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Exclusive: Scientists closer to determining conditions in which our planetary system formed

Chondrules

The following is an interview with Dr. Jay Melosh University Distinguished Professor of Earth, Atmospheric and Planetary Sciences, Purdue University. Dr. Melosh is one of a team of Scientists studying the composition of meteorites.

1. What do you believe could be the origin of the material, or “chondrules” that forms asteroids?

We believe that chondrules, millimeter-size spherical grains that form the major component of most stony meteorites, were created in low-velocity impacts (that is, a few kilometers/sec, many times faster than a rifle bullet, but slow compared to present day collision speeds in the solar system). They formed when baby planets, “planetesimals” a few hundred km in diameter, collided during the first few million years after the birth of the sun. During the early phase of such collisions the process of “jetting”, which occurs when solid surfaces merge obliquely, ejected melted droplets of rock and metal at high speed. These droplets escaped the colliding bodies and eventually were slowed by hydrogen and helium gas still remaining around the sun. They then collected into loose masses of spherical droplets along with unmelted “matrix” and eventually were compacted by internal heat and other impacts into the asteroids we find today.

2. How is this method of chondrule creation different from currently held beliefs regarding chondrule creation?

Previous theories posited that chondrules were the major building blocks of the planets and had to find some way to process essentially all of the mass of Earth, Venus, Mars and Mercury into mm-size droplets of formerly molten rock. Their formation was attributed to strong shock waves in the gas and dust of the solar nebula (the disk of gas and dust circulating around the newly-born sun). Other theories supposed that strong lightening discharges in the solar nebula could have melted most of the dust and metal grains into chondrules. In our theory we suppose that only a small fraction of the material that went into making up the planets was processed into chondrules, but that this material preferentially ended up in the smaller bodies that now inhabit our asteroid belt.

3. How did you come to your findings?

I have long been impressed that the droplets produced by large impacts on the Earth, such as the Chicxulub impact that extinguished the dinosaurs, were nearly indistinguishable in size and internal texture from the chondrules in meteorites. The problem was to find some way in which impacts in the early solar system could produce a dense accumulation of molten droplets without diluting them in the much larger volume of broken rock that typically forms in an impact crater. Our final answer resembles that found at terrestrial craters: The huge mass of broken rock stays near the site of the impact because of gravity, which holds the slow-moving broken rock near the impact site, while the high-speed melted ejecta fly far away. On the Earth, the melted droplets are deposited in a layer far from the crater, where they form most of the material in that deposit. During collisions among nascent planets, the broken rock stays behind on the largish (moon-sized and bigger) planetary “embryos” because of their gravity, while the melted rock and metal is flies free at high speed, leaving most of the broken debris behind.

4. How is your study different from past studies?

Others have proposed that chondrules were created in impacts, but to explain why meteorites are made mainly of chondrules without the usual broken rock, they proposed that the embryo planets were entirely molten, so any collision would create a spray of pure droplets from deep within the molten embryos. The problem with this theory is that chondrules often contain blebs of iron-nickel metal, which would have sunk to the core of any completely molten proto-planet and would thus not be included in the ejected droplets. Our model explains the presence of metal as the natural result of ejection from just the surfaces of growing planets, on which the iron has not has a chance to separate from the rocky components.

5. What do you feel are the long-term implications of this study?

We believe that the impact scenario constrains the time, size and density of the growing planetary embryos as well as the duration of remnant gas in the solar nebula and thus reveals the exact conditions under which our planetary system formed.

6. What is the next step in your research?

There are many details known about chondrules that are not directly explained by our present model, which is pretty broad-brush at present. Future research will focus on finding out whether all that is known about chondrules can be fit into our model.