Organic Matter In Comets May Predate Our Sun
By Judith E Braffman-Miller
Comets are icy invaders from the darkest and most distant domains of our Solar System, and they carry within their frozen hearts, remnants of the primordial ingredients that went into the ancient birth of our Sun and its family of planets, moons, and smaller objects. These beautiful, breathtaking visitors from our Solar System’s outer limits migrate into the brilliant light and melting heat of the inner Solar System, with their famous tails flashing as they soar above us in Earth’s sky. Planetary scientists think that by obtaining an understanding of the composition of these fragile, ephemeral frozen invaders from our Solar System’s deep freeze, they can likewise attain insight into what mysterious ingredients contributed to the recipe that cooked up our Sun and its family 4.56 billion years ago. Indeed, the European Space Agency’s (ESA’s) Rosetta space probe discovered a large amount of organic material in the nucleus of comet “Chury”. In an article appearing in the August 31, 2017 issue of the Monthly Notices of the Royal Astronomical Society (UK), two French astronomers proposed that this organic matter originated in interstellar space and predates the birth of our Solar System.
The ESA’s Rosetta mission, which ended in September 2016, discovered that organic matter accounted for 40% (by mass) of the nucleus of comet 67P Churyumov-Gerasimenko–sometimes referred to as “Chury”, for short. Organic compounds, combining carbon, hydrogen, nitrogen, and oxygen, are the substances that enabled life to emerge on our own planet. However, according to Dr. Jean-Loup Bertaux and Dr. Rosine Lallement–of the Laboratoire Atmospheres, Milieux, Observations Spatiales (CNRS/UPMC/Universitie de Versailles Saint-Quentin-em-Yvelines and the Galaxies, Etoiles, Physique et Instrumentation department of the Paris Observatory (Observatoire de Paris/CNRS/Universite Paris Diderot, respectively–these organic molecules were born in the space between stars, long before our Sun and its family had emerged from their natal cold, dark, giant molecular cloud. Indeed, Dr. Bertaux and Dr. Lallement note that astronomers are already well aware of the interstellar source of this organic matter.
Launched aboard an Ariane 5 rocket in March 2004, the Rosetta spacecraft shot high into the skies above Kourou in French Guiana, carrying along with it the Philae lander. The three ton probe sported huge wing-like solar panels, and in May 2014, it conducted the first of a series of major burns of its rocket thrusters to stalk its target–the comet called “Chury”. “Chury” travels an elliptical orbit around our Star, sweeping out from beyond the orbit of the gas-giant Jupiter–which is almost 500 million miles from our planet–and then traveling a lengthy trek between Earth and Mars.
On November 12, 2014, Philae performed the first successful landing on a comet. Unfortunately, Philae’s battery power ran out two days after making its historic landing on “Chury”. Communications with Philae were briefly restored in June and July 2015, but because of diminishing solar power, Rosetta’s communications module with the lander was turned off on July 27, 2016. On September 30, 2016, the Rosetta spacecraft ended its mission by hard-landing on comet “Chury’s” Ma’at region.
The probe is named after the Rosetta stone, which is a stele of ancient Egyptian origin featuring a decree in three scripts. The lander is named after the Philae obelisk, which displays a bilingual Egyptian and Greek hieroglyphic inscription.
Comets are relic icy planetsimals. This means that they are what is left of a vast population of primordial building blocks that formed the quartet of giant, gaseous planets dwelling in the outer regions of our Solar System: Jupiter, Saturn, Uranus, and Neptune. On the other hand, rocky planetesimals, similar to the asteroids, are the remnants of the ancient building blocks that went into the construction of the four inner, solid planets: Mercury, Venus, Earth, and Mars. Planetesimals, of both the icy and rocky kind, bumped into one another and frequently merged, forming ever larger and larger objects, when our Solar System was young and first taking shape billions of years ago.
The icy, dusty comets fly, with sparkling tails flashing and thrashing, into Earth’s well-lit and toasty inner domain around our Sun. These strange frozen objects come soaring towards our Sun from their frozen homes located in cold, twilight outer reservoirs that they share with a myriad of others of their frigid kind in our Solar System’s deep freeze. The comets originate in the Kuiper Belt, Scattered Disc, and Oort Cloud. Of these distant domains, that host a multitude of comet nuclei, the Kuiper Belt and Scattered Disc reside beyond the orbit of the outermost major planet, the ice-giant Neptune. The still-hypothetical Oort Cloud is much more remote, and is thought to form a gigantic sphere around our entire Solar System–extending to (at least) 10% of the distance to the nearest star beyond our Sun. Short-period comets originate in the Kuiper Belt and Scattered Disc, and they come screaming into the inner Solar System more frequently than every two hundred years. The Oort Cloud contains the most distant comets–the long-period comets–that invade Earth’s inner kingdom at a minimum of every two hundred years. Because the Kuiper Belt and Scattered Disc are much closer to Earth, short-period comets have played a more important role in our planet’s history than long-period comets.
Every time a migrating comet zips into the warm inner Solar System, it loses some of its mass as a result of sublimation of its ices to gas. This means that these sparkling visitors are doomed. For example, the very well-known Halley’s Comet is predicted to have a life expectancy of less than 100,000 years. The comets that we can see today, as they streak though the sky above us, are destined to evaporate and vanish as a result of sublimation of their ices into gas. However, these ill-fated objects will inevitably be replaced by fresh, new comets that will eventually journey into the melting, merciless heat of the inner Solar System.
The nucleus, or core, of a comet is composed primarily of ice and dust that is trapped within a coating of dark organic material. The ice is mostly frozen water, but other types of ice are trapped within the nucleus, as well–such as methane, carbon dioxide, ammonia, and carbon monoxide ice. As the glittering comet soars inward towards our fiery Sun, the ice that envelopes its nucleus morphs into a gas, and this is what creates a comet’s cloud called a coma. Radiation flowing out from our Star shoves the very small motes of dust away from the coma, and this creates the brilliant, dusty tails that comets are so famous for.
The nucleus of a comet is usually only about 10 miles–or less. However, some comets dramatically display truly awe-inspiring comas, that can be over 1 million miles wide! Some particularly glitzy comets show off amazing tails that extend for 100 million miles.
Leaving a trail of debris behind them, as they make their incredible journey into the inner Solar System, comets have been known to be the source of meteor showers on Earth–such as they Perseid meteor shower that lights up our night sky every year between August 9 and 13. The Perseid meteor shower occurs when our planet passes through the orbit of the Swift-Tuttle comet.
Organic Matter In Comets May Predate Our Sun
Astronomers have known for 70 years that an analysis of a star’s spectra can suggest some mysterious absorptions throughout interstellar space when viewed at specific wavelengths. These are termed diffuse interstellar bands (DIBs). DIBs are attributed to complex organic molecules that the American astrophysicist, Dr. Theodore Snow of the University of Colorado, believes may compose the largest known reservoir of organic matter in the Universe. This interstellar organic material is normally discovered in the same proportions. However, extremely dense clouds of matter like presolar nebulae are exceptions. In the heart of these nebulae, where matter is even denser, DIB absorptions plateau or even plummet. This happens because the organic molecules that are responsible for the formation of DIBs clump together. The clumps of matter absorb less radiation than when it flowed freely through interstellar space.
Such primitive, primordial nebulae frequently contract to create a solar system like our own–hosting planets, moons, asteroids, and comets. Stars like our Sun are born in the secretive depths of these extremely dense blobs embedded within the billowing, swirling folds of giant, dark, cold molecular clouds that inhabit our Milky Way Galaxy in huge numbers. Even though it may seem counter intuitive, things have to get very cold in order for a searing-hot baby star to be born. This is because stars are born tucked within relatively dense concentrations of gas and dust, and these regions are extremely frigid, with temperatures of only 10 to 20 Kelvin–only a bit above absolute zero. At these temperatures, gases become molecular, causing atoms to merge together, thus making the gas clump to very high densities. When this density reaches a certain point, stars are born.
All stars are huge spheres composed of searing-hot, glaring, roiling, gas. The billions upon billions of stars that dwell in the observable Universe are all primarily composed of hydrogen–which is both the most abundant atomic element listed in the familiar Periodic Table, as well as the lightest. Stars transform hydrogen fuel deep within their hot nuclear-fusing cores into progressively heavier and heavier atomic elements. The only atomic elements that formed in the Big Bang birth of the Cosmos about 13.8 billion years ago, were hydrogen, helium, and small quantities of beryllium, and lithium (Big Bang nucleosynthesis). All of the other atomic elements listed in the Periodic Table were formed deep within the seething, secretive hearts of the stars, their glaring-hot interiors progressively fusing the nuclei of atoms into heavier and heavier things (stellar nucleosynthesis).
Even though giant molecular clouds are mainly composed of gas, with smaller quantities of dust, they also contain large populations of sparkling, newborn stars. The material within the ghostly, billowing, dark clouds clumps together in an assortment of sizes, with the smaller clumps extending approximately one light-year across. The dense clumps eventually collapse to form protostars. The entire star-birthing process lasts for about 10 million years.
The sparkling myriad of stars inhabiting the Cosmos are kept bouncy and fluffy as a result of the energy that is manufactured by the process of nuclear fusion that is occurring within their cores. The stars are able to maintain a necessary–and very delicate–equilibrium between the powerful squeezing crush of their own relentless gravity–which tries to pull everything in–and their enormous energy output, which churns out radiation pressure, that tries to push everything out and away from the star. This immense production of energy is the result of stellar nucleosynthesis that creates heavier atomic elements out of lighter ones. The precious balance between gravity and radiation pressure is maintained from the “birth” of the star until its “death”–the entire “lifetime” of the star–which it spends on the hydrogen-burning main sequence of the Hertzsprung-Russell Diagram of Stellar Evolution. Alas, the inevitable comes when the star has finally managed to burn its necessary supply of hydrogen fuel–and gravity wins the ancient battle against pressure. At this tragic point, the star’s core collapses, and the star perishes. Small stars, like our own Sun, meet their doom with relative peacefulness, as well as great beauty–puffing off their varicolored outer gaseous layers into the space between stars. Larger, heavier stars, on the other hand, do not go gentle into that good night. More massive stars end their stellar “lives” by blasting themselves to pieces in the catastrophic and violent rage of a supernova explosion, which effectively destroys the star. Where there was once a star, there is a star no more. Therefore, the mass of a star is what determines its fate.
The Rosetta mission taught astronomers that comet nuclei form as the result of gentle accretion of grains progressively larger and larger in size. First, small particles bump into one another and then stick together to create larger grains. These then go on to combine to form ever larger chunks–and so on, and on, until a comet nucleus forms that is a few miles wide. The frozen comet nucleus, in this way, becomes one of the multitude icy inhabitants of the Kuiper Belt, Scattered Disc, or Oort Cloud –where it will remain unless some gravitational interaction evicts it from its frigid, twilight home and sends it screaming towards the melting fires of our Sun.
The comet’s organic molecules, that once drifted throughout the primordial, primitive solar nebula–and are responsible for DIBs–were probably not destroyed, but instead were incorporated into the grains composing cometary nuclei, where they have remained for 4.6 billion years. A sample return mission would allow laboratory analysis of cometary organic material and, at long last, uncover the hidden identity of the mysterious interstellar matter causing the observed strange patterns in stellar spectra.
If cometary organic molecules were indeed manufactured in the space between stars–and if they played an important part in the emergence of life on Earth, as many scientists believe today–is it possible that they might also have seeded life on many other planets in our Galaxy?
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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