CapitalistImperialistPig

Mike Brown's penultimate lecture in his Caltech/Coursera Solar System course concerns what he calls "the best experiment to find life in the Solar System" yet. On it's trip to Jupiter, the Galileo spacecraft flew by a couple of planets to get a slingshot boost on its way, taking some data in the process. The data consisted of infrared spectra, imagery at multiple filtered wavelengths, and radio data. Some of the data was highly suggestive: oxygen in the atmosphere, presence of liquid water, very abundant methane in the atmosphere, and some peculiar features in the wavelength filtered data. All of the above were suggestive of the presence of life, but hardly conclusive. You have perhaps guessed the identity of the planet in question: Earth. The data, analyzed by Carl Sagan and colleagues, also included imagery with a resolution of roughly 1 kilometer. No definitive signs of human construction were found at that scale. The only definitive evidence of life, and...

Or, at least, less than 100. We know that, says Mike Brown, because the level of methane on Mars is less than 1 part per billion - about the amount that would be produced by 100 cows, burping. It's not exactly a shock that Mars is not prime cattle country, but the aforementioned result is also a major blow to one of Mike's favorite theories: that some kind of life on Mars was all but certain since life from Earth was very likely transported to Mars in the Early days by meteorites blown off the Earth in major collision, to land there when the planet was more hospitable. Even today, it seems likely that Mars has locales where methanogens would survive and reproduce. Of course the reverse voyage scenario is also possible. In which case it would be a bit tragic that the planet that (perhaps) gave us birth, no longer supports life.

Dynamical friction plays an important role in planetary formation. The basic notion is equipartition of energy - not by collisions, but by gravitational interactions. When the protoplanetary disk has reached the stage of being populated by a variety of bodies of various masses, random gravitational encounters will tend to slow down the big ones and speed up the small ones - relative to the mean orbital speed. Being slower is an advantage from the standpoint of growth, as larger bodies with smaller relative velocities merge more readily, so the big get bigger faster than the small. The fast moving smaller planetesimals either get ejected or ultimately crash into bodies large enough to hang on to them. Encounters between the large bodies, whether planets or protoplanets, and the smaller, statistically tend to change the semi-major axis of the little guys, but unless the little guys are ejected completely, they are doomed to return to the point of encounter with the big guy. Comets...