Cassini at Saturn: The planet as we know it now and what we hope to discover in the final three years

Abstract:

Saturn objectives for the remainder of the Cassini mission fall into four broad themes: (1) aftermath of a giant storm, (2) seasonal and temporal atmospheric change, (3) polar latitudes, and (4) Saturn’s interior. We list of the questions that Cassini will address in order below.

(1) Aftermath of a giant storm. Planet-encircling storms occur every 20-30 years. The latest was in 2010-2011. Smaller storms occur every few months. The questions are: How deep are the roots of the storms? What chemicals do they dredge up from below? Do they change the winds in their vicinity? How do storms lead to ammonia depletion and holes in the clouds? What is the depth of the lightning flashes, and is it related to the freezing level of the water cloud? How do storms in the troposphere heat the stratosphere?

(2) Seasonal and temporal atmospheric change. The 30-year seasonal cycle and the 11-year solar activity cycle are externally driven. In addition, Saturn generates its own 15-year cycle of equatorial winds and temperatures. The questions are: What are the sources and sinks of trace species (water, CO2, acetylene, ethane, diacetylene)? Will the equatorial atmosphere continue to oscillate with a 15-year period? How does Saturn's atmosphere respond to the solar cycle and the solar maximum?

(3) Polar latitudes are the sites of energetic auroral activity, and they have a giant hexagon-shaped cloud feature and hurricane “eyes” that are centered on the poles. The questions are: What maintains the northern hexagon for more than 30 years? Are hurricane-like features unique to the polar regions? Is the atmosphere "slipping" relative to the magnetosphere? What does the aurora tell us about coupling between the magnetic field and atmosphere?

(4) Saturn’s interior. The interior structure provides information about how the planet formed and evolved. The questions are: What does the gravity field reveal about the winds, composition, and equation of state of the interior? What does the internal magnetic field reveal about planetary dynamos and the internal rate of rotation? Has helium settled toward the center, and how has that affected the planet's thermal evolution? What do we learn from Saturn’s internal oscillations, which have been detected using the rings as a giant seismometer?