Fig.2: Orbital eccentricity (the “noncircularity” of Io’s orbit) and surface heatflow are plotted versus time.
Dr. Ojakangas’ doctoral research, under the direction of professor D.J. Stevenson, resulted in three highly acclaimed papers, collectively referenced by over 550 other
journal publications as of November, 2020. The first paper describes a mathematical model of the heat flow emanating from Jupiter’s volcanically active moon Io (Fig.1) — by far the most volcanically active body in the solar system, having turned itself inside out well over a dozen times during our solar system’s history. The model predicts that Io may be volcanically active for nearly 10 million years, followed by quiescent periods lasting as much as 100 million years (Fig.2). This is due to a feedback mechanism acting between the thermal energy within Io and the energy stored in its orbit. As the orbit becomes more eccentric, energy is stored in the orbit, but tidal flexing and associated friction also increase, eventually leading to volcanic activity and collapse of the eccentricity. Then the process repeats. These complex dynamics, intimately tied to the remarkable Laplace Resonance involving Io, Europa and Ganymede, help explain the enormous amount of thermal energy presently being emitted from the surface of Io.
The second paper that resulted from Dr. Ojakangas’ doctoral research describes the nature of the shell of water ice covering Jupiter’s second large moon, Europa (Figure 3), which is covered with a complex network of global fractures. Due to global variations of solar radiation and internal tidal heating within this shell, this work showed that a vast ocean of liquid water should exist beneath the shell, decoupling its motion from the moon’s rocky interior. This ocean has since been confirmed. Furthermore, the shell’s thickness was predicted (see figure on right) to vary in such a way that as it approaches thermal equilibrium, it may become dynamically unstable. Following on logically from these results, the third paper describes the dynamics of this shell as it is predicted to reorient itself by 90 degrees about the direction toward Jupiter. In the process of reorientation, it behaves like a pendulum falling from the top of its support to the bottom, while immersed in a viscous fluid. Such reorientation would create enormous stresses in the ice as the shell exhibits such polar wander. This process is predicted to repeat perhaps every 10 million years. Fracture patterns in Europa’s ice, with the geometries expected from this phenomenon, were later discovered by analysis of spacecraft images.
In the leading story of the December 2008, issue of Scientific American, Dr. Ojakangas is mentioned, since his Ph.D publications are relevant to the mysterious issues behind the tidal heating of tiny Enceladus. Enceladus was discovered to be volcanically active when Cassini arrived in 2004.
