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Building blocks of Life in Titan’s Atmosphere – NASA

Building blocks of Life in Titan’s Atmosphere – NASA



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A simulation of the atmosphere of the moon of Saturn, Titan, has shown that complex organic blocks that could lead to creating life have been found lower in the atmosphere of the moon than previously estimated.

The research that has been done by Scientists at the Jet Propulsion Laboratory of NASA, shows that the upper atmosphere of Titan is hospitable to the formation of complex organic molecules, as principal investigator Mark Allen suggests. Furthermore he suggests that sun on the lower atmosphere can ignited reactions that could lead to the formation of liquids and solids and not just gases.

You can read more here.


    NASA Scientists Discover 'Weird' Molecule in Titan's Atmosphere

    Until now, cyclopropenylidene has been detected only in molecular clouds of gas and dust, such as the Taurus Molecular Cloud, which is a stellar nursery in the constellation Taurus more than 400 light years away. Recently, NASA Goddard scientist Conor Nixon, along with his team, found this unique molecule in Titan's atmosphere the first time it has been detected outside of a molecular cloud. Cyclopropenylidene is the only other closed-loop molecule besides benzene to have been detected at Titan. Closed-loop molecules are important because they form the backbone rings for the nucleobases of DNA, the complex chemical structure that carries the genetic code of life, and RNA, another critical compound for life's functions. Credits: Conor Nixon/NASA's Goddard Space Flight Center

    NASA scientists identified a molecule in Titan's atmosphere that has never been detected in any other atmosphere. In fact, many chemists have probably barely heard of it or know how to pronounce it: cyclopropenylidene, or C3H2.

    Scientists say that this simple carbon-based molecule may be a precursor to more complex compounds that could form or feed possible life on Titan.

    Researchers found C3H2 by using a radio telescope observatory in northern Chile known as the Atacama Large Millimeter/submillimeter Array (ALMA). They noticed C3H2, which is made of carbon and hydrogen, while sifting through a spectrum of unique light signatures collected by the telescope these revealed the chemical makeup of Titan's atmosphere by the energy its molecules emitted or absorbed.

    "When I realized I was looking at cyclopropenylidene, my first thought was, 'Well, this is really unexpected,'" said Conor Nixon, a planetary scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, who led the ALMA search. His team's findings were published on October 15 in the Astronomical Journal.

    Though scientists have found C3H2 in pockets throughout the galaxy, finding it in an atmosphere was a surprise. That's because cyclopropenylidene can react easily with other molecules it comes into contact with and form different species. Astronomers have so far found C3H2 only in clouds of gas and dust that float between star systems -- in other words, regions too cold and diffuse to facilitate many chemical reactions.

    But dense atmospheres like Titan's are hives of chemical activity. That's a major reason scientists are interested in this moon, which is the destination of NASA's forthcoming Dragonfly mission. Nixon's team was able to identify small amounts of C3H2 at Titan likely because they were looking in the upper layers of the moon's atmosphere, where there are fewer other gases for C3H2 to interact with. Scientists don't yet know why cyclopropenylidene would show up in Titan's atmosphere but no other atmosphere. "Titan is unique in our solar system," Nixon said. "It has proved to be a treasure trove of new molecules."

    The largest of Saturn's 62 moons, Titan is an intriguing world that's in some ways the most similar one to Earth we have found. Unlike any other moon in the solar system -- there are more than 200 -- Titan has a thick atmosphere that's four times denser than Earth's, plus clouds, rain, lakes and rivers, and even a subsurface ocean of salty water.

    Titan's atmosphere is made mostly of nitrogen, like Earth's, with a hint of methane. When methane and nitrogen molecules break apart under the glare of the Sun, their component atoms unleash a complex web of organic chemistry that has captivated scientists and thrust this moon to the top of the list of the most important targets in NASA's search for present or past life in the solar system.

    "We're trying to figure out if Titan is habitable," said Rosaly Lopes, a senior research scientist and Titan expert at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. "So we want to know what compounds from the atmosphere get to the surface, and then, whether that material can get through the ice crust to the ocean below, because we think the ocean is where the habitable conditions are."

    The types of molecules that might be sitting on Titan's surface could be the same ones that formed the building blocks of life on Earth. Early in its history, 3.8 to 2.5 billion years ago, when methane filled Earth's air instead of oxygen, conditions here could have been similar to those on Titan today, scientists suspect.

    "We think of Titan as a real-life laboratory where we can see similar chemistry to that of ancient Earth when life was taking hold here," said Melissa Trainer, a NASA Goddard astrobiologist. Trainer is the Dragonfly mission's deputy principal investigator and lead of an instrument on the Dragonfly rotorcraft that will analyze the composition of Titan's surface.

    "We'll be looking for bigger molecules than C3H2," Trainer said, "but we need to know what's happening in the atmosphere to understand the chemical reactions that lead complex organic molecules to form and rain down to the surface.

    Cyclopropenylidene is the only other "cyclic," or closed-loop, molecule besides benzene to have been found in Titan's atmosphere so far. Although C3H2 is not known to be used in modern-day biological reactions, closed-loop molecules like it are important because they form the backbone rings for the nucleobases of DNA, the complex chemical structure that carries the genetic code of life, and RNA, another critical compound for life's functions. "The cyclic nature of them opens up this extra branch of chemistry that allows you to build these biologically important molecules," said Alexander Thelen, a Goddard astrobiologist who worked with Nixon to find C3H2.

    Scientists like Thelen and Nixon are using large and highly sensitive Earth-based telescopes to look for the simplest life-related carbon molecules they can find in Titan's atmosphere. Benzene was considered to be the smallest unit of complex, ringed hydrocarbon molecules found in any planetary atmosphere. But now, C3H2, with half the carbon atoms of benzene, appears to have taken its place.

    Nixon's team used the ALMA observatory to peer at Titan in 2016. They were surprised to find a strange chemical fingerprint, which Nixon identified as cyclopropenylidene by searching through a database of all known molecular light signatures.

    To double check that the researchers were actually seeing this unusual compound, Nixon pored through research papers published from analyses of data from NASA's Cassini spacecraft, which made 127 close flybys of Titan between 2004 and 2017. He wanted to see if an instrument on the spacecraft that sniffed out the chemical compounds around Saturn and Titan could confirm his new result. (The instrument - called a mass spectrometer - picked up hints of many mysterious molecules at Titan that scientists are still trying to identify.) Indeed, Cassini had spotted evidence for an electrically charged version of the same molecule, C3H3+.

    Given that it's a rare find, scientists are trying to learn more about cyclopropenylidene and how it might interact with gases in Titan's atmosphere.

    "It's a very weird little molecule, so it's not going be the kind you learn about in high school chemistry or even undergraduate chemistry," said Michael Malaska, a JPL planetary scientist who worked in the pharmaceutical industry before falling in love with Titan and switching careers to study it. "Down here on Earth, it's not going be something you're going to encounter."

    But, Malaska said, finding molecules like C3H2 is really important in seeing the big picture of Titan: "Every little piece and part you can discover can help you put together the huge puzzle of all the things going on there."


    NASA Discovers “Very Weird” Molecule in Titan’s Atmosphere

    These infrared images of Saturn’s moon Titan represent some of the clearest global views of the icy moon’s surface. The views were created using 13 years of data acquired by the Visual and Infrared Mapping Spectrometer instrument onboard NASA’s Cassini spacecraft. Credit: NASA/JPL-Caltech/University of Nantes/University of Arizona

    NASA scientists identified a molecule in Titan’s atmosphere that has never been detected in any other atmosphere. In fact, many chemists have probably barely heard of it or know how to pronounce it: cyclopropenylidene, or C3H2. Scientists say that this simple carbon-based molecule may be a precursor to more complex compounds that could form or feed possible life on Titan.

    This image was returned January 14, 2005, by the European Space Agency’s Huygens probe during its successful descent to Titan’s surface. This is the colored view that’s been processed to add reflection spectra data to give better indication of the actual color of Titan’s surface.
    Credit: NASA/JPL/ESA/University of Arizona

    Researchers found C3H2 by using a radio telescope observatory in northern Chile known as the Atacama Large Millimeter/submillimeter Array (ALMA). They noticed C3H2, which is made of carbon and hydrogen, while sifting through a spectrum of unique light signatures collected by the telescope these revealed the chemical makeup of Titan’s atmosphere by the energy its molecules emitted or absorbed.

    “When I realized I was looking at cyclopropenylidene, my first thought was, ‘Well, this is really unexpected,’” said Conor Nixon, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who led the ALMA search. His team’s findings were published on October 15, 2020, in the Astronomical Journal.

    Though scientists have found C3H2 in pockets throughout the galaxy, finding it in an atmosphere was a surprise. That’s because cyclopropenylidene can react easily with other molecules it comes into contact with and form different species. Astronomers have so far found C3H2 only in clouds of gas and dust that float between star systems — in other words, regions too cold and diffuse to facilitate many chemical reactions.

    But dense atmospheres like Titan’s are hives of chemical activity. That’s a major reason scientists are interested in this moon, which is the destination of NASA’s forthcoming Dragonfly mission. Nixon’s team was able to identify small amounts of C3H2 at Titan likely because they were looking in the upper layers of the moon’s atmosphere, where there are fewer other gases for C3H2 to interact with. Scientists don’t yet know why cyclopropenylidene would show up in Titan’s atmosphere but no other atmosphere. “Titan is unique in our solar system,” Nixon said. “It has proved to be a treasure trove of new molecules.”

    The largest of Saturn’s 62 moons, Titan is an intriguing world that’s in some ways the most similar one to Earth we have found. Unlike any other moon in the solar system — there are more than 200 — Titan has a thick atmosphere that’s four times denser than Earth’s, plus clouds, rain, lakes and rivers, and even a subsurface ocean of salty water.

    Titan’s atmosphere is made mostly of nitrogen, like Earth’s, with a hint of methane. When methane and nitrogen molecules break apart under the glare of the Sun, their component atoms unleash a complex web of organic chemistry that has captivated scientists and thrust this moon to the top of the list of the most important targets in NASA’s search for present or past life in the solar system.

    “We’re trying to figure out if Titan is habitable,” said Rosaly Lopes, a senior research scientist and Titan expert at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California. “So we want to know what compounds from the atmosphere get to the surface, and then, whether that material can get through the ice crust to the ocean below, because we think the ocean is where the habitable conditions are.”

    The types of molecules that might be sitting on Titan’s surface could be the same ones that formed the building blocks of life on Earth. Early in its history, 3.8 to 2.5 billion years ago, when methane filled Earth’s air instead of oxygen, conditions here could have been similar to those on Titan today, scientists suspect.

    “We think of Titan as a real-life laboratory where we can see similar chemistry to that of ancient Earth when life was taking hold here,” said Melissa Trainer, a NASA Goddard astrobiologist. Trainer is the Dragonfly mission’s deputy principal investigator and lead of an instrument on the Dragonfly rotorcraft that will analyze the composition of Titan’s surface.

    “We’ll be looking for bigger molecules than C3H2,” Trainer said, “but we need to know what’s happening in the atmosphere to understand the chemical reactions that lead complex organic molecules to form and rain down to the surface.


    Dragonfly is a NASA mission that aims to explore the chemistry and habitability of Saturn’s largest moon, Titan. Credit: NASA’s Goddard Space Flight Center/Johns Hopkins University Applied Physics Laboratory

    Cyclopropenylidene is the only other “cyclic,” or closed-loop, molecule besides benzene to have been found in Titan’s atmosphere so far. Although C3H2 is not known to be used in modern-day biological reactions, closed-loop molecules like it are important because they form the backbone rings for the nucleobases of DNA, the complex chemical structure that carries the genetic code of life, and RNA, another critical compound for life’s functions. “The cyclic nature of them opens up this extra branch of chemistry that allows you to build these biologically important molecules,” said Alexander Thelen, a Goddard astrobiologist who worked with Nixon to find C3H2.

    Scientists like Thelen and Nixon are using large and highly sensitive Earth-based telescopes to look for the simplest life-related carbon molecules they can find in Titan’s atmosphere. Benzene was considered to be the smallest unit of complex, ringed hydrocarbon molecules found in any planetary atmosphere. But now, C3H2, with half the carbon atoms of benzene, appears to have taken its place.

    Nixon’s team used the ALMA observatory to peer at Titan in 2016. They were surprised to find a strange chemical fingerprint, which Nixon identified as cyclopropenylidene by searching through a database of all known molecular light signatures.

    Until now, cyclopropenylidene has been detected only in molecular clouds of gas and dust, such as the Taurus Molecular Cloud, which is a stellar nursery in the constellation Taurus more than 400 light years away. Recently, NASA Goddard scientist Conor Nixon, along with his team, found this unique molecule in Titan’s atmosphere the first time it has been detected outside of a molecular cloud. Cyclopropenylidene is the only other closed-loop molecule besides benzene to have been detected at Titan. Closed-loop molecules are important because they form the backbone rings for the nucleobases of DNA, the complex chemical structure that carries the genetic code of life, and RNA, another critical compound for life’s functions. Credit: Conor Nixon/NASA’s Goddard Space Flight Center

    To double check that the researchers were actually seeing this unusual compound, Nixon pored through research papers published from analyses of data from NASA’s Cassini spacecraft, which made 127 close flybys of Titan between 2004 and 2017. He wanted to see if an instrument on the spacecraft that sniffed out the chemical compounds around Saturn and Titan could confirm his new result. (The instrument – called a mass spectrometer – picked up hints of many mysterious molecules at Titan that scientists are still trying to identify.) Indeed, Cassini had spotted evidence for an electrically charged version of the same molecule, C3H3 + .

    Given that it’s a rare find, scientists are trying to learn more about cyclopropenylidene and how it might interact with gases in Titan’s atmosphere.

    “It’s a very weird little molecule, so it’s not going be the kind you learn about in high school chemistry or even undergraduate chemistry,” said Michael Malaska, a JPL planetary scientist who worked in the pharmaceutical industry before falling in love with Titan and switching careers to study it. “Down here on Earth, it’s not going be something you’re going to encounter.”

    But, Malaska said, finding molecules like C3H2 is really important in seeing the big picture of Titan: “Every little piece and part you can discover can help you put together the huge puzzle of all the things going on there.”

    Reference: “Detection of Cyclopropenylidene on Titan with ALMA” by Conor A. Nixon, Alexander E. Thelen, Martin A. Cordiner, Zbigniew Kisiel, Steven B. Charnley, Edward M. Molter, Joseph Serigano, Patrick G. J. Irwin, Nicholas A. Teanby and Yi-Jehng Kuan, 15 October 2020, Astronomical Journal.
    DOI: 10.3847/1538-3881/abb679


    Contents

    The presence of a significant atmosphere was first suspected by Spanish astronomer Josep Comas i Solà, who observed distinct limb darkening on Titan in 1903, [8] and confirmed by Gerard P. Kuiper in 1944 using a spectroscopic technique that yielded an estimate of an atmospheric partial pressure of methane of the order of 100 millibars (10 kPa). [9] Subsequent observations in the 1970s showed that Kuiper's figures had been significantly underestimated methane abundances in Titan's atmosphere were ten times higher, and the surface pressure was at least double what he had predicted. The high surface pressure meant that methane could only form a small fraction of Titan's atmosphere. [10] In 1980, Voyager 1 made the first detailed observations of Titan's atmosphere, revealing that its surface pressure was higher than Earth's, at 1.5 bars (about 1.48 times that of Earth's). [11]

    The joint NASA/ESA Cassini-Huygens mission provided a wealth of information about Titan, and the Saturn system in general, since entering orbit on July 1, 2004. It was determined that Titan's atmospheric isotopic abundances were evidence that the abundant nitrogen in the atmosphere came from materials in the Oort cloud, associated with comets, and not from the materials that formed Saturn in earlier times. [12] It was determined that complex organic chemicals could arise on Titan, [13] including polycyclic aromatic hydrocarbons, [14] propylene, [15] and methane. [16] [17]

    The Dragonfly mission by NASA is planning to land a large aerial vehicle on Titan in 2034. [18] The mission will study Titan's habitability and prebiotic chemistry at various locations. [19] The drone-like aircraft will perform measurements of geologic processes, and surface and atmospheric composition. [20]

    Observations from the Voyager space probes have shown that the Titanean atmosphere is denser than Earth's, with a surface pressure about 1.48 times that of Earth's. [11] Titan's atmosphere is about 1.19 times as massive as Earth's overall, [21] or about 7.3 times more massive on a per surface area basis. It supports opaque haze layers that block most visible light from the Sun and other sources and renders Titan's surface features obscure. The atmosphere is so thick and the gravity so low that humans could fly through it by flapping "wings" attached to their arms. [22] Titan's lower gravity means that its atmosphere is far more extended than Earth's even at a distance of 975 km, the Cassini spacecraft had to make adjustments to maintain a stable orbit against atmospheric drag. [23] The atmosphere of Titan is opaque at many wavelengths and a complete reflectance spectrum of the surface is impossible to acquire from the outside. [24] It was not until the arrival of Cassini–Huygens in 2004 that the first direct images of Titan's surface were obtained. The Huygens probe was unable to detect the direction of the Sun during its descent, and although it was able to take images from the surface, the Huygens team likened the process to "taking pictures of an asphalt parking lot at dusk". [25]

    Titan's vertical atmospheric structure is similar to Earth. They both have a troposphere, stratosphere, mesosphere, and thermosphere. However, Titan's lower surface gravity creates a more extended atmosphere, [26] with scale heights of 15–50 km (9–31 mi) in comparison to 5–8 km (3.1-5 mi) on Earth. [6] Voyager data, combined with data from Huygens and radiative-convective models provide increased understanding of Titan's atmospheric structure. [27]

    • Troposphere: This is the layer where a lot of the weather occurs on Titan. Since methane condenses out of Titan's atmosphere at high altitudes, its abundance increases below the tropopause at an altitude of 32 km (20 mi), leveling off at a value of 4.9% between 8 km (5 mi) and the surface. [28][29] Methane rain, haze rainout, and varying cloud layers are found in the troposphere.
    • Stratosphere: The atmospheric composition in the stratosphere is 98.4% nitrogen—the only dense, nitrogen-rich atmosphere in the Solar System aside from Earth's—with the remaining 1.6% composed mostly of methane (1.4%) and hydrogen (0.1–0.2%). [28] The main tholin haze layer lies in the stratosphere at about 100–210 km (62–130 mi). In this layer of the atmosphere there is a strong temperature inversion caused by the haze due to a high ratio of shortwave to infrared opacity. [2]
    • Mesosphere: A detached haze layer is found at about 450–500 km (280-310 mi), within the mesosphere. The temperature at this layer is similar to that of the thermosphere because of the cooling of hydrogen cyanide (HCN) lines. [30]
    • Thermosphere: Particle production begins in the thermosphere [6] This was concluded after finding and measuring heavy ions and particles. [31] This was also Cassini's closest approach in Titan's atmosphere.
    • Ionosphere: Titan's ionosphere is also more complex than Earth's, with the main ionosphere at an altitude of 1,200 km (750 mi) but with an additional layer of charged particles at 63 km (39 mi). This splits Titan's atmosphere to some extent into two separate radio-resonating chambers. The source of natural extremely-low-frequency (ELF) waves on Titan, as detected by Cassini–Huygens, is unclear as there does not appear to be extensive lightning activity.

    Titan's atmospheric chemistry is diverse and complex. Each layer of the atmosphere has unique chemical interactions occurring within that are then interacting with other sub layers in the atmosphere. For instance, the hydrocarbons are thought to form in Titan's upper atmosphere in reactions resulting from the breakup of methane by the Sun's ultraviolet light, producing a thick orange smog. [32] The table below highlights the production and loss mechanisms of the most abundant photochemically produced molecules in Titan's atmosphere. [6]

    Chemistry in Titan's atmosphere
    Molecule Production Loss
    Hydrogen Methane photolysis Escape
    Carbon Monoxide O + CH 3 ⟶ H 2 CO + H >>
    H 2 CO + h ν ⟶ CO + H 2 / 2 H >> CO + OH ⟶ CO 2 + H >>Ethane 2 CH 3 + M ⟶ C 2 H 6 + M >> CondensationAcetylene C 2 H + CH 4 ⟶ C 2 H 2 + CH 3 >> C 2 H 2 + h ν ⟶ C 2 H + H >> CondensationPropane CH 3 + C 2 H 5 + M ⟶ C 3 H 8 + M >> CondensationEthylene CH + CH 4 ⟶ C 2 H 4 + H >> CH 2 + CH 3 ⟶ C 2 H 4 + H >> C 2 H 4 + h ν ⟶ C 2 H 2 + H 2 / 2 H >>Hydrogen Cyanide N + CH 3 ⟶ H 2 CN + H >> H 2 CN + H ⟶ HCN + H 2 >> CondensationCarbon Dioxide CO + OH ⟶ CO 2 + H >> CondensationMethylacetylene CH + C 2 H 4 ⟶ CH 3 CCH + H >> CH 3 CCH + h ν ⟶ C 3 H 3 + H >> H + CH 3 CCH ⟶ C 3 H 5 >>Diacetylene C 2 H + C 2 H 2 ⟶ C 4 H 2 + H >> C 4 H 2 + h ν ⟶ C 4 H + H >> Magnetic field Edit Titan has no magnetic field, although studies in 2008 showed that Titan retains remnants of Saturn's magnetic field on the brief occasions when it passes outside Saturn's magnetosphere and is directly exposed to the solar wind. [33] This may ionize and carry away some molecules from the top of the atmosphere. Titan's internal magnetic field is negligible, and perhaps even nonexistent. [34] Its orbital distance of 20.3 Saturn radii does place it within Saturn's magnetosphere occasionally. However, the difference between Saturn's rotational period (10.7 hours) and Titan's orbital period (15.95 days) causes a relative speed of about 100 km/s between the Saturn's magnetized plasma and Titan. [34] That can actually intensify reactions causing atmospheric loss, instead of guarding the atmosphere from the solar wind. [35] Chemistry of the ionosphere Edit In November 2007, scientists uncovered evidence of negative ions with roughly 13 800 times the mass of hydrogen in Titan's ionosphere, which are thought to fall into the lower regions to form the orange haze which obscures Titan's surface. [36] The smaller negative ions have been identified as linear carbon chain anions with larger molecules displaying evidence of more complex structures, possibly derived from benzene. [37] These negative ions appear to play a key role in the formation of more complex molecules, which are thought to be tholins, and may form the basis for polycyclic aromatic hydrocarbons, cyanopolyynes and their derivatives. Remarkably, negative ions such as these have previously been shown to enhance the production of larger organic molecules in molecular clouds beyond our Solar System, [38] a similarity which highlights the possible wider relevance of Titan's negative ions. [39] Atmospheric circulation Edit There is a pattern of air circulation found flowing in the direction of Titan's rotation, from west to east. In addition, seasonal variation in the atmospheric circulation has also been detected. Observations by Cassini of the atmosphere made in 2004 also suggest that Titan is a "super rotator", like Venus, with an atmosphere that rotates much faster than its surface. [40] The atmospheric circulation is explained by a big Hadley circulation that is occurring from pole to pole. [2] Methane cycle Edit Energy from the Sun should have converted all traces of methane in Titan's atmosphere into more complex hydrocarbons within 50 million years — a short time compared to the age of the Solar System. This suggests that methane must be somehow replenished by a reservoir on or within Titan itself. Most of the methane on Titan is in the atmosphere. Methane is transported through the cold trap at the tropopause. [41] Therefore the circulation of methane in the atmosphere influences the radiation balance and chemistry of other layers in the atmosphere. If there is a reservoir of methane on Titan, the cycle would only be stable over geologic timescales. [6] Evidence that Titan's atmosphere contains over a thousand times more methane than carbon monoxide would appear to rule out significant contributions from cometary impacts, because comets are composed of more carbon monoxide than methane. That Titan might have accreted an atmosphere from the early Saturnian nebula at the time of formation also seems unlikely in such a case, it ought to have atmospheric abundances similar to the solar nebula, including hydrogen and neon. [42] Many astronomers have suggested that the ultimate origin for the methane in Titan's atmosphere is from within Titan itself, released via eruptions from cryovolcanoes. [43] [44] [45] Daytime and Twilight (sunrise/sunset) Skies Edit Sky brightness and viewing conditions are expected to be quite different from Earth and Mars due to Titan's farther distance from the Sun ( 10 AU) and complex haze layers in its atmosphere. The sky brightness model videos show what a typical sunny day may look like standing on the surface of Titan based on radiative transfer models. [46] For astronauts who see with visible light, the daytime sky has a distinctly dark orange color and appears uniform in all directions due to significant Mie scattering from the many high-altitude haze layers. [46] The daytime sky is calculated to be 100-1000 times dimmer than an afternoon on Earth, [46] which is similar to the viewing conditions of a thick smog or dense fire smoke. The sunsets on Titan are expected to be "underwhelming events", [46] where the Sun disappears about half-way up in the sky ( 50° above the horizon) with no distinct change in color. After that, the sky will slowly darken until it reaches night. However, the surface is expected to remain as bright as the full Moon up to 1 Earth day after sunset. [46] In near-infrared light, the sunsets resemble a Martian sunset or dusty desert sunset. [46] Mie scattering has a weaker influence at longer infrared wavelengths, allowing for more colorful and variable sky conditions. During the daytime, the Sun has a noticeable solar corona that transitions color from white to "red" over the afternoon. [46] The afternoon sky brightness is 100 times dimmer than Earth. [46] As evening time approaches, the Sun is expected to disappear fairly close to the horizon. Titan's atmospheric optical depth is the lowest at 5 microns. [47] So, the Sun at 5 microns may even be visible when it is below the horizon due to atmospheric refraction. Similar to images to Martian sunsets from Mars rovers, a fan-like corona is seen to develop above the Sun due to scattering from haze or dust at high-altitudes. [46] In regards to Saturn, the planet is nearly fixed in its position in the sky because Titan's orbit is tidally locked around Saturn. However, there is a small 3° east-to-west motion over a Titan year due to the orbital eccentricity, [48] similar to the analemma on Earth. Sunlight reflected off of Saturn, Saturnshine, is about 1000 times weaker than solar insolation on the surface of Titan. [48] Even though Saturn appears several times bigger in the sky than the Moon in Earth's sky, the outline of Saturn is masked out by the brighter Sun during the daytime. Saturn may only become discernible at night, but only at a wavelength of 5 microns. This is due to two factors: the small optical depth of Titan's atmosphere at 5 microns [47] [49] and the strong 5 μm emissions from Saturn's night side. [50] In visible light, Saturn will make the sky slightly brighter, similar to an overcast night with a full moon on Earth. [48] Saturn's rings are hidden from view owing to the alignment of Titan's orbital plane and the plane of the rings. [48] Saturn is expected to show phases, akin to the phases of Venus on Earth, that partially illuminate the surface of Titan at night, except for eclipses. [48] From outer space, Cassini images from near-infrared to UV wavelengths have shown that the twilight periods (sunrise/sunset) are brighter than the daytime on Titan. [51] [52] Scientists expect that planetary brightness will weaken going from the day to night side of the planetary body, known as the terminator. This paradoxical observation has not been observed on any other planetary body with a thick atmosphere. [52] The Titanean twilight outshining the dayside is likely due to a combination of Titan's atmosphere extending hundreds of kilometers above the surface and intense forward Mie scattering from the haze. [52] Radiative transfer models have not reproduced this effect. [46] The persistence of a dense atmosphere on Titan has been enigmatic as the atmospheres of the structurally similar satellites of Jupiter, Ganymede and Callisto, are negligible. Although the disparity is still poorly understood, data from recent missions have provided basic constraints on the evolution of Titan's atmosphere. Roughly speaking, at the distance of Saturn, solar insolation and solar wind flux are sufficiently low that elements and compounds that are volatile on the terrestrial planets tend to accumulate in all three phases. [53] Titan's surface temperature is also quite low, about 94 K. [54] [55] Consequently, the mass fractions of substances that can become atmospheric constituents are much larger on Titan than on Earth. In fact, current interpretations suggest that only about 50% of Titan's mass is silicates, [56] with the rest consisting primarily of various H2O (water) ices and NH3·H2O (ammonia hydrates). NH3, which may be the original source of Titan's atmospheric N2 (dinitrogen), may constitute as much as 8% of the NH3·H2O mass. Titan is most likely differentiated into layers, where the liquid water layer beneath ice Ih may be rich in NH3. [ jargon ] Tentative constraints are available, with the current loss mostly due to low gravity [57] and solar wind [58] aided by photolysis. The loss of Titan's early atmosphere can be estimated with the 14 N– 15 N isotopic ratio, because the lighter 14 N is preferentially lost from the upper atmosphere under photolysis and heating. Because Titan's original 14 N– 15 N ratio is poorly constrained, the early atmosphere may have had more N2 by factors ranging from 1.5 to 100 with certainty only in the lower factor. [57] Because N2 is the primary component (98%) of Titan's atmosphere, [59] the isotopic ratio suggests that much of the atmosphere has been lost over geologic time. Nevertheless, atmospheric pressure on its surface remains nearly 1.5 times that of Earth as it began with a proportionally greater volatile budget than Earth or Mars. [55] It is possible that most of the atmospheric loss was within 50 million years of accretion, from a highly energetic escape of light atoms carrying away a large portion of the atmosphere (hydrodynamic escape). [58] Such an event could be driven by heating and photolysis effects of the early Sun's higher output of X-ray and ultraviolet (XUV) photons. Because Callisto and Ganymede are structurally similar to Titan, it is unclear why their atmospheres are insignificant relative to Titan's. Nevertheless, the origin of Titan's N2 via geologically ancient photolysis of accreted and degassed NH3, as opposed to degassing of N2 from accretionary clathrates, may be the key to a correct inference. Had N2 been released from clathrates, 36 Ar and 38 Ar that are inert primordial isotopes of the Solar System should also be present in the atmosphere, but neither has been detected in significant quantities. [60] The insignificant concentration of 36 Ar and 38 Ar also indicates that the 40 K temperature required to trap them and N2 in clathrates did not exist in the Saturnian sub-nebula. Instead, the temperature may have been higher than 75 K, limiting even the accumulation of NH3 as hydrates. [61] Temperatures would have been even higher in the Jovian sub-nebula due to the greater gravitational potential energy release, mass, and proximity to the Sun, greatly reducing the NH3 inventory accreted by Callisto and Ganymede. The resulting N2 atmospheres may have been too thin to survive the atmospheric erosion effects that Titan has withstood. [61] NASA Team Investigates Complex Chemistry at Titan

    A laboratory experiment at JPL simulating the atmosphere of Saturn's moon Titan suggests another region in the atmosphere that could brew up prebiotic materials.

    "Scientists previously thought that as we got closer to the surface of Titan, the moon's atmospheric chemistry was basically inert and dull," said Murthy Gudipati, the paper's lead author at JPL. "Our experiment shows that's not true. The same kind of light that drives biological chemistry on Earth's surface could also drive chemistry on Titan, even though Titan receives far less light from the sun and is much colder. Titan is not a sleeping giant in the lower atmosphere, but at least half awake in its chemical activity."

    Scientists have known since NASA's Voyager mission flew by the Saturn system in the early 1980s that Titan, Saturn's largest moon, has a thick, hazy atmosphere with hydrocarbons, including methane and ethane. These simple organic molecules can develop into smog-like, airborne molecules with carbon-nitrogen-hydrogen bonds, which astronomer Carl Sagan called "tholins."

    "We've known that Titan's upper atmosphere is hospitable to the formation of complex organic molecules," said co-author Mark Allen, principal investigator of the JPL Titan team that is a part of the NASA Astrobiology Institute, headquartered at Ames Research Center, Moffett Field, Calif. "Now we know that sunlight in the Titan lower atmosphere can kick-start more complex organic chemistry in liquids and solids rather than just in gases."

    The team examined an ice form of dicyanoacetylene -- a molecule detected on Titan that is related to a compound that turned brown after being exposed to ambient light in Allen's lab 40 years ago.

    In this latest experiment, dicyanoacetylene was exposed to laser light at wavelengths as long as 355 nanometers. Light of that wavelength can filter down to Titan's lower atmosphere at a modest intensity, somewhat like the amount of light that comes through protective glasses when Earthlings view a solar eclipse, Gudipati said. The result was the formation of a brownish haze between the two panes of glass containing the experiment, confirming that organic-ice photochemistry at conditions like Titan's lower atmosphere could produce tholins.

    The complex organics could coat the "rocks" of water ice at Titan's surface and they could possibly seep through the crust, to a liquid water layer under Titan's surface. In previous laboratory experiments, tholins like these were exposed to liquid water over time and developed into biologically significant molecules, such as amino acids and the nucleotide bases that form RNA.

    "These results suggest that the volume of Titan's atmosphere involved in the production of more complex organic chemicals is much larger than previously believed," said Edward Goolish, acting director of NASA's Astrobiology Institute. "This new information makes Titan an even more interesting environment for astrobiological study."

    The team included Isabelle Couturier of the University of Provence, Marseille, France Ronen Jacovi, a NASA postdoctoral fellow from Israel and Antti Lignell, a Finnish Academy of Science postdoctoral fellow from Helsinki at JPL.


    Building blocks of Life in Titan’s Atmosphere – NASA - History

    Scientists analysing data gathered by Cassini have confirmed the presence of heavy negative ions in the upper regions of Titan’s atmosphere. These particles may act as building blocks for more complicated organic molecules.

    The discovery was completely unexpected because of the chemical composition of the atmosphere (which lacks oxygen - responsible for forming negative ions in the lower ionosphere of the Earth - and mainly consists of nitrogen and methane). The observation has now been verified on 16 different encounters.

    Prof Andrew Coates, researcher at University College London’s Mullard Space Science Laboratory and lead author of the paper, says: “Cassini’s electron spectrometer has enabled us to detect negative ions which have 10 000 times the mass of hydrogen. Additional rings of carbon can build up on these ions, forming molecules called polycyclic aromatic hydrocarbons, which may act as a basis for the earliest forms of life.

    Saturn's moon Titan is the second largest in the solar system - and the only one with a dense atmosphere. The atmosphere, nitrogen and methane, resembles that of the early Earth. A detector led by the University College London (the Electron Spectrometer, part of the CAPS instrument) on the orbiter detects an unexpected component in Titan's high atmosphere - extremely heavy hydrocarbon-based negative ions. Their mass is at least 10 000 times that of a hydrogen atom, detected at 953 km above the surface about the distance from London to Milan. The image shows Titan’s haze and the heavy ions. These form part of the haze in the atmosphere, and may fall towards Titan’s surface as organic gunk. They may become Carl Sagan’s tholins a brown residue appearing in the Miller-Urey experiment, where a spark excites a mixture of gases resembling that of Earth’s early atmosphere.

    The right hand side of the image shows the negative ion signature on the T16 encounter where CAPS-ELS sees the ions. The vertical stripes show the ions seen as the instrument is scanned through Cassini's direction of travel. Increasing numbers of ions are shown by redder colours as they are rammed into the sensor. Energy, and the mass of ions, increases vertically. Credits: Right panel: UCL-MSSL (A. Coates), Left panel: NASA/JPL/Space Science Institute

    Coates added, “Their existence poses questions about the processes involved in atmospheric chemistry and aerosol formation and we now think it most likely that these negative ions form in the upper atmosphere before moving closer to the surface, where they probably form the mist which shrouds the planet and which has hidden its secrets from us in the past. It was this mist which stopped the Voyager mission from examining Titan more closely in 1980 and was one of the reasons that Cassini was launched.”

    The new paper builds on work published in Science on 11 May where the team found smaller tholins, up to 8000 times the mass of hydrogen, forming away from the surface of Titan.

    Dr Hunter Waite of the South West Research Institute in Texas and author of the earlier study, said: “Tholins are very large, complex, organic molecules thought to include chemical precursors to life. Understanding how they form could provide valuable insight into the origin of life in the solar system."

    Article: Geophysical Research Letters, 'Discovery of heavy negative ions in Titan's ionosphere', A. Coates, F. Crary, G. Lewis, D. Young, J. Waite Jr. and E. Sittler Jr.


    Cassini's search for the building blocks of life on Titan

    Cassini captures Saturn’s largest moon, Titan. Credit: NASA/JPL-Caltech/SSI

    Lakes and seas of liquid methane, rain from hydrocarbon clouds, and evidence of poisonous hydrogen cyanide in the atmosphere of Titan were just some of the discoveries the Cassini probe made of Saturns's largest moon.

    The space probe has now made its final pass of Titan as it heads towards its grand finale plunge into the ringed planet later this week.

    Dubbed Cassini's "goodbye kiss" by NASA, Titan has been the subject of much scrutiny by the probe, with 127 flybys on its 13-year mission exploring the planetary system.

    One of Cassini's greatest feats is its contribution to untangling the complicated chemistry of Titan, no doubt one of the more chemically diverse objects in our Solar System.

    We have known for some time that the combination of ultraviolet rays from the Sun and particle bombardment has altered the mainly nitrogen and methane atmosphere over time.

    This chemistry has sustained a thick, orange smog layer surrounding the entire body, shrouding Titan's oceans and landscape from view prior to Cassini's arrival.

    With Cassini's toolkit of advanced sensing instruments – combined with atmospheric sampling by the Huygens probe during its 2005 descent to the surface – the mission has developed a comprehensive picture of Titan's chemistry.

    The murky orange disk of Saturn’s moon Titan. Credit: NASA/JPL/Space Science Institute

    Intriguingly, on top of the hundreds of molecules accounted for, chemical models developed here on Earth incorporating Cassini data predict the existence of even more complex material.

    Of potential significance to biochemistry, these molecules have evaded observation over the relatively short Cassini mission, being either out of view or present at levels below the detection limits of the equipment.

    Even if only formed in small quantities in the atmosphere it is plausible that these life-bearing species have built up on the surface over Titan's history.So what are these chemicals and how do they come to be?

    Unlike Earth, oxygen atoms are rather scarce in Titan's atmosphere. Water is locked as surface ice and there appear to be no abundant sources of O₂ gas.

    In oxygen's place, we see nitrogen play a more significant role in Titan's atmospheric chemistry.

    Here, common products of nitrogen reactions are the cyanide family of compounds, of which hydrogen cyanide (HCN) is the simplest and most abundant.

    As the numbers of cyanide molecules build up at lower, colder altitudes they form cloud layers of large floppy polymers (tholins) and budding ice aerosols.

    As the aerosols descend to the surface, shells of methane and ethane ice form further layers on the exterior. This acts to protect the inner organic material on its descent to the surface before being dispersed in hydrocarbon lakes and seas.

    Surprisingly it is these cyanide compounds, chemicals closely associated with toxicity and death to Earthly lifeforms, that may actually provide avenues for life-bearing biomolecules to form in space environments.

    Some simulations predict that cyanides trapped in ices and exposed to space radiation can lead to the synthesis of amino acids and DNA nucleobase structures – the building blocks of life on Earth.

    Excited by these predictions and their implications toward astrobiology, chemists have rushed to explore these reactions in the laboratory.

    This composite image shows an infrared view of Saturn’s moon Titan from Cassini’s flyby in November 2015. The near-infrared wavelengths in this image allow Cassini’s vision to penetrate the haze and reveal the moon’s surface. Credit: NASA/JPL/University of Arizona/University of Idaho

    Synchrotron experiments: Titan-in-a-can

    Our contributions to astrochemistry have focused on simulating the atmosphere of Titan and its cyanide haze.

    With a specialised gas cell installed at the Australian Synchrotron, we are able to replicate the cold temperatures associated with Titan's cloud layers.

    By injecting cyanides (the friendlier variety) into our cell we can determine the size, structure and density of Titan aerosols as they grow over time probing with infrared light from the facility.

    These results have provided us with a list of signatures for which we can locate cyanide aerosols using infrared astronomy.

    The next step will be to seed these aerosols with organic species to determine if they can be identified in extraterrestrial atmospheres.

    Cassini’s view of Titan’s high northern latitudes in May 2012, the lakes on the left are full of liquid hydrocarbons while those on the top right are only partially filled, or represent saturated ground or mudflat. Credit: NASA/JPL-Caltech/ASI/Cornell

    Perhaps these signals will act as a beacon for future explorations designed to search for complex organic material in more remote space locations – potentially even on the "giant Earth" exoplanets in distant star systems.

    Space provides us a unique perspective to turn back the pages of chemistry. Among the planets, moons and stars - and the not quite emptiness between - we can study the initial reactions thought to have started chemistry here on Earth.

    Using ever more sensitive telescopes and advanced spacecraft, we have uncovered chemical nurseries - pockets of gas and ice exerted to harsh space radiation - in our Solar System and beyond.

    Such cold, icy objects as Titan, the moons of Jupiter, Trans-Neptunian Objects (such as Pluto and other minor bodies in the Kuiper belt and beyond), as well as microscopic interstellar dust particles, all generate higher-order organic molecules from simple chemical ingredients.

    As far as we know, the lack of heat and liquid water precludes life to exist at these worlds.

    Cassini’s spectrum view of the southern polar vortex shows a signature of frozen hydrogen cyanide molecules (HCN). Credit: NASA/JPL-Caltech/ASI/University of Arizona/SSI/Leiden Observatory and SRON

    However, we can look for clues regarding life's origins on a primitive Earth. Were life-bearing chemicals delivered via comet impact, or made in-house near the early ocean shores or deep sea volcanoes? Observing the chemistry of distant objects could one day provide the answers.

    These forays into our chemical history have been enabled by the significant steps we have taken in our exploration of space including, as a glowing example, the resounding success of Cassini's exploration of Titan.

    This article was originally published on The Conversation. Read the original article.


    NASA Selects Flying Mission to Study Titan for Origins, Signs of Life

    Taking advantage of Titan’s dense atmosphere and low gravity, Dragonfly will explore dozens of locations across the icy world, sampling and measuring the compositions of Titan’s organic surface materials to characterize the habitability of Titan’s environment and investigate the progression of prebiotic chemistry. Image Credit: NASA/JHU-APL.

    NASA has announced that our next destination in the solar system is the unique, richly organic world Titan. Advancing our search for the building blocks of life, the Dragonfly mission will fly multiple sorties to sample and examine sites around Saturn’s icy moon.

    Dragonfly will launch in 2026 and arrive in 2034. The rotorcraft will fly to dozens of promising locations on Titan looking for prebiotic chemical processes common on both Titan and Earth. Dragonfly marks the first time NASA will fly a multi-rotor vehicle for science on another planet it has eight rotors and flies like a large drone. It will take advantage of Titan’s dense atmosphere — four times denser than Earth’s — to become the first vehicle ever to fly its entire science payload to new places for repeatable and targeted access to surface materials.

    Titan is an analog to the very early Earth, and can provide clues to how life may have arisen on our planet. During its 2.7-year baseline mission, Dragonfly will explore diverse environments from organic dunes to the floor of an impact crater where liquid water and complex organic materials key to life once existed together for possibly tens of thousands of years. Its instruments will study how far prebiotic chemistry may have progressed. They also will investigate the moon’s atmospheric and surface properties and its subsurface ocean and liquid reservoirs. Additionally, instruments will search for chemical evidence of past or extant life.

    “With the Dragonfly mission, NASA will once again do what no one else can do,” said NASA Administrator Jim Bridenstine. “Visiting this mysterious ocean world could revolutionize what we know about life in the universe. This cutting-edge mission would have been unthinkable even just a few years ago, but we’re now ready for Dragonfly’s amazing flight.”

    Dragonfly took advantage of 13 years’ worth of Cassini data to choose a calm weather period to land, along with a safe initial landing site and scientifically interesting targets. It will first land at the equatorial “Shangri-La” dune fields, which are terrestrially similar to the linear dunes in Namibia in southern Africa and offer a diverse sampling location. Dragonfly will explore this region in short flights, building up to a series of longer “leapfrog” flights of up to 8 kilometers (5 miles), stopping along the way to take samples from compelling areas with diverse geography. It will finally reach the Selk impact crater, where there is evidence of past liquid water, organics — the complex molecules that contain carbon, combined with hydrogen, oxygen, and nitrogen — and energy, which together make up the recipe for life. The lander will eventually fly more than 175 kilometers (108 miles) — nearly double the distance traveled to date by all the Mars rovers combined.

    “Titan is unlike any other place in the solar system, and Dragonfly is like no other mission,” said Thomas Zurbuchen, NASA’s associate administrator for science at the agency’s headquarters in Washington. “It’s remarkable to think of this rotorcraft flying miles and miles across the organic sand dunes of Saturn’s largest moon, exploring the processes that shape this extraordinary environment. Dragonfly will visit a world filled with a wide variety of organic compounds, which are the building blocks of life and could teach us about the origin of life itself.”

    Titan has a nitrogen-based atmosphere like Earth. Unlike Earth, Titan has clouds and rain of methane. Other organics are formed in the atmosphere and fall like light snow. The moon’s weather and surface processes have combined complex organics, energy, and water similar to those that may have sparked life on our planet.

    Titan is larger than the planet Mercury and is the second largest moon in our solar system. As it orbits Saturn, it is about 1.4 billion kilometers (886 million miles) away from the Sun, about 10× further than Earth. Because it is so far from the Sun, its surface temperature is around –179°C (–290°F). Its surface pressure is also 50% higher than Earth’s.

    Dragonfly was selected as part of the agency’s New Frontiers program, which includes the New Horizons mission to Pluto and the Kuiper Belt, Juno to Jupiter, and OSIRIS-REx to the asteroid Bennu. Dragonfly is led by Principal Investigator Elizabeth Turtle, who is based at Johns Hopkins University’s Applied Physics Laboratory in Laurel, Maryland. New Frontiers supports missions that have been identified as top solar system exploration priorities by the planetary community. The program is managed by the Planetary Missions Program Office at NASA’s Marshall Space Flight Center in Huntsville, Alabama, for the agency’s Planetary Science Division in Washington.

    “The New Frontiers program has transformed our understanding of the solar system, uncovering the inner structure and composition of Jupiter’s turbulent atmosphere, discovering the icy secrets of Pluto’s landscape, revealing mysterious objects in the Kuiper belt, and exploring a near-Earth asteroid for the building blocks of life,” said Lori Glaze, director of NASA’s Planetary Science Division. “Now we can add Titan to the list of enigmatic worlds NASA will explore.”


    Contents

    Titan's consideration as an environment for the study of prebiotic chemistry or potentially exotic life stems in large part due to the diversity of the organic chemistry that occurs in its atmosphere, driven by photochemical reactions in its outer layers. The following chemicals have been detected in Titan's upper atmosphere by Cassini's mass spectrometer:

    Study Magee, 1050 km Cui, 1050 km Cui, 1077 km Waite et al., 1000–1045 km
    Density (cm −3 ) (3.18±0.71) x 10 9 (4.84±0.01) x 10 9 (2.27±0.01) x 10 9 (3.19, 7.66) x 10 9
    Proportions of different species
    Nitrogen (96.3±0.44)% (97.8±0.2)% (97.4±0.5)% (95.5, 97.5)%
    14 N 15 N (1.08±0.06)%
    Methane (2.17±0.44)% (1.78±0.01)% (2.20±0.01)% (1.32, 2.42)%
    13 CH4 (2.52±0.46) x 10 −4
    Hydrogen (3.38±0.23) x 10 −3 (3.72±0.01) x 10 −3 (3.90±0.01) x 10 −3
    Acetylene (3.42±0.14) x 10 −4 (1.68±0.01) x 10 −4 (1.57±0.01) x 10 −4 (1.02, 3.20) x 10 −4
    Ethylene (3.91±0.23) x 10 −4 (5.04±0.04) x 10 −4 (4.62±0.04) x 10 −4 (0.72, 1.02) x 10 −3
    Ethane (4.57±0.74) x 10 −5 (4.05±0.19) x 10 −5 (2.68±0.19) x 10 −5 (0.78, 1.50) x 10 −5
    Hydrogen cyanide (2.44±0.10) x 10 −4
    40 Ar (1.26±0.05) x 10 −5 (1.25±0.02) x 10 −5 (1.10±0.03) x 10 −5
    Propyne (9.20±0.46) x 10 −6 (9.02±0.22) x 10 −6 (6.31±0.24) x 10 −6 (0.55, 1.31) x 10 −5
    Propene (2.33±0.18) x 10 −6 (0.69, 3.59) x 10 −4
    Propane (2.87±0.26) x 10 −6 <1.84 x 10 −6 <2.16e-6(3.90±0.01) x 10 −6
    Diacetylene (5.55±0.25) x 10 −6 (4.92±0.10) x 10 −6 (2.46±0.10) x 10 −6 (1.90, 6.55) x 10 −6
    Cyanogen (2.14±0.12) x 10 −6 (1.70±0.07) x 10 −6 (1.45±0.09) x 10 −6 (1.74, 6.07) x 10 −6
    Cyanoacetylene (1.54±0.09) x 10 −6 (1.43±0.06) x 10 −6 <8.27 x 10 −7
    Acrylonitrile (4.39±0.51) x 10 −7 <4.00 x 10 −7 <5.71 x 10 −7
    Propanenitrile (2.87±0.49) x 10 −7
    Benzene (2.50±0.12) x 10 −6 (2.42±0.05) x 10 −6 (3.90±0.01) x 10 −7 (5.5, 7.5) x 10 −3
    Toluene (2.51±0.95) x 10 −8 <8.73 x 10 −8 (3.90±0.01) x 10 −7 (0.83, 5.60) x 10 −6

    As mass spectrometry identifies the atomic mass of a compound but not its structure, additional research is required to identify the exact compound that has been detected. Where the compounds have been identified in the literature, their chemical formula has been replaced by their name above. The figures in Magee (2009) involve corrections for high pressure background. Other compounds believed to be indicated by the data and associated models include ammonia, polyynes, amines, ethylenimine, deuterium hydride, allene, 1,3 butadiene and any number of more complex chemicals in lower concentrations, as well as carbon dioxide and limited quantities of water vapour. [2] [3] [4]

    Due to its distance from the Sun, Titan is much colder than Earth. Its surface temperature is about 94 K (−179 °C, or −290 °F). At these temperatures, water ice—if present—does not melt, evaporate or sublime, but remains solid. Because of the extreme cold and also because of lack of carbon dioxide (CO2) in the atmosphere, scientists such as Jonathan Lunine have viewed Titan less as a likely habitat for extraterrestrial life, than as an experiment for examining hypotheses on the conditions that prevailed prior to the appearance of life on Earth. [5] Even though the usual surface temperature on Titan is not compatible with liquid water, calculations by Lunine and others suggest that meteor strikes could create occasional "impact oases"—craters in which liquid water might persist for hundreds of years or longer, which would enable water-based organic chemistry. [6] [7] [8]

    However, Lunine does not rule out life in an environment of liquid methane and ethane, and has written about what discovery of such a life form (even if very primitive) would imply about the prevalence of life in the universe. [9]

    Past hypothesis about the temperature Edit

    In the 1970s, astronomers found unexpectedly high levels of infrared emissions from Titan. [10] One possible explanation for this was the surface was warmer than expected, due to a greenhouse effect. Some estimates of the surface temperature even approached temperatures in the cooler regions of Earth. There was, however, another possible explanation for the infrared emissions: Titan's surface was very cold, but the upper atmosphere was heated due to absorption of ultraviolet light by molecules such as ethane, ethylene and acetylene. [10]

    In September 1979, Pioneer 11, the first space probe to conduct fly-by observations of Saturn and its moons, sent data showing Titan's surface to be extremely cold by Earth standards, and much below the temperatures generally associated with planetary habitability. [11]

    Future temperature Edit

    Titan may become warmer in the future. [12] Five to six billion years from now, as the Sun becomes a red giant, surface temperatures could rise to

    200 K (−70 °C), high enough for stable oceans of a water–ammonia mixture to exist on its surface. As the Sun's ultraviolet output decreases, the haze in Titan's upper atmosphere will be depleted, lessening the anti-greenhouse effect on its surface and enabling the greenhouse effect created by atmospheric methane to play a far greater role. These conditions together could create an environment agreeable to exotic forms of life, and will persist for several hundred million years. [12] This was sufficient time for simple life to evolve on Earth, although the presence of ammonia on Titan could cause the same chemical reactions to proceed more slowly. [12]

    The lack of liquid water on Titan's surface was cited by NASA astrobiologist Andrew Pohorille in 2009 as an argument against life there. Pohorille considers that water is important not only as the solvent used by "the only life we know" but also because its chemical properties are "uniquely suited to promote self-organization of organic matter". He has questioned whether prospects for finding life on Titan's surface are sufficient to justify the expense of a mission that would look for it. [13] However, his claims go against the idea that life on Earth is not the only type of life possible.

    Laboratory simulations have led to the suggestion that enough organic material exists on Titan to start a chemical evolution analogous to what is thought to have started life on Earth. While the analogy assumes the presence of liquid water for longer periods than is currently observable, several hypotheses suggest that liquid water from an impact could be preserved under a frozen isolation layer. [14] It has also been proposed that ammonia oceans could exist deep below the surface [15] [16] one model suggests an ammonia–water solution as much as 200 km deep beneath a water ice crust, conditions that, "while extreme by terrestrial standards, are such that life could indeed survive". [17] Heat transfer between the interior and upper layers would be critical in sustaining any sub-surface oceanic life. [15] Detection of microbial life on Titan would depend on its biogenic effects. For example, the atmospheric methane and nitrogen could be examined for biogenic origin. [17]

    Data published in 2012 obtained from NASA's Cassini spacecraft, have strengthened evidence that Titan likely harbors a layer of liquid water under its ice shell. [18]

    Titan is the only known natural satellite (moon) in the Solar System that has a fully developed atmosphere that consists of more than trace gases. Titan's atmosphere is thick, chemically active, and is known to be rich in organic compounds this has led to speculation about whether chemical precursors of life may have been generated there. [19] [20] [21] The atmosphere also contains hydrogen gas, which is cycling through the atmosphere and the surface environment, and which living things comparable to Earth methanogens could combine with some of the organic compounds (such as acetylene) to obtain energy. [19] [20] [21]

    The Miller–Urey experiment and several following experiments have shown that with an atmosphere similar to that of Titan and the addition of UV radiation, complex molecules and polymer substances like tholins can be generated. The reaction starts with dissociation of nitrogen and methane, forming hydrogen cyanide and acetylene. Further reactions have been studied extensively. [22]

    In October 2010, Sarah Hörst of the University of Arizona reported finding the five nucleotide bases—building blocks of DNA and RNA—among the many compounds produced when energy was applied to a combination of gases like those in Titan's atmosphere. Hörst also found amino acids, the building blocks of protein. She said it was the first time nucleotide bases and amino acids had been found in such an experiment without liquid water being present. [23]

    In April 2013, NASA reported that complex organic chemicals could arise on Titan based on studies simulating the atmosphere of Titan. [24] In June 2013, polycyclic aromatic hydrocarbons (PAHs) were detected in the upper atmosphere of Titan. [25]

    Research has suggested that polyimine could readily function as a building block in Titan's conditions. [26] Titan's atmosphere produces significant quantities of hydrogen cyanide, which readily polymerize into forms which can capture light energy in Titan's surface conditions. As of yet, the answer to what happens with Titan's cyanide is unknown while it is rich in the upper atmosphere where it is created, it is depleted at the surface, suggesting that there is some sort of reaction consuming it. [27]

    Hydrocarbons as solvents Edit

    Although all living things on Earth (including methanogens) use liquid water as a solvent, it is conceivable that life on Titan might instead use a liquid hydrocarbon, such as methane or ethane. [28] Water is a stronger solvent than hydrocarbons [29] however, water is more chemically reactive, and can break down large organic molecules through hydrolysis. [28] A life-form whose solvent was a hydrocarbon would not face the risk of its biomolecules being destroyed in this way. [28]

    Titan appears to have lakes of liquid ethane or liquid methane on its surface, as well as rivers and seas, which some scientific models suggest could support hypothetical non-water-based life. [19] [20] [21] It has been speculated that life could exist in the liquid methane and ethane that form rivers and lakes on Titan's surface, just as organisms on Earth live in water. [30] Such hypothetical creatures would take in H2 in place of O2, react it with acetylene instead of glucose, and produce methane instead of carbon dioxide. [30] By comparison, some methanogens on Earth obtain energy by reacting hydrogen with carbon dioxide, producing methane and water.

    In 2005, astrobiologists Chris McKay and Heather Smith predicted that if methanogenic life is consuming atmospheric hydrogen in sufficient volume, it will have a measurable effect on the mixing ratio in the troposphere of Titan. The effects predicted included a level of acetylene much lower than otherwise expected, as well as a reduction in the concentration of hydrogen itself. [30]

    Evidence consistent with these predictions was reported in June 2010 by Darrell Strobel of Johns Hopkins University, who analysed measurements of hydrogen concentration in the upper and lower atmosphere. Strobel found that the hydrogen concentration in the upper atmosphere is so much larger than near the surface that the physics of diffusion leads to hydrogen flowing downwards at a rate of roughly 10 25 molecules per second. Near the surface the downward-flowing hydrogen apparently disappears. [29] [30] [31] Another paper released the same month showed very low levels of acetylene on Titan's surface. [29]

    Chris McKay agreed with Strobel that presence of life, as suggested in McKay's 2005 article, is a possible explanation for the findings about hydrogen and acetylene, but also cautioned that other explanations are currently more likely: namely the possibility that the results are due to human error, to a meteorological process, or to the presence of some mineral catalyst enabling hydrogen and acetylene to react chemically. [1] [32] He noted that such a catalyst, one effective at −178 °C (95 K), is presently unknown and would in itself be a startling discovery, though less startling than discovery of an extraterrestrial life form. [1]

    The June 2010 findings gave rise to considerable media interest, including a report in the British newspaper, the Telegraph, which spoke of clues to the existence of "primitive aliens". [33]

    Cell membranes Edit

    A hypothetical cell membrane capable of functioning in liquid methane was modeled in February 2015. [34] The proposed chemical base for these membranes is acrylonitrile, which has been detected on Titan. [35] Called an "azotosome" ('nitrogen body'), formed from "azoto", Greek for nitrogen, and "soma", Greek for body, it lacks the phosphorus and oxygen found in phospholipids on Earth but contains nitrogen. Despite the very different chemical structure and external environment, its properties are surprisingly similar, including autoformation of sheets, flexibility, stability, and other properties. According to computer simulations azotosomes could not form or function under the weather conditions found on Titan. [36]

    An analysis of Cassini data, completed in 2017, confirmed substantial amounts of acrylonitrile in Titan's atmosphere. [37] [38]

    Comparative habitability Edit

    In order to assess the likelihood of finding any sort of life on various planets and moons, Dirk Schulze-Makuch and other scientists have developed a planetary habitability index which takes into account factors including characteristics of the surface and atmosphere, availability of energy, solvents and organic compounds. [39] Using this index, based on data available in late 2011, the model suggests that Titan has the highest current habitability rating of any known world, other than Earth. [39]

    Titan as a test case Edit

    While the Cassini–Huygens mission was not equipped to provide evidence for biosignatures or complex organics, it showed an environment on Titan that is similar, in some ways, to ones theorized for the primordial Earth. [40] Scientists think that the atmosphere of early Earth was similar in composition to the current atmosphere on Titan, with the important exception of a lack of water vapor on Titan. [41] Many hypotheses have developed that attempt to bridge the step from chemical to biological evolution.

    Titan is presented as a test case for the relation between chemical reactivity and life, in a 2007 report on life's limiting conditions prepared by a committee of scientists under the United States National Research Council. The committee, chaired by John Baross, considered that "if life is an intrinsic property of chemical reactivity, life should exist on Titan. Indeed, for life not to exist on Titan, we would have to argue that life is not an intrinsic property of the reactivity of carbon-containing molecules under conditions where they are stable. " [42]

    David Grinspoon, one of the scientists who in 2005 proposed that hypothetical organisms on Titan might use hydrogen and acetylene as an energy source, [43] has mentioned the Gaia hypothesis in the context of discussion about Titan life. He suggests that, just as Earth's environment and its organisms have evolved together, the same thing is likely to have happened on other worlds with life on them. In Grinspoon's view, worlds that are "geologically and meteorologically alive are much more likely to be biologically alive as well". [44]

    Panspermia or independent origin Edit

    An alternate explanation for life's hypothetical existence on Titan has been proposed: if life were to be found on Titan, it could have originated from Earth in a process called panspermia. It is theorized that large asteroid and cometary impacts on Earth's surface have caused hundreds of millions of fragments of microbe-laden rock to escape Earth's gravity. Calculations indicate that a number of these would encounter many of the bodies in the Solar System, including Titan. [45] [46] On the other hand, Jonathan Lunine has argued that any living things in Titan's cryogenic hydrocarbon lakes would need to be so different chemically from Earth life that it would not be possible for one to be the ancestor of the other. [47] In Lunine's view, presence of organisms in Titan's lakes would mean a second, independent origin of life within the Solar System, implying that life has a high probability of emerging on habitable worlds throughout the cosmos. [48]

    The proposed Titan Mare Explorer mission, a Discovery-class lander that would splash down in a lake, "would have the possibility of detecting life", according to astronomer Chris Impey of the University of Arizona. [49]

    The planned Dragonfly rotorcraft mission is intended to land on solid ground and relocate many times. [50] Dragonfly will be New Frontiers program Mission #4. Its instruments will study how far prebiotic chemistry may have progressed. [51] Dragonfly will carry equipment to study the chemical composition of Titan's surface, and to sample the lower atmosphere for possible biosignatures, including hydrogen concentrations. [51]


    Weird ring-shaped molecule on Titan could be a building block to life

    A circular molecule spotted on Saturn’s moon Titan may help form precursors to life. This compound hasn’t been seen in the atmosphere of any planet or moon before.

    The molecule is called cyclopropenylidene and is made up of three carbon atoms in a ring with two hydrogen atoms attached. Conor Nixon at NASA’s Goddard Space Flight Center in Maryland and his colleagues spotted it floating in Titan’s thick atmosphere using the Atacama Large Millimeter/submillimeter Array in Chile.

    Finding this molecule on Titan was a surprise. It is extremely reactive – if it bumps into any other particles, it tends to be quick to chemically react with them to form new compounds. Because of this, it had previously only ever been seen in tenuous clouds of gas and dust in interstellar space. Somehow, it lasts in the upper layers of Titan’s skies.

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    Read more: Return to Titan: Why this icy world is our best bet to find life

    Ring-shaped molecules like this tend to act as the building blocks of molecules necessary for life, such as DNA and RNA. “This is a really small building block, but you can build bigger and bigger things with it,” says Nixon. “I don’t think anyone necessarily believes that there’s microbes on Titan, but the fact that we can form complex molecules like this on Titan could help tell us things like how life got started on Earth.”

    Conditions on Titan now may be similar to those on Earth early in the planet’s history, when the air was dominated by methane instead of oxygen. Studying its potential for life could help us learn about the beginnings of life here as well.

    Titan has the biggest variety of molecules on any moon or planet we have investigated, says Nixon. ”It’s sort of this happy hunting ground for new things,” he says. “Molecules like this are almost an early warning sign that there’s more exciting chemistry to be found.”

    Right now, we can only look for that from Earth, but the Dragonfly spacecraft, planned to launch in 2027, will examine Titan’s surface up close.

    Journal reference: The Astronomical Journal, DOI: 10.3847/1538-3881/abb679

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