Titan's South Polar Vortex

These two views of Saturn's moon Titan show the southern polar vortex, a huge, swirling cloud that was first observed by NASA's Cassini spacecraft in 2012.

The view at left is a spectral map of Titan obtained with the Cassini Visual and Infrared Mapping Spectrometer (VIMS) on November 29, 2012. The inset image is a natural-color close-up of the polar vortex taken by Cassini's wide-angle camera (part of the view previously released as PIA14925).

Three distinct components are evident in the VIMS image, represented by different colors: the surface of Titan (orange, near center), atmospheric haze along the limb (light green, at top) and the polar vortex (blue, at lower left).

To the VIMS instrument, the spectrum of the southern polar vortex shows a remarkable difference with respect to other portions of Titan's atmosphere: a signature of frozen hydrogen cyanide molecules (HCN). This discovery has suggested to researchers that the atmosphere of Titan's southern hemisphere is cooling much faster than expected. Observing seasonal shifts like this in the moon's climate is a major goal for Cassini's current extended mission.

Image credit: NASA/JPL-Caltech/ASI/University of Arizona/SSI/Leiden Observatory and SRON

Note: For more information, see Titan's Swirling Polar Cloud is Cold and Toxic (ESA) and Swirling Cloud at Titan's Pole is Cold and Toxic (JPL).

Titan's Subsurface Reservoirs

Hundreds of lakes and seas are spread across the surface of Saturn's moon Titan. These lakes are filled with hydrocarbons, a form of organic compound that is also found naturally on Earth and includes methane.

While most of the liquid in the lakes is thought to be replenished by rainfall from clouds in Titan's atmosphere, the cycling of liquid between the subsurface, surface and atmosphere is still not well understood.

Scientists have modeled how a subsurface reservoir ('alkanofer') of liquid hydrocarbons, filled with rainfall runoff, would diffuse throughout Titan's porous icy crust. They found that this diffusion could cause a new reservoir – formed from clathrates - to form where the bottom of the original reservoir meets layers of non-porous ice.

Clathrates are compounds that form a crystal structure with small cages that trap other substances like methane and ethane. Titan's subsurface clathrate reservoirs would interact with and fractionate (separate) the liquid phase within the original underground hydrocarbon lake, slowly changing its composition. Eventually, subsurface lakes that had come into contact with the clathrate layer would mainly be composed of either propane or ethane, depending on the type of clathrate that had formed.

Importantly, this would continue up to Titan's surface. Lakes fed by these propane or ethane subsurface reservoirs would show the same kind of composition, whereas those fed by rainfall would be different and contain methane, nitrogen, and trace amounts of argon and carbon monoxide. The composition of the lake would indicate what was happening deep underground.

Illustration credit: ESA/ATG medialab

Note: For more information, see PIA18417: Titan's Subsurface Reservoirs (Artist's Concept) and Icy Aquifers on Titan Transform Methane Rainfall.

Methane Clouds Over Titan's Ligeia Mare

This animated sequence of Cassini images shows methane clouds moving above the large methane sea on Saturn's moon Titan known as Ligeia Mare.

The spacecraft captured the views between July 20 and July 22, 2014, as it departed Titan following a flyby. Cassini tracked the system of clouds as it developed and dissipated over Ligeia Mare during this two-day period. Measurements of the cloud motions indicate wind speeds of around 7 to 10 miles per hour (3 to 4.5 meters per second).

The timing between exposures in the sequence varies. In particular, there is a 17.5-hour jump between the second and third frames. Most other frames are separated by one to two hours.

A separate view, PIA18421, shows the location of these clouds relative to features in Titan's north polar region.

Animation credit: NASA/JPL-Caltech/Space Science Institute

Note: For more information, see Cassini Tracks Clouds Developing Over a Titan Sea.

Titan's Ocean Perhaps as Salty as the Dead Sea

Researchers found that Titan's ice shell, which overlies a very salty ocean, varies in thickness around the moon, suggesting the crust is in the process of becoming rigid.

Scientists analyzing data from NASA's Cassini mission have firm evidence the ocean inside Saturn's largest moon, Titan, might be as salty as Earth's Dead Sea.

The new results come from a study of gravity and topography data collected during Cassini's repeated flybys of Titan during the past 10 years. Using the Cassini data, researchers presented a model structure for Titan, resulting in an improved understanding of the structure of the moon's outer ice shell. The findings are published in this week's edition of the journal Icarus.

"Titan continues to prove itself as an endlessly fascinating world, and with our long-lived Cassini spacecraft, we're unlocking new mysteries as fast as we solve old ones," said Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory in Pasadena, California, who was not involved in the study.

Additional findings support previous indications the moon's icy shell is rigid and in the process of freezing solid. Researchers found that a relatively high density was required for Titan's ocean in order to explain the gravity data. This indicates the ocean is probably an extremely salty brine of water mixed with dissolved salts likely composed of sulfur, sodium and potassium. The density indicated for this brine would give the ocean a salt content roughly equal to the saltiest bodies of water on Earth.

"This is an extremely salty ocean by Earth standards," said the paper's lead author, Giuseppe Mitri of the University of Nantes in France. "Knowing this may change the way we view this ocean as a possible abode for present-day life, but conditions might have been very different there in the past."

Cassini data also indicate the thickness of Titan's ice crust varies slightly from place to place. The researchers said this can best be explained if the moon's outer shell is stiff, as would be the case if the ocean were slowly crystallizing and turning to ice. Otherwise, the moon's shape would tend to even itself out over time, like warm candle wax. This freezing process would have important implications for the habitability of Titan's ocean, as it would limit the ability of materials to exchange between the surface and the ocean.

A further consequence of a rigid ice shell, according to the study, is any outgassing of methane into Titan's atmosphere must happen at scattered "hot spots" -- like the hot spot on Earth that gave rise to the Hawaiian Island chain. Titan's methane does not appear to result from convection or plate tectonics recycling its ice shell.

How methane gets into the moon's atmosphere has long been of great interest to researchers, as molecules of this gas are broken apart by sunlight on short geological timescales. Titan's present atmosphere contains about five percent methane. This means some process, thought to be geological in nature, must be replenishing the gas. The study indicates that whatever process is responsible, the restoration of Titan's methane is localized and intermittent.

"Our work suggests looking for signs of methane outgassing will be difficult with Cassini, and may require a future mission that can find localized methane sources," said Jonathan Lunine, a scientist on the Cassini mission at Cornell University, Ithaca, New York, and one of the paper's co-authors. "As on Mars, this is a challenging task."

Image credit: NASA/JPL-Caltech/SSI/Univ. of Arizona/G. Mitri/University of Nantes

Note: For more information, see Saturn's Moon Titan Has a Very Salty Ocean.

Water's Early Journey in a Solar System

The building blocks of comets, and apparently Saturn's largest moon, Titan, formed under similar conditions in the disk of gas and dust that formed the sun.

NASA's Spitzer Space Telescope observed a fledgling solar system like the one depicted in this artist's concept, and discovered deep within it enough water vapor to fill the oceans on Earth five times. This water vapor starts out in the form of ice in a cloudy cocoon (not pictured) that surrounds the embryonic star, called NGC 1333-IRAS 4B (buried in center of image). Material from the cocoon, including ice, falls toward the center of the cloud. The ice then smacks down onto a dusty pre-planetary disk circling the stellar embryo (doughnut-shaped cloud) and vaporizes. Eventually, this water might make its way into developing planets.

Illustration credit: NASA/JPL-Caltech

Note: For more information, see Titan's Building Blocks Might Pre-date Saturn.

Titan

Only a sharp and careful eye can make out the subtle variations in Titan's clouds when viewed in visible light. However, these subtle features sometimes become more readily apparent when imaged at other wavelengths of light. This infrared image clearly reveals a band around the Titan's north pole.

Cassini scientists are regularly monitoring Titan, hoping to understand more about Titan's dense atmosphere and clouds.

This view looks toward the leading side of Titan. North on Titan is up and rotated 31 degrees to the left. The image was taken with the Cassini spacecraft narrow-angle camera on January 26, 2014 using a spectral filter which preferentially admits wavelengths of near-infrared light centered at 889 nanometers.

The view was acquired at a distance of approximately 1.5 million miles (2.4 million kilometers) from Titan. Image scale is 9 miles (14 kilometers) per pixel.

Image credit: NASA/JPL-Caltech/Space Science Institute

Radio Occultation of Titan During Cassini Flyby

Cassini will attempt to bounce signals off of Saturn's moon Titan once more during a flyby on June 18, 2014, revealing important details about the moon's surface.

As NASA's Cassini spacecraft zooms toward Saturn's smoggy moon Titan for a targeted flyby on June 18, mission scientists are excitedly hoping to repeat a scientific tour de force that will provide valuable new insights into the nature of the moon's surface and atmosphere.

For Cassini's radio science team, the last flyby of Titan, on May 17, was one of the most scientifically valuable encounters of the spacecraft's current extended mission. The focus of that flyby, designated "T-101," was on using radio signals to explore the physical nature of Titan's vast northern seas and probe the high northern regions of its substantial atmosphere.

The Cassini team hopes to replicate the technical success of that flyby during the T-102 encounter, slated for June 18, during which the spacecraft will attempt similar measurements of Titan. During closest approach, the spacecraft will be just 2,274 miles (3,659 kilometers) above the surface of the moon while traveling at 13,000 miles per hour (5.6 kilometers per second).

During the upcoming flyby, if all goes well as before, Cassini's radio science subsystem will bounce signals off the surface of Titan, toward Earth, where they will be received by the ground stations of NASA's Deep Space Network. This sort of observation is known as a bistatic scattering experiment and its results can yield clues to help answer a variety of questions about large areas of Titan's surface: Are they solid, slushy or liquid? Are they reflective? What might they be made of?

During the May encounter, Cassini beamed radio signals over the two largest bodies of liquid on Titan, seas named Ligeia Mare and Kraken Mare. During that first attempt, scientists could not be certain the signals would successfully bounce off the lakes to be received on Earth. They were thrilled when ground stations received specular reflections -- essentially the glint -- of the radio frequencies as they ricocheted off Titan.

"We held our breath as Cassini turned to beam its radio signals at the lakes," said Essam Marouf, a member of the Cassini radio science team of San Jose State University in California. "We knew we were getting good quality data when we saw clear echoes from Titan's surface. It was thrilling."

A second technical accomplishment -- an experiment to send precision-tuned radio frequencies through Titan's atmosphere -- also makes the May and June flybys special. The experiment, known as a radio occultation, provides information about how temperatures vary by altitude in Titan's atmosphere. Preparing for these experiments tested just how thoroughly the Cassini team has come to understand the structure of Titan's atmosphere during nearly a decade of study by the mission.

During this type of radio occultation, a signal is beamed from Earth through the atmosphere of Titan toward the Cassini spacecraft, which responds back to Earth with an identical signal. Information about Titan is imprinted in the signal as it passes through the moon's atmosphere, encountering differences in temperature and density. The trick is that the transmitted signal must be varied during the experiment so that it remains nearly constant when received by the spacecraft.

In order to give the occultation experiments any chance of success, the team has to account for not only the relative motions of the spacecraft and the transmitting antennas on the rotating planet Earth, but also the ways the signal is bent by different layers in Titan's atmosphere.

While this procedure has been used successfully for several Saturn occultations in the past two years, it had not yet been tried at Titan. And since the Titan occultations last just a few minutes, the team was concerned about how quickly the frequency lockup between ground and spacecraft could be established, if at all. For comparison, NASA's Magellan mission tried the technique at Venus in the 1990s, without success.

As they waited for signs of confirmation during the May encounter, the team saw the signal lock occur in only a few seconds, indicating that their predictions were spot-on. Data on Titan's atmosphere flowed in, adding new information to the mission's campaign to monitor the changing of the seasons on this alien moon.

"This was like trying to hit a hole-in-one in golf, except that the hole is close to a billion miles away, and moving," said Earl Maize, Cassini project manager at NASA's Jet Propulsion Laboratory in Pasadena, California. "This was our first attempt to precisely predict and compensate for the effect of Titan's atmosphere on the uplinked radio signal from Earth, and it worked to perfection."

Illustration credit: NASA/JPL-Caltech

Crescent Titan and Its South Polar Vortex

Titan's polar vortex stands illuminated where all else is in shadow. Scientists deduce that the vortex must extend higher into Titan's atmosphere than the surrounding clouds because it is still lit in images like this. Although the south polar region is now in winter, the Sun can still reach high features like the vortex.

Titan (3,200 miles, or 5,150 kilometers across) is Saturn's largest moon. For a color image of the south polar vortex on Titan, see PIA14919. For a movie of the vortex, see PIA14920.

This view looks toward the Saturn-facing hemisphere of Titan. North on Titan is up and rotated 32 degrees to the right. The image was taken with the Cassini spacecraft wide-angle camera on February 3, 2014 using a spectral filter which preferentially admits wavelengths of near-infrared light centered at 742 nanometers.

The view was obtained at a distance of approximately 134,000 miles (215,000 kilometers) from Titan. Image scale is 8 miles (13 kilometers) per pixel.

Image credit: NASA/JPL-Caltech/Space Science Institute

Artist's Conception of Cassini Observing Sunsets on Titan

Using data collected by Cassini's Visual and Infrared Mapping Spectrometer, or VIMS, while observing Titan's sunsets, researchers created simulated spectra of Titan as if it were a planet transiting across the face of a distant star. The research helps scientists to better understand observations of exoplanets with hazy atmospheres.

Image Credit: NASA/JPL-Caltech

Note: For more information, see Sunsets on Titan Reveal the Complexity of Hazy Exoplanets.

Titanic Crescents

Titan's south polar vortex mimics the moon itself, creating an elegant crescent within a crescent. Situated above the surrounding polar atmosphere, the raised walls along the sunward side of the vortex just catch the grazing sunlight, creating a crescent of its own. Titan (3,200 miles, or 5,150 kilometers across) is Saturn's largest moon and possesses a dense and dynamic atmosphere. For a color image of the south polar vortex on Titan, see PIA14919. For a movie of the vortex, see PIA14920.

This view looks toward the trailing hemisphere of Titan. North on Titan is up. The image was taken with the Cassini spacecraft wide-angle camera on December 1, 2013 using a spectral filter which preferentially admits wavelengths of near-infrared light centered at 939 nanometers.

The view was obtained at a distance of approximately 108,000 miles (174,000 kilometers) from Titan. Image scale is 6 miles (10 kilometers) per pixel.

Image credit: NASA/JPL-Caltech/Space Science Institute

Titan

Titan's atmosphere puts on a display with the detached haze to the north (top of image) and the polar vortex to the south.

Titan (3,200 miles, or 5,150 kilometers across) is Saturn's largest moon. For a color image of the south polar vortex on Titan, see PIA14919. For a movie of the vortex, see PIA14920.

This view looks toward the Saturn-facing hemisphere of Titan. North on Titan is up and rotated 24 degrees to the left. The image was taken with the Cassini spacecraft narrow-angle camera on August 20, 2013 using a spectral filter sensitive to wavelengths of near-infrared light centered at 889 nanometers.

The view was obtained at a distance of approximately 1.6 million miles (2.5 million kilometers) from Titan. Image scale is 9 miles (15 kilometers) per pixel.

Photo credit: NASA/JPL-Caltech/Space Science Institute

Titan's Dark Side

NASA's Cassini spacecraft looks towards the dark side of Saturn's largest moon, Titan, capturing the blue halo caused by a haze layer that hovers high in the moon's atmosphere. The haze that permeates Titan's atmosphere scatters sunlight and produces the orange color seen here. More on Titan's orange and blue hazes can be found at PIA14913.

This view looks towards the side of Titan (3,200 miles or 5,150 kilometers across) that leads in its orbit around Saturn. North on Titan is up and rotated 40 degrees to the left. Images taken using red, green and blue spectral filters were combined to create this natural-color view. The images were taken with the Cassini spacecraft narrow-angle camera on November 3, 2013.

The view was acquired at a distance of approximately 2.421 million miles (3.896 million kilometers) from Titan. Image scale is 14 miles (23 kilometers) per pixel.

Photo credit: NASA/JPL-Caltech/Space Science Institute

Note: For more information, see Cassini Sees Saturn and Moons in Holiday Dress.

Titan's South Polar Vortex

Slipping into shadow, the south polar vortex at Saturn's moon Titan still stands out against the orange and blue haze layers that are characteristic of Titan's atmosphere. Images like this, from NASA's Cassini spacecraft, lead scientists to conclude that the polar vortex clouds form at a much higher altitude -- where sunlight can still reach -- than the lower-altitude surrounding haze.

For another color image of the south polar vortex on Titan, see PIA14919. For a movie of the vortex, see PIA14920.

This view looks towards the trailing hemisphere of Titan (3,200 miles or 5,150 kilometers across). North on Titan is up and rotated 17 degrees to the left. Images taken using red, green and blue spectral filters were combined to create this natural-color view. The image was taken with the Cassini spacecraft narrow-angle camera on July 30, 2013.

The view was acquired at a distance of approximately 895,000 miles (1.441 million kilometers) from Titan. Image scale is 5 miles (9 kilometers) per pixel.

Photo credit: NASA/JPL-Caltech/Space Science Institute

Note: For more information, see Cassini Sees Saturn and Moons in Holiday Dress.

Titan and Rhea

Saturn's largest and second largest moons, Titan and Rhea, appear to be stacked on top of each other in this true-color scene from NASA's Cassini spacecraft.

The north polar hood can be seen on Titan (3,200 miles or 5,150 kilometers across) appearing as a detached layer at the top of the moon on the top right. See PIA08137 to learn more about Titan's atmosphere and the north polar hood.

This view looks toward the Saturn-facing side of Rhea (949 miles or 1528 kilometers across). North on Rhea is up and rotated 35 degrees to the right.

Images taken using red, green and blue spectral filters were combined to create this natural-color view. The images were acquired with the Cassini spacecraft narrow-angle camera on June 16, 2011, at a distance of approximately 1.1 million miles (1.8 million kilometers) from Rhea and 1.5 million miles (2.5 million kilometers) from Titan. Image scale is 7 miles (11 kilometers) per pixel on Rhea and 9 miles (15 kilometers) on Titan.

Photo credit: NASA/JPL-Caltech/Space Science Institute

Note: For more information, see Cassini Sees Saturn and Moons in Holiday Dress.

Titan's South Polar Vortex

The sunlit edge of Titan's south polar vortex stands out distinctly against the darkness of the moon's unilluminated hazy atmosphere. The Cassini spacecraft images of the vortex led scientists to conclude that its clouds form at a much higher altitude -- where sunlight can still reach -- than the surrounding haze.

Titan (3,200 miles, or 5,150 kilometers across) is Saturn's largest moon. For a color image of the south polar vortex on Titan, see PIA14919. For a movie of the vortex, see PIA14920.

This view looks toward the trailing hemisphere of Titan. North on Titan is up and rotated 32 degrees to the left. The image was taken with the Cassini spacecraft narrow-angle camera on July 14, 2013 using a spectral filter sensitive to wavelengths of near-infrared light centered at 938 nanometers.

The view was obtained at a distance of approximately 808,000 miles (1.3 million kilometers) from Titan and at a Sun-Titan-spacecraft, or phase, angle of 82 degrees. Image scale is 5 miles (8 kilometers) per pixel.

Photo credit: NASA/JPL-Caltech/Space Science Institute

Cassini Orbits Through September 2017

This picture traces Cassini's orbits from Saturn orbit insertion, on 1 July 2004, through the planned end of the mission, on 15 September 2017.

Saturn is in the center, with the orbit of its largest moon Titan in red and the orbits of its six other inner satellites in white.

Cassini's prime mission, completed in 2008, is shown in green. Its first mission extension, which was known as the Equinox Mission and ended in 2010, is shown in orange. The completed orbits of its second mission extension, known as the Solstice Mission, are shown in purple. Orbits after Cassini's 15th anniversary of launch, on 15 October 2012, appear in dark grey. These include orbits that pass inside Saturn's innermost ring, which start in April 2017.

Flybys are a major element of Cassini's tour. The spacecraft's looping, elliptical path around Saturn is carefully designed to enable occasional visits to the many moons in the system. All flybys provide an opportunity to learn more about Saturn's icy satellites, and encounters with giant Titan are actually used to navigate the spacecraft, changing its orbit or setting up future flybys.

Many of the most exciting encounters are "targeted" flybys, for which Cassini's flight path is steered so the spacecraft will pass by a specific moon at a predetermined distance, referred to as "closest approach". Cassini's targeted flybys have yielded incredible close-up views and many groundbreaking science results.

Image credit: NASA/JPL-Caltech

Note: For more information, see The Active Sun Boosts Titan's Outer Atmosphere

NASA's Cassini Spacecraft Finds Ingredient of Household Plastic in Space

NASA's Cassini spacecraft looks toward the night side of Saturn's largest moon and sees sunlight scattering through the periphery of Titan's atmosphere and forming a ring of color.

NASA's Cassini spacecraft has detected propylene, a chemical used to make food-storage containers, car bumpers and other consumer products, on Saturn's moon Titan.

This is the first definitive detection of the plastic ingredient on any moon or planet, other than Earth.

A small amount of propylene was identified in Titan's lower atmosphere by Cassini's composite infrared spectrometer (CIRS). This instrument measures the infrared light, or heat radiation, emitted from Saturn and its moons in much the same way our hands feel the warmth of a fire.

Propylene is the first molecule to be discovered on Titan using CIRS. By isolating the same signal at various altitudes within the lower atmosphere, researchers identified the chemical with a high degree of confidence. Details are presented in a paper in the September 30 edition of the Astrophysical Journal Letters.

"This chemical is all around us in everyday life, strung together in long chains to form a plastic called polypropylene," said Conor Nixon, a planetary scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and lead author of the paper. "That plastic container at the grocery store with the recycling code 5 on the bottom -- that's polypropylene."

CIRS can identify a particular gas glowing in the lower layers of the atmosphere from its unique thermal fingerprint. The challenge is to isolate this one signature from the signals of all other gases around it.

The detection of the chemical fills in a mysterious gap in Titan observations that dates back to NASA's Voyager 1 spacecraft and the first-ever close flyby of this moon in 1980.

Voyager identified many of the gases in Titan's hazy brownish atmosphere as hydrocarbons, the chemicals that primarily make up petroleum and other fossil fuels on Earth.

On Titan, hydrocarbons form after sunlight breaks apart methane, the second-most plentiful gas in that atmosphere. The newly freed fragments can link up to form chains with two, three or more carbons. The family of chemicals with two carbons includes the flammable gas ethane. Propane, a common fuel for portable stoves, belongs to the three-carbon family.

Previously, Voyager found propane, the heaviest member of the three-carbon family, and propyne, one of the lightest members. But the middle chemicals, one of which is propylene, were missing.

As researchers continued to discover more and more chemicals in Titan's atmosphere using ground- and space-based instruments, propylene was one that remained elusive. It was finally found as a result of more detailed analysis of the CIRS data.

"This measurement was very difficult to make because propylene's weak signature is crowded by related chemicals with much stronger signals," said Michael Flasar, Goddard scientist and principal investigator for CIRS. "This success boosts our confidence that we will find still more chemicals long hidden in Titan's atmosphere."

Cassini's mass spectrometer, a device that looks at the composition of Titan's atmosphere, had hinted earlier that propylene might be present in the upper atmosphere. However, a positive identification had not been made.

"I am always excited when scientists discover a molecule that has never been observed before in an atmosphere," said Scott Edgington, Cassini's deputy project scientist at NASA's Jet Propulsion Laboratory in Pasadena, California. "This new piece of the puzzle will provide an additional test of how well we understand the chemical zoo that makes up Titan's atmosphere."

Image credit: NASA/JPL-Caltech/Space Science Institute

Titan's North Polar Collar

Titan's polar collar -- previously seen by Voyager 2 and the Hubble Space Telescope -- has now been observed by the Cassini spacecraft, seen here in ultraviolet light. The collar is believed to be seasonal in nature. Researchers are still studying its cause and evolution.

This view looks toward the Saturn-facing hemisphere of Titan. North on Titan is up and rotated 32 degrees to the right. The image was taken with the Cassini spacecraft narrow-angle camera on April 13, 2013 using a spectral filter sensitive to wavelengths of ultraviolet light centered at 338 nanometers.

The view was acquired at a distance of approximately 1.1 million miles (1.8 million kilometers) from Titan and at a Sun-Titan-spacecraft, or phase, angle of 4 degrees. Image scale is 7 miles (11 kilometers) per pixel.

Photo credit: NASA/JPL-Caltech/Space Science Institute

The Formation of Aerosols in Titan's Haze

This illustration shows the various steps that lead to the formation of the aerosols that make up the haze on Titan, Saturn's largest moon.

When sunlight or highly energetic particles from Saturn's magnetosphere hit the layers of Titan's atmosphere above 1000 km, the nitrogen and methane molecules there are broken up. This results in the formation of massive positive ions and electrons, which trigger a chain of chemical reactions that produce a variety of hydrocarbons. Many of these hydrocarbons have been detected in Titan's atmosphere, including Polycyclic Aromatic Hydrocarbons (PAHs), which are large carbon-based molecules that form from the aggregation of smaller hydrocarbons. Some of the PAHs detected in the atmosphere of Titan also contain nitrogen atoms.

PAHs are the first step in a sequence of increasingly larger compounds. Models show how PAHs can coagulate and form large aggregates, which tend to sink, due to their greater weight, into the lower atmospheric layers. The higher densities in Titan's lower atmosphere favor the further growth of these large conglomerates of atoms and molecules. These reactions eventually lead to the production of carbon-based aerosols, large aggregates of atoms and molecules that are found in the lower layers of the haze that enshrouds Titan, well below 500 km.

The formation scenario of aerosols in Titan's atmosphere depicted in this illustration is based on the simulations described by Lavvas et al., 2011 (The Astrophysical Journal, 728, 80); doi:10.1088/0004-637X/728/2/80.

Illustration credit: ESA/ATG medialab

Note: For more information, see Cassini Sees Precursors to Aerosol "Snow" on Titan.

Titan's South Polar Vortex

The recently formed south polar vortex stands out in the color-swaddled atmosphere of Saturn's largest moon, Titan, in this natural color view from NASA's Cassini spacecraft.

See PIA14919 and PIA14920 to learn more about this mass of swirling gas around the pole in the atmosphere.

Images taken using red, green and blue spectral filters were combined to create this natural color view. The images were acquired with the Cassini spacecraft wide-angle camera on July 25, 2012, at a distance of approximately 64,000 miles (103,000 kilometers) from Titan. Image scale is 4 miles (6 kilometers) per pixel.

Photo credit: NASA/JPL-Caltech/Space Science Institute

Note: For more information, see Ice Cloud Heralds Fall at Titan's South Pole.