For the past fifty years, scientists who study meteorites have been attempting to solve a mystery concerning the origin of chondrules which are tiny millimeter-size glassy droplets of once-molten rock that meteoriticist Henry Sorby described as “drops of fiery rain” in 1877. These mysterious little droplets are seen in 85% of the meteorites that shower down upon our planet. Solving this great meteorite mystery is difficult, because chondrule formation has not been observed in modern times–the right conditions for their formation probably have not existed for over 4.5 billion years in our Solar System, when a primordial disk of dust and gas (called a protoplanetary disk) circled our newborn Sun. In July 2013, scientists suggested that they may have discovered the strange origin of these mysterious “drops of fiery rain”!
When a dense blob embedded within a cold, dark interstellar molecular cloud collapses under its own weight to form a star, it tends to leave in its wake a disk of dust particles that readily glue themselves together to form progressively larger and larger objects, that eventually may grow into mature planets. The birth and evolution of our own Solar System is believed to have begun about 4.568 billion years ago with the gravitational collapse of a dense portion of a giant molecular cloud. Most of the collapsing material from the dense blob formed our Star, the Sun, while the rest of it flattened out into a pancake-like protoplanetary disk composed of gas and sticky dust particles. From this primordial, swirling disk of dust and gas, came the planets, moons, asteroids, comets, and assorted other small Solar System bodies that we are now familiar with.
Protoplanetary disks have been spotted swirling around a number of young stars inhabiting young stellar clusters. These searing-hot and massive disks form at the same time that their baby star is born, and they proceed to nourish the central neonatal protostar with a rich formula composed of gas and dust. The extremely hot temperatures that characterize the inner regions of the protoplanetary disk vaporize most of the volatile material, such as water, organics, and some rocks, leaving behind only the most refractory elements such as iron. Ice is able to survive only in the outer limits of the whirling disk.
Protoplanetary disks can hang around and nourish their baby stars for about 10 million years. By the time the young star has entered the terrible T Tauri stage of its now-toddler life, the nourishing disk has become thinner–and cooler. A T Tauri star is very active–like most toddlers–and at less than 10 million years old, sports a mass that is about equal to or a bit less than that of our Sun. T Tauris possess diameters that are about four times that of our own Star, but they are still in the process of shrinking down to a more mature size. By the time the toddler star has reached this stage, less volatile materials have begun to condense near the center of its nourishing disk, creating extremely tiny dust grains that harbor crystalline silicates. The movement of material from the outer limits of the disk can mix these recently formed sticky dust grains with primordial ones, which contain organic matter, as well as other volatiles. This mixing and shaking can explain some of the peculiarities observed in the composition of Solar System bodies, such as the presence of interstellar grains in primitive meteorites and refractory inclusions in comets. Our middle-aged Sun was a T Tauri very long ago.
Scientists have long thought that the secret ingredients of the rich, rocky stew that cooked up the four terrestrial planets haunting the inner regions of our Solar System–Mercury, Venus, our Earth, and Mars–consisted primarily of chondritic rock. However, they were unable to determine how this mysterious ingredient came into existence. Chondrules, which are tiny spherical granules, are composed of either pyroxene or olivine. Sorby suggested back in the 19th century that these glassy little droplets might have somehow condensed out of the swirling dust and gas laden protoplanetary disk that did a lazy merry-go-round around our Star when it was young.
In July 2013, scientists finally proposed what has been called a “radical” solution to the cosmochemical puzzle of how these numerous glassy little blobs became embedded within chondritic meteorites–which are the largest class of meteorites.
A Fiery Rain!
Meteoriticists have long thought that chondrules were originally liquid droplets that were dancing around in Space before becoming rapidly cooled. But how did the liquid form? “There’s a lot of data that have been puzzling to people,” explained Dr. Lawrence Grossman to the press on July 8, 2013. Dr. Grossman, of the University of Chicago, is senior author of a study published in the July 2013 issue of Geochimica et Cosmochimica Acta.
Dr. Grossman and his team’s study reconstructs the sequence of minerals that condensed out of the primordial protoplanetary disk over 4 billion years ago– the ancient dusty gas-filled cloud that ultimately gave birth to our brilliant Sun and its enchanting retinue of planets and other lovely objects. The authors conclude that a condensation process cannot account for the production of the mysterious glassy droplets. Instead, they now favor a “radical” theory that their formation involves collisions between planetesimals–the building blocks of planets–which gravitationally coalesced in our primordial Solar System.
“That’s what my colleagues found so shocking, because they had considered the idea so kooky,” Dr. Grossman added.
Dr. Grossman continued to study this “kooky” theory after Dr. Conel Alexander of the Carnegie Institution in Washington, and his colleagues, provided a missing piece to the mysterious puzzle. Dr. Alexander’s team found a tiny amount of sodium hiding secretly within the cores of the olivine crystals that were embedded within the tiny glassy chondrules.
Dr. Alexander’s discovery was of critical importance because when olivine crystallizes out of a liquid that is of chondrule composition, at temperatures of about 2,000 degrees Kelvin, most of the sodium stays in the liquid if it doesn’t entirely evaporate! However, in spite of the great volatility of sodium, a sufficient quantity of it remained in the liquid to be seen, buried like a treasure, inside the olivine–a result of the evaporation suppression that resulted from either high dust concentration or high pressure. No more than a mere 10% of the secret sodium evaporated out of the solidifying tiny droplets of “fiery rain”!
Dr. Grossman and his colleagues reconstructed the conditions that would be necessary to stop any greater amount of evaporation. The scientists devised their calculation in terms of total dust enrichment and pressure in the ancient protoplanetary disk of dust and gas from which some ingredients of the chondrites emerged.
“You can’t do it in the (protoplanetary disk). That’s what led… to planetesimal impacts. That’s where you get high dust enrichments. That’s where you can generate high pressures,” Dr. Grossman continued to explain to the press on July 9, 2013.
Dr. Grossman and his colleague Dr. Alexei Fedkin, who is also at the University of Chicago, have devised a scenario in which planetesimals made up of water ice, magnesium silicates, and metallic nickel-iron, condensed out of the protoplanetary disk long before the chondrules formed. The decay of radioactive elements within the tumbling horde of planetesimals produced sufficient heat to melt the water ice. The water then bubbled through the planetesimals, did a crazy interactive dance with the metal, and ultimately oxidized the iron. With still more heating–produced either before or during the very frequent planetesimal collisions–the magnesium silicates re-formed, incorporating iron oxide this time. When the crashing planetesimals blasted into each other, producing extremely high pressures, tiny liquid droplets containing iron oxide showered out.
“That’s where your first iron oxide comes from, not from what I’ve been studying my whole career,” Dr. Grossman continued to comment.
Dr. Alexander noted in the July 12, 2013 issue of the journal Science that “Sooner or later, someone’s going to come up with a mechanism that solves it all. I’m an optimist.”
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various newspapers, magazines, and journals. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.
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