Cassini Data: Titan May Have a Rigid Ice Shell

An analysis of gravity and topography data from the Saturnian moon Titan obtained by NASA's Cassini spacecraft suggests there could be something unexpected about the moon's outer ice shell. The findings, published on August 28 in the journal Nature, suggest that Titan's ice shell could be rigid, and that relatively small topographic features on the surface could be associated with large ice "roots" extending into the underlying ocean.

The study was led by planetary scientists Douglas Hemingway and Francis Nimmo at the University of California, Santa Cruz, who used data from Cassini. The researchers were surprised to find a counter-intuitive relationship between gravity and topography.

"Normally, if you fly over a mountain, you expect to see an increase in gravity due to the extra mass of the mountain," said Nimmo, a Cassini participating scientist. "On Titan, when you fly over a mountain, the gravity gets lower. That's a very odd observation."

One potential explanation is that each bump in the topography on the surface of Titan is offset by a deeper "root" that is big enough to overwhelm the gravitational effect of the bump on the surface. The root could act like an iceberg extending below the ice shell into the ocean underneath it. In this model, Cassini would detect less gravity wherever there is a big chunk of ice rather than water because ice is less dense than water.

"It's like a big beach ball under the ice sheet pushing up on it, and the only way to keep it submerged is if the ice sheet is strong," said Hemingway, the paper's lead author and a Cassini team associate. "If large roots under the ice shell are the explanation, this means that Titan's ice shell must have a very thick rigid layer."

If these findings are correct, a thick rigid ice shell makes it very difficult to have ice volcanoes, which some scientists have proposed to explain other features seen on the surface. They also suggest that convection or plate tectonics are not recycling Titan's ice shell, as they do with Earth's geologically active crust.

Video credit: ESA/NASA/JPL/University of Arizona

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

Earhart Propeller in the A-Ring

Cassini scientists continue their quest to understand the origin and evolution of the newly discovered features observed in Saturn's A-ring which have become known as "propellers." In this image, the propeller which scientists have dubbed "Earhart" (at the lower left of the image) has been re-acquired.

Scientists hope to understand how the bodies which generate the features -- themselves too small to be seen, yet significantly larger than a typical ring particle -- move around the ring over time. It is hoped that these features may provide insights about how forming planets move around their solar systems. For more on Earhart, see PIA12790.

This view looks toward the sunlit side of the rings from about 48 degrees above the ringplane. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on May 11, 2013.

The view was acquired at a distance of approximately 250,000 miles (400,000 kilometers) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 99 degrees. Image scale is 1 mile (2 kilometers) per pixel.

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

Abisme Crater

From the USGS Astrogeology Science Center:

The IAU Working Group for Planetary System Nomenclature has approved the name Abisme for a crater on Iapetus. For more information, see the map of Iapetus in the Gazetteer of Planetary Nomenclature.

Epimetheus and the A and F Rings

Although Epimetheus appears to be orbiting between the A and F rings in this image, it's just an illusion! Epimetheus, which orbits Saturn well outside of the F ring's orbit, is actually on the near side of Saturn to Cassini while the rings seen here are on the far side of the planet. Whew, that's a relief!

This view looks toward the unilluminated side of the rings from about 3 degrees below the ringplane. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on April 15, 2013.

The view was acquired at a distance of approximately 700,000 miles (1.1 million kilometers) from Epimetheus and at a Sun-Epimetheus-spacecraft, or phase, angle of 30 degrees. Image scale is 4 miles (7 kilometers) per pixel.

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

Saturn's North Polar Hexagon

The weather forecast for Saturn's north pole: storms. Lots and lots of storms. Here, the area within Saturn's north polar hexagon is shown to contain myriad storms of various sizes, not the least of which is the remarkable and imposing vortex situated over the planet's north pole.

The north polar hexagon was first observed by Voyager. To see more of the hexagon, see PIA10486 and PIA11682.

This view is centered on Saturn's north pole. North is up and rotated 33 degrees to the left. The image was taken with the Cassini spacecraft wide-angle camera on June 14, 2013 using a spectral filter sensitive to wavelengths of near-infrared light centered at 752 nanometers.

The view was obtained at a distance of approximately 476,000 miles (766,000 kilometers) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 45 degrees. Image scale is 26 miles (42 kilometers) per pixel.

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

Squeezing and Releasing Enceladus

This set of images from NASA's Cassini mission shows how the gravitational pull of Saturn affects the amount of spray coming from jets at the active moon Enceladus. Enceladus has the most spray when it is farthest away from Saturn in its orbit (inset image on the left) and the least spray when it is closest to Saturn (inset image on the right).

Water ice and organic particles gush out of fissures known as "tiger stripes" at Enceladus' south pole. Scientists think the fissures are squeezed shut when the moon is feeling the greatest force of Saturn's gravity. They theorize the reduction of that gravity allows the fissures to open and release the spray. Enceladus' orbit is slightly closer to Saturn on one side than the other. A simplified version of that orbit is shown as a white oval.

Scientists correlate the brightness of the Enceladus plume to the amount of solid material being ejected because the fine grains of water ice in the plume are very bright when lit from behind. Between the dimmest and brightest images, they detected a change of about three to four times in brightness, approximately the same as moving from a dim hallway to a brightly lit office.

This analysis is the first clear finding that shows the jets at Enceladus vary in a predictable manner. The background image is a mosaic made from data obtained by Cassini's imaging science subsystem in 2006. The inset image on the left was obtained on October 1, 2011. The inset image on the right was obtained on January 30, 2011.

A related image, PIA17039, shows just the Enceladus images. The Saturn system mosaic was created from data obtained by Cassini's imaging cameras in 2006.

Image credit: NASA/JPL-Caltech/University of Arizona/Cornell/SSI

Note: For more information, see PIA17039: Enceladus "On" and "Off"; also, NASA's Cassini Sees Forces Controlling Enceladus Jets.