NASA’s Cassini spacecraft is monitoring the evolution of a mysterious feature in a large hydrocarbon sea on Saturn’s moon Titan. The feature covers an area of about 100 square miles (260 square kilometers) in Ligeia Mare, one of the largest seas on Titan. It has now been observed twice by Cassini’s radar experiment, but its appearance changed between the two apparitions.
The mysterious feature, which appears bright in radar images against the dark background of the liquid sea, was first spotted during Cassini’s July 2013 Titan flyby. Previous observations showed no sign of bright features in that part of Ligeia Mare. Scientists were perplexed to find the feature had vanished when they looked again, over several months, with low-resolution radar and Cassini’s infrared imager. This led some team members to suggest it might have been a transient feature. But during Cassini’s flyby on August 21, 2014, the feature was again visible, and its appearance had changed during the 11 months since it was last seen.
Scientists on the radar team are confident that the feature is not an artifact, or flaw, in their data, which would have been one of the simplest explanations. They also do not see evidence that its appearance results from evaporation in the sea, as the overall shoreline of Ligeia Mare has not changed noticeably.
The team has suggested the feature could be surface waves, rising bubbles, floating solids, solids suspended just below the surface, or perhaps something more exotic.
The researchers suspect that the appearance of this feature could be related to changing seasons on Titan, as summer draws near in the moon’s northern hemisphere. Monitoring such changes is a major goal for Cassini’s current extended mission.
“Science loves a mystery, and with this enigmatic feature, we have a thrilling example of ongoing change on Titan,” said Stephen Wall, the deputy team lead of Cassini’s radar team, based at NASA’s Jet Propulsion Laboratory in Pasadena, California. “We’re hopeful that we’ll be able to continue watching the changes unfold and gain insights about what’s going on in that alien sea.”
Cassini’s Arrival to Saturn
More Cassini News
Bright Clumps in Saturn Ring Now Mysteriously Scarce
Compared to the age of the solar system — about four-and-a-half billion years — a couple of decades are next to nothing. Some planetary locales change little over many millions of years, so for scientists who study the planets, any object that evolves on such a short interval makes for a tempting target for study. And so it is with the ever-changing rings of Saturn.
Image Credit:
NASA/JPL-Caltech/SSI
Case in point: Saturn’s narrow, chaotic and clumpy F ring. A recent NASA-funded study compared the F ring’s appearance in six years of observations by the Cassini mission to its appearance during the Saturn flybys of NASA’s Voyager mission, 30 years earlier. The study team found that, while the overall number of clumps in the F ring remained the same, the number of exceptionally bright clumps of material plummeted during that time. While the Voyagers saw two or three bright clumps in any given observation, Cassini spied only two of the features during a six-year period. What physical processes, they wondered, could cause only the brightest of these features to decline sharply?
While a variety of features in Saturn’s many rings display marked changes over multiple years, the F ring seems to change on a scale of days, and even hours. Trying to work out what is responsible for the ring’s tumultuous behavior is a major goal for ring scientists working on Cassini.
“Saturn’s F ring looks fundamentally different from the time of Voyager to the Cassini era,” said Robert French of the SETI Institute in Mountain View, California, who led the study along with SETI Principal Investigator Mark Showalter. “It makes for an irresistible mystery for us to investigate.”
The researchers hypothesize that the brightest clumps in the F ring are caused by repeated impacts into its core by small moonlets up to about 3 miles (5 kilometers) wide, whose paths around Saturn lie close to the ring and cross into it every orbit. They propose that the diminishing number of bright clumps results from a drop in the number of these little moonlets between the Voyager and Cassini eras.
As for what might have caused the moonlets to become scarce, the team has a suspect: Saturn’s moon Prometheus. The F ring encircles the planet at a special location, near a place called the Roche limit — get any closer to Saturn than this, and tidal forces from the planet’s gravity tear apart smaller bodies. “Material at this distance from Saturn can’t decide whether it wants to remain as a ring or coalesce to form a moon,” French said. Prometheus orbits just inside the F ring, and adds to the pandemonium by stirring up the ring particles, sometimes leading to the creation of moonlets, and sometimes leading to their destruction.
Every 17 years, the orbit of Prometheus aligns with the orbit of the F ring in such a way that its influence is particularly strong. The study team thinks this periodic alignment might spur the creation of many new moonlets. The moonlets would then crash repeatedly through the F ring, like cars in a Hollywood high-speed chase, creating bright clumps as they smash across lanes of ring material. Fewer clumps would be created as time goes by, because the moonlets themselves are eventually destroyed by all the crashes.
As with any good scientific hypothesis, the researchers offer a way to test their ideas. It happens that the Voyager encounters with Saturn occurred a few years after the 1975 alignment between Prometheus and the F ring, and Cassini was present for the 2009 alignment. If the moon’s periodic influence is indeed responsible for creating new moonlets, then the researchers expect that Cassini would see the F ring return to a Voyager-like number of bright clumps in the next couple of years.
“Cassini’s continued presence at Saturn gives us an interesting opportunity to test this prediction,” said Linda Spilker, Cassini project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California, who was not involved in the study. “Whatever the result, we’re certain to learn something valuable about how rings, as well as planets and moons, form and evolve.”
The study by French and colleagues was published in the online edition of the Journal Icarus on July 15, 2014.
Swirling Cloud at Titan’s Pole is Cold and Toxic
Image Credit:
NASA/JPL-Caltech/ASI/Univ. of Arizona/SSI/Leiden Obsv./SRON
Scientists analyzing data from NASA’s Cassini mission have discovered that a giant, toxic cloud is hovering over the south pole of Saturn’s largest moon, Titan, after the atmosphere there cooled dramatically.
The scientists found that this giant polar vortex contains frozen particles of the toxic compound hydrogen cyanide, or HCN.
“The discovery suggests that the atmosphere of Titan’s southern hemisphere is cooling much faster than we expected,” said Remco de Kok of Leiden Observatory and SRON Netherlands Institute for Space Research, lead author of the study published today in the journal Nature
Titan is the only moon in the solar system that is cloaked in a dense atmosphere. Like our home planet, Earth, Titan experiences seasons. As it makes its 29-year orbit around the sun along with Saturn, each season lasts about seven Earth years. The most recent seasonal switch occurred in 2009, when winter gave way to spring in the northern hemisphere, and summer transitioned to autumn in the southern hemisphere.
In May 2012, while Titan’s southern hemisphere was experiencing autumn, images from Cassini revealed a huge swirling cloud, several hundred miles across, taking shape above Titan’s south pole. This polar vortex appears to be an effect of the change of season.
A puzzling detail about the swirling cloud is its altitude, some 200 miles (about 300 kilometers) above Titan’s surface, where scientists thought the temperature was too warm for clouds to form. “We really didn’t expect to see such a massive cloud so high in the atmosphere,” said de Kok.
Keen to understand what could give rise to this mysterious cloud, the scientists dove into Cassini’s observations and found an important clue in the spectrum of sunlight reflected by Titan’s atmosphere.
A spectrum splits the light from a celestial body into its constituent colors, revealing signatures of the elements and molecules present. Cassini’s visual and infrared mapping spectrometer (VIMS) maps the distribution of chemical compounds in Titan’s atmosphere and on its surface.
“The light coming from the polar vortex showed a remarkable difference with respect to other portions of Titan’s atmosphere,” says de Kok. “We could clearly see a signature of frozen HCN molecules.”
As a gas, HCN is present in small amounts in the nitrogen-rich atmosphere of Titan. Finding these molecules in the form of ice was surprising, as HCN can condense to form frozen particles only if the atmospheric temperature is as cold as minus 234 degrees Farenheit (minus 148 degrees Celsius). This is about 200 degrees Fahrenheit (about 100 degrees Celsius) colder than predictions from current theoretical models of Titan’s upper atmosphere.
To check whether such low temperatures were actually possible, the team looked at observations from Cassini’s composite infrared spectrometer (CIRS), which measures atmospheric temperature at different altitudes. Those data showed that the southern hemisphere of Titan has been cooling rapidly, making it possible to reach the cold temperature needed to form the giant toxic cloud seen on the south pole.
Atmospheric circulation has been drawing large masses of gas towards the south since the change of season in 2009. As HCN gas becomes more concentrated there, its molecules shine brightly at infrared wavelengths, cooling the surrounding air in the process. Another factor contributing to this cooling is the reduced exposure to sunlight in Titan’s southern hemisphere as winter approaches there.
“These fascinating results from a body whose seasons are measured in years rather than months provide yet another example of the longevity of the remarkable Cassini spacecraft and its instruments,” said Earl Maize, Cassini project manager at NASA’s Jet Propulsion Laboratory in Pasadena, California. “We look forward to further revelations as we approach summer solstice for the Saturn system in 2017.”
Cassini Tracks Clouds Developing Over a Titan Sea
NASA’s Cassini spacecraft recently captured images of clouds moving across the northern hydrocarbon seas of Saturn’s moon Titan. This renewed weather activity, considered overdue by researchers, could finally signal the onset of summer storms that atmospheric models have long predicted.
A movie showing the clouds’ movement is available here
The Cassini spacecraft obtained the new views in late July, as it receded from Titan after a close flyby. Cassini tracked the system of clouds developing and dissipating over the large methane sea known as Ligeia Mare for more than two days. Measurements of cloud motions indicate wind speeds of around 7 to 10 mph (3 to 4.5 meters per second).
For several years after Cassini’s 2004 arrival in the Saturn system, scientists frequently observed cloud activity near Titan’s south pole, which was experiencing late summer at the time. Clouds continued to be observed as spring came to Titan’s northern hemisphere. But since a huge storm swept across the icy moon’s low latitudes in late 2010, only a few small clouds have been observed anywhere on the icy moon. The lack of cloud activity has surprised researchers, as computer simulations of Titan’s atmospheric circulation predicted that clouds would increase in the north as summer approached, bringing increasingly warm temperatures to the atmosphere there.
“We’re eager to find out if the clouds’ appearance signals the beginning of summer weather patterns, or if it is an isolated occurrence,” said Elizabeth Turtle, a Cassini imaging team associate at the Johns Hopkins University Applied Physics Lab in Laurel, Maryland. “Also, how are the clouds related to the seas? Did Cassini just happen catch them over the seas, or do they form there preferentially?”
A year on Titan lasts about 30 Earth years, with each season lasting about seven years. Observing seasonal changes on Titan will continue to be a major goal for the Cassini mission as summer comes to Titan’s north and the southern latitudes fall into winter darkness.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory (JPL) in Pasadena, California, manages the mission for NASA’s Science Mission Directorate in Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team consists of scientists from the United States, England, France and Germany. The imaging team is based at the Space Science Institute in Boulder, Colorado.
Cassini Watches Mysterious Feature Evolve in Titan Sea
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