The Interiors of Jupiter & Saturn

Abstract:
This talk will review our current knowledge of how our solar system formed and then discuss the recent gravity measurements of Saturn and Jupiter by the Cassini and Juno spacecrafts. During the Grand Finale phase, the Cassini spacecraft traveled inside Saturn’s rings and measured the planet’s gravitational field with high precision. The magnitudes of gravity coefficients J6, J8, and J10 were unexpectedly large and could not be explained with traditional interior models that assumed uniform rotation. So we introduced differential rotation on cylinders and showed that all even coefficients J2 through J10 can be matched.

Since its arrival at Jupiter in 2016, the Juno spacecraft has measured the planet’s gravity field with every flyby. The interpretation of these measurements has again been challenging but in this case because the magnitudes of the gravity coefficients J4 and J6 were smaller than predicted by traditional interiors models that included a dense inner core composed of rock and ice. Here we instead present models with dilute cores and deep winds.

Bio:
Burkhard Militzer is professor of planetary science at the University of California, Berkeley. He is the director of the Center for Integrative Planetary Science. Since 2007, he has been on the faculty of the Department of Earth and Planetary Science and the Department of Astronomy. He has a background in condensed matter physics and received his PhD from the University of Urbana-Champaign in 2000. Today he works on understanding the interiors of giant planets with NASA missions Juno and Cassini. He also studies matter at extreme conditions with computer simulations.

Summary:

Model of planetary formation
- Accretion of rocks and gas into planetary cores
- Model 1: Once core grows to 8 Earth masses, it starts accumulating gasses and turns into very massive gas giants
- Model 2 (less likely): Gas giants collapse as a single step and then grow a core inside.
- Typical model of gas giants:
  - Molecular hydrogen
  - Helium rain
  - Metallic hydrogen
  - Rocky core (like Earth’s)
Experiments
- High density/temperature experiments
  - LLNL: lasers
  - Sandia: magnetic pinch
  - Measure properties of highly dense materials
- Spacecraft:
  - Cassini probe flies around Saturn, turns off engine and measures the shape of Saturn’s gravitational field over its surface based on the acceleration of Cassini
  - Dataset: 30 fly-bys near the equator
  - Gravity field represented as expansion of Legendre polynomial
    - Traditional approximation that leverages prior knowledge about planet structure like hydrostatic equilibrium, which ensures north-south symmetry
    - Too few data points to use more flexible models
    - Other work has tried to measure the gravity field of local structures like the Big Red Spot
  - Gravity field is then fed into their simulations
    - Model planetary interiors
    - Use interiors to predict the shape of gravity field, compare to data.
    - Disagreement between models and measurement is ~mm/sec
Modeling Saturn:
- Before Cassini arrived at Saturn they had simulations of interior and predicted gravity field
- Data was very notably different from predictions. What’s wrong?!
- Idea:
  - What if the equator is spinning faster than other latitudes (4%)?
    - That would push extra mass around the equator
    - Corollary: there is shear inside the planet as different latitudes are dragging against each other
  - Saturn’s clouds are moving very fast but they’re just clouds
  - Idea is that the clouds that we see are actually representative of the motion of deeper structures (structures we see as clouds are 9000km deep)
  - Models now agree with measurement
- Saturn’s rings
  - Measured mass of Saturn’s rings: .41 Mimas masses
  - Implies that rings formed 10-100 million years ago
  - Argument:
    - Rings have a level of brightness today
    - Assume that they were fully light originally
    - Dark material being added on regular basis
    - From the current mass of rings, can work back to their formation time.
    - What created the rings?
      - Kuyper belt object hit a moon
      - 3 Moons go into resonance, causing a collision between them
Modeling Jupiter:
- Juno spacecraft orbiting Jupitor
- Their simulation disagreed with measurements of gravity field from Juno
- Adjustments:
  - Make the core more diluted (extra helium in the core, not just pure rocky core)
    - Such a core would convect internally
    - Jupiter’s magnetic field would then be driven by convection in 3 separate layers: molecular hydrogen, metallic hydrogen and core.
      - Data: Jupiter’s magnetic field is highly variable around the surface of the planet
  - Add corresponding effects on wind