“In all of my life, So much more I see, Like a glittering prize, I saw you up on a clear day.” from Glittering Prize by Charles Burchill (1959- ), Derek Forbes (1956- ), James Kerr (1959- ), and Michael Joseph MacNeil (1958- ).
Truly magnificent things can be built in the tiniest of increments.
A recent ice storm showed this to be true.
The storm worked its magic under the cover of night – with a gentle, seemingly deliberate, calmness. It gilded every surface with a weighty load, saddling everything with a hefty burden. During the darkness of the night, the only evidence of its passage was the loud cracking of tree branches as they succumbed to the weight of the ice.
In the daytime, the artistry of the storm was in evidence everywhere – it had transformed the very essence of the outdoors into a dazzling beauty. The dull greys and browns of bare tree branches sparkled like the finest and clearest of crystals. Tree buds and overwintering berries were rendered into glistening, lacquered jewels. It was a beautiful, gleaming sight to behold. In the clear light of the day, it was a glittering prize.
The ice storm created this glazed landscape through accretion. Accretion is a gradual process that builds up layers of a material, in this case, ice, over time. At near-zero Celsius, the freezing, misty rain adhered to all surfaces, gradually adding layer-upon-layer of ice – eventually enveloping them in a centimetres-thick crystal sheath.
The air of the ice-storm, saturated with water provided the building materials for the accretion. Surfaces at or below the temperature that water changes state from liquid to solid provided the foundation upon which accretion occurred. As the liquid water touched these surfaces, they instantly crystallised. As each new molecule of water encountered the surface, it likewise crystallised, so that each successive crystal layer encased that which was already there.
A closer look at the ice casings tells the story of their origins. Gorgeous, distinct, large water crystals could be made out on the surface of the frozen sheath. Each crystal shared the same 6-pointed symmetry characteristic of water crystals. The crystallisation of water molecules, with their single atom of oxygen bonded to two hydrogen atoms, produces the distinct six-point structure arising from the optimal packing of each molecule into the crystal.
Notably, each crystal has individual features that are contingent on the specific conditions of its solidification. The random collision of water molecules with the surface of the ice sheath creates this individuality. Each crystal is formed under slightly different conditions – at a different location on the over icy coating, or at a different time. Each crystal is similar in its structure, but unique in its specific features. In some cases the six-pointed crystal is tightly compact, in other instances it is long, elaborated, outstretched.
The individual ice crystals speckle the surfaces of the ice sheath, barely visible to the naked eye, like distant stars in a night sky. Similar, yet unique.
It is fitting that these stunning crystals should conjure up an image of celestial bodies, because they share common origin – accretion.
Just as the water-saturated air enabled the accretion of crystals during the ice storm, clouds of gases across the universe have been, and continue to be, ripe for accretion to give rise to stars and planets. These clouds, or nebulae, are dense aggregates of molecular gases, like hydrogen gas, H2. High densities of molecular gases in nebulae provide the raw materials to build solar systems – stars with their individual planets – through accretion. Accretion can be thought of as the “birthing” process for stars in these stellar nurseries.
In the process of building solar systems, there are two kinds of accretion. The first is involved in the creation of the star itself. The other is the process that may give rise to planets. In both instances, the process is gradual – over time objects are drawn together, and adhere to give rise to new, larger structures.
In the case of star formation, accretion is a gravity-driven process. When conditions are ripe within a nebula, molecular gases aggregate and interact. When these gases aggregate, they create bodies with ever-increasing mass and gravity of their own. They have become protostars.
As the nascent star increases in mass, its gravity increases, attracting objects with lower mass toward its surface. Over time, these objects encircle the star, forming what is known as an accretion disk. Just as was the case with the icy casings that emerged from the ice storm, each star, and the accretion disk that surrounds it, have distinctive features that are contingent on the conditions at the time of their emergence.
A significant subset of these unique stars, surrounded by their associated accretion disk, are suitable for the creation of a solar system – a sun with its satellites. These stars belong to the T Tuari class, named after the prototype T Tauri, and included, in its early genesis, our own sun. They are characterised by having a star surrounded by an accretion disk rich in gas and dust, known as the protoplanetary disk.
Planets are thought to emerge in such systems through one of two mechanisms.
In the first mechanism, gravitational instabilities arise in the disk of gas and dust surrounding the star. This instability results in formation of a clump of gas with its own gravity – a protoplanet. As this protoplanet circles its star, it sweeps up gas and dust in it path, as they are drawn in by the protoplanet’s gravity. The dust aggregates to form the planet’s core, and the planet grows until it clears the path of its orbit. This mechanism, known as the gas-instability process, creates a planet that has a chemistry that matches that of the entire solar system, as it emerged by sweeping a path through the entire protoplanetary disk. What’s more, the process can occur very rapidly, with the planet growing immensely after the original gravitational instability.
The second mechanism is a true accretion process. In this mechanism, dust particles within the protoplanetary disk collide with each other at random and adhere, forming aggregates known as planetisimals. Over time, planetisimals collide with each other as they are both drawn toward the sun by its gravity, while being pushed away from it by its solar wind. The embryonic planets then undergo one of two paths – they accrete gas to form gas giants like Jupiter, or they continue to aggregate to form rocky planets like Mars and Earth. In either case, the trajectory to planethood is a much slower one than gravitational instability, The random collision to form planetisimals itself could take incredibly long times, and then growth beyond the embryonic stage is thought to take up to 10 million years to create a planet the size of Jupiter. As the accretion process is contingent on random collisions, it produces planets that have distinct chemistry relative to system as a whole.
The giant gas planets in our solar system appear to be products of the accretion process. For example, the atmosphere of Jupiter has a 3-fold greater abundance carbon, nitrogen, and sulfur than the sun. By contrast, less massive Saturn has more carbon than the sun by a factor of 7. These variations relative to the sun suggests that proto-Jupiter and proto-Saturn did not merely uniformly sweep the protoplanetary disk, which would result in an atomospheric composition aligned with the sun. Instead, random accretion can account for their unique atmospheric chemistry. This said, our solar system is very ancient, at 4.6 billion years of age. Therefore, it is difficult to be certain about its early history, as we can only observe a snapshot of its existence at a relatively mature state.
These notions of solar system formation owe their origins to the 18th century astronomers Emanuel Swedenborg and Pierre-Simon Laplace, who formulated what was to become known as Nebula Hypothesis. Of course, this hypothesis was just that, hypothetical, as we could never actually observe the process at work. It had to be inferred that these processes were at work based on our own solar system. At least this was the case, until we were able to observe solar systems other than our own.
In the 20th century, planets outside of our own solar system, exoplanets, were observed for the first time. Today, there are over 1000 confirmed exploplanets. If one was able to measure the atmospheric chemistry of these planets relative to their stars, particularly for a young solar system, we might better determine if accretion or gravitational instability were at work to create planets elsewhere in the universe.
Taking the measurement of exoplanet atmospheric chemistry is a tall order. While space probes like Galileo have directly sampled the atmosphere of planets within our solar system, no probes are able to sample the atmospheres of planets that are light years away. Fortunately, astronomers have a trick to examine the chemistry in the atmosphere of distant stars and planets.
Astronomers can take advantage of the way chemicals interact with light to measure the abundance of those chemicals. When light interacts with chemicals, some of that light is absorbed so that the light that emerges has a different spectrum than the original transmitted light. By analysing the spectrum of light that has passed through a gas, like an atmosphere, astronomers can determine the chemicals that must be present in that atmosphere, and their abundance relative to each other.
The detection and analysis of chemical spectra, known as spectroscopy, has been validated for planetary atmospheric chemistry by comparing spectra for planets in our solar system with the direct samples of their atmospheres that were obtained by space probes. With this confirmation of the validity of the technique, astronomers can confidently collect spectra from distant exoplanets to measure their atmospheric chemistry. Of course, this is dependent on being able to observe those exoplanets directly, with a high degree of accuracy.
HR 8799c is a gas giant planet that orbits a nearby star, HR 8799, in the company of three other gas giant planets. The entire system HR 8799 is about 130 light years away, which is considered nearby by astronomy standards (but is still more than 1000 trillion kilometres away).
The HR 8799 system is relatively young. It is only 30 million years old, in comparison to our solar system’s lifetime of 4.6 billion years. Consequently, the planets in this system are relatively hot (between 900 and 1200 K). They are hot due to their youth, as the heat remains from the gravitational energy released during their formation. All told, they provide an excellent target to test hypotheses related to the mechanisms of planet formation.
Analysis of the atmospheric spectrum for HR 8799c revealed the presence of water (H2O) and carbon monoxide (CO). It was expected that the planet might also possess methane (CH4), but none could be detected. Importantly, the ratio of water and CO in the atmosphere of HR 8799c was at odds with that of the planet’s star, HR 8799. The results were consistent with the hypothesis that this planet, like our own gas giants, formed by accretion as opposed to gravitational instability.
Whether the prominence of accretion as a planet-forming mechanism applies to other solar systems, beyond ours and that of HR 8799, remains to be determined. What we do know is that the prevailing evidence points to a key role for accretion in making the gas giants that populate our universe.
Here, in our own tiny corner of the universe, we owe our existence to accretion. The slow, gradual workings of accretion have provided us with a place to stand, a sun to warm our faces, and planets and stars that twinkle in our night-time skies.
Accretion shows the patient workings of the universe – the gradual build-up that occurs over vast amounts of time, and the great wonders that can produce. There is a tendency to think of things as always wearing away, eroding. Life all too often appears to be a process of incremental loss. Accretion is the counter to that. Life can also be a process of accretion – random, incremental gains that ultimately shape the individual. Like the stars and planets above, or the glazed landscape after an ice storm, it can produce something worthy of awe and fascination. A glittering prize if ever there was one.
Images: All photographs by Malcolm M. Campbell.
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