Chondrules 2: Hot and dusty, with a chance of lightning!

hartmanNow, don’t get me wrong, the study of chondrules is a serious business. Understanding how these mm-sized objects formed is at the very heart of meteorite science. But just because a topic is important doesn’t mean it can’t be fun too! I’ve just spent two very enjoyable and thought provoking days at the Workshop on Chondrules and the Protoplanetary Disk, held at the Natural History Museum in London. Unlike bigger conferences, which can sometimes feel a bit intimidating, I am a big fan of this sort of focussed workshop format.

So, what were the hot topics being aired at the chondrules workshop?

Well, the first day was really dominated by the topic of “chondrule-matrix complementarity”. Wow, that’s a bit of a mouthful, but people were really getting pretty excited by it. In essence, it’s the idea that chondrules formed in relatively limited regions of the early solar nebula, such that the chondrules and the dusty material in which they are enclosed, are both from the same source region and were not randomly mixed together. It’s an idea that has a lot going for it, because primitive meteorites show great compositional diversity and each group must have formed in quite different environments. Although I am a bit of an agnostic on this topic, I found myself flip-flopping on both sides of the argument, as the Monday morning session progressed. There was a lot of passion expressed on the subject and I took away from the discussion some new ideas about how this concept might be tested further.

The first day also included some very detailed presentations on the ages of chondrules. These are very old objects, but not the oldest. That accolade goes to another bizarre set of objects that go by the acronym CAIs, standing for Calcium Aluminium-rich Inclusions.  CAIs are the earliest dated Solar System solids and formed in a short interval of several hundred thousand years. CAIs date the formation of our Solar System at around 4,567 million years ago. In contrast, chondrules probably formed over a longer period of three to four million years, that may overlap CAIs, or have started about a million years later.  There was some debate about this at the workshop.

Chondrules have a lot to tell us about the conditions that existed in the early Solar System, so much of the Tuesday morning session was taken up with presentations about the physical and thermal environment in which these objects might have formed. While there is excellent agreement about the compositional and isotopic characteristics of chondrules, there is a lot of debate (disagreement) about what it all means. This is not surprising. The environment in which chondrules must have formed is one that we can have no experience of. Astronomical observations have been made of gas clouds around newly formed stars, but we still know very little about what conditions might have been like in such a hostile and alien environment. In fact, most of the evidence we have comes from primitive meteorites. So, it was not surprising that the debate got a bit lively at times.  It was mainly good natured though!


Finally, the Tuesday afternoon session looked at how chondrules might have formed. There are a lot of models. A range of processes were suggested for forming chondrules in the dusty nebula that preceded planet formation. These include various types of shock waves and perhaps also lightning discharge. Small planets probably formed rapidly in the early Solar System, so another class of chondrule-forming models invoke various processes related to these so-called planetesimals and include splashing and jetting during impact between both hot and cold bodies. Finally, and a new one on me, was heating of pre-existing dust as it flew over hot lava lakes on early-formed planetesimals.

At the end of the workshop a three-way vote was taken on what process the participants felt was the most likely one to form chondrules: 1) nebular, 2) planetary, or 3) don’t know. The split was fairly even, so it seems another workshop is needed. In fact, another one is taking place shortly in Vancouver. If it goes as well as the one at the Natural History Museum it will be a lively, fun and very informative experience.

PS I voted nebular!





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