Tuesday, December 30, 2008

Into the Fire

Closer to the protosun, in the hottest regions of the forming solar system, it was the heaviest elements, not ices and gases, that survived to form the planets. Thus the terrestrial planets are rich in the elements silicon, iron, magnesium, and aluminum. The dust grains and then planetesimals from which these planets were formed were rocky rather than icy. It is fortunate that water ice and organic compounds later rained down on the early Earth, or the present-day planet would be as lifeless as the moon.

Out of the Frying Pan

As the solar nebula contracted and flattened into its pancake-like shape, gravitational energy was released in the form of heat, increasing its temperature. Due to the inverse-square law of gravitational attraction, matter piled up mostly at the center of the collapsing cloud. The density of matter and the temperature were highest near the center of the system, closest to the protosun, and gradually dropped farther out into the disk.
At the very center of the nascent solar system, where heat and density were greatest, the solar mass coalesced. In this very hot region, the carefully assembled interstellar dust was pulled apart into its constituent atoms, while the dust in the outer regions of the disk remained intact. Once the gravitational collapse from a cloud to a disk was complete, the temperatures began to fall again, and new dust grains condensed out of the vaporized material toward the center of the solar system. This vaporization and recondensation process was an important step in the formation of the solar system, because it chemically differentiated the dust grains that would go on to form the planets. These grains originally had a uniform composition. In the regions nearest the protosun—where temperatures were highest—metallic grains formed, because metals survived the early heat. Moving farther out, silicates (rocky material), which could not survive intact close to the protosun, were condensed from the vapor. Farther out still, there were water ice grains, and, even farther, ammonia ice grains. What is fascinating to realize is that the heat of the protosun depleted the inner solar system (which is home to the earth) of water ice and organic carbon compounds. These molecules, as we will see, survived in the outer solar system and later rained onto the surfaces of the inner planets, making one of them habitable.
The composition of the surviving dust grains determined the type of planet that would form. Farthest from the sun, the most common substances in the preplanetary dust grains were water vapor, ammonia, and methane, in addition to the elements hydrogen, helium, carbon, nitrogen, and oxygen—which were distributed throughout the solar system. The jovian planets, therefore, formed around mostly icy material. And in the cooler temperatures farthest from the protosolar mass, greater amounts of material were able to condense, so the outer planets tended to be very massive. Their mass was such that, by gravitational force, they accreted hydrogen-rich nebular gases in addition to dust grains. Hydrogen and helium piled onto the outer planets, causing them to contract and heat up. Their central temperatures rose, but never high enough to trigger fusion, the process that produces a star’s enormous energy. Thus the jovian worlds are huge, but also gaseous.

Whipping Up the Recipe

While there is substantial variety among the nine planets, they tend to fall into two broad categories: the large gaseous outer planets, known as the jovians, and the smaller rocky inner planets, the terrestrials. Why this particular differentiation?
As with just about any recipe in any kitchen, part of the difference is caused by heat.

Tuesday, December 16, 2008

Accretion and Fragmentation

The preplanetary clumps grew by accretion from objects that might be imagined to be the size of baseballs and basketballs to planetesimals, embryonic protoplanets several hundred miles across. The early solar system must have consisted of millions of planetesimals. While smaller than mature planets, the planetesimals were large enough to have sufficiently powerful gravitational forces to affect each other. The result was near misses and collisions that merged planetesimals into bigger objects, but also fragmentation, as collisions resulted in chunks of some planetesimals breaking off. As we saw in the last chapter, the formation of the moon likely happened at this point in the history of the solar system.
The larger planetesimals, with their proportionately stronger gravitational fields, captured the lion’s share of the fragments, growing yet larger, while the smaller planetesimals joined other planets or were “tossed out.” A certain number of fragments escaped capture to become asteroids and comets.
Unlike the planets, whose atmospheres and internal geological activity (volcanism and tectonics) would continue to evolve matter (the earth, for example, has rocks and minerals that vary greatly in age), asteroids and comets remained geologically static, dead; therefore, their matter, unchanging, marks well the date of solar system birth.

Birth of the Planets

Let’s put the nebular theory and the condensation theory together, as most current astronomers do.
Here is a possible portrait of the formation of our solar system: A cloud of interstellar dust, measuring about a light-year across, begins to contract, rotating more rapidly the more it contracts. With the accelerating rotation comes a flattening of the cloud into a pancake-like disk, perhaps 100 A.U. across—100 times the current distance between the earth and the sun.
The original gases and dust grains that had formed the nebular cloud have contracted into condensation nuclei, which begin to attract additional matter, forming clumps that rotate within the disk.
The clumps encounter other clumps and more matter, growing larger by accretion. Accretion is the gradual accumulation of mass, and usually refers to the building up of larger masses from smaller ones through the mutual gravitational attraction of matter.

Pearls the Size of Worlds

Beginning in the 1940s, astronomers returned to the idea of the solar nebula to create a modification of it called the condensation theory.
There were critics of the nebular theory in the nineteenth century, among them James Clerk Maxwell, who had figured out the fundamentals of electromagnetic radiation. What Maxwell and the other critics of the Kant-Laplace theory didn’t know about was interstellar dust. Microscopic dust grains—ice crystals and rocky matter—formed in the cooling atmosphere of dying stars, then grew by attracting additional atoms and molecules of various gases. These dust grains served two purposes in the formation of planets:
  1. The presence of grains hastened the collapse of the nebular cloud by promoting the radiation of heat from it. This radiation of heat cooled the cloud, accelerating its collapse.
  2. Each grain acted as a condensation nucleus, like the grain of sand in an oyster that eventually becomes a pearl. These grains eventually grew into pearls the size of worlds. In effect, these grains were planetary seeds.

Angular Momentum Explained

Most importantly, Laplace introduced conservation of angular momentum to the discussion of planetary formation. He demonstrated mathematically that the solar nebula—the gaseous mass that would become the solar system—would spin faster as it contracted. Anyone who has watched an ice skater spinning knows this is true. As a skater pulls in his arms, bringing his mass closer to his axis of rotation, he will spin faster. If he were to put his arms out at his side, his rotation would slow. Newton described how all objects with mass were mutually attracted. As the cloud of gas that eventually formed the solar system started to collapse, it would have to rotate faster and faster to conserve angular momentum. And, as the speed of rotation increased, the shape of the solar nebula would change, becoming the pancake-like disk Kant had first pictured. Think of that the next time you watch the local pizza maker throw dough in the air, making it spin, flatten, and strech all at once.
Laplace theorized that as the spinning disk contracted, it would form concentric rings, each of which would clump together into a “protoplanet” (a sort of embryonic planet), which ultimately developed into a mature planet. The center of the disk (in this picture) would coalesce into a hot, gaseous “protosun,” which ultimately became the sun.