Wednesday at AGU

So far this morning I've seen a bunch of fantastic images from Mars and some remarkable data on seismic tremor. The most striking thing from Mars is widespread traces of riverbeds, raised above the surrounding topography as the uncemented sediments around them have eroded away. They're textbook examples of inverted topography, and Mars scientist Ken Edgett told the audience that "short of being on Mars, picking up a piece of rock and seeing fossil impressions of raindrops on it, this is the best evidence for actual rainfall on Mars in the past."

Also this morning I took in some talks on two thorny, mystifying problems: the deep structure of the Sierra Nevada and various slow earthquakes. The Sierra is interesting as a prime candidate for crustal delamination, the process (a hypothesis, really) in which the bottom half of the crust grows heavy and breaks off, sinking into the mantle. That's like taking off the bottom of a double boiler and putting your recipe directly on the burner. Seismic pictures are our best clues to what's going on, but interpreting them is an intricate and ambiguous art. I guess that's the best thing to say about the slow earthquakes too—nonvolcanic tremor, episodic tremor, transient slip events and other events that don't fit the usual criteria of earthquakes.

This afternoon I've been sitting in on two sessions about the atmospheres of planets. Best punchline so far has been Kevin Zahnle's, on the question of why planets of a certain size even have atmospheres: "Why is there air? Because it hasn't escaped yet." There is quite a prejudice in the universe against air, as young Sol-type stars burn very hot in the ultraviolet and want to disassociate common gases. One speaker put forth a model in which Mars lost everything very early, then grew a new atmosphere once the Sun cooled a bit. I love watching minds grapple with grand questions like these.

By the way, there's lots of news coming out of AGU. Particularly active is the AGU coverage on the Wired Science group blog by Alexis Madrigal and Betsy Mason. They are very busy here in the pressroom.