News | December 30, 2015

Cassini Top 10 Science Highlights – 2015

First Deep Seafloor Hydrothermal Vents Found Beyond Earth
Global Ocean Beneath Enceladus’ Surface
Titan Observed Outside of Saturnian Magnetosphere
Density of A Ring Particles May Indicate Recent Origins
Titan Southern Polar Ice Cloud
Curtain Vents on Enceladus?
Discovery of Tethys Red Arcs
Saturn’s 30-year Giant Storms Powered by Water Convection
Seasonal Change Seen at Saturn’s Poles
Huygens Probe Imaging Mosaic of Titan’s Surface and Descent Movie

1. First Deep Seafloor Hydrothermal Vents Found Beyond Earth

Cassini finds first evidence of active hot-water chemistry beyond planet Earth in two separate scientific papers.
An extensive, four-year analysis of data from the spacecraft, computer simulations and laboratory experiments led researchers to the conclusion the tiny silica (SiO2) grains most likely form when hot water containing dissolved minerals from the moon's rocky interior travels upward, coming into contact with cooler water. Temperatures required for the interactions that produce the tiny silica grains would be at least 194 degrees Fahrenheit (90 degrees Celsius).
In another parallel analysis, Cassini scientists identified hydrothermal activity as one of two likely sources of the excess methane in the plume of gas and ice particles that erupt from Enceladus.

2. Global Ocean Beneath Enceladus’ Surface

A global ocean lies beneath the icy crust of Saturn's geologically active moon Enceladus based on two independent lines of evidence.
Researchers found the magnitude of the moon's very slight wobble as it orbits Saturn, known as a libration, can only be accounted for if its outer ice shell is not frozen solid to its core, meaning a global ocean must be present.
Cassini scientists analyzed more than seven years' worth of images of Enceladus taken by the spacecraft, which has been orbiting Saturn since mid-2004. They carefully mapped the positions of features on Enceladus -- mostly craters -- across hundreds of images in order to measure changes in the moon's rotation with extreme precision. As a result, they found Enceladus has a tiny, but measurable wobble as it orbits Saturn.
If the surface and core were rigidly connected, the core would provide so much dead weight the wobble would be far smaller than we observe it to be. This proves that there must be a global layer of liquid separating the surface from the core.

3. Titan Observed Outside of Saturnian Magnetosphere

Analysis of Cassini’s December 2013 Titan flyby showed that the encounter was unique within Cassini's mission, as it was the only that the spacecraft has observed Titan outside the region of space dominated by Saturn's magnetic field, called its magnetosphere.
During a Cassini flyby on Dec. 1, 2013, the giant moon happened to be on the sunward side of Saturn when a powerful outburst of solar activity reached the planet. The strong surge in the solar wind so compressed the sun-facing side of Saturn's magnetosphere that the bubble's outer edge was pushed inside the orbit of Titan. This left the moon exposed to, and unprotected from, the raging stream of energetic solar particles.
The influence of Saturn’s magnetosphere on Titan was not present, allowing Cassini's magnetometer to observe Titan’s atmosphere as it interacted directly with the solar wind. The special circumstance allowed scientists to study the shockwave that formed around Titan where the full-force solar wind rammed into the moon's atmosphere.

4. Density of A Ring Particles May Indicate Recent Origins

Saturn’s A ring was found to be warmer than expected at the planet’s equinox, and also had an unusually large thermal asymmetry about the equinox.
Models indicate this could be due to the A ring being mostly composed of denser particles made primarily of solid ice, with a thin top layer of fluffy regolith.
Some mechanisms may be confining dense particles in the middle A ring, and keeping them from diffusing across the ring. Alternatively, the (middle) A ring might have “recently” formed (< 100 million years ago) by destruction of an icy satellite, so that dense particles have not yet diffused over the A ring and regolith mantles of particles have not grown thick.
Portions of the A ring could be much younger than other parts of Saturn’s ring system.

5. Titan Southern Polar Ice Cloud

Scientists have detected a monstrous new cloud of frozen compounds in Titan’s low- to mid-stratosphere – a stable atmospheric region above the troposphere, or active weather layer.
A large cloud, first seen in 2012 at higher altitudes, turned out to be just the tip of the iceberg. A much more massive ice cloud system has now been found lower in the stratosphere, peaking at an altitude of about 124 miles (200 kilometers).
For the past few years, Cassini has been catching glimpses of the transition from fall to winter at Titan’s south pole – the first time any spacecraft has seen the onset of a Titan winter.

6. Curtain Vents on Enceladus?

New research using data from Cassini suggests most of the eruptions from Saturn's moon Enceladus might actually be diffuse curtains rather than discrete jets. Many features that appear to be individual jets of material erupting along the length of prominent “tiger stripe” fractures in the moon's south polar region might be phantoms created by an optical illusion, according to the new study.
Researchers modeled eruptions on Enceladus as uniform curtains along the tiger stripe fractures. They found that phantom brightness enhancements appear in places where the viewer is looking through a "fold" in the curtain. The folds exist because the fractures in Enceladus' surface are more wavy than perfectly straight. Researchers think this optical illusion is responsible for most of what appear to be individual jets.
Curtain eruptions occur on Earth where molten rock, or magma, gushes out of a deep fracture. These eruptions, which often create spectacular curtains of fire, are seen in places such as Hawaii, Iceland and the Galapagos Islands.

7. Discovery of Tethys Red Arcs

Like graffiti sprayed by an unknown artist, unexplained arc-shaped, reddish streaks are visible on the surface of Saturn's icy moon Tethys.
Revealed using infrared-enhanced images, the red arcs are narrow, curved lines on the moon's surface, and are among the most unusual color features on Saturn's moons to
The origin of the features and their reddish color is a mystery to Cassini scientists. Possibilities being studied include ideas that the reddish material is exposed ice with chemical impurities, or the result of outgassing from inside Tethys. They could also be associated with features like fractures that are below the resolution of the available images.

8. Saturn’s 30-year Giant Storms Powered by Water Convection

Changes in temperature and the composition of the hydrogen-laden air within the remnants of a giant storm system on Saturn reveal that air was lofted more than 200 km (120 miles) in altitude from the deeper water condensation levels.
New analysis confirm that Saturn’s giant storms are hundreds of times more powerful than those on Earth, yet are powered ultimately by the same source: water.
Water vapor is heavier than the hydrogen and helium that make up the bulk of Saturn's atmosphere. Once each giant storm dumps its huge mass of rain, the air within the clouds is left lighter than the atmosphere below. For a time, this situation shuts off the process of convection -- in which warm, moist air rises, and cool, dense air sinks -- that creates new clouds and storms.
The air above has to cool off, radiating its heat to space, before its density is greater than that of the hot, wet air below. This cooling process takes about 30 years, the frequency of the giant storms.

9. Seasonal Change Seen at Saturn’s Poles

Saturn's polar regions have displayed extreme seasonal changes during Cassini's decade-long watch, providing the most comprehensive view ever obtained of seasonal change on a giant planet.
Saturn’s polar stratosphere features large warm vortices (a polar hood) during the summer that changed substantially over the last decade.
The North Pole warmed by about 36 degrees F (20 kelvins) during spring. Cassini is still waiting for emergence of a seasonal vortex expected to appear before the end of the mission. The South Pole cooled by about 63 degrees F (35 kelvins) during its fall. Shifting polar temperatures depend not only on sunlight, but also on enormous global circulation patterns.

10. Huygens Probe Imaging Mosaic of Titan’s Surface and Descent Movie

Ten years ago, an explorer from Earth, the Huygens probe, was released from the Cassini spacecraft and parachuted into the haze of an alien moon toward an uncertain fate. After a gentle descent lasting more than two hours, it landed with a thud on a frigid floodplain on Titan, surrounded by icy cobblestones.
To celebrate the feat of humanity's first landing on a moon in the outer solar system, images from Huygens’ Descent Imager/Spectral Radiometer were combined into a mosaic at resolutions as low as 50 cm near the landing site. A movie of the descent was also produced.
Comparison of images from different emission angles and altitudes yielded topographic information, including reflectivity and roughness of the terrain.