NASA Cassini: Saturn's shadows

Credit: NASA /JPL /Space Science Institute (SSI)

It may seem odd to think of planets casting shadows out in the inky blackness of space, but it is a common phenomenon.

Earth's shadow obscures the Moon during a lunar eclipse, and Jupiter's moons cast small shadows onto their parent planet.

One of the best places in our Solar System to spot intriguing and beautiful celestial shadows is at Saturn.

On 1 July, the international Cassini mission celebrates 10 years of exploring Saturn, its rings and its moons, an endeavour that has produced invaluable science but also stunning images like this.

Drifting along in the foreground, small and serene, is Saturn's icy moon Mimas.

The blue backdrop may at first appear to be the gas giant's famous and impressive set of rings, with pale and dark regions separated by long inky black slashes, but it is actually the northern hemisphere of Saturn itself.

The dark lines slicing across the frame are shadows cast by the rings onto the planet.

Although we may not associate the colour blue with Saturn, when Cassini arrived at the planet the northernmost regions displayed the delicate blue palette shown in this image.

As this region of Saturn is generally quite free of cloud, scattering by molecules in the atmosphere causes sunlight to take a longer path through the atmosphere.

The light is scattered predominantly at shorter, bluer, wavelengths. This is similar to why the sky on Earth appears blue to our eyes.

Seasonal changes over the years since this photo was taken have turned the blue into Saturn's more familiar golden hue. The reverse is occurring in the south, which is slowly becoming bluer.

This image is composed of infrared, optical and ultraviolet observations from Cassini's narrow-angle camera on 18 January 2005. The colours closely match what the scene would look like in true colour.

Global map of Ganymede, Jupiter's biggest moon

Making the map of Ganymede was a long and complex task. 

Some of the scientists behind the map were graduate students and postdocs at Brown University when the Galileo data began to arrive in the 1990s. 

Image courtesy U.S. Geological Survey.

Scientists, including Brown University geologists and students, have completed the first global geological map of Ganymede, Jupiter's largest moon and the largest in the solar system.

With its varied terrain and possible underground ocean, Ganymede is considered a prime target in the search for habitable environments in the solar system, and the researchers hope this new map will aid in future exploration.

Geoffrey Collins

The work, led by Geoffrey Collins, a Ph.D. graduate of Brown now a professor at Wheaton College in Massachusetts, took years to complete.

"It is very rewarding to see the results of all of our efforts here at Brown come together into this integrated global compilation that will now be used to plan the next phase of scientific exploration of the Galilean satellites," said Jim Head, the Scherck Distinguished Professor of Geological Sciences at Brown and one of the map's co-authors.

Global geologic map of Ganymede. Image credit: Collins G.C. et al.

The researchers combined images from the Voyager and Galileo spacecraft to put the map together. Voyager was the first mission to fly through the Jupiter satellite system and passed by the icy surface of Ganymede in 1979.

Those first images revealed a complex surface, segmented and fractured into dark and light terrain.

In 1995, the Galileo spacecraft was placed in orbit around Jupiter and began to return high-resolution images of the surface that help to understand many of the features seen at low-resolution by Voyager.

Jim Head

Head was a co-investigator on the Galileo's Solid State Imaging (SSI) experiment.

In that role, he and his team were responsible for planning the imaging sequences for Ganymede in order to identify and investigate the scientific targets of highest priority.

The team worked for several years to obtain the data necessary to make the global map.

"This was an amazing time," Head said. "Brown graduate and undergraduate students worked shoulder-to-shoulder in the Planetary Geosciences Laboratory in Lincoln Field Building, studying the newly acquired images and choosing new sites of scientific interest."

"The discoveries were daily and the adrenaline was surging as we rushed to collect our thoughts and plans, review them with the SSI Team, and get them uploaded to the spacecraft in time for the next encounter."

"I'm so glad all that work has paid off in the form of this detailed global map," Head said.

"It is equally rewarding to see that the Brown team has now moved on to positions of leadership in the planetary exploration research community."

The new geological map of Ganymede, published yesterday by the U.S. Geological Survey

NASA Cassini: Infrared Image of Saturn's Rings

Although it may look to our eyes like other images of the rings, this infrared image of Saturn's rings was taken by the Cassini spacecraft using a special filter that will only admit light polarised in one direction. 

Scientists can use these images to learn more about the nature of the particles that make up Saturn's rings.

The bright spot in the rings is the "opposition surge" where the Sun-Ring-Spacecraft angle passes through zero degrees. 

Ring scientists can also use the size and magnitude of this bright spot to learn more about the surface properties of the ring particles.

This view looks toward the sunlit side of the rings from about 19 degrees above the ringplane. 

The image was taken with the Cassini spacecraft wide-angle camera on Aug. 18, 2013 using a spectral filter sensitive to wavelengths of near-infrared light centered at 705 nanometers.

The view was acquired at a distance of approximately 712,000 miles (1.1 million kilometers) from Saturn and at a Sun-rings-spacecraft, or phase, angle of 7 degrees. 

Image scale is 43 miles (68 kilometers) per pixel.

Image Credit: NASA /JPL-Caltech /Space Science Institute (SSI)

NASA's Cassini Spacecraft: Saturn's rings cast shadows on the planet

The spectacular rings of Saturn cast dark shadows on the ringed planet as the winter season approaches in Saturn's southern hemisphere in this view from the Cassini spacecraft. 

With the cold season comes a blue hue on Saturn that is likely caused by a drop in ultraviolet sunlight and haze it produces. This image was taken on July 29, 2013 and released on Dec. 23.

Credit: NASA /JPL-Caltech /Space Science Institute

NASA's Cassini spacecraft has capped 2013 with a spectacular new collection of Saturn photos showcasing the planet's beauty, as well with its trademark rings and strange moons.

The newly released Saturn photos by Cassini include two views of Enceladus, Saturn's sixth-largest moon. Enceladus is a winter-appropriate ice world.

Geysers at its poles shoot ice particles into space, some of which make it into orbit around Saturn. Some of this space "snow" becomes part of Saturn's E ring, Saturn's second outermost ring that is made of microscopic particles.

Other images highlight Saturn's largest moon, Titan. There are no jolly elves at Titan's north pole; liquid methane and ethane seas appear as splotchy features near the moon's poles.

At the south pole, a high-altitude vortex swirls. The hazy orange atmosphere of Titan is thought to resemble the atmosphere of early Earth.

NASA Cassini sees precursors to aerosol haze on Titan

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. 

Titan's north polar hood can be seen at the top of this view, and a hint of the south polar vortex can be detected at the bottom. 

This view looks toward the Saturn-facing side of Titan (3,200 miles, or 5,150 kilometers across). 

North on Titan is up and rotated 9 degrees to the right. 

Images taken using red, green and blue spectral filters were combined to create this natural colour view. 

The images were acquired with the Cassini spacecraft wide-angle camera on June 6, 2012, at a distance of approximately 134,000 miles (216,000 kilometers) from Titan. 

Image scale is 8 miles (13 kilometers) per pixel. 

Credit: NASA/JPL-Caltech/Space Science Institute

Scientists working with data from NASA's Cassini mission have confirmed the presence of a population of complex hydrocarbons in the upper atmosphere of Saturn's largest moon, Titan, that later evolve into the components that give the moon a distinctive orange-brown haze.

The presence of these complex, ringed hydrocarbons, known as polycyclic aromatic hydrocarbons (PAHs), explains the origin of the aerosol particles found in the lowest haze layer that blankets Titan's surface.

Scientists think these PAH compounds aggregate into larger particles as they drift downward.

"With the huge amount of methane in its atmosphere, Titan smog is like L.A. smog on steroids," said Scott Edgington, Cassini deputy project scientist based at NASA's Jet Propulsion Laboratory.

"These new papers using Cassini data shed light on how the heavy, complex hydrocarbon molecules that make up Titan's smog came to form out of the simpler molecules in the atmosphere. Now that they have been identified, the longevity of Cassini's mission will make it possible to study their variation with Titan seasons."

Of all the bodies in the solar system, Saturn's largest moon, Titan, has the atmosphere most resembling that of Earth.

Like that of our planet, Titan's atmosphere is largely composed of molecular nitrogen. Unlike Earth's atmosphere, however, Titan's contains only small traces of oxygen and water.

Another molecule, methane, plays a similar role to that of water in Earth's atmosphere, and makes up about 2 percent of Titan's atmosphere.

Scientists have speculated that the atmosphere of this moon may resemble that of our planet in its early days, before primitive living organisms enriched it with oxygen via photosynthesis.

When sunlight or highly energetic particles from Saturn's magnetic bubble hit the layers of Titan's atmosphere above about 600 miles (1,000 kilometers), 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, producing a variety of hydrocarbons—a wide range of which have been detected in Titan's atmosphere.

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 300 miles (500 kilometers).

The process is similar to Earth, where smog starts with sunlight breaking up hydrocarbons that are emitted into the air. The resulting pieces recombine to form more complex molecules.

Aerosols in Titan's lower haze have been studied using data from the descent of the European Space Agency's Huygens probe, which reached the surface in 2005, but their origin remained unclear.

New studies analyzing data from Cassini's visual and infrared mapping spectrometer (VIMS) gathered in July and August 2007 might solve the problem.

One new study of Titan's upper atmosphere in the Astrophysical Journal describes the detection of the PAHs, which are large carbon-based molecules that form from the aggregation of smaller hydrocarbons.

"We can finally confirm that PAHs play a major role in the production of Titan's lower haze, and that the chemical reactions leading to the formation of the haze start high up in the atmosphere," said this paper's lead author Manuel López-Puertas from the Astrophysics Institute of Andalucia in Granada, Spain.

"This finding is surprising: we had long suspected that PAHs and aerosols were linked in Titan's atmosphere, but didn't expect we could prove this with current instruments."

The team of scientists had been studying the emission from various molecules in Titan's atmosphere when they stumbled upon a peculiar feature in the data. One of the characteristic lines in the spectrum—from methane emissions—had a slightly anomalous shape, and the scientists suspected it was hiding something.

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 about 600 miles (1,000 kilometers), 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 about 300 miles (500 kilometers). Credit: ESA/ATG medialab

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