The opinions expressed here are well-reasoned and insightful -- needless to say they are not the opinions of my employers

14 December 2009

AGU, monday night

I attended a session on early solar system dynamics this morning. It included a talk on orbital eccentricity and distance (and how they change as the sun loses mass) in which the author concluded that the Oort cloud cannot exist, but I need to look more closely at his numbers before I comment on any of it. Suffice it to say I'm a fan of the Oort cloud with its long period comets and highly inclined orbits...

One interesting talk was Age of the Solar System Revisited (Wadhwa and Bouvier). Someone once suggested that the age of earth is a function of time. In his Universe Song, Eric Idle quotes some numbers about size, velocity, mass, and sheer number of stars that reflected the knowledge at that time. In a recent interview his co-author, acknowledging that some of the numbers were understood to be different now, offered that "the facts have changed." "No", Idle replied, "the facts haven't changed -- our understanding of the facts has changed."

Our understanding of the age of the earth is based on the age of meteorites. Radiometric dating of meteorites depends on the same key assumptions as dating of other types of rocks, the key assumption being that we're dealing with a system that has been closed since the event we're trying to date.

The formation of planets from smaller planetessimals implies a fair amount of alteration, and at least part of our understanding of earth's composition is based on the existence of meteorites derived from asteroids that went partially down the planet-forming path -- that is, they became large enough and hot enough to differentiate into a core and mantle. So dating any old meteorite fragment won't do.

A significant fraction, perhaps 90%, of interplanetary debris contain chondrules (from the Greek chondros, meaning grain). these tiny particles consist of mostly devitrified silica-glass droplets, and the meteoritic bodies that contain them (called chondrites) have the appearance of sedimentary rocks. The cooling history of the grains supports the idea that chondrites are formed by the agglomeration of smaller solid particles, as the chondrules cannot have been cooled from a melt to form a rounded droplet within the meteoroid. Chondrules, in fact, appear to be among the oldest solid particles within the solar system. I say among the oldest because chondritic meteorites also contain Calcium-Aluminum-rich inclusions (CAIs, which include, but are not limited to, anorthite and pyroxene) that formed at higher temperatures, and pre-date the formation of the chondrules by perhaps 2 million years.

In this talk the authors look at the results from sampling CAIs from several carbonaceous chondrites (these are chondrites that not only contain small amounts of volatiles, but their refractory composition is close to 1:1 with that of the sun). these samples give dates varying from 4.5676 +- 0.001 to 4.5687 +- 0.002 billion years based on Pb-Pb isochrons. the same meteorites yielded more consistent model ages using Al-Mg and Hf-W systematics. The group claims that there is a yet unidentified problem with Pb-Pb system in these meteorites, suggesting that perhaps the problem is that the initial 238U:235U ratios are not as consistent from sample to sample as has been assumed.

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