Burkhard Militzer, U.C. Berkeley
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