Читать книгу Exploring the Solar System - Peter Bond - Страница 26

1 A molecular cloud made up of gas and dust begins to collapse.

2 A protostar begins to form at the core of the collapsing nebula.

3 A disk‐shaped nebula of orbiting dust and gas develops in the protostar's equatorial plane.

4 Dust grains in the disk collide and merge.

5 Large (1 mm) dust grains fall into a thin, dusty sheet.

6 Collisions produce planetesimals 1 m to 1 km across.

7 More collisions between planetesimals produce planetary embryos.

8 Planetary embryos stir up the leftover planetesimals.

9 Planetesimals then collide and fragment.

10 A cascade of collisions reduces fragments to dust.

11 Planets sweep up some of the dust.

12 Radiation and a “wind” of charged particles from the central star remove the remaining gas and dust.

While the largest protoplanets continue to grow, the remaining rocky planetesimals grind each other into dust. Some of this dust is drawn in by the surviving planets, while much of the remainder is swept out of the Solar System when the Sun evolves into a hydrogen‐burning star. (A cloud of micron‐sized dust particles still exists in the ecliptic plane of the Solar System. Known as the zodiacal cloud, it is composed of silicate particles that are largely derived from collisions between main belt asteroids.)

One of the problems that must be solved by Solar System theorists is an explanation for the silicate and metal‐rich nature of the terrestrial planets and the dominance of hydrogen and helium in the outer planets (Box 1.4). Clearly, the marked difference in composition between the inner and outer planets must be related to the materials that made up different regions of the disk.

The dense, rocky nature of the Earth and its neighbors suggests that they simply formed through the accretion of dust grains in the solar nebula. However, studies of primitive chondritic meteorites show the presence of millimeter‐sized droplets (chondrules) that were once liquid.

It seems that, before they amalgamated to form the meteorites, these existed for a brief period as independent spheroids at temperatures above 1,500°C. Some chondrules seem to include other chondrules, indicative of being exposed to high temperatures on more than one occasion (see Chapter 13). The source of the heating is uncertain, although shock waves, solar heating, and collisions between planetesimals have been suggested.

Laboratory experiments indicate that these molten globules were cooled very rapidly, within 10 million years of the collapse of the molecular cloud. The cause of such sudden cooling events remains unclear. What does seem certain is that the chondrules and dust began to stick together and grow in size, creating chunks of chondritic material. Drag from gas in the nebula encouraged the pebble‐sized objects to creep inward, all the time gathering in more material.

Once a population of large planetesimals evolved, their destiny was determined largely by chance. A fast, head‐on collision caused the objects to break apart. A slow, gentler encounter enabled the participants to merge into an even larger object. In this way, the terrestrial planets grew to more or less their current size over a period of some 10 million years.

The huge amounts of kinetic energy dumped in the planets by frequent, massive impacts caused partial or total melting and the creation of magma oceans. This led to internal differentiation, with the denser elements, such as iron, sinking to the core and the lighter ones rising to the surface to create silicate crusts.

Early atmospheres were generated by outgassing of volatile molecules such as water, methane, ammonia, hydrogen, nitrogen, and carbon dioxide. A final heavy bombardment, which ended about 3.8 billion years ago, is clearly marked in the crater record of the Moon, and this has been applied to other planets and satellites.

Occasionally a satellite was created as the by‐product of a major impact. Such is thought to be the case with Earth and its Moon. Debris from an ancient collision between the young Earth and a Mars‐sized planetesimal created a ring of debris that eventually came together to form the Moon. A similar explanation has been put forward for the satellites of Mars and the Pluto‐Charon system (see Chapters 3 and 12).