We Are All Made of Stardust

We Are All Made of Stardust 

Where Do Elements Come From? 

Element factories in space.

Nebulae from which the first-generation stars formed consisted entirely of the lightest atoms, because only these atoms were generated by Big Bang nucleosynthesis. In contrast, the Universe of today contains 92 naturally occurring elements. Where did the other 87 elements come from? In other words, how did elements with larger atomic numbers (such as carbon, sulphur, silicon, iron, gold, and uranium), which are common on Earth, form? Physicists have shown that these elements form during the life cycle of stars, by the process of stellar nucleosynthesis. Because of stellar nucleosynthesis, we can consider stars to be “element factories,” constantly fashioning larger atoms out of smaller atoms. 

What happens to the atoms formed in stars? Some escape into space during the star’s lifetime, simply by moving fast enough to overcome the star’s gravitational pull. The stream of atoms emitted from a star during its lifetime is a stellar wind (figure above a). Some escape only when a star dies. A small or medium star (like our Sun) releases a large shell of gas as it dies, ballooning into a “red giant” during the process, whereas a large star blasts matter into space during a supernova explosion (figure above b). Most very heavy atoms (those with atomic numbers greater than that of iron) require even more violent circumstances to form than generally occurs within a star. In fact, most very heavy atoms form during a supernova explosion. Once ejected into space, atoms from stars and supernova explosions form new nebulae or mix back into existing nebulae.

When the first generation of stars died, they left a legacy of new, heavier elements that mixed with residual gas from the Big Bang. A second generation of stars and associated planets formed out of these compositionally more diverse nebulae. Second-generation stars lived and died, and contributed heavier elements to third-generation stars. Succeeding generations contain a greater proportion of heavier elements. Because not all stars live for the same duration of time, at any given moment the Universe contains many different generations of stars. Our Sun may be a third-, fourth-, or fifth-generation star. Thus, the mix of elements we find on Earth includes relicts of primordial gas from the Big Bang as well as the disgorged guts of dead stars. Think of it the elements that make up your body once resided inside a star!

The Nebular Theory for Forming  the Solar System 

In recent posts, we introduced scientific concepts of how stars form from nebulae. But we delayed our discussion of how the planets and other objects in our Solar System originated until we had discussed the production of heavier atoms such as carbon, silicon, iron, and uranium, because planets consist predominantly of these elements. Now that we’ve discussed stars as element factories, we return to the early  history of the Solar System and introduce the nebular theory, an explanation for the origin of planets, moons, asteroids, and comets. According to the nebular theory, these objects formed from the material in the flattened outer part of the disk, the material that did not become part of the star. This outer part is called the protoplanetary disk. 

What did the protoplanetary disk consist of? The disk from which our Solar System formed contained all 92 elements, some as isolated atoms, and some bonded to others in molecules. Geologists divide the material formed from these atoms and molecules into two classes. Volatile materials such as hydrogen, helium, methane, ammonia, water, and carbon dioxide are materials that can exist as gas at the Earth’s surface. In the pressure and temperature conditions of space, all volatile materials remain in a gaseous state closer to the Sun. But beyond a distance called the “frost line,” some volatiles condense into ice. (Note that we do not limit use of the word “ice” to water alone.) Refractory materials are those that melt only at high temperatures, and they condense to form solid soot-sized particles of “dust” in the coldness of space. As the proto-Sun began to form, the inner part of the disk became hotter, causing volatile elements to evaporate and drift to the outer portions of the disk. Thus, the inner part of the disk ended up consisting predominantly of refractory dust, whereas the outer portions accumulated large quantities of volatile materials and ice. As this was happening, the protoplanetary disk evolved into a series of concentric rings in response to gravity.

The grainy interior of this meteorite may resemble the texture of a small planetesimal.

How did the dusty, icy, and gassy rings transform into planets? Even before the proto-Sun ignited, the material of the surrounding rings began to clump and bind together, due to gravity and electrical attraction. First, soot-sized particles merged to form sand- to marble-sized grains that resembled “dust bunnies.” Then, these grains stuck together to form grainy basketball-sized blocks (figure above), which in turn collided. If the collision was slow, blocks stuck together or simply bounced apart. If the collision was fast, one or both of the blocks shattered, producing smaller fragments that recombined later. Eventually, enough blocks coalesced to form planetesimals, bodies whose diameter exceeded about 1 km. Because of their mass, the planetesimals exerted enough gravity to attract and pull in other objects that were nearby. Figuratively, planetesimals acted like vacuum cleaners, sucking in small pieces of dust and ice as well as smaller planetesimals that lay in their orbit, and in the process they grew progressively larger. Eventually, victors in the competition to attract mass grew into protoplanets, bodies approaching the size of today’s planets. Once a protoplanet succeeded in incorporating virtually all the debris within its orbit, it became a full-fledged planet. 

Early stages in Earth’s planet-forming process probably occurred very quickly some computer models suggest that it may have taken less than a million years to go from the dust and gas stage to the large planetesimal stage. Planets may have grown from planetesimals in 10 to 200 million years. In the inner orbits, where the protoplanetary disk consisted mostly of dust, small terrestrial planets composed of rock and metal formed. In the outer part of the Solar System, where significant amounts of ice existed, protoplanets latched on to vast amounts of ice and gas and evolved into the giant planets. Fragments of materials that were not incorporated in planets remain today as asteroids and comets.

When did the planets form? Geologists have found that special types of meteorites thought to be leftover planetesimals formed at 4.57 Ga, and thus consider that date to be the birth date of the Solar System. If this date is correct, it means that the Solar System formed about 9 billion years after the Big Bang, and thus is only about a third as old as the Universe. 

Differentiation of the Earth  and Formation of the Moon 

When planetesimals started to form, they had a fairly homogeneous distribution of material throughout, because the smaller pieces from which they formed all had much the same composition and collected together in no particular order. But large planetesimals did not stay homogeneous for long, because they began to heat up. The heat came primarily from three sources: the heat produced during collisions (similar to the phenomenon that happens when you bang on a nail with a hammer and they both get warm), the heat produced when matter is squeezed into a smaller volume, and the heat produced from the decay of radioactive elements. In bodies whose temperature rose sufficiently to cause internal melting, denser iron alloy separated out and sank to the centre of the body, whereas lighter rocky materials remained in a shell surrounding the centre. By this process, called differentiation, protoplanets and large planetesimals developed internal layering early in their history. As we will see later, the central ball of iron alloy constitutes the body’s core and the outer shell constitutes its mantle.

In the early days of the Solar System, planets continued to be bombarded by meteorites (solid objects, such as fragments of planetesimals, falling from space that land on a planet) even after the Sun had ignited and differentiation had occurred (see Meteors and Meteorites). Heavy bombardment in the early days of the Solar System pulverized the surfaces of planets and eventually left huge numbers of craters. Bombardment also contributed to heating the planets. 

Based on analysis and the dating of Moon rocks, most geologists have concluded that at about 4.53 Ga, a Mars-sized protoplanet slammed into the newborn Earth. In the process, the colliding body disintegrated and melted, along with a large part of the Earth’s mantle. A ring of debris formed around the remaining, now-molten Earth, and quickly coalesced to form the Moon. Not all moons in the Solar System necessarily formed in this manner. Some may have been independent protoplanets or comets that were captured by a larger planet’s gravity. 

Meteors and Meteorites 

During the early days of the Solar System,  the Earth collided with and incorporated countless planetesimals and smaller fragments of solid material lying in its path. Intense bombardment ceased about  3.9 Ga, but even today collisions with space objects continue, and over 1,000 tons of material (rock, metal, dust, and ice) fall to Earth, on average, every year. The vast majority of this material consists of fragments derived from comets and asteroids sent careening into the path of the Earth after billiard-ball-like collisions with each other out in space, or because of the gravitational pull of a passing planet deflected their orbit. Some of the material, however, consists of chips of the Moon or Mars, ejected into space when large objects collided with those bodies. 

Meteors and meteorites.

Astronomers refer to any object from space that enters the Earth’s atmosphere as a meteoroid. Meteoroids move at speeds of 20 to 75 km/s (over 45,000 mph), so fast that when they reach an altitude of about 150 km, friction with the atmosphere causes them to heat up and vaporize, leaving a streak of bright, glowing gas. The glowing streak, an atmospheric  phenomenon, is a meteor (also known colloquially, though incorrectly, as a “falling star”) (figure above a). Most visible meteors completely vaporize by an altitude of about 30 km. But dust-sized ones may slow down sufficiently to float to Earth, and larger ones (fist-sized or bigger) can survive the heat of entry to reach the surface of the planet. In some cases, meteoroids explode in brilliant fireballs. 

Objects that strike the Earth are called meteorites. Although almost all meteorites are small and have not caused notable damage on Earth during human history, a very few have smashed through houses, dented cars, and bruised people. During the longer term of Earth history, however, there have been some catastrophic collisions that left huge craters (figure above b). 

Most meteorites are asteroidal or planetary fragments, for icy material is too fragile to survive the fall. Researchers recognize three basic classes of meteorites: iron (made of iron-nickel alloy), stony (made of rock), and stony iron (rock embedded in a matrix of metal). Of all known meteorites, about 93% are stony and 6% are iron (figure above c). Researchers have concluded that some meteors (a special subcategory of stony meteorites called carbonaceous chondrites, because they contain carbon and small spherical nodules called chondrules) are asteroids derived from planetesimals that never underwent differentiation into a core and mantle. Other stony meteorites and all iron meteorites are asteroids derived from planetesimals that had differentiated into a metallic core and a rocky mantle early in Solar System history but later shattered into fragments during collisions with other planetesimals. Most meteorites appear to be about 4.54 Ga, but carbonaceous chondrites are as old as 4.57 Ga and are the oldest solar system materials ever measured.

Since meteorites represent fragments of undifferentiated and differentiated planetesimals, geologists consider the average composition of meteorites to be representative of the average composition of the whole Earth. In other words, the estimates that geologists use for the proportions of different elements in the Earth are based largely on studying meteorites. Stony meteorites are probably similar in composition to the mantle, and iron meteorites are probably similar in composition to the core.

Why Are Planets Round? 

Small planetesimals were jagged or irregular in shape, and asteroids today have irregular shapes. Planets, on the other hand, are more or less spherical. Why? Simply put, when a protoplanet gets big enough, gravity can change its shape. To picture how, imagine a block of cheese sitting outside on a hot summer day. As the cheese gets softer and softer, gravity causes it to spread out in a pancake-like blob. This model shows that gravitational force alone can cause material to change shape if the material is soft enough. Now let’s apply this model to planetary growth. 

The rock composing a small planetesimal is cool and strong enough so that the force of gravity is not sufficient to cause the rock to flow. But once a planetesimal grows beyond a certain critical size (about 1,000 km in diameter), its interior becomes warm and soft enough to flow in response to gravity. As a consequence, protrusions are pulled inward toward the centre, and the planetesimal re-forms into a special shape that permits the force of gravity to be nearly the same at all points on its surface. This special shape is spherical because in a sphere mass is evenly distributed around the centre.
Credits: Stephen Marshak (Essentials of Geology)

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