surface could form amino acids, if not self-replicating proteins. Studying Titan’s prebiotic chemistry can, therefore, facilitate the understanding of how life may have originated in the universe.

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