planets.agu.org/Interview-with-Dr-David-Stevenson.php Planetary Origins and Evolution An Interview with Dr. David Stevenson by Mark Hilverda. Published on April 26, 2011. David Stevenson David Stevenson in the foothills of the Southern Alps, New Zealand. Image credit: David Stevenson. David Stevenson is the George Van Osdol Professor of Planetary Science at the California Institute of Technology. His research interests include internal structure and evolution of both major and terrestrial planets, application of fluid dynamics and magnetohydrodynamics [dynamics of electrically conducting fluids] to planetary interiors, and the origin of the solar system. I recently caught up with him to discuss his early career, his thoughts on the best way to send a probe to the Earths core, and his involvement with the upcoming Juno mission to Jupiter, among other topics. Every person seems to enter the field of planetary science via different avenues. How did you first become interested in planetary science? As a high school student and even as an undergraduate in New Zealand, I was unaware that planetary science existed as a career. It seemed more like a hobby. I knew about the Apollo program, of course, but the kind of science possible with Apollo did not appeal all that much when I was young -- I appreciate it much more now. When it came time to decide where to go for graduate work, I had no doubt that I wanted to do theoretical physics and indeed my thesis is in that area. I chose Cornell for my doctoral studies and that was fortuitous in two ways: It led me to take a course from Carl Sagan and it led me to work with Ed Salpeter, a very distinguished astrophysicist who happened to be temporarily interested in planets at that time. But the avenue was really condensed matter physics (an area where Cornell excelled): I became interested in planets because of the interesting high pressure physics of planets, especially Jupiter. What led you to focus on planetary interiors as one of your primary research areas? Although this interest developed from condensed matter physics, I was also fascinated by the connection between planetary interiors and some basic questions such as: Why do some planets have magnetic fields while others do not? Why do planets have cores? What is the source of heat coming from the deep interiors of planets? These questions could be tackled through a combination of my primary expertise in condensed matter physics and my self-taught expertise in fluid dynamics. (I have no formal training in fluid dynamics but a lot of my work involves that subject). I suppose that I could have had just as much fun working in astrophysics but by chance ended up in planetary work. Young planetary system Artist concept of a young star system. Image credit: NASA/JPL-Caltech/T. Pyle (SSC). In 2003 you wrote a modest proposal for sending a probe to Earths core, which gained widespread circulation and seemed to capture a lot of peoples imagination. This sounds like a paper that was a lot of fun to write - could you guide us through the process of inspiration to publication? The essential idea of my modest proposal had actually been in my head for at least a decade before I wrote it down. However, I was asked to comment on the script for a movie called The Core (this was after the script had been finalized but before the film was released). Although I did not think much of the scientific content of the movie (especially the idea of stopping the rotation of Earths core, which is silly), it did set me thinking anew about my old idea. I wrote the paper up in one afternoon on New Years Eve (even the calculations took only a few hours). I fear the paper is still somewhat misunderstood: The notion that you might actually do what I proposed was offered somewhat tongue in cheek (hence the allusion to Jonathan Swifts satirical essay, A Modest Proposal) but the notion that one should contemplate the importance of downward exploration was a very serious one, even though the difficulties are immense. I also frequently encounter people who confuse my idea with a different and much older idea of melting ones way down into the interior (the China Syndrome method). My idea involved rapid cracking driven by gravity, not the slower process of melting, though it did indeed also lead to melting, both to start the process and because of the large release of gravitational energy. Planetary cores Generalized planetary interiors of terrestrial planets. Image credit: NASA Earlier in 1999 you also published a paper speculating about the potential for interstellar planets. There has been debate about whether Cha 110913, which was discovered in 2004, constitutes an interstellar planet. As technology progresses do you expect to find a significant number of these potential worlds or does current theory indicate they should be uncommon? This was certainly a more serious paper and probably at least partly correct, though the catchy inclusion of habitability was very speculative. We now know that planets are very common and expect that a fraction (probably a small fraction) wander interstellar space. But I have in mind a very specific notion, which is that these planets arise by ejection from embryonic planetary systems forming around stars. There might also be planets at the low mass end of the brown dwarf sequence that form in much the same way as stars form. Cha 110913 is likely to be of that kind. People often get exercised about definitions (i.e., is this a planet?), and I dont care much for such terminological disputes. But I do care about process and origin. The problem is that the kind of body I have in mind (plausibly an Earth mass, though larger or smaller are possible) will be very hard to detect. It might be detected serendipitously or maybe through much larger technological developments than those envisaged for the next decade.
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