A New Theory Of Planet Birth Muddies The Issue
By Judith E Braffman-Miller
Our Solar System’s Main Asteroid Belt, located between the orbits of Mars and Jupiter, is the home of a swarming sea of dancing rocky worldlets, that are generally thought to be the relics of an ancient era when our Star and its family of orbiting objects were first forming. According to this scenario, the rocky asteroids, that heavily populate this Belt, resemble the thundering horde of primordial planetesimals–the building blocks of planets–that contributed to the formation of the quartet of solid Earth-like planets that circle our Sun in the warm and well-lit inner regions of our Solar System. Most scientists propose that the planetesimals that built up the inner planets were rocky, but a new theory challenges that scenario, and instead suggests that the most ancient asteroids were made of mud–not rock. In July 2017, a team of planetary scientists published new research showing that the original planetary building blocks in our Solar System were enormous balls of warm mud. According to this model, radioactive heat in the primordial Solar System could have melted these balls of ice and dust long before they had sufficient time to experience a sea-change into rock.
The new research is published in the July 14, 2017 issue of the journal Science Advances, a publication of the American Association for the Advancement of Science (AAAS). The findings could potentially solve several nagging mysteries about the composition of meteorites found on Earth, and could also explain why asteroids are different from comets. Carbonaceous meteorites are thought to be chunks of the very first asteroids–and most of our scientific knowledge about the first solid bodies to do their mysterious dance within our ancient Solar System is derived from carbonaceous meteorites.
Both asteroids and comets are leftover planetesimals. While asteroids are similar to rocky planetesimals, comets are akin to the delicate, icy, and dirty ancient planetesimals that contributed to the construction of the four majestic, gaseous, giant denizens of our Solar System’s outer limits. In our Solar System’s distant, dimly-lit deep-freeze, there is a twilight, shadowy region called the Kuiper Belt that exists beyond Neptune’s orbit around our Star. In addition to the Kuiper Belt there are two other remote regions hosting frozen comet nuclei–the Scattered Disk and the Oort Cloud. The Oort Cloud is very far away, and many planetary scientists propose that it creates an enormous shell surrounding our entire Solar System. The Oort Cloud extends about half way to the nearest star beyond our Sun, which is Proxima Centauri.
Fragile, ephemeral comets are the icy leftover planetesimals that built up the majestic, gaseous, giant planets: Jupiter, Saturn, Uranus, and Neptune. Comets come screaming into the golden, glowing light and melting heat of the inner Solar System from their frozen home–with their flashing tails thrashing. These delicate refugees from the frigid twilight zone, beyond the ice giant Neptune, differ dramatically from the rocky and metallic asteroids that inhabit the Main Asteroid Belt. Asteroid-like planetesimals are thought to have created the four rocky, solid inner planets: Mercury, Venus, Earth, and Mars.
The most common type of asteroid, termed carbonaceous asteroids, possibly delivered both water and organic molecules to Earth, setting the stage for life to emerge and evolve on our planet. The carbonaceous asteroids formed ice, dust, and mineral grains–termed chondrules–in the swirling dense disk of gas and dust that gave birth to our Star and its retinue of planets, moons, and smaller objects.
However, little is known about the history of carbonaceous asteroids–and they have some intriguingly mysterious attributes. These space-rocks seem to have experienced changes at relatively low and uniform temperatures. This indicates that they must have, in some way, lost heat from within. Many astronomers have suggested that water streaming around inside these ancient asteroids made them cool off, but soluble elements appear not to have moved around–as would be expected if water had been flowing. Furthermore, the compositions of the first asteroids are almost identical to our Sun’s. Indeed, if all of the hydrogen and helium were removed from our Sun, what would be left are the mineral ratios found in these chunks of primordial rock. This means that the first asteroids were born directly from the swirling disk composed of gas and dust that existed before the planets of our Solar System emerged. In addition, their chemical composition indicates that these ancient rocky chunks formed in the presence of water–and at relatively cool temperatures of about 150 degrees Celsius.
Swirling Protoplanetary Accretion Disks
As primordial, primitive leftovers from our Solar System’s formation, asteroids and comets provide precious information about the chemical brew from which the planets of our Sun’s family were born. If planetary scientists want to learn about the composition of the ancient mixture from which the planets emerged, they must identify the chemical constituents of the relic ancient debris left behind to tell the story of that long ago and vanished era.
Our Solar System was born in the billowing, undulating depths of one of the many giant, cold, and dark molecular clouds that haunt our Milky Way Galaxy like immense phantoms. Our Sun and its family emerged as the result of the collapse of a relatively small, but superdense, blob tucked snugly within one of the swirling folds of just such a beautiful and enormous cloud. Most of the material belonging to the collapsing blob gathered at the center and ultimately caught fire as a result of the process of nuclear fusion–giving birth to our Sun. The leftover material flattened out and evolved into what is called a protoplanetary accretion disk, out of which the planets, moons, asteroids, comets and other small Solar System objects eventually formed.
Protoplanetary accretion disks have been observed surrounding many baby stars within young stellar clusters. The whirling, flat disks of gas and dust take shape at approximately the same time that their star is born, but their initial stages of development are difficult to observe because they are enshrouded within thick blankets of obscuring dust. The nurturing accretion disk feeds the hungry newborn star (protostar) a nourishing meal composed of gas and dust. At this stage, the protoplanetary accretion disk is both extremely massive and seething-hot. These whirling disks can hang around their young star for as long as 10 million years.
By the time the stellar tot has grown to reach the T Tauri phase of its development, the nutritious protoplanetary accretion disk has thinned out considerably, and cooled off. A T Tauri is an active, young variable star that is less than 10 million years old–which makes it a mere toddler in star-years. T Tauris sport diameters that are several times greater than that of our own Sun–which is a relatively small star entering mid-life. However, T Tauris, unlike human children, shrink as they grow up. By the time the youthful, active star has reached this stage, less volatile materials have started to condense close to the center of the protoplanetary accretion disk, forming very “sticky” extremely fine grains of dust. These very tiny dust motes contain crystalline silicates.
Within the disk, the floating grains of “sticky” dust bump into one another frequently, and merge together to create ever larger and larger objects within the crowded environment of the accretion disk. The tiny dust motes ultimately form objects up to several centimeters in size, and these proceed to merge together to form the planetesimals–which can grow to become 1 kilometer across, or even larger. The planetesimals represent a very abundant population of objects, and they can travel throughout the entire disk. Some of these very ancient objects can survive long enough to tell the story of that vanished era when our Star and its family were first forming.
A New Theory Of Planet Birth Muddies The Issue
Modelling primordial asteroids as enormous warm mud balls makes the most sense, according to Dr. Philip Bland of Curtin University in Perth, Australia, and his colleague Dr. Bryan Travis of the Planetary Science Institute (PSI) in Tucson, Arizona. Dr. Bland began his research on the giant ancient mud ball model in order to understand the nature of the ancient planetesimal precursors that built up the quartet of larger terrestrial planets that inhabit our inner Solar System today–Earth included.
“The assumption has been that hydrothermal alteration was occurring in certain classes of rocky asteroids with material properties similar to meteorites. However, these bodies would have accreted as a high-porosity aggregate of igneous clasts and fine-grained primordial dust, with ice filling much of the pore space. Mud would have formed when the ice melted from heat released from decay of radioactive isotopes, and the resulting water mixed with fine-grained dust,” explained Dr. Bryan Travis in a July 14, 2017 PSI Press Release. Dr. Travis is a senior planetary scientist at the PSI, and a co-author of the new paper titled Giant convecting mud balls of the early Solar System.
Dr. Travis used his Mars and Asteroids Global Hydrology Numerical Model (MAGHNUM) to carry out computer simulations. adapting MAGHNUM to be able to simulate movement of a distribution of rock grain sizes and flow of mud in carbonaceous chondrite asteroids.
According to the new model, when the dust, ice and chondrules mixed together, they would not have been compacted under pressure into rock immediately. Instead, the ice would be melted into its liquid water phase by decaying radioactive atoms lurking with the gas and dust. This ancient mixture would have resulted in a muddy sludge.
The model also indicates that these warm mud ball asteroids probably were born from relic dusty material still lingering long after our Sun’s birth, and that convection would also occur. Convection would cause the interior of the warm mud ball to readily cool off. Both soluble and insoluble elements would then combine, thus preserving the primordial, primitive chemistry of the ancient asteroid. In this way, the new model explains a number of puzzling features of interest better than rock. However, as time passed, the mud would have eventually changed to rock. This metamorphosis may have been helped by the gravitational pressure that occurred when the asteroid had finally grown big enough–or, alternatively, by collisions with other objects.
This finding could provide a new scientific approach for later research into the evolution of water and organic material in our Solar System, and generate new approaches to how and when we continue on our quest to discover habitable worlds beyond our own.
Therefore, the results of the new model show that a large number of the first asteroids–those that delivered water and organic material to the terrestrial planets, including Earth–may have been born as giant convecting mud balls–instead of solid rock.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, newspapers, and journals. Although she has written on a variety of topics, she 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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