Astropedia Textbook

Artist's conception of the solar nebula.

The initial stage in the history of the Solar System is the collapse and rotation of a large, diffuse cloud. After the cloud collapses to a stable configuration with a young Sun and a surrounding disk of gas and dust, we are ready to account for the properties of the planets.  The next stage in the solar nebula was the slow and steady formation of planets from the microscopic particles in the solar nebula. What clues to this process do we get from the chemical composition of the Solar System? We need to explain why the planets vary in composition, and why the composition depends on their distance from the Sun.

Helmholtz contraction heated the outer, dusty regions of the solar nebula to at least 2000 K. Once the Sun formed and the contraction stopped, there was no new source of gravitational energy. As a result, the gas in the nebula began to cool. At such a high starting temperature, virtually all elements were in gaseous form. Like all cosmic gas, most of the solar nebula was hydrogen and helium, but a few percent of the atoms were heavier elements, such as silicon, iron, and other rock-forming material.

How did solid particles of planet-forming rock arise in this gas? A similar process occurs on the Earth's atmosphere. When an air mass cools, water molecules in the air condense to form particles. In a cooling cloud, water vapor condenses into snowflakes, raindrops, hailstones, or the ice crystals that make up cirrus clouds. Condensation occurs more easily when there is a grain of dust or some solid material to start with. Similarly, as the solar nebula cooled, different molecules condensed into droplets. As it cooled even more, the droplets solidified into tiny solid particles.

Different mineral compounds condense at different temperatures — this is called a condensation sequence. The condensation sequence is important in understanding the differences among planets. Knowing that the gas in the solar nebula cooled from about 2000 K, let's look at what minerals form, and in what order. At temperatures of about 1600 K, heavy elements such as aluminum and titanium condensed. They formed microscopic particles of metallic oxides. At about 1400 K a more important constituent, iron, condensed. Microscopic bits of nickel-iron alloy formed as mineral grains, or perhaps as coatings on the earlier aluminum and titanium grains. Still more important, at about 1300 K, abundant silicates began to appear in solid form. These silicate minerals are the common rock-forming materials, containing complex mixtures of magnesium, calcium and iron. All of them are bound to oxygen. Black carbonaceous minerals condensed at the much lower temperature of 300 K. Finally, at temperatures in the range of 100 to 200 K, hydrogen-rich molecules condensed into ices, primarily water ice, frozen methane, and frozen ammonia. This sequence can be confirmed in laboratory tests by cooling mixtures of elements in the same proportions as they occurred in the solar nebula.

Some carbonaceous chondrites. From left to right: Allende, Tagish Lake and Murchison.

The condensation sequence shows a clear relationship between temperature and the atomic or molecular weight of the materials that become solid. Above 1000 K, we see oxides of aluminum (molecular weight 102), pure flakes of iron and nickel (atomic weights of 56 and 59), and silicate rocks like enstatite and olivine (molecular weights of 100 and 172). At 300 K and below, we see carbon in the form of pure soot (atomic weight 12), water ice (molecular weight 18), and ices of methane and ammonia (molecular weights 16 and 17). 

If the solar nebula had cooled in the same way throughout, there would be no composition variations in the Solar System. The same minerals would have condensed everywhere. Instead, the nebula stayed hotter near the center, due to the energetic young Sun, and it cooled more rapidly on the edge.

The orbits of the bodies in the Solar System to scale (clockwise from top left)

Why are the inner and outer Solar System bodies so different? The inner nebula formed dust grains that were complex mixtures of magnesium-, calcium-, and iron-rich silicate minerals, the basic materials of rocks. The Sun's radiation drove lighter gases and molecules outward. The outer parts of the nebula, far from the Sun, cooled to much lower temperatures and formed other types of solid grains. At distances corresponding to the middle of the asteroid belt, black carbonaceous minerals condensed. Beyond the outer asteroid belt, water ice condensed. This explains the "soot line" and the "frost line." Ammonia and methane ices condensed at 100 to 200 K in the outermost nebula. In the outer Solar System, these ices remain solid even in direct sunlight. They survive today in comets and in the atmospheres and satellites of the giant planets. The condensation sequence explains why there are icy worlds in the outer Solar System, but not in the inner Solar System.

Nucleus of Halley's comet.

 

The small, solid bodies that formed in the primordial Solar System are called planetesimals. Comets represent one type of survivor from the ancient population of planetesimals. The icy material in most comets hasn't been heated significantly since the early days of planetesimal formation, so comets preserve clues to that era. Interplanetary dust grains may be debris from comets, undisturbed since the formation of the Solar System. These tiny particles (about the size of white blood cells) filter into the Earth's upper atmosphere. There, U.S. Air Force sampling programs have captured them for study — imagine a high altitude aircraft trawling something like an air filter to scoop up the particles.

Asteroids are larger remnants of planetesimals. In the asteroid belt, planetesimals never completed the process of accretion, because nearby Jupiter exerted enough gravity to disrupt the process. Meteorites that are fragments of asteroids therefore give us evidence about planetesimals and conditions during the early Solar System.

What do scientists find when they analyze these meteorites? Primitive carbonaceous meteorites never completely melted, so they retain much of their original structure. Their composition supports the accretion process described above. For instance, some carbonaceous meteorites contain inclusions of material believed to be among the earliest solid objects in the Solar System. These pebble-like inclusions are rich in the elements that condensed first (at the highest temperatures), such as osmium and tungsten. These minerals formed at temperatures of about 1450 to 1840 K. Yet the surrounding carbonaceous matrix of these meteorites formed at lower temperatures. The high-temperature minerals must have condensed first and aggregated into pebble-like objects. These were then surrounded by sooty carbonaceous material, which condensed later, after the nebula had cooled. Just as in archaeology, we can trace a complex history in a single rock from space.