Cassini Top 10 Science Highlights - 2017

• Cassini found evidence that hydrogen gas, which could potentially provide a chemical energy source for life, is pouring into the subsurface ocean of Enceladus from hydrothermal activity on the seafloor.

• The presence of ample hydrogen in the moon's ocean means that microbes -- if any exist there -- could use it to obtain energy by combining the hydrogen with carbon dioxide dissolved in the water. This chemical reaction is known as "methanogenesis.

• The discovery was made during Cassini's deepest-ever dive through the plume on Oct. 28, 2015. " See "NASA Missions Provide New Insights into 'Ocean Worlds' in Our Solar System" (/news/3024/nasa-missions-provide-new-insights-into-ocean-worlds-in-our-solar-system/ ) from April 12, 2017. less

• Ring scientists were puzzled that the narrow gap between Saturn and its rings appeared to be relatively dust-free. This assessment was based on data Cassini collected during its first dive through the region on April 26, 2017.

Cassini's Radio and Plasma Wave Science instrument detected hundreds of ring particles per second when it crossed the ring plane just outside of Saturn's main rings, but only detected a few pings on April 26.

• A dustier environment in the gap might have meant the spacecraft's saucer-shaped main antenna would be needed as a shield during most of the subsequent dives through the ring plane.

• The Radio and Plasma Wave (RPWS) team's analysis suggested that the RPWS antennas, capable of detecting ring particle impacts, only encountered a few particles as Cassini crossed the gap - none were larger than those in smoke (about 1 micron across).

• However, in this same region, the Cosmic Dust Analyzer (CDA) detected a large number of tiny particles that were much smaller than those could be detected by the RPWS antennas. One big question is what happened to all of the larger particles that models predicted would be there. Fortunately, these tiny ring particles did not represent a hazard to the Cassini spacecraft.

• See Cassini Finds 'The Big Empty' Close to Saturn from April 30, 2017 and Saturn Surprises As Cassini Continues its Grand Finale from July 24, 2017. less

• Several Grand Finale passages allowed for imaging "noodles" of Saturn at high resolution from pole to pole.

• Cassini successfully imaged all six of the small ring features called "propellers" whose orbits were being tracked over the last several years of the mission, further revealing insights into their properties and interaction with the rings.

• Early estimates from the Grand Finale imply that Saturn's magnetic field tilt is less than 0.06 degrees. Thus, the precise length of a day on Saturn still eludes scientists.

• Cassini saw evidence that molecules from the rings are raining down onto the atmosphere. This influx of material from the rings was expected, but data show hints of ingredients more complex than just water, which makes up the bulk of the rings' composition. In particular, the instrument detected methane, a volatile molecule that scientists did not expect to be abundant in the rings nor found so high in Saturn's atmosphere.

• New analyses reveal that the gravitational influences of seven moons (including Pan, Atlas, Prometheus, Pandora, Janus, Epimetheus and Mimas) combine to keep the A ring from spreading out and dissipating.

See Saturn Surprises As Cassini Continues its Grand Finale from July 24 and Fresh Findings from Cassini from Oct. 16, 2017. less

• The region of space between Saturn and its rings that is shaded by the rings themselves has less ionized gas, or plasma, than regions in sunlight.

• Scientists hypothesize that the reduced sunlight in the shadowed region leads to less ionization of any gas that happens to be in that space.

• While there is less plasma in this region, there is still activity that may be related to a phenomenon called "ring rain," where charged water molecules migrate from the innermost ring to the planet.

See Watch rain and shadows from Saturn's rings give the planet a funky ionosphere from Dec. 11. less

• Cassini's Ring Grazing Orbits (Nov. 2016 - April 2017) allowed for the closest-ever imaging observations of the ring moons Atlas, Daphnis and Pan.

• Observations of Daphnis also revealed its gravitational interactions with the nearby rings.

• Pan and Atlas were revealed to have ridges around their equators and a fairly smooth mantle of material on their surfaces -- likely an accumulation of fine particles from the neighboring rings. Only few craters are obvious on their surfaces.

• See Small Wonders from June 28, Cassini Sees 'Flying-Saucer' Moon Atlas Up Close from April 12, Cassini Reveals Strange Shape of Saturn's Moon Pan from March 8, Daphnis Up Close from Jan. 18, 2017. less

• Cassini's highest resolution ring images reveal the rings to have varied structure down to the smallest scales observable by the spacecraft's cameras.

• Although Cassini saw these features with names like "straw" and "propellers" earlier in the mission, the spacecraft's final, ultra-close orbits provided opportunities to see them in greater detail. Some images resolved details as small as 0.3 miles (550 meters), which is on the scale of Earth's tallest buildings. The features are caused by clumping ring particles and small, embedded moonlets.

• Images of features in Saturn's C ring called "plateaus" have deepened the mystery surrounding them. These bright bands have a streaky texture that is very different from the textures of the regions around them.

• See Textures in the C Ring and More Textures in the C Ring from July 24 and Close Views Show Saturn's Rings in Unprecedented Detail" from Jan. 29. less

• A modeling study showed that heat from friction could power hydrothermal activity on Enceladus for billions of years if the moon has a highly porous core.

• Simulations show that as Enceladus orbits Saturn, rocks in the core flex and rub together, generating heat. The loose interior also allows water from the ocean to percolate deep down, where it heats up, then rises, interacting chemically with the rocks.

• This activity should be at a maximum at the moon's poles, where plumes of the warm, mineral-laden water gush from the seafloor and travel upward, thinning the moon's ice shell from beneath to just half a mile to 3 miles (1 to 5 kilometers) at the south pole.

• Using microwave measurements from Cassini's radar antenna, scientists determined Enceladus' south polar region is warmer than expected just a few feet below its icy surface. This suggests that Enceladus' ocean of liquid water might be only a couple of miles beneath this region -- closer to the surface than previously thought.

• Excess heat is especially pronounced over three fractures that are similar to the "tiger stripes" -- prominent, actively venting fractures that slice across the pole -- except that they don't appear to be active at the moment. Seemingly dormant fractures lying above the moon's warm, underground sea point to the dynamic character of Enceladus' geology, suggesting the moon might have experienced several episodes of activity in different places on its surface.

• See Powering Saturn's Active Ocean Moon from Nov. 6 and Cassini Sees Heat Below the Icy Surface of Enceladus from March 12. less

• Titan's atmosphere has been known to contain exotic molecules produced by both sun-driven and energetic particle-driven chemistry. Recent discoveries have revealed that Titan's chemical makeup is even stranger than previously thought.

• Experiments simulating the frigid surface conditions on Titan revealed that significant amounts of nitrogen can be dissolved in the extremely cold liquid methane that rains from the skies and collects in rivers, lakes and seas. Slight changes in temperature, air pressure or composition can cause the nitrogen to rapidly separate out of solution, like the fizz that results when opening a bottle of carbonated soda.

• Cassini scientists discovered a particular type of negatively charged molecule at Titan. Negatively charged species -- or 'anions' -- were not something scientists expected to find because they are highly reactive and should not last long in Titan's atmosphere before combining with other materials. Their detection is reshaping current understanding of how hazes form in the moon's atmosphere.

• Scientists detected the chemical acrylonitrile in Titan's atmosphere, also known as vinyl cyanide. On Earth, this chemical is useful in the manufacture of plastics. Under the frigid conditions of Titan, it's thought to be capable of forming stable, flexible structures similar to cell membranes.

• Exotic ice in a cloud in Titan's stratosphere was determined to be a combination of the simple organic molecule hydrogen cyanide together with the large ring-shaped chemical benzene. The two chemicals appear to have condensed at the same time to form ice particles, rather than one being layered on top of the other.

• See NASA Team Finds Noxious Ice Cloud on Saturn's Moon Titan from Oct. 18, NASA Finds Moon of Saturn Has Chemical That Could Form 'Membranes' from July 31, New Insights into Titan's Complex Chemistry from July 26, and Experiments Show Titan Lakes May Fizz with Nitrogen from March 14. less

• Saturn's Great Storm of 2010-2011 generated disturbances that were felt many thousands of kilometers away.

• Storms like this one have been observed to erupt on Saturn every 20 to 30 years.

• Patterns of temperatures and winds in Saturn's stratosphere normally repeat about every 15 years in a cycle called the Quasi-Periodic Oscillation (QPO). Scientists think of the pattern as something like a planetary heartbeat.

• The Great Storm interrupted this cycle via the infusion of waves of energy into the stratosphere.

• Temperatures dropped about 10 Kelvin and disrupted wind patterns in the remote regions affected by the storm.

• See Giant Storms Cause Palpitations in Saturn's Atmospheric Heartbeat from Dec. 13. less

• Cassini scientists found evidence that the moon's spin axis -- the line through the north and south poles -- has reoriented, possibly due to a collision with a smaller body, such as an asteroid.

• Examination of features on Enceladus indicates that it may have tipped away from its original axis by about 55 degrees -- more than halfway toward rolling completely onto its side.

• Scientists speculate that the area around the icy moon's currently active south pole may have been struck in the past by an asteroid when the region was closer to the equator.

• The resulting unstable, wobbly rotation would have eventually stabilized, likely taking more than a million years.

• See Cassini Finds Saturn Moon May Have Tipped Over from May 30. less